First author: Belkhiri, L, The actinide-imide bonding revisited: A relativistic DFT study, JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 757, 155, (2005)
Abstract: The electronic structure of mono and bis(imide) cyclopentadienyl thorium and uranium complexes have been investigated using density functional theory (DFT) calculations in the framework of the relativistic zero order regular approximation (ZORA) implemented in the ADF (Amsterdam density functional) program. In the case of N-phenyl imido ligands, these complexes exhibit both sigma and pi interaction between the central metal atom and the three electron pairs borne by the nitrogen atom. These electron pairs interact strongly with the uranium orbitals, especially the 5f ones, insuring a great stability to these species. A triple bond is thus expected for the uranium-nitrogen interaction in these complexes. The geometry of the bis(imide) species is very peculiar. It is characterized by coplanar Nph phenylimide groups, very short U-N bond lengths and a linear U-N-C-ph coordination angle. These features could not be observed for a transition metal complex, as this has been demonstrated on the example of molybdenum Mo(VI) species which exhibit a bent Mo-N-C-ph coordination angle. We show that the 5f uranium orbitals are mainly responsible for this peculiar geometry.

First author: Liao, MS, Effects of peripheral substituents and axial ligands on the electronic, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 11996, (2005)
Abstract: The effects of peripheral substituents and axial ligands (L) on the electronic structure and properties of cobalt tetraphenylporphyrin (CoTPP) have been studied using DFT methods. Various density functionals were tested, and the ground state of each system was determined by considering several possible low-lying states. The ground states of the fully fluorinated CoTPPF28(L)(2) complexes with L == THF, Py, and Im were identified to be high-spin (E-4(g)) by the meta-GGA functional tau-HCTH, which contains the kinetic energy density tau, in agreement with experimental measurements. All the pure GGA functionals, including the recently developed mPBE, OPBE, and HCTH/407, show more or less overestimation of the relative energies of the high-spin states. The energy gap between the (2)A(1g) and E-4(g) states is insignificant (similar to 0.1 eV) and varies in the order L = Py < L = THF < L = Im. The results and their trend are consistent with F-19 NMR studies which show partial population of the E-4(g) state in CoTPPF28(THF)(2) and CoTPPF28(Py)(2) and a complete conversion to the high-spin state in CoTPPF28(1-Melm)(2). Upon coordination by two very strong field axial CO ligands, CoTPPF28(CO)(2) becomes low-spin, as in unligated (CoTPPFx. The influence of the peripheral substituents and axial ligands on the ionization potentials, electron affinities, and CoTPPFx-(L)(2) binding strength was also investigated in detail.

First author: Malkin, I, Relativistic two-component calculations of electronic g-tensors that include spin polarization,JOURNAL OF CHEMICAL PHYSICS, 123, 11996, (2005)
Abstract: The first two-component relativistic density-functional approach for the calculation of electronic g-tensors is reported that includes spin polarization using noncollinear spin-density functionals. The method is based on the relativistic Douglas-Kroll-Hess Hamiltonian and has been implemented into the ReSpect program package. Using three self-consistent-field calculations with orthogonal orientations of total magnetization J, the full g-matrix may be obtained. In contrast to previous spin-restricted two-component treatments, results with the new approach agree excellently with spin-polarized one-component calculations for light-atom radicals. Additionally, unlike one-component approaches, the method also reproduces successfully the negative Delta g(parallel to)-values of heavy-atom (2)Sigma radicals and the negative Delta g(perpendicular to) components in cysteinyl. The new method removes effectively the dilemma existing up to now regarding the simultaneous inclusion of spin polarization and higher-order spin-orbit effects in g-tensor calculations. It is straightforwardly applicable to higher than doublet spin multiplicities and has been implemented with hybrid functionals.

First author: Wang, FY, Controlling ligand substitution reactions of organometallic complexes: Tuning cancer cell cytotoxicity, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF, 102, 18269, (2005)
Abstract: Organometallic compounds offer broad scope for the design of therapeutic agents, but this avenue has yet to be widely explored. A key concept in the design of anticancer complexes is optimization of chemical reactivity to allow facile attack on the target site (e.g., DNA) yet avoid attack on other sites associated with unwanted side effects. Here, we consider how this result can be achieved for monofunctional “piano-stool” ruthenium(II) arene complexes of the type [(n(6)-arene)Ru(ethylenediamine)(X)](n+). A potentially important activation mechanism for reactions with biomolecules is hydrolysis. Density functional calculations suggested that aquation (substitution of X by H2O) occurs by means of a concerted ligand interchange mechanism. We studied the kinetics and equilibria for hydrolysis of 21 complexes, containing, as X, halides and pseudohalides, pyridine (py) derivatives, and a thiolate, together with benzene (bz) or a substituted bz as arene, using UV-visible spectroscopy, HPLC, and electrospray MS. The x-ray structures of six complexes are reported. In general, complexes that hydrolyze either rapidly {e.g., X = halide [arene = hexamethylbenzene (hmb)]} or moderately slowly [e.g., X = azide, dichloropyridine (arene = hmb)] are active toward A2780 human ovarian cancer cells, whereas complexes that do not aquate (e.g., X = py) are inactive. An intriguing exception is the X = thiophenolate complex, which undergoes little hydrolysis and appears to be activated by a different mechanism. The ability to tune the chemical reactivity of this class of organometallic ruthenium arene compounds should be useful in optimizing their design as anticancer agents.

First author: Garda, GR, Chemisorption of NCO on Cu(100): A density functional theory study, SURFACE SCIENCE, 598, 57, (2005)
Abstract: The isocyanate group adsorption on sites of different coordination of Cu(100) was theoretically studied considering the cluster approach. The site of four-fold symmetry is the most favored. When NCO adsorbs on Cu, it charges negatively. This electron transfer from the substrate is greater for the site of lowest coordination. The projected DOS curves for the most important valence molecular orbitals of isocyanate group indicate a strong mixing between its 2 pi orbital and 3d(xz) and 3d(yz) AOs of Cu. The predicted asymmetric mode at 2187 cm(-1) for NCO adsorbed on the hollow site agrees very well with the experimental observed values of 2162-65 cm(-1).

First author: Harding, CJ, Circular dichroism in the angle-resolved C 1s photoemission spectra of gas-phase carvone enantiomers, JOURNAL OF CHEMICAL PHYSICS, 123, 57, (2005)
Abstract: The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the (C) under bar = O and (C) under barH(2)= carbon 1s orbitals were determined to be 292.8 ± 0.2 and 289.8 ± 0.2 eV, respectively. The remaining (C) under bar – H C 1s levels substantially overlap nder an intense central peak centered at 290.5 ± 0.2 eV. The angle-resolved photoemission from the carbonyl carbon (C) under bar = O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of similar to 6% at the magic angle of 54.7 to the light beam propagation direction. This corresponds to an expected 0 degrees – 180 degrees forward-backward electron emission asymmetry of similar to 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.

First author: Shamov, GA, Density functional studies of actinyl aquo complexes studied using small-core effective core potentials and a scalar four-component relativistic method, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 10961, (2005)
Abstract: The title compounds, [AnO(2)(H2O)5](n+), n = 1 or 2 and An = U, Np, and Pu, are studied using relativistic density functional theory (DFT). Three rather different relativistic methods are used, small-core effective core potentials (SC-ECP), a scalar four-component all-electron relativistic method, and the zeroeth-order regular approximation. The methods provide similar results for a variety of properties, giving confidence in their accuracy. Spin-orbit and multiplet corrections to the An(VI)/An(V) reduction potential are added in an approximate fashion but are found to be essential. Bulk solvation effects are modeled with continuum solvation models (CPCM, COSMO). These models are tested by comparing explicit (cluster), continuum, and mixed cluster/ continuum solvation models as applied to various properties. The continuum solvation models are shown to accurately account for the effects of the solvent, provided that at least the first coordination sphere is included. Reoptimizing the structures in the presence of the bulk solvent is seen to be important for the equatorial bond lengths but less relevant for energetics. Explicit inclusion of waters in the second coordination sphere has a modest influence on the energetics. For the first time, free energies of solvation are calculated for all six [AnO(2)(H2O)(5)](n+) species. The calculated numbers are within the experimental error margins, and the experimental trend is reproduced correctly. By comparison of different relativistic methods, it is shown that an accurate relativistic description leads to marked improvements over the older large-core ECP (LC-ECP) method for bond lengths, vibrational frequencies, and, in particular, the An(VI)/An(V) reduction potential. Two approximate DFT methods are compared, B3LYP, a hybrid DFT method, and PBE, a generalized gradient approximation. Either method yields An(VI)/An(V) reduction potentials of comparable quality. Overall, the experimental reduction potentials are accurately reproduced by the calculations.

First author: Goumans, TPM, Toward the catalytic synthesis of phosphiranes. A computational study, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 690, 5517, (2005)
Abstract: Reaction pathways for the formation of zirconocene phosphinidene complex Cp2Zr(PR3)=PR from Cp2ZrCl2 and LiH and LiPRH and its reactivity to 1,2-dichloroethane are explored with density functional theory using model structures that are devoid of substituents. After the initial Cp2Zr(Cl)PH2 is generated with LiPH2 reaction with LiH is likely to eliminate HCl in a single step to give directly the 16-electron complex Cp2Zr=PH, which is stabilized by the PH3 phosphine ligand. The intermediate formation of a phosphine hydride complex, Cp2Zr(H)PH2 resulting from hydride substitution, is unlikely both on the basis of unfavorable reaction energies and calculated P-31 NMR chemical shifts that indicate that such a species cannot have been observed experimentally. It is likely that a diphosphine complex, Cp2Zr(PH2)(2), results on using an excess of the lithium phosphide, which on H-transfer gives directly the phosphine-stabilized phosphinidene complex. The reactivity of this species is dominated by the release of its stabilizing phosphine ligand to give a highly reactive 16-electron phosphinidene complex, Cp2Zr=PH, which reacts with 1,2-dichloroethane after coordination to one of the chlorine atoms in two asynchronous metathesis steps to the three-membered phosphirane ring. In this process, ZrCl2 is reformed enabling its recycling to regenerate the phosphinidene complex. This study highlights the special reactivity of the 16-electron Cp2Zr=PH and suggests that related complexes may be generated similarly, thereby expanding the synthetic potential of these nucleophilic reagents.

First author: Badiei, YM, Electronic structure and electrophilic reactivity of discrete copper diphenylcarbenes, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 690, 5989, (2005)
Abstract: The beta-diketiminato Cu(I) arene adduct {[Me3NN]Cu}(2)(mu-toluene) (3) is prepared in 62% isolated yield by addition of the neutral beta-diketimine H(Me3NN] to copper t-butoxide in toluene. An X-ray structure of 3 shows that the bridging toluene ligand exhibits eta(2)- bonding to each Cu center via four contiguous C atoms. Reaction of the dicopper 3 with 1 equiv. N2CPh2 provides {[Me3NN]-Cu }2(mu-CPh2) (4) as purple crystals in 70% isolated yield. Dicopper carbene 4 possesses a Cu-Cu distance of 2.485(l) A in the solid state and dissociates a [Me3NN]Cu fragment in arene solvents to provide low concentrations Of [Me3NN]Cu=CPh2 (2) and (Me3NN]Cu(arene). DFT calculations performed on terminal carbene 2 and dicopper carbene 4 illustrate relationships between these two bonding modes and suggest electrophilic reactivity at the carbene carbon atom bound to Cu. Dicopper carbene 4 undergoes efficient carbene transfer to HC&3bond; CPh and PPh3 resulting in the formation of 1,3,3-triphenylcyclopropene and Ph(3)p=CPh2 while reaction with the isocyanide CNAr (Ar = 2,6-Me2C6H3) results in loss of the carbene as Ph2C=CPh2. In each case, the [Me3NN]Cu fragment is trapped by the incoming nucleophile as the three-coordinate [Me3NN]Cu(L). Reaction of 4 with 0, rapidly generates benzophenone and {[Me3NN]Cu}(2)(mu-OH)(2)

First author: Jacobsen, H, Bonding aspects of P-heterocyclic carbene transition metal complexes. A computational assessment, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 690, 6068, (2005)
Abstract: Gradient-corrected (BP86) density-functional calculations were used to study the chemical bond between transition-metal complexes and N- and P-heterocyclic carbenes EHC (EHC = imidazolin-2-ylidene; 1,3-dimethylimidazolin-2-ylidene; 1,3-dihydro-1,3-diphosphol-2-ylidene; 1,3-dimethyl-1,3-diphosphol-2-ylidene). Forty two complexes of the type [M] <- EHC, [M] = CuCl, AgCl, AuCl, BeCl+, Cu+, Ag+, Au+, have been studied. Both electrostatic contributions as well as pi-back-donation are of special importance for the [M] <- EHC bond. The metal-ligand bond strengths are comparable for NHC and PHC complexes. Whereas the former undergo stronger electrostatic interactions, the latter show a higher degree of pi-bonding. When considering NHC and PHC as ligands for transition-metal-based catalysts, the results of the present study suggest that PHC both compete with NHC – in terms of metal-to-ligand bond strength – as well as complement NHC – in terms of the nature of the metal ligand bond.

First author: Frenking, G, Chemical bonding in transition metal carbene complexes, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 690, 6178, (2005)
Abstract: In this work, we summarize recent theoretical studies of our groups in which modern quantum chemical methods are used to gain insight into the nature of metal-ligand interactions in Fischer- and Schrock-type carbene complexes. It is shown that with the help of charge- and energy-partitioning techniques it is possible to build a bridge between heuristic bonding models and the physical mechanism which leads to a chemical bond. Questions about the bonding situation which in the past were often controversially discussed because of vaguely defined concepts may be addressed in terms of well defined quantum chemical expressions. The results of the partitioning analyses show that Fischer and Schrock carbenes exhibit different bonding situations, which are clearly revealed by the calculated data. The contributions of the electrostatic and the orbital interaction are estimated and the strength of the sigma donor and pi acceptor bonding in Fischer complexes are discussed. We also discuss the bonding situation in complexes with N,N-heterocyclic carbene ligands.

First author: Yan, LK, Density functional theory study on the first hyperpolarizabilities of organoimido derivatives of hexamolybdates, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 22332, (2005)
Abstract: The first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of arylimido molybdate derivatives have been investigated by density functional theory (DFT). The molecular orbital character analysis reveals that organoimido-to-polyanion charge transfer may be responsible for the NLO properties of this kind of molybdate derivatives. The NLO study shows intra-ion charge transfer is helpful to increase the first hyperpolarizability of arylimido molybdate derivatives. The lengthening of organoirrrido pi-conjugation enhances the beta(vec) value. System 4 has the largest beta(vec) value at the static electronic field, 1.238 x 10(-27)esu. Orbital analysis shows that the degree of charge transfer between polyanion cluster and organic segment was increased when the second organoimido polyanion was introduced. The present investigation provides important insight into NLO origin and properties of polyanion arylimido molybdate derivatives.

First author: Gahungu, G, Molecular geometry, electronic structure and optical properties study of meridianal tris(8-hydroxyquinolinato)gallium(III) with ab initio and DFT methods, JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM,755, 19, (2005)
Abstract: We have investigated the ability of a number of chemical models to describe the geometry and optical properties of meridianal tris(8-hydroxyquinolinato)gallium(III) mer-Gaq3. It was found that the flexible basis set with diffuse and polarized orbitals (6-31+G*), in combination with electron correlation (DFTB3LYP) is required for an accurate description of the molecular geometry for the ground state. The best agreement between simulated and experimental spectra, both in terms of maximum absorption and emission wavelengths (respectively. 426 and 531 nm versus experimental values of similar to 393 and similar to 526 nm) was achieved with TD-B3LYP/3-21G* and that for the Stokes Shift (122 versus 133 nm) achieved using TD-B3LYP/6-31 +G*. Furthermore, the Al/Ga substitution effects on the optical properties were ascribed to the induced changes in the geometrical parameters involving the metal center.

First author: Brooks, AJ, Co-C bond activation in methylmalonyl-CoA mutase by stabilization of the post-homolysis product CO2+ cobalamin, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 16522, (2005)
Abstract: Despite decades of research, the mechanism by which coenzyme B-12 (adenosylcobalamin, AdoCbl)-dependent enzymes promote homolytic cleavage of the cofactor’s Co-C bond to initiate catalysis has continued to elude researchers. In this work, we utilized magnetic circular dichroism spectroscopy to explore how the electronic structure of the reduced B12 cofactor (i.e., the post-homolysis product Co2+CbI) is modulated by the enzyme methylmalonyl-CoA mutase. Our data reveal a fairly uniform stabilization of the Co 3d orbitals relative to the corrin pi/pi*-based molecular orbitals when Co2+CbI is bound to the enzyme active site, particularly in the presence of substrate. Contrastingly, our previous studies (Brooks, A. J.; Vlasie, M.; Banerjee, R.; Brunold, T. C. J. Am. Chem. Soc. 2004, 126, 8167-8180.) showed that when AdoCbl is bound to the MMCM active site, no enzymatic perturbation of the CO(3+)Cbl electronic structure occurs, even in the presence of substrate (analogues). Collectively, these observations provide direct evidence that enzymatic Co-C bond activation involves stabilization of the post-homolysis product, CO2+- Cbl, rather than destabilization of the CO(3+)Cbl “ground” state.

First author: Tangen, E, The challenge of being straight: Explaining the linearity of a low-spin {FeNO}(7) unit in a tropocoronand complex, INORGANIC CHEMISTRY, 44, 8699, (2005)
Abstract: We have carried out a density functional theory study of the S = 1/2 {FeNO}(7) tropocoronand complex, Fe(5,5-TC)NO, as well as of some simplified models of this compound. The calculations accurately reproduce the experimentally observed trigonal-bipyramidal geometry of this complex, featuring a linear NO in an equatorial position and a very short Fe-N-NO distance. Despite these unique structural features, the qualitative features of the bonding turn out to be rather similar for Fe(5,5-TC)NO and {FeNO}(7) porphyrins. Thus, there is a close correspondence between the molecular orbitals (MOs) in the two cases. However, there is a critical, if somewhat subtle, difference in the nature of the singly occupied MOs (SOMOs) between the two. For square-pyramidal heme-NO complexes, the SOMO is primarily Fe d(z)(2)-based, which favors sigma-bonding interactions with an NO pi* orbital, and hence a bent FeNO unit. However, for trigonal-bipyramidal Fe(5,5-TC)(NO), the SOMO is best described as primarily Fe d(x)(2)-(2)(z) in character, with the Fe-N-NO vector being identified as the z direction. Apparently, such a d orbital is less adept at sigma bonding with NO and, as such, pi bonding dominates the Fe-NO interaction, leading to an essentially linear FeNO unit and a short Fe-N-NO distance.

First author: Pietrzyk, P, Relativistic density functional calculations of EPR g tensor for eta(1){CuNO}(11) species in discrete and zeolite-embedded states, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 10571, (2005)
Abstract: Spin-unrestricted zeroth order regular approximation (ZORA) and the scalar relativistic method based on Pauli Hamiltonian implemented in the Amsterdam Density Functional suite were used to calculate the electronic g tensor for isolated covalent {CuNO}(11) and electrostatic {q-NO}(1) species and for various model molecular and nonmolecular {CuNO}(11)-containing systems, epitomizing copper nitrosyl cage adducts in the ZSM-5 zeolite. The predicted g tensor values using the ZORA/VWN scheme were in satisfactory agreement with experimental EPR results. Relativistic, diamagnetic, and paramagnetic contributions to the calculated g tensor were quantified. The nature of the observed Delta g shifts was discussed in terms of the molecular orbital contributions due to the magnetic field-induced couplings and their structure sensitivity. The influence of basis set and exchange-correlation functional on the results was also briefly evaluated.

First author: Zein, S, Energetics of binuclear spin transition complexes, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 16197, (2005)
Abstract: The electronic structures of five binuclear iron(II) complexes, four of which display spin transitions between the low-spin (LS) and high-spin (HS) electronic states, are studied by density functional theory (DFT) calculations. Three electronic states, corresponding to [LS-LS], [LS-HS], and [HS-HS] electronic configurations, are characterized. The nature of the ground state agrees with the experimentally observed magnetic state of complexes stabilized at low temperatures. The results of the calculations agree with the conclusion of the phenomenological model, that the enthalpy of the [LS-HS] state must be lower than the average enthalpy of the [LS-LS] and [HS-HS] states, to create conditions for a two-step spin transition. The exchange parameters between Fe(II) ions in the [HS-HS] states are evaluated. It is shown that all complexes are weakly antiferromagnetic and the synergy between two spin transition centers is mainly of elastic origin.

First author: Brooks, AJ, Electronic structure studies of the adenosylcobalamin cofactor in glutamate mutase,BIOCHEMISTRY, 44, 15167, (2005)
Abstract: Glutamate mutase (GM) is a cobalamin-dependent enzyme that catalyzes the reversible interconversion of L-glutamate and L-threo-3-methylaspartate via a radical-based mechanism. To initiate catalysis, the 5′-deoxyadenosylcobalamin (AdoCbl) cofactor’s Co-C bond is cleaved homolytically to generate an adenosyl radical and Co(2+)Cbl. In this work, we employed a combination of spectroscopic and computational tools to evaluate possible mechanisms by which the Co-C bond is activated for homolysis. Minimal perturbations to the electronic absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectra of AdoCbl are observed upon formation of holoenzyme, even in the presence of substrate (or a substrate analogue), indicating that destabilization of the Co(3+)Cbl “ground state” is an unlikely mechanism for Co-C bond activation. In contrast, striking alterations are observed in the spectroscopic data of the post-homolysis product Co(2+)Cbl when bound to glutamate mutase in the presence of substrate (or a substrate analogue) as compared to unbound Co(2+)Cbl. These enzymatic perturbations appear to most strongly affect the metal-to-ligand charge-transfer transitions of Co(2+)Cbl. suggesting that the cofactor/active-site interactions give rise to a fairly uniform stabilization of the Co 3d orbitals. Remarkable similarities between the results obtained in this study and those reported previously for the related Cbl-dependent isomerase methylmalonyl-CoA mutase indicate that a common mechanism by which the cofactor’s Co-C bond is activated for homolytic cleavage may be operative for all base-off/His-on Cbl-dependent isomerases.

First author: Ghosh, A, Norcorrole and dihydronorcorrole: A predictive quantum chemical study, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 44, 4479, (2005)
Abstract: DFT calculations indicate norcorrole (NCH2), a 16 pi-electron doubly contracted porphyrin, to be a promising synthetic tar(jet, especially when complexed to a small cation such as Ni-II. The relative stability of norcorrole derivatives appears to reflect the fact that they are not antiaromatic, but should rather be viewed as bis(dipyrrins). However, norcorrole (NC) complexes are expected to be prone to reduction and we predict that high-valent transition metal dihydronorcorrole (DHNC) derivatives should exist as reasonably stable substances. Our calculations further predict that both NC and DHNC complexes should exhibit short metal-nitrogen distances and moderate to strong doming of the macrocycle. Overall, a number of calculated results suggest that NC and DHNC derivatives should be of unique electronic-structural interest, as conceptual links between aromatic tetrapyrroles such as porphyrin and corrole on the one hand and open-chain pyrrole-based ligands such as dipyrrins and biliverdines on the other.

First author: Han, WG, Active site structure of class I ribonucleotide reductase intermediate X: A density functional theory analysis of structure, energetics, and spectroscopy, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 15778, (2005)
Abstract: Several models for the active site structure of class I ribonucleotide reductase (RNR) intermediate X have been studied in the work described in this paper, using broken-symmetry density functional theory (DFT) incorporated with the conductor-like screening (COSMO) solvation model. The calculated properties, including geometries, spin states, Fe-57, H-1, and O-17 hyperfine tensors, Mossbauer isomer shifts, and quadrupole splittings, and the estimation of the Fe(IV) d-d transition energies have been compared with the available experimental values. On the basis of the detailed analysis and comparisons, we propose a definite form for the active site structure of class I RNR intermediate X, which contains an Fe1(III)Fe2(IV) center (where Fe1 is the iron site closer to Tyr122, and the two iron sites are high-spin antiferromagnetically coupled to give a total (1)/(2) net spin), two mu-oxo bridges, one terminal water which binds to Fe1(III) and also H-bonds to both side chains of Asp84 and Glu238, and one bidentate carboxylate group from the side chain of Glu115.

First author: Vargas, A, DFT and ATR-IR insight into the conformational flexibility of cinchonidine adsorbed on platinum: Proton exchange with metal, JOURNAL OF CATALYSIS, 236, 1, (2005)
Abstract: The adsorption mode of cinchonidine on platinum is discussed in the light of new computational studies in which the alkaloid is adsorbed on a lame metal cluster. Previous computations focused on the role of 1(S)-(4-quinolinyl)ethanol as anchoring group, but in the present study the role of the quinuclidine ring is analyzed, leading to a consistent view of the roles of both moieties of the alkaloid in the adsorption process. The tertiary nitrogen of the quinuclidine moiety can participate in the anchoring of the alkaloid and can be protonated by surface hydrogen. The conformational flexibility of the quinuclidine moiety was investigated by attenuated total reflection infrared experiments under nearly in Situ conditions, by comparing the adsorption behavior of cinchonidine and O-phenyl-cinchonidine on platinum. Close agreement was found between experimental observations and theoretical calculations. A mechanism is proposed whereby the tertiary nitrogen can promote charge polarization of hydrogen and its transfer to the substrate.

First author: Wang, F, Theoretical study of the electronic spectra of square-planar platinum(II) complexes based on the two-component relativistic time-dependent density-functional theory, JOURNAL OF CHEMICAL PHYSICS, 123, 1, (2005)
Abstract: In the present work the electronic spectra of [PtCl4](2-), [PtBr4](2-), and [Pt(CN)(4)](2-) are studied with a recently proposed relativistic time-dependent density-functional theory (TDDFT) based on the two-component zeroth-order regular approximation and a noncollinear exchange-correlation (XC) functional. The contribution to the double group excited states in terms of singlet and triplet single group excited states is estimated through the inner product of the transition density matrix obtained from two-component and scalar relativistic TDDFT calculations to better understand the double group excited states. Spin-orbital coupling effects are found to be very important in order to simulate the electronic spectra of these complexes. The results show that the two-component TDDFT formalism can afford excitation energies with high accuracy for the transition-metal systems studied here when use is made of a proper XC potential.

First author: Bento, AP, Ab initio and DFT benchmark study for nucleophilic substitution at carbon (S(N)2@C) and Silicon (S(N)2@Si), JOURNAL OF COMPUTATIONAL CHEMISTRY, 26, 1497, (2005)
Abstract: To obtain a set of consistent benchmark potential energy surfaces (PES) for the two archetypal nucleophilic substitution reactions of the chloride anion at carbon in chloromethane (S(N)2@C) and at silicon in chlorosilane (S(N)2@Si), we have explored these PESes using a hierarchical series of ab initio methods [HF, MP2, MP4SDQ, CCSD, CCSD(T)] in combination with a hierarchical series of six Gaussian-type basis sets, up to g polarization. Relative energies of stationary points are converged to within 0.01 to 0.56 kcal/mol as a function of the basis-set size. Our best estimate, at CCSD(T)CCSD(T)/aug-cc-pVQZ, for the relative energies of the [Cl-, CH3Cl] reactant complex, the [Cl-CH3-Cl](-) transition state and the stable [Cl-SiH3-Cl](-) transition complex is -10.42, +2.52, and -27.10 kcal/mol, respectively. Furthermore, we have investigated the performance for these reactions of four popular density functionals, namely, BP86, BLYP, B3LYP, and OLYP, in combination with a large doubly polarized Slater-type basis set of triple-zeta quality (TZ2P). Best overall agreement with our CCSD(T)/aug-cc-pVQZ benchmark is obtained with OLYP and B3LYP. However, OLYP performs better for the S(N)2@C overall and central barriers, which it underestimates by 2.65 and 4.05 kcal/mol, respectively. The other DFT approaches underestimate these barriers by some 4.8 (B3LYP) to 9.0 kcal/mol (BLYP).

First author: Flisak, Z, DFT study of ethylene and propylene copolymerization over a heterogeneous catalyst with a coordinating Lewis base, MACROMOLECULES, 38, 9865, (2005)
Abstract: The copolymerization of ethylene and propylene over a heterogeneous Ti(III) catalyst containing tetrahydrofuran (THF) as a Lewis base and MgCl2 as a support has been studied by means of DFT. Two feasible models of active sites have been examined thoroughly, and one of them turned out to be favorable in terms of both catalytic activity and the microstructure of the resulting polymer. The external barriers of olefin insertion for this model range from 3.1 to 16.0 kcal/mol and are influenced by a variety of factors, such as the structure of the growing polymer chain and the nature of the incoming olefin as well as the orientation of the ligands around the titanium atom. Stochastic simulations performed on the basis of insertion and termination barriers provided us with the insight into the composition and microstructure of the copolymer as well as its molecular weight as a function of comonomer partial pressures. It is demonstrated that the reactivity of ethylene in the copolymerization process is significantly higher than that of propylene, which is consistent with known experimental data. Our results also indicate moderate regiospecificity and stereoselectivity toward propylene, depending on the partial pressures of the comonomers.

First author: Stephens, FH, Mechanism of white phosphorus activation by three-coordinate Molybdenum(III) complexes: A thermochemical, kinetic, and quantum chemical investigation, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY,127, 15191, (2005)
Abstract: White phosphorus (P-4) reacts with three-coordinate molybdenum(III) trisamides or molybdaziridine hydride complexes to produce either bridging or terminal phosphide (P3-) species, depending upon the ancillary ligand steric demands. Thermochemical measurements have been made that place the Mo equivalent to P triple bond dissociation enthalpy at 92.2 kcal center dot mol(-1). Thermochemical measurements together with computational analysis rule out simple P-atom abstraction from P-4 as a step in the phosphorus activation mechanism. Kinetic measurements made by the stopped-flow method show that the reaction between the monomeric molybdenum complexes and P-4 is first-order both in metal complex and in P-4. cyclo-P-3 complexes can be obtained when ancillary ligand steric demands are small, but kinetic measurements rule them out as monometallic intermediates in the P-4 activation mechanism. Also studied by calorimetric, kinetic, and in one case variable-temperature NMR methods is the process of p-phosphide bridge formation. Post-rate-determining steps of the P-4 activation process were examined in a search for minima on the reaction’s potential energy surface, leading to the proposal of two plausible, parallel, bimetallic reaction channels.

First author: Berger, JA, A physical model for the longitudinal polarizabilities of polymer chains, JOURNAL OF CHEMICAL PHYSICS, 123, 15191, (2005)
Abstract: The aim of this work is to provide a physical model to relate the polarizability per unit cell of oligomers to that of their corresponding infinite polymer chains. For this we propose an extrapolation method for the polarizability per unit cell of oligomers by fitting them to a physical model describing the dielectric properties of polymer chains. This physical model is based on the concept of a dielectric needle in which we assume a polymer chain to be well described by a cylindrically shaped nonconducting rod with a radius much smaller than its length. With this model we study in which way the polarizability per unit cell approaches the limit of the infinite chain. We show that within this model the macroscopic contribution of the induced electric field to the macroscopic electric field vanishes in the limit of an infinite polymer chain, i.e., there is no macroscopic screening. The macroscopic electric field becomes equal to the external electric field in this limit. We show that this identification leads to a relation between the polarizability per unit cell and the electric susceptibility of the infinite polymer chain. We test our dielectric needle model on the polarizability per unit cell of oligomers of the hydrogen chain and polyacetylene obtained earlier using time-dependent current-density-functional theory in the adiabatic local-density approximation and with the Vignale-Kohn functional. We also perform calculations using the same theory on truly infinite polymer chains by employing periodic boundary conditions. We show that by extrapolating the oligomer results according to our dielectric needle model we get good agreement with our results from calculations on the corresponding infinite polymer chains.

First author: Jacob, CR, Orbital-free embedding applied to the calculation of induced dipole moments in CO2 center dot center dot center dot X (X=He, Ne, Ar, Kr, Xe, Hg) van der Waals complexes, JOURNAL OF CHEMICAL PHYSICS, 123, 15191, (2005)
Abstract: The orbital-free frozen-density embedding scheme within density-functional theory [T. A. Wesolowski and A. Warshel, J. Phys. Chem. 97, 8050 (1993)] is applied to the calculation of induced dipole moments of the van der Waals complexes CO(2)center dot X (X=He, Ne, Ar, Kr, Xe, Hg). The accuracy of the embedding scheme is investigated by comparing to the results of supermolecule Kohn-Sham density-functional theory calculations. The influence of the basis set and the consequences of using orbital-dependent approximations to the exchange-correlation potential in embedding calculations are examined. It is found that in supermolecular Kohn-Sham density-functional calculations, different common approximations to the exchange-correlation potential are not able to describe the induced dipole moments correctly and the reasons for this failure are analyzed. It is shown that the orbital-free embedding scheme is a useful tool for applying different approximations to the exchange-correlation potential in different subsystems and that a physically guided choice of approximations for the different subsystems improves the calculated dipole moments significantly.

First author: Jensen, L, Theory and method for calculating resonance Raman scattering from resonance polarizability derivatives, JOURNAL OF CHEMICAL PHYSICS, 123, 15191, (2005)
Abstract: We present a method to calculate both normal Raman-scattering (NRS) and resonance Raman-scattering (RRS) spectra from the geometrical derivatives of the frequency-dependent polarizability. In the RRS case, the polarizability derivatives are calculated from resonance polarizabilities by including a finite lifetime of the electronic excited states using time-dependent density-functional theory. The method is a short-time approximation to the Kramers, Heisenberg, and Dirac formalism. It is similar to the simple excited-state gradient approximation method if only one electronic excited state is important, however, it is not restricted to only one electronic excited state. Since the method can be applied to both NRS and RRS, it can be used to obtain complete Raman excitation profiles. To test the method we present the results for the S-2 state of uracil and the S-4, S-3, and S-2 states of pyrene. As expected, the results are almost identical to the results obtained from the excited-state gradient approximation method. Comparing with the experimental results, we find in general quite good agreement which enables an assignment of the experimental bands to bands in the calculated spectrum. For uracil the inclusion of explicit waters in the calculations was found to be necessary to match the solution spectra. The calculated resonance enhancements are on the order of 10(4) – 10(6), which is in agreement with experimental findings. For pyrene the method is also able to distinguish between the three different electronic states for which experimental data are available. The neglect of anharmonicity and solvent effects in the calculations leads to some discrepancy between theory and experiment.

First author: Green, JC, Electronic structure of M(BH4)(4), M = Zr, Hf, and U, by variable photon-energy photoelectron spectroscopy and density functional calculations, INORGANIC CHEMISTRY, 44, 7781, (2005)
Abstract: Photoelectron (PE) spectra have been obtained for the M(BH4)(4) (M = Zr, Hf and U) molecules in the 20-60 eV photon-energy range, and for M = U, also in the 90-120 eV region. Derived branching ratios (BR) and relative parlial-photoionization cross sections (RPPICS) of the valence bands are used to confirm band assignment and demonstrate d-orbital covalency for all three compounds and f-orbital covalency for U(BH4)(4). Core ionizations are identified and used to confirm resonance features in the RPPICS. The absorption spectrum of Zr(BH4)4 between 20 and 60 eV shows 4p absorption at 35.5 eV, coincident with the 4p-4d resonance in the RPPICS of the I e and 2t(2) ionization bands of Zr(BH4)4. Less intense absorption bands at 32.5 and 33.8 eV correspond with shape resonance features in the la, and 1t(2) PE bands. The RPPICS of the f band of U(BH4)(4) shows two strong resonant features between 95 and 120 eV. Direct photoemission of the 5f electrons from U(BH4)4 results only in the observation of the F-2(5/2) ion state of [U(BH4)(4)](+), but in the 5d-5f resonant region, a weak band corresponding to the F-2(7/2) ion state is also observed. The splitting of the 1t2 band of U(BH4)4 is attributed to a small contribution of U 6p semi-core electrons to this MO. Density functional calculations give a good estimate of the pattern of ionization energies, although the calculated absolute values are lower than the experimental values, the first IE by 0.5 eV for Zr(BH4)(4) and Hf(BH4)(4) and 1.0 eV for U(BH4)(4). The MO compositions are in very good agreement with the deductions made from the BR and RPPICS analyses.

First author: Tangen, E, Toward modeling H-NOX domains: A DFT study of heme-NO complexes as hydrogen bond acceptors, INORGANIC CHEMISTRY, 44, 7802, (2005)
Abstract: Density functional theory calculations (PW91/STO-TZP, including basis-set superposition error corrections) have been used to evaluate hydrogen bond energies of five- and six-coordinate heme-NO complexes with phenol and imidazole, chosen as models for distal pocket tyrosine and histidine residues, The calculated interaction energies are approximately 2 kcal/mol for phenol and 3-4 kcal/mol for imidazole, which are 2-4 times smaller than the energies calculated for heme-O-2 complexes hydrogen-bonding with a distal histidine. Interestingly, the hydrogen bond energies are found to be very similar for five- and six-coordinate heme-NO complexes, which may be viewed as contrary to the interpretation of a recent observation on a bacterial H-NOX (Heme-Nitric oxide/OXygen-binding) protein with sequence homology to mammalian-soluble guanylate cyclase.

First author: Minkin, VI, Spectroscopic and theoretical evidence for the elusive intermediate of the photoinitiated and thermal rearrangements of photochromic spiropyrans, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 9605, (2005)
Abstract: Spectral properties and photochromic behavior of a series of novel 1′,3′,3′-trimethyl-1,2-tetramethylenespiro-[7H-furo(3,2-f)-(2H-1)-benzop yran-7,2′-indolines] 1-4 have been studied. The mechanism of the photoinitiated ring-opening reaction involves the formation of an acoplanar cis-cisoid intermediate, the lifetime of which in the case of 6-(tert-butyl) derivative 4 is long enough to observe its absorption and fluorescence spectra under conditions of continuous irradiation. The occurrence of the intermediate on the reaction paths of the thermal and photochemical ring-opening processes has been also shown by the DFT and CIS calculations. The TD-B3LYP/6-31G**//HF/6-31G** calculated spectrum of the intermediate well matches that observed experimentally. For spiropyran 3 with a 6-NO2 group, kinetic and activation parameters of the photoinitiated coloration and dark bleaching reactions have been determined.

First author: de Jong, GT, Oxidative addition of the fluoromethane C-F bond to Pd. An ab initio benchmark and DFT validation study, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 9685, (2005)
Abstract: We have computed a state-of-the-art benchmark potential energy surface (PES) for two reaction pathways (oxidative insertion, OxIn, and S(N)2) for oxidative addition of the fluoromethane C-F bond to the palladium atom and have used this to evaluate the performance of 26 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing these reactions. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods (HF, MP2, CCSD, CCSD(T)) in combination with a hierarchical series of seven Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -5.3 (-6.1) kcal/mol for the formation of the reactant complex, 27.8 (25.4) kcal/mol for the activation energy for oxidative insertion (OxIn) relative to the separate reactants, 37.5 (31.8) kcal/mol for the activation energy for the alternative S(N)2 pathway, and -6.4 (-7.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.4-2.7 kcal/mol and errors in activation energies ranging from 0.3 to 2.8 kcal/mol. The B3LYP functional compares very well with a slight underestimation of the overall barrier for OxIn by -0.9 kcal/mol. For comparison, the well-known BLYP functional underestimates the overall barrier by -10.1 kcal/mol. The relative performance of these two functionals is inverted with respect to previous findings for the insertion of Pd into the C-H and C-C bonds. However, all major functionals yield correct trends and qualitative features of the PES, in particular, a clear preference for the OxIn over the alternative S(N)2 pathway.

First author: Senthilkumar, K, Absolute rates of hole transfer in DNA, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 14894, (2005)
Abstract: Absolute rates of hole transfer between guanine nucleobases separated by one or two A:T base pairs in stilbenedicarboxamide-linked DNA hairpins were obtained by improved kinetic analysis of experimental data. The charge-transfer rates in four different DNA sequences were calculated using a density-functional-based tight-binding model and a semiclassical superexchange model. Site energies and charge-transfer integrals were calculated directly as the diagonal and off-diagonal matrix elements of the Kohn-Sham Hamiltonian, respectively, for all possible combinations of nucleobases. Taking into account the Coulomb interaction between the negative charge on the stilbenedicarboxamide linker and the hole on the DNA strand as well as effects of base pair twisting, the relative order of the experimental rates for hole transfer in different hairpins could be reproduced by tight-binding calculations. To reproduce quantitatively the absolute values of the measured rate constants, the effect of the reorganization energy was taken into account within the semiclassical superexchange model for charge transfer. The experimental rates could be reproduced with reorganization energies near 1 eV. The quantum chemical data obtained were used to discuss charge carrier mobility and

First author: Gudat, D, On the borderline between cis and trans in organometallic (phosphane)platinum(II) complexes,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 127, 4056, (2005)
Abstract: The borderline between the cis and trans configurations in square-planar diarylplatinum(ii) complexes with triethylphosphane ligands [Pt(Ar)(2)(PEt3)(2)] [Ar = 2,4,6-trimethylphenyl (mesityl), 2,6-dimethylphenyl, 2-methylphenyl, 3-methylphenyl, 4-methylphenyl and phenyl] has been investigated by a combination of multinuclear (H-1, C-13, P-31 and Pt-195) NMR spectroscopy, X-ray crystallography and quantum chemical (DFT) calculations. When formed under thermodynamic conditions, the complexes show a clear cutoff between cis (tolyl and phenyl complexes) and trans (2,6-xylyl and mesityl complexes). The syn and anti stereoisomers were identified for the 2-methylphenyl and 3-methylphenyl derivatives. Calculated data (structural isomers or NMR) are in excellent agreement with experimental findings.

First author: Bucknum, MJ, A chemically intuitive proposal for the structure of n-diamond, MOLECULAR PHYSICS, 103, 2707, (2005)
Abstract: Structural analysis on carbonaceous samples produced from high temperature – pressure conditions by Palatnik et al. in 1984 indicate the existence of a metallic allotrope of carbon with a diffraction pattern closely matching that of cubic diamond. A structure proposed for this phase by Konyashin et al. and others suggests that the four interstitial carbon atoms occupying 1/2 the tetrahedral holes in the ordinary cubic diamond lattice are vacant in the new structure. This leads to a transformation of the Fd3m, ordinary cubic diamond structure- type, to a simple face centred cubic carbon lattice in space group Fm3m, with a lattice parameter of 3.56 angstrom, identical with that of cubic diamond. The new structure supports the diffraction evidence accumulated for this so-called n-diamond’ phase, but does not hold up to a first principles total energy optimization at the DFT level of theory for the fcc lattice, which reports the Konyashin et al. structure to be unstable. Here we report an alternative tetragonal carbon structural-type, which we have called glitter’, that explains the observed diffraction pattern of n-diamond reasonably well, and that is stabilized by extensive spiroconjugation in three dimensions leading to a metallic status for the carbonaceous structural-type.

First author: Prakash, S, Magnetic field dependence of photo-CIDNP MAS NMR on photosynthetic reaction centers of Rhodobacter sphaeroides WT, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 14290, (2005)
Abstract: Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in frozen and quinone depleted photosynthetic reaction centers of the purple bacteria Rhodobacter sphaeroides wild type (WT) by C-13 solid-state NMR at three different magnetic fields. All light-induced signals appear to be emissive at all three fields. At 4.7 T (200 MHz proton frequency), the strongest enhancement of NMR signals is observed, which is more than 10 000 above the Boltzmann polarization. At higher fields, the enhancement factor decreases. At 17.6 T, the enhancement factor is about 60. The field dependence of the enhancement appears to be the same for all nuclei. The observed field dependence is in line with simulations that assume two competing mechanisms of polarization transfer from electrons to nuclei, three-spin mixing (TSM) and differential decay (DD). These simulations indicate a ratio of the electron spin density on the special pair cofactors is 3:2 in favor of the L-BChl during the radical cation state. The good agreement of simulations with the experiments raises expectations that artificial solid reaction centers can be tuned to show photo-CIDNP in the near future.

First author: Roger, M, Lanthanide(III)/actinide(III) differentiation in mixed cyclopentadienyl/dithiolene compounds from X-ray diffraction and density functional theory analysis, ORGANOMETALLICS, 24, 4940, (2005)
Abstract: Treatment Of [U(CP*)(2)Cl-2] with Na(2)dddt in thf afforded the “ate” complex [U(CP*)(2)Cl-(dddt)Na(thf)(2)] (1), and the salt-free compound [U(CP*)(2)(dddt)] (2) could be extracted from 1 with toluene (Cp* = eta-C5Me5; dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). Reduction of 2 with Na(Hg) or addition of Na(2)dddt to [U(CP*)(2)Cl2Na(thf)(x)] in the presence of 18-crown-6 gave the first uranium(III) dithiolene compound, [Na(18-crown-6)(thf)(2)] [U(CP*)(2)(dddt)] (4). The dimeric lanthanide complexes [{Ln(Cp*)(2)(dddt)K(thf)(2)}(2)] (Ln = Ce (5), Nd (6)) were prepared by reaction of [Ln(Cp*)(2)Cl2K] with K(2)dddt, and in the presence of 15-crown-5, they were transformed into the cation-anion pairs [K(15-crown-5)2][Ln(Cp*)2(dddt)] (Ln = Ce (7), Nd (8)). The crystal structures of 2, 4(.)thf, 5-7, 7(.)0.5(pentane), and 8(.)0.5(pentane) were determined by X-ray diffraction analysis. Comparison of the structural parameters of the anions [M(CP*)(2)(dddt)](-) (M = U, Ce, Nd) revealed that the U-S and U-C(Cp*) distances are shorter than those expected from a purely ionic bonding model; the relatively small folding of the dddt ligand suggests that the interaction between the C = C double bond and the metal center is weak, in agreement with the NMR observations in solution. The structural data obtained from molecular geometry optimizations on the complexes [M(CP*)(2)(dddt)](-,0) (M = Ce, U) using relativistic density functional theory (DFT) calculations reproduce experimental trends. A detailed orbital analysis shows that the contraction of the metal-sulfur bond lengths when passing from [Ce(Cp*)(2)(dddt)] – to [U(CP*)(2)(dddt)](-) is partly related to the uranium 5f orbital-ligand mixing, which is greater than the cerium 4f orbital-ligand mixing. The comparison of the two [U(CP*)(2)(dddt)](-,0) species reveals a higher ligand-to-metal donation in the case of the U(IV) complex.

First author: Seth, M, Calculation of excitation energies of open-shell molecules with spatially degenerate ground states. I. Transformed reference via an intermediate configuration Kohn-Sham density-functional theory and applications to d(1) and d(2) systems with octahedral and tetrahedral symmetries, JOURNAL OF CHEMICAL PHYSICS, 123, 4940, (2005)
Abstract: A method for calculating the UV-vis spectra of molecules with spatially degenerate ground states using time-dependent density-functional theory (TDDFT) is proposed. The new transformed reference via an intermediate configuration Kohn-Sham TDDFT (TRICKS-TDDFT) method avoids the difficulties caused by the multireference nature of spatially degenerate states by rather than utilizing the ground state instead taking a nondegenerate excited state with desirable properties as the reference for the TDDFT calculation. The scope and practical application of the method are discussed. Like all open-shell TDDFT calculations this method at times suffers from the inability to produce transitions to states that are eigenfunctions of the total spin operator. A technique for alleviating this difficulty to some extent is proposed. The applicability and accuracy of the TRICKS-TDDFT method is demonstrated through example calculations of several d(1) and d(2) transition metal complexes with tetrahedral and octahedral symmetries. For the most part, the results of these calculations are similar in quality to to those obtained from standard TDDFT calculations.

First author: Verbraak, H, High-resolution infrared spectroscopy of the charge-transfer complex [Ar-N-2](+center dot): A combined experimental/theoretical study, JOURNAL OF CHEMICAL PHYSICS, 123, 4940, (2005)
Abstract: A combined experimental and theoretical study of the charge-transfer complex [Ar-N-2](+center dot) is presented. Nearly 50 transitions split by spin-rotation interaction have been observed by means of infrared diode laser absorption spectroscopy in a supersonic planar plasma expansion. The band origin is at 2272.2563(18) cm(-1) and rotational constants in the ground and vibrationally (NN-stretch) excited state amount to 0.128 701(8) cm(-1) and 0.128 203(8) cm(-1), respectively. The interpretation of the data in terms of a charge switch upon complexation is supported by new ab initio calculations. The best estimate for a linear equilibrium structure yields R-e(NN)=1.102 A and R-e(Ar-N)=2.190 A. Predictions for molecular parameters not directly available from the experimental results are presented as well. Furthermore, the electronic structure and Ar-N bonding mechanism of [Ar-N-2](+center dot) have been analyzed in detail. The Ar-N bond is a textbook example of a classical 2-center-3-electron bond.

First author: Farrugia, LJ, Experimental X-ray charge density studies on the binary carbonyls, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 8834, (2005)
Abstract: The experimental charge densities in the binary carbonyls Cr(CO)(6) (1), Fe(CO)(5) (2), and Ni(CO)(4) (3) have been investigated on the basis of high-resolution X-ray diffraction data collected at 100 K. The nature of the metal-ligand interactions has been studied by means of deformation densities and by topological analyses using the Atoms in Molecules (AIM) approach of Bader. A detailed comparison between the experimental results and theoretical results from previous work and from gas-phase and periodic DFT/B3LYP calculations shows excellent agreement, both on a qualitative and quantitative level. An examination of the kappa-restricted multipole model (KRMM) for Cr(CO)(6), using theoretically derived structure factors, showed it to provide a somewhat worse fit than a model with freely refined kappa’ values. The experimental atomic graphs for the metal atoms in 2 and 3 were found to be dependent on the muitipole model used for that atom. In the case of compound 2, restriction of the multipole populations according to idealized site symmetry of D-3h gave an atomic graph in essential agreement with the theoretical gas-phase study. For compound 3, all multipole models fail to reproduce the atomic graph obtained from the theoretical gas-phase study. The atomic quadrupole moments for the C atoms in all compounds were consistent with significant pi back-donation from the metal atoms.

First author: Prins, P, Charge transport in self-organized pi-stacks of p-phenylene vinylene oligomers, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 18267, (2005)
Abstract: We have studied the mobility of charge carriers along self-organizing pi-stacks of hydrogen-bonded phenylene vinylene oligomers in solution, by time-resolved microwave conductivity measurements. The value deduced for the mobility along the stacks is 3 x 10(-3) and 9 X 10(-3) cm(2)/(V s) for holes and electrons, respectively. Additionally, we have calculated the mobility along the,pi-stacks using a hopping model based on parameters from density functional theory. The mobility values obtained from these calculations are in good agreement with the experimental values if it is assumed that there are relatively large twist angles between neighboring molecules in the stack. It is shown that a significantly higher mobility can be attained if the twist angle between neighboring oligomers is reduced.

First author: Zhang, W, Influence of pi-stacking on the resonant enhancement of the second-order nonlinear optical response of dipolar chromophores, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 18378, (2005)
Abstract: The wavelength-dependent second-harmonic generation (SHG) efficiency of two simple dipolar chromophores, 4-NO2C6H4N(H)BUn (1) and 4-NO2C6H4SN(H)Bu-t (2), was compared in solution and in the solid state. Hyper-Rayleigh scattering measurements at 532 nm provided comparable molecular first hyperpolarizabilities. Both compounds crystallize in non-centrosymmetric space groups, but a more efficient arrangement of dipole moments results in a significantly larger d(eff) value for 2. Kurtz-Perry experiments from 450 to 700 nm revealed an important difference in the resonant component of the nonlinear optical responses of these compounds; the SHG efficiency of crystalline 1 depends more strongly on the incident wavelength than that of 2. This would be in contradiction with the TD-DFT excitation energies calculated for these molecules, but the observation can be explained by the resonant contribution from low-energy interchromophore excitations enabled by pi-stacking in the crystal of 1.

First author: Yan, LK, Electronic properties of Strandberg anions: A DFT study of [X2Mo5O23](n-), (X = P-V, S-VI, As-V, Se-VI), and [(RP)(2)Mo5O21](4-) (R = H, CH3, C2H5), INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 105, 37, (2005)
Abstract: The electronic properties of the anions mentioned in the title polyanions were calculated by means of Density Functional Theory (DFT). The redox properties and the basicity of the external oxygen sites of those polyanions were analyzed. The results show that the redox properties of Strandberg anions depend on the nature of heteroatom X. The organic group bonded to the heteroatom modifies the redox property of the cluster. The oxygen basicities of the polyanions were analyzed by virtue of molecular electrostatic potential (MEP). The MEP distribution suggests that the most basic centers are triple-bridging oxygen atoms, one of which is shared with two metal atoms and one heteroatom X in [P2Mo5O23](6-) and [As2Mo5O23](6-). In [(RP)(2)Mo5O21](4-), the triple-bridging oxygen atoms and the double-bridging oxygen atoms bonded to two Mo atoms identified as the most basic centers.

First author: McKee, ML, Study of Hg-2(2+) and complexes of NpO2+ and UO22+ in solution. Examples of cation-cation interactions, INORGANIC CHEMISTRY, 44, 6975, (2005)
Abstract: Density functional theory (BPW91/TZ2P) is used to explore the nature of cation-cation interactions (CCls) that exist between two actinyl cations in solution. Solvation, which is modeled using COSMO, favors the complexes (ONpO-ONpO)(2+) and (ONpO-OUO)(3+) over separated NpO2+(aq) and UO22+(aq) cations because of the quadratic dependence of solvation on charge. For (OUO-OUO)(4+), solvation effects, even though very large, are unable to overcome intrinsic electrostatic repulsion between the units. The actinyl-actinyl complexes are T-shaped, with the oxygen of one unit coordinated to the actinide metal of the other unit. The association free energies of (ONpO-ONpO)(2+) and (ONpO-OUO)(1+) are calculated as -42.1 and -29.2 kcal/mol. Explicit consideration of the first solvation shell at the B3LYP/LANL2DZ level suggests that the free energies of binding may be overestimated. The Hg-2(2+) dication, though not considered a “traditional” CCl, is very similar to the actinyl-actinyl interaction. The binding free energy of Hg-2(2+) in solution is calculated as -16.0 kcal/mol.

First author: Diez, RP, A density-functional study on the formation of Mo-2(2+), JOURNAL OF CHEMICAL PHYSICS, 123, 6975, (2005)
Abstract: The presence of metastable states in the doubly ionized molybdenum dimer is studied using gradient-corrected scalar-relativistic density-functional theory. Seventeen metastable states are found within an energy range of less than 6.5 eV. All those states show lifetimes large enough to assure experimental detection. The calculation of the second adiabatic ionization potential of the neutral molybdenum dimer seems to confirm that the doubly ionized dimer is produced by the electron-capture process Mo-2(+)+Ar+-> Mo-2(2+)+Ar, in which the ionization potentials of Ar and Mo-2(+) play a crucial role [K. Franzreb, R. C. Sobers, Jr., J. Lorincik, and P. Williams, J. Chem. Phys. 120, 7983 (2004)]. Moreover, the present results indicate that other species having ionization potentials between 13.01 and 15.34 eV could be used as projectiles to produce Mo-2(2+). It is also shown that Xe+ ions could not react with Mo-2(+) to produce double ionized dimers. A simple thermodynamic argument is also proposed that seems to increase the possibilities of forming Mo-2(2+) from Mo-2(+) by using Ar+ as projectile ions.

First author: Simpson, SH, Calculation of the birefringences of nematic liquid crystals at optical and infrared wavelengths,JOURNAL OF CHEMICAL PHYSICS, 123, 6975, (2005)
Abstract: In this paper we critically examine computational methods for predicting the birefringences of nematic liquid crystals, with a view to screening molecules for potential use in infrared applications. Using the liquid-crystal 5CB as a test molecule, we calculate molecular electronic polarizabilities using ab initio quantum-mechanical techniques and a wide range of basis sets. We show that the polarizabilities tend to a limiting value as the quality of the basis set is improved. However, the biggest hurdle remains the determination of the refractive index from the polarizability data. We examine several methods for performing this conversion and conclude that the simplest equation, due to Vuks, is adequate for predicting the birefringence, given the uncertainties involved in other parameters. The agreement between calculation and experiment is best described as “semiquantitative.” We perform similar calculations for a wide range of nematic liquid crystals at both 589 and 1550 nm, taking into account the likely impact of molecular vibrations at the longer wavelength. We demonstrate that there is a simple scale factor, for conventional nematics, between the birefringence at visible wavelengths and in the infrared. Thus knowledge of the birefringence at optical wavelengths, as widely available in the literature, is a good guide to the usefulness of conventional nematic liquid crystals as active elements for optical switching in the telecommunications industry.

First author: Ayuela, A, Charging mechanism for the bond elongation observed in suspended chains of gold atoms – art. no. 161402, PHYSICAL REVIEW B, 72, 6975, (2005)
Abstract: Abnormally large bond lengths (3.5-5.0 angstrom) between gold atoms forming small chains suspended between two electrodes have been observed in some experiments. Using the density functional theory we explore the possibility that the elongation could arise from the electrical charging of the chains induced in nonequilibrium by the electron beam of the transmission electron microscope used to image the nanowires in those experiments.

First author: Garcia-Fernandez, P Sousa, C, Coherent tunneling in Cu2+- and Ag2+-doped MgO and CaO : Cu2+ explored through ab initio calculations, PHYSICAL REVIEW B, 72, 6975, (2005)
Abstract: The observation of coherent tunnelling in Cu2+- and Ag2+-doped MgO and CaO:Cu2+ was a crucial discovery in the realm of the Jahn-Teller (JT) effect. The main reasons favoring this dynamic behavior are now clarified through ab initio calculations on Cu2+- and Ag2+-doped cubic oxides. Small JT distortions and an unexpected low anharmonicity of the e(g) JT mode are behind energy barriers smaller than 25 cm(-1) derived through CASPT2 calculations for Cu2+- and Ag2+-doped MgO and CaO:Cu2+. The low anharmonicity is shown to come from a strong vibrational coupling of MO610- units (M=Cu,Ag) to the host lattice. The average distance between the d(9) impurity and ligands is found to vary significantly on passing from MgO to SrO following to a good extent the lattice parameter.

First author: Ciofini, I, Effect of self-interaction error in the evaluation of the bond length alternation in trans-polyacetylene using density-functional theory, JOURNAL OF CHEMICAL PHYSICS, 123, 6975, (2005)
Abstract: The calculation of the bond-length alternation (BLA) in trans-polyacetylene has been chosen as benchmark to emphasize the effect of the self-interaction error within density-functional theory (DFT). In particular, the BLA of increasingly long acetylene oligomers has been computed using the Moller-Plesset wave-function method truncated at the second order and several DFT models. While local-density approximation (LDA) or generalized gradient corrected (GGA) functionals strongly underestimate the BLA, approaches including self-interaction corrections (SIC) provide significant improvements. Indeed, the simple averaged-density SIC scheme (ADSIC), recently proposed by Legrand [J. Phys. B 35, 1115 (2002)], provides better results for the structure of large oligomers than the more complex approach of Krieger [Phys. Rev. A 45, 101 (1992)]. The ADSIC method is particularly promising since both the exchange-correlation energy and potential are improved with respect to standard LDA/GGA using a physically appealing correction, through a different route than the more popular approach through the Hartree-Fock exchange inclusion within the hybrid functionals.

First author: Rosa, A, Structural, optical, and photophysical properties of nickel(II) alkylthioporphyrins: Insights from experimental and DFT/TDDFT studies, INORGANIC CHEMISTRY, 44, 6609, (2005)
Abstract: The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio, group, NiOMTP (OMTP = 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D-2d and hybrid (ruffled with a modest degree of saddling) D-2 conformations, both characterized by an up-down (ud) orientation of the vicinal methylthic, groups are by far the preferred conformations in the “gas phase”. In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and beta-alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthic, groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway (1)(pi,pi) -> (3)(d(x2),d(x2-y2)) -> ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands.

First author: Wolff, SK, Analytical second derivatives in the Amsterdam density functional package, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 104, 645, (2005)
Abstract: Recently, analytical second derivatives with respect to nuclear coordinates have been implemented in the Amsterdam density functional (ADF) package. This article presents the detailed formalism of that implementation. Calculations on small molecules such as methane show good agreement between the analytical and numerical frequencies. Calculations on benzene and larger molecules show that the analytical second derivatives code is 2 to 3 times faster than the numerical.

First author: Neugebauer, J, Modeling solvent effects on electron-spin-resonance hyperfine couplings by frozen-density embedding, JOURNAL OF CHEMICAL PHYSICS, 123, 645, (2005)
Abstract: In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc’s) of the H2NO molecule. The hfcc’s for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle.

First author: de Montigny, F, Electron transfer and electron exchange between [Cp*(dppe)Fe](n+) (n=0, 1) building blocks mediated by the 9,10-bis(ethynyl)anthracene bridge, ORGANOMETALLICS, 24, 4558, (2005)
Abstract: A novel bis(iron) alkynyl-bridged complex, Cp*(dppe)Fe(C&3bond; C-9,10-ant-C&3bond; C)Fe(dppe)Cp*, 10 (ant = anthracene), and its oxidized forms, 10(.)PF(6), 10-TCNQ, and 10(.)2PF(6), were synthesized and characterized by X-ray crystal structures. The cyclic voltammogram of 10 shows two well-reversible redox couples at -0.40 and -0.04 V (vs SCE) and a third redox process close to the solvent edge. Density-functional theory (DFT) calculations carried out on the substituted model complex (eta(5)-C5H5)(eta(2) -dpe)Fe-C&3bond; C-9,10-ant-C&3bond; C-Fe(eta(2)-dpe)(eta(5)-C5H5), 10-H (dpe = H2P-(CH2)(2)-PH2), suggest that the HOMO, which is depopulated upon oxidation, has a dominant anthracene character. The H-1 NMR and the magnetic susceptibility measurements indicate that the complex 10(.)2PF(6) is diamagnetic, in contrast with that of its congeners of the bis(iron) series, which do not contain the anthracene fragment in the bridge. Experimental measurements and DFT calculations reveal a large energy gap between the singlet ground state and the triplet excited state (Delta E-ST(exp) < -1200 cm(-1)). Mossbauer spectroscopy reveals that the two iron centers are spectroscopically equivalent for the three oxidation states. The parameters found for 10(.)2PF(6) are not typical of an iron(III) center, but rather characteristic of iron(II). The spectrum of 10(.)TCNQ is in accord with a detrapped mixed-valence (AN) electronic structure with a large contribution of the delocalized impaired electron on the carbon bridge. In addition, Mossbauer and LTV-vis data and DFT calculations indicate also that the electronic structure of the MV complex is much closer to the dication 10(.)2PF(6) than the parent complex 10. The analysis of the NIR absorption band allows the determination of a large electronic coupling parameter, as expected for a class III MV complex (V-ab = 2180 cm(-1)). Taken as a whole, the experimental and theoretical data emphasize the specific role of the anthracene fragment inserted in the carbon bridge, which allows good electronic communication between the iron centers, but significantly contributes to the displacement of the spin density from the metal centers onto the alpha and beta sp carbon atoms in the vicinity of the metal and the ipso carbon of the anthracene.

First author: Santi, S, Designing molecules for metal-metal electronic communication: Synthesis and molecular structure of the couple of heterobimetallic isomers [eta(6)-(2-ferrocenyl)indenel-Cr(CO)(3) and [eta(6)-(3-ferrocenyl)indenel-Cr(CO)(3), ORGANOMETALLICS, 24, 4691, (2005)
Abstract: The heterobinuclear isomers [eta(6)-(2-ferrocenybindene]-Cr(CO)(3) (1) and [eta(6)-(3-/errocenyl)indene]-Cr(CO)(3) (2) have been prepared and the crystal structure determination showed that the Fe(C5H5) and Cr(CO)(3) groups in the two molecules are disposed in different conformations with respect to the Cp-indene bridging ligand, cisoid in 1 and transoid in 2. Preliminary electrochemical (CV) and spectroscopic (IR and near-IR) results obtained for the corresponding monooxidized 1(+) and 2(+) demonstrate the existence of stronger electronic coupling in 1(+) than in 2(+).

First author: Haneline, MR, Coordination of nitroxide and nitronyl-nitroxide organic radicals to trimeric perfluoro-o-phenylene mercury, INORGANIC CHEMISTRY, 44, 6248, (2005)
Abstract: The interaction of trimeric perfluoro-o-phenylene mercury (1) with TEMPO (1 11, 5,5-tetramethylpentamethylene nitroxide) in CH2Cl2 leads to the formation of the 1:1 adduct [1 center dot TEMPO] (2). The same reaction carried out with NIT-Ph (2-(phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) leads to the formation of either [1 center dot NIT center dot Ph center dot 1] (3) or [1 center dot NITPh](n) (4), depending on the amount of NIT-Ph present in solution. Adducts 2, 3, and 4 have been fully characterized and their crystal structures determined. The solid-state structure of 2 contains molecules of [1 center dot TEMPO] in which the nitroxide oxygen atom is triply coordinated to the mercury centers of 1. A similar situation is encountered in the structure of 3 where each oxygen atom of the NIT-Ph molecule interacts with the mercury centers of an adjacent molecule of 1. The structure of 4 consists of extended helical polymeric chains that contain alternating molecules of 1 and NIT-Ph. As in 2 and 3, the interactions responsible for the formation of these chains involve the triple coordination of the oxygen atoms of the NIT-Ph molecule to the mercury centers of 1. DFT calculations suggest that the bonding in adducts such as 2, 3, and 4 is most likely dominated by electrostatic rather than covalent interactions. In agreement with this view, magnetic susceptibility measurements carried out on these adducts indicate that 1 does not mediate significant coupling between organic radicals coordinated on either side of the trinuclear core.

First author: Atanasov, M, Modeling properties of molecules with open d-shells using density functional theory, COMPTES RENDUS CHIMIE, 8, 1421, (2005)
Abstract: An overview of the theory and applications of a recently proposed ligand-field density functional theory (LFDFT) is given. We describe a procedure based on DFT allowing to deduce the parameters of this non-empirical LF approach consisting of the following steps: (i) an average of configuration (AOC) DFT calculation, With equal occupancies of the d-orbitals is carried out (ii) with these orbitals kept frozen, the energies of all single determinants (SD) within the whole LF-manifold are calculated and used as a data base in a further step to provide all the Racah- and LF-parameters needed in a conventional LF-calculation. A more rigorous analysis of this approach in terms of Lowdin’s energy partitioning and effective Hamiltonians is used to provide explicit context for its applicability and to set more rigorous criteria for its limitations. The formalism has been extended to account for spin-orbit coupling as well. Selected applications cover tetrahedral CrX4 (X = Cl, Br) and FeO42- and octahedral CrX63- (X = F-, Cl-, Br-) complexes. Transition energies are calculated with an accuracy of 2000 cm(-1), deviations being larger for spin-forbidden transitions and smaller for spin-allowed ones. Analysis show, that ligand field parameters deduced from experiment are well reproduced, while interelectronic repulsion parameters are calculated systematically to be by 30-50% of lower in energy. A prieralization of the LFDFT theory to dimers of transition metals allows to calculate exchange coupling integrals in reasonable agreement with experiment and with comparable success to the broken symmetry approach; in addition they allow to judge ferromagnetic contributions to exchange coupling integral which have been ignored before.

First author: Mitoraj, M, DFT studies on isomerization reactions in the copolymerization of ethylene and methyl acrylate catalyzed by Ni-diimine and Pd-diimine complexes, JOURNAL OF MOLECULAR MODELING, 11, 341, (2005)
Abstract: Gradient corrected density functional theory (DFT) has been used to investigate the isomerization reactions in the process of the ethylene/methyl acrylate copolymerization catalyzed by Pd-dimine and Ni-dimine complexes, modeled by a generic system N<^>N-M-(CH3)(+); N<^>N = -N(H)-C(H)-C(H)-N(H)-. The influence of the polar group and of the metal on the isomerization mechanism was studied. The results show that for the Pd-catalyst the isomerization follows the standard mechanism observed in homopolymerization processes, with the beta-hydrogen-transfer to the metal and formation of a pi-olefin-hydride complex. Electron withdrawing character of the polar group results in an increase of the hydride energy and the isomerization barrier. For the Ni-catalyst the overall isomerization picture is modified by the formation of a sigma-olefin-hydride complex, in which the olefin is coordinated to the metal by the oxygen atom of the polar group. Such a sigma-olefin-hydride is lower in energy for the Ni catalyst than the pi-olefin-hydride complex by 9.6 kcal mol(-1). The latter is preferred by 2.6 kcal mol(-1) for the Pd-based system. The calculated isomerization barriers are 20.9 and 24.0 kcal mol(-1) (with respect to the initial 4-member chelate) for the Pd-catalyst and Ni-catalyst, respectively. This can result in a larger fraction of ester group directly connected to the copolymer backbone observed experimentally for the Ni-catalyst.

First author: Neugebauer, J, An explicit quantum chemical method for modeling large solvation shells applied to aminocoumarin C151, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 7805, (2005)
Abstract: The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. The computer time used in this new implementation scales approximately linearly (with a low prefactor) with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved.

First author: Garcia-Lastra, JM, Origin of the different color of ruby and emerald, PHYSICAL REVIEW B, 72, 7805, (2005)
Abstract: The different color exhibited by ruby and emerald is a fundamental but still unsolved question. According to recent EXAFS measurements, such a difference can hardly be explained on the basis of a different average distance between Cr3+ and the six oxygen ligands. The puzzling difference in color between the two gemstones is shown in this work to arise essentially from the distinct electrostatic potential imposed by the rest of lattice ions upon the active electrons of the CrO69- unit. Main effects are shown to come from the electric field generated in the neighborhood of the Cr3+ site in ruby which is absent in the case of emerald due to symmetry.

First author: Apitz, D, Investigation of chromophore-chromophore interaction by electro-optic measurements, linear dichroism, x-ray scattering, and density-functional calculations, PHYSICAL REVIEW E, 72, 7805, (2005)
Abstract: Free-beam interferometry and angle-resolved absorption spectra are used to investigate the linear electro-optic coefficients and the linear dichroism in photoaddressable bis-azo copolymer thin films. From the first- and second order parameters deduced, the chromophore orientation distribution is calculated and displayed for several poling temperatures and chromophore concentrations. The influence of dipole-dipole interaction on the overall polymer dynamics is discussed. The first order parameter, and therefore the Pockels effect, peaks for a poling temperature of around 10 degrees C above the glass transition. The decrease of the Pockels effect above this temperature region is triggered by a head-to-tail chromophore orientation, i.e., a transition to a microcrystalline phase, increasing the second order parameter. Comparison of the experimentally observed absorption spectra and those obtained by density-functional calculations support the picture of differently aligned bis-azo dye molecules in a trans,trans configuration. Complementary wide-angle x-ray scattering is recorded to confirm the various kinds of ordering in samples poled at different temperatures.

First author: Luo, Y, DFT study of the tetranuclear lutetium and yttrium polyhydride cluster complexes [(C5Me4SiMe3)(4)Ln(4)Hs(8)] (Ln = Lu, Y) that contain a four-coordinate hydrogen atom, ORGANOMETALLICS, 24, 4362, (2005)
Abstract: DFT calculations revealed some unprecedented aspects of the structure of the tetranuclear lutetium and yttrium polyhydride complexes [(C5Me4SiMe3)(4)Ln(4)H(8)] (Ln = Lu, Y). In contrast with the previously described X-ray analysis of (C5Me4SiMe3)(4)LU4H8, which suffered from a serious disorder problem and showed a C-2v-symmetrical structure with one body-centered mu(4)-H, two face-capped mu(3)-H, and five edge-bridged mu(2)-H atoms, the present DFT studies indicated that the optimized Ln(4)H(8) core prefers a pseudo-C-3v-symmetrical structure with one body-centered mu(4)-H, one face-capped mu(3)-H, and six edge-bridged mu(2)-H atoms. Metal-metal orbital interactions via the hydride bridges were also observed in these complexes. The,mu(4)-H bonding fashion, a new bonding mode for hydrogen, was well proved by the Wiberg bond indexes and linear overlap bond orders. The X-ray structure of (C5Me4SiMe3)(4)Y4H8, which was solved without suffering from the disorder problems, showed excellent agreement with the theoretical calculations.

First author: Bernskoetter, WH, C-H bond activation reactions with ligand adducts of beta-diiminate iridium dihydride,ORGANOMETALLICS, 24, 4367, (2005)
Abstract: Addition of neutral ligands to the trigonal prismatic iridium tetrahydride ((BDI)-B-iPr)IrH4 (BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6-(Pr2C6H3)-Pr-i) induces reductive elimination of dihydrogen to afford ((BDI)-B-iPr)IrH2(L) (L = phosphine, tetrahydrothiophene, cyclohexene) compounds. A combination of solution NMR data and infrared spectroscopic studies have established that the phosphine dihydride compounds and the cyclohexene adduct are best described as classical iridium(III) dihydride or “stretched dihydrogen” complexes. The corresponding tetrahydrothiophene adduct exhibits spectroscopic features and reactivity patterns consistent with increased iridium(I) dihydrogen character. In complexes containing large cone angle phosphines such as PCy3 and (PPr3)-Pr-i, ligand dissociation is facile and isotopic exchange in arene substrates is observed. The experimental data support an Ir(III)-Ir(V) oxidative addition-reductive elimination sequence for C-H bond activation, in contrast to more traditional coordinatively saturated precursors, where Ir(l)-Ir(III) couples are preferred.

First author: Hart, RT, On the spectral similarity of bridging and nonbridging oxygen in tellurites, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 7636, (2005)
Abstract: We show by high field O-17 solid-state nuclear magnetic resonance (NMR) and by A initio calculations of both the NMR and the oxygen Is photoelectron spectra that the oxygen sites in tellurite glasses show no spectroscopic distinction, even when comparing bridging and nonbridging sites. This is remarkable because two such sites differ formally by a full electronic charge, and they are readily distinguished by these same methods in silicates. We argue that this similarity arises from the symmetry breaking that occurs when the original TeO2 crystal solid forms, due to the pseudo-Jahn-Teller distortion induced by the two additional valence electrons present in Te-IV as compared to Si-IV.

First author: Xifra, R, Fine-tuning the electronic properties of highly stable organometallic Cu-III complexes containing monoanionic macrocyclic ligands, CHEMISTRY-A EUROPEAN JOURNAL, 11, 5146, (2005)
Abstract: A family of highly stable organometallic Cu-III complexes with monoanionic triazamacrocyclic ligands (L’) with general formula [CuL’](+) have been prepared and isolated, and their structural, spectroscopic, and redox properties thoroughly investigated. The HLi ligands have been designed in order to understand and quantify the electronic effects exerted by electron donor and electron-withdrawing groups on either the aromatic ring or the central secondary amine or on both. In the solid state the Cu-III complexes were mainly characterized by single-crystal X-ray diffraction analysis, whereas in solution their structural characterization was mainly based on H-1 NMR spectroscopy given the diamagnetic nature of the d(8) square-planar Cu-III complexes. Cyclic voltammetry together with H-1 NMR and UV/Vis spectroscopy have allowed us to quantify the electronic effects exerted by the ligands on the Cu-III metal center. A theoretical analysis of this family of Cu-III complexes has also been undertaken by DFT calculations to gain a deeper insight into the electronic structure of these complexes, which has in turn allowed a greater understanding of the nature of the UV/Vis transitions as well as the molecular orbitals involved.

First author: Pacco, A, Lanthanide(III)-induced conversion of 12-metallacrown-4 to 5-metallacrown-5 complexes in solution, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 11, 3303, (2005)
Abstract: The conversion of 12-metallacrown-4 complexes with copper(II) in the central cavity to 15-metallacrown-5 complexes with lanthanide(iii) ions in the central cavity upon addition of trivalent lanthanide ions was analysed in solution by electrospray ionisation mass spectrometry (ESI-MS), absorption spectrophotometry, circular dichroism and proton NMR spectroscopy. In all cases, copper(II) ions were the ring metals. The lanthanide-induced shifts of the proton resonances in the NMR spectra of different lanthanide (m)-containing 15-metallacrown-5 complexes with tyrosinehydroximate ligands were studied. The dynamics of ligand exchange of alpha-aminohydroximate-based lanthanide (iii) -containing 15-metallacrown-5 complexes were studied in methanol solutions. The ring structure of the 12-metallacrown-4 complex that acts as starting compound for the formation of the 15-metallacrown-5 complexes was optimized by DFT methods using alanine-hydroximate as a model ligand.

First author: Jensen, L, The first hyperpolarizability of p-nitroaniline in 1,4-dioxane: A quantum mechanical/molecular mechanics study, JOURNAL OF CHEMICAL PHYSICS, 123, 3303, (2005)
Abstract: In this work we have investigated the first hyperpolarizability of pNA in 1,4-dioxane solution using a quantum mechanics/molecular mechanics (QM/MM) model. The particular model adopted is the recently developed discrete solvent reaction field (DRF) model. The DRF model is a polarizable QM/MM model in which the QM part is treated using time-dependent density-functional theory and local-field effects are incorporated. This allows for direct computation of molecular effective properties which can be compared with experimental results. The solvation shift for the first hyperpolarizability is calculated to be 30% which is in good agreement with the experimental results. However, the calculated values, both in the gas phase and in solution, are by a factor of 2 larger than the experimental ones. This is in contrast to the calculation of the first hyperpolarizability for several small molecules in the gas phase where fair agreement is found with experimental. The inclusion of local-field effects in the calculations was found to be crucial and neglecting them led to results which are significantly larger. To test the DRF model the refractive index of liquid 1,4-dioxane was also calculated and found to be in good agreement with experiment.

First author: Klein, A, Multiple isomerism (cis/trans; syn/anti) in [(dmso)(2)Pt(aryl)(2)] complexes: A combined structural, spectroscopic, and theoretical investigation, ORGANOMETALLICS, 24, 4125, (2005)
Abstract: The occurrence of cis or trans configurations in square planar diarylbis(dimethyl sulfoxide)platinum(II) complexes with S-bonded dmso ligands and aryl = 2,3,4,5,6-pentamethylphenyl, 2,4,6-trimethylphenyl, 2,6-dimethylphenyl, 2-, 3-, and 4-methylphenyl, and phenyl has been investigated by multinuclear (H-1, C-13, and Pt-195) NMR spectroscopy and crystal structure analysis. Both methods confirm the cis configuration for complexes with the smaller phenyl and methylphenyl (tolyl) ligands and the trans configuration for the compounds with the bulkier ligands, starting from 2,6-dimethylphenyl. Spectroscopic criteria could thus be established to identify configurational isomers. For the 2-tolyl complex an additional kind of isomerism arises from the relative orientation of the two methyl substituents on the same side (syn) or on different sides of the coordination plane (anti). 2D-NMR spectroscopy allowed us to identify the conformers and to study isomerization mechanisms. DFT calculation results agree well with the experimental structures and with the spectroscopic data.

First author: Slootweg, JC, Methylene-azaphosphirane as a reactive intermediate, CHEMISTRY-A EUROPEAN JOURNAL, 11, 4808, (2005)
Abstract: Reaction of the transient phosphinidene complexes R-P=W(CO)5 with N-substituted-diphenyl-ketenimines leads unexpectedly to the novel 2-aminophosphindoles, as confirmed by an X-ray crystal structure determined for one of the derivatives. Experimental evidence for a methylene-azaphosphirane intermediate was found by using the iron-complexed phosphinidene iPr(2)N-P-Fe(CO)(4), which affords the 2-aminophosphindole together with the novel triethylene-2,3-dihydro-1H-benzo[1,3]azaphosphole. Analysis of the reaction pathways with DFT indicates that the initially formed yields both phosphorous heterocycles by way of a [1,5]- or [1,3]sigmatropic shift, respectively followed by a II-shift. Strain underlies both rearrangements. which causes these remarkably selective conversions that can be lulled by changing the substituents.

First author: Schneider, SK, Extending the NHC concept: C-C coupling catalysis by a Pd-II carbene (rNHC) complex with remote heteroatoms, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 11, 2973, (2005)
Abstract: A palladium complex, trans-Cl(PPh3)(2)Pd{Ca-cyclo-C6H4-O-N(Me) C=(CH)-H-b}(C-a-C-b), that contains a carbene ligand with remote heteroatoms (rNHC) is much more active in certain C-C coupling reactions than comparably simple NHC- and phosphane-containing precatalysts. The rNHC ligand binds strongly by electrostatic as well as orbital interaction to the metal.

First author: Pink, SMM, DL-Proline, ACTA CRYSTALLOGRAPHICA SECTION C-CRYSTAL STRUCTURE COMMUNICATIONS, 61, O506, (2005)
Abstract: In the structure of DL-proline, C5H9NO2, the molecules are connected via classical intermolecular N-H center dot center dot center dot O hydrogen bonds involving the amine and carboxyl groups [N center dot center dot center dot O=2.7129 (15) and 2.8392 (16) angstrom], and form chains along the b-axis direction and parallel to ((1) over bar 01). The chains are linked into sheets via weak non-classical hydrogen bonds. The conformation of the molecule and its packing are notably different from the monohydrated DL-proline form.

First author: Tomasi, J, Quantum mechanical continuum solvation models, CHEMICAL REVIEWS, 105, 2999, (2005)
Abstract: In the structure of DL-proline, C5H9NO2, the molecules are connected via classical intermolecular N-H center dot center dot center dot O hydrogen bonds involving the amine and carboxyl groups [N center dot center dot center dot O=2.7129 (15) and 2.8392 (16) angstrom], and form chains along the b-axis direction and parallel to ((1) over bar 01). The chains are linked into sheets via weak non-classical hydrogen bonds. The conformation of the molecule and its packing are notably different from the monohydrated DL-proline form.

First author: Ouddai, N, Theoretical aspects of the bonding in acetylide-bridged organometallic dinuclear complexes,COMPTES RENDUS CHIMIE, 8, 1336, (2005)
Abstract: The electronic and geometrical structures of a variety of acetylide-bridged organometallic dinuclear complexes (MLn)(2)(mu-C-2) (M = Sc, Ti, W, Ru, Pt) are analysed and compared by use of extended Huckel and density functional-theory calculations. It is established that all the studied complexes feature the ethyne valence structure, except the titanium species for which resonance forms of ethyne-type and cumulenic-type must be invoked to rationalise its structure. Results indicate that the a-bonded framework is similar in all complexes and mainly governs the metal-carbon bonding in these species. In all cases but the titanium compound, the a-type M-C is hardly complemented by pi-back-donation from the metals into acceptor C, orbitals. The peculiar energy and composition of the HOMOs, which are in general rnetal-carbon antibonding and carbon-carbon bonding for the metals at the right of the periodic table, can allow the observation of cumulenic structures upon oxidation.

First author: Peacock, CL, Surface complexation model for multisite adsorption of copper(II) onto kaolinite, GEOCHIMICA ET COSMOCHIMICA ACTA, 69, 3733, (2005)
Abstract: We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO4Hn)(n-6) and binuclear (Cu2O6)(n-8) inner-sphere complexes on variable-charge equivalent to AlOH sites and as Cu2+ on ion exchangeable equivalent to X–H+ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)(6) polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (equivalent to AlOH)(2)Cu(OH)(2)(0), tridentate (equivalent to Al3O(OH)(2))Cu-2(OH)(3)(0) and equivalent to X–Cu2+ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions 2 equivalent to AlOH + Cu2+ + 2H(2)O = (equivalent to AlOH)(2)Cu(OH)(2)(0)+ 2H(+) 3(equivalent to AlOH) + 2Cu(2+) + 3H(2)O = (equivalent to Al3O(OH)(2))Cu-2(OH)(3)(0)+ 4H(+) and equivalent to X—H+ + Cu2+ = equivalent to X—Cu2+ + X+. (Al)

First author: Giorgi, G, The existence of atomic-like states of silicon in siloxanic networks: A DFT approach,MICROELECTRONIC ENGINEERING, 81, 448, (2005)
Abstract: In this paper, we intend to demonstrate the possible presence of atomic-silicon cryptates in siloxanic networks by using a density functional approach. Since model molecules have been employed to model the candidate sites to host atomic silicon, we found that metastable adducts can be formed only in regions where the siloxanic network is not subjected to steric constraints; differently, stationary states are unstable in highly reticulated siloxanic networks. Our conclusions reveal that Si atom is kept at the surface in atomic-like configuration by a charge donation from oxo-oxygen atoms into the empty Si p(pi) orbital and by pi charge back-donation from singly occupied Si 3p, orbitals into empty sigma* model molecule orbitals.

First author: Bruce, MI, Iron versus ruthenium: Dramatic changes in electronic structure result from replacement of one Fe by Ru in [{Cp*(dppe)Fe}-CC-CC-{Fe(dppe)Cp*}](n+) (n=0, 1, 2), ORGANOMETALLICS, 24, 3864, (2005)
Abstract: The reactions of FeCl(dppe)Cp* and Ru(C equivalent to CC equivalent to CH)L2Cp’ with Na[BPh4] and 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu; 2 equiv) in a mixed thf/NEt3 solvent afford {Cp*(dppe)-Fe}(C equivalent to CC equivalent to C){Ru(PP)Cp’} (PP = dppe, Cp’ = Cp*, 7; PP = (PPh3)(2), Cp’ = Cp, 8). Cyclic voltammetry shows that these mixed Fe/Ru complexes undergo sequential loss of up to three electrons, with the mono- and dioxidized species being isolated following chemical oxidation. Computational (DFT) and spectroscopic (IR, NMR, ESR, Mossbauer) studies are consistent with a polarized ground-state structure with oxidation leading to the gradual evolution of cumulenic character in the FeC4Ru moiety and a greater degree of orbital mixing between the Fe, C, and Ru centers than found in the related heterometallic complex [{Cp*(dppe)-Fe}(C equivalent to CC equivalent to C){Re(NO)(PPh3)Cp*}](n+) ([6](n+)). In contrast to the two-electron oxidation products derived from the diiron complex {Cp*(dppe)Fe}(C equivalent to CC equivalent to C){Fe(dppe)Cp*} (1) and iron/rhenium complex 6, the dications [7](2+) and [8]2+ feature a dominant contribution from a singlet ground state. Thus, while 6 behaves in a manner closely related to 1, 7 and 8 are more closely related to {Cp(Ph3P)(2)Ru}(C equivalent to CC equivalent to C){Ru(PPh3)(2)Cp} (2) and {Cp*(dppe)Ru}-(C equivalent to CC equivalent to C){Ru(dppe)Cp*} (3), clearly demonstrating the pronounced role that choice of metal as well as formal electron count can play in tuning the electronic and magnetic properties of this fascinating class of compound.

First author: Cedeno, DL, Metal-olefin interactions in M(CO)(5)(cycloolefin) (M = Cr, Mo, W; Cycloolefin = cyclopropene to cyclooctene): Strain relief and metal-olefin bond strength, ORGANOMETALLICS, 24, 3882, (2005)
Abstract: Density functional theory calculations on the title compounds indicate that metal-olefin bond strengths follow the trend for cyclic olefin strain energies. It was found, however, that the proportionality between metal-olefin bond energy and strain energy is not evenly distributed throughout the olefin series. For instance, cyclopropene and cyclobutene are expected to bind to the metal much more weakly than would be anticipated on the basis of their strain energies. A bond energy decomposition analysis reveals that the metal-olefin interaction is responsible for strain relief in the cycloolefins by means of the rehybridization of the olefinic carbons. However, the geometrical changes accompanying this rehybridization, namely C=C elongation and olefin pyramidalization, involve an energetic cost that is paid at the expense of the bonding interaction energy. Nonpyramidalized strained olefins such as cyclopropene and cyclobutene undergo large conformational changes, to the detriment of their large attractive interaction energies. It was found that a cyclic olefin that is already deformed, such as trans-cyclooctene, interacts strongly with a metal to relieve strain but does not suffer much energy-costly reorganizations. This, thus, constitutes an energy benefit to the metal-trans-cyclooctene bond strength.

First author: De Jong, GT, Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study, JOURNAL OF COMPUTATIONAL CHEMISTRY, 26, 1006, (2005)
Abstract: We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol.

First author: Zuidema, E, Reductive elimination of organic molecules from palladium-diphosphine complexes,ORGANOMETALLICS, 24, 3703, (2005)
Abstract: The effects governing the rate of reductive elimination of dimethyl ether, acetonitrile, vinyl cyanide, and methyl ethanoate from palladium diphosphine complexes were studied by means of a density functional theory method. Energy barriers, computed as the difference in energy between the reactant and the corresponding transition state using H2P(CH2)(2)PH2 as model for diphosphine ligands, varied from 38 kcal mol(-1) (dimethyl ether) to barrierless elimination of methyl ethanoate, in good agreement with experimental results. MO analysis reveals striking differences that are related to the donor/acceptor capabilities of the reacting moieties. For the elimination of acetonitrile, the bite angle effect on the reaction rate, observed by Moloy when different diphosphine ligands were used, was studied in depth. We considered (R2PXPR2)Pd(CH3)(CN) complexes for a number of different ligand backbones (X = (CH2)(n), n = 1-4; X = cis-, trans-but-2-ene) which span a large bite angle range and for a number of different phosphine substituents (R = H, Me, Ph). With the use of QM/MM strategies, steric and electronic effects were separated and evaluated, and the results indicate that the rate enhancement is electronic in nature, steric effects being negligible. The analysis reveals that wide bite angle ligands destabilize the reactant and stabilize the transition state, thus accelerating the reaction.

First author: Saladino, AC, DFT calculations of EPR parameters of transition metal complexes: Implications for catalysis,CATALYSIS TODAY, 105, 122, (2005)
Abstract: Transition metal and ligand hyperfine coupling constants for paramagnetic vanadium and copper model complexes have been calculated using DFT methods that are available in commercial software packages. Variations in EPR parameters with ligand identity and ligand orientation are two of the trends that have been investigated with DFT calculations. For example, the systematic variation of the vanadium hyperfine coupling constant with orientation for an imiclazole ligand in a VO2+ complex has been observed experimentally and has also been reproduced by DFT calculations. Similarly, changes in the vanadium hyperfine coupling constant with ligand binding have been calculated using model complexes and DFT methods. DFT methods were also used to calculate ligand hyperfine coupling constants in transition metal systems. The variation of the proton hyperfine coupling constant with water ligand orientation was investigated for [VO(H2O)(5)](2+) and the results were used to interpret high resolution EPR data of VO2+-exchanged zeolites. Nitrogen hyperfine and quadrupole coupling constants for VO2+ model complexes were calculated and compared with experimental data. The computational results were used to enhance the interpretation of the EPR data for vanadium-exchanged zeolites which are promising catalytic materials. The implications of the DFT calculations of EPR parameters with respect to catalysis will be discussed.

First author: Cavigliasso, G, Metal-metal interactions in mixed-valence [M2Cl9](2-) species: Electronic structure of d(1)d(2) (V, Nb, Ta) and d(4)d(5) (Fe, Ru, Os) face-shared systems, INORGANIC CHEMISTRY, 44, 5081, (2005)
Abstract: The molecular and electronic structures of mixed-valence d(1)d(2) (V, Nb, Ta) and d(4)d(5) (Fe, Ru, Os) face-shared [M2Cl9](2-) dimers; have been calculated by density functional methods in order to investigate metal-metal bonding in this series. General similarities are observed between d(1)d(2) and d(4)d(5) systems and can be considered to reflect the electron-hole equivalence of the individual d(1)-d(5) and d(2)-d(4) configurations. The electronic structures of the dimers have been analyzed using potential energy curves for the broken-symmetry and other spin states resulting from the d(1)d(2) and d(4)d(5) coupling modes. In general, a spin-doublet (S = 1/2) state, characterized by delocalization of the metal-based electrons in a metal-metal bond with a formal order of 1.5, is favored in the systems containing 4d and 5d metals, namely, the Nb, Ta, Ru, and Os dimers. In contrast, the calculated ground structures for [V2Cl9](2-) and [Fe2Cl9](2-) correspond to a spin-quartet (S = 3/2) state involving weaker coupling between the metal centers and electron localization. In the case of [Ru2Cl9](2-), both the spin-doublet and spin-quartet states are predicted to be energetically favored suggesting that this species may exhibit double-minima behavior. A comparison of computational results across the (d(1)d(1), d(1)d(2), d(2)d(2)) [Nb2Cl9](z-) and [Ta2Cl9](z-) and (d(4)d(4), d(1)d(5), d(5)d(5)) [Ru2Cl9](z-) and [Os2Cl9](z-) series has revealed that, in all four cases, the shortening of the metal-metal distances correlates with an increase in formal metal-metal bond order.

First author: Atanasov, M, The stability and eventual lone pair distortion of the hexahalide complexes and molecules of the fifth to eighth main-group elements with one lone pair, as isolated entities or in oligomeric clusters: A vibronic coupling and DFT study, INORGANIC CHEMISTRY, 44, 5092, (2005)
Abstract: The introduced DF-supported vibronic coupling model together with the hardness rule indicates, for the title compounds, that the tendency toward lone pair (LP) distortions decreases with increasing coordination number (CN) and upon proceeding from fluoride to iodide as the ligands. Thus, only some hexafluoro complexes and molecules are calculated to actually undergo LIP deformations; here, from the possible highest-symmetry deformations, those with C-4v geometry possess the lowest energy, leading to the complete ablation of one ligand and, hence, explaining the nonexistence of the complexes AsF63-, SbF63-, and SF62-. If a lower-symmetry strain is imposed on the octahedral species, for example, induced by the simultaneous presence of terminal and bridging ligands in oligomers, the vibronic energy potential is activated. It may induce pronounced distortions, which are much larger than those of analogous clusters with central ions lacking the LP. Dimers and tetramers with common edges and faces are investigated, with the predicted and calculated deformations of the constituting octahedra again following the stability sequence C-4v > C-2v > C-3v The model nicely accounts for the observed trends, as well as reproduces the experimental structures and energy balances, at least semiquantitatively; its predictive power exceeds that of the valence-shell electron-pair repulsion concept.

First author: Bangesh, M, TD-DFT studies on the electronic structure of imidazole bound vanadate in vanadium containing haloperoxidases (VHPO), JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 725, 163, (2005)
Abstract: Time dependent density functional theory (TD-DFT) is utilized to study the electronic structure of the active site of vanadium containing haloperoxidases (VHPOs). Calculations on the species, modeled for pentacoordination of the vanadium atom as found in the active site of the enzyme are performed for different geometries and protonation states of the vanadate moiety. Calculations are also carried out on structures in which vanadate is surrounded by all of the seven amino acid residues which constitute the active site of the enzyme. Two such structures with mono and doubly protonated vanadate were considered. Excitations calculated by TD-DFT, using statistical average of the orbital model exchange-correlation potential (SAOP), reveal that the excitation behavior of doubly protonated vanadate species located in the protein model matrix closely resembles the experimentally observed optical transition for the holoenzyme. Implications of the hydrogen bonding interactions through the protein environment as well as the variation of the protonation state of the active site vanadate are discussed. For the vanadate a rather large distortion from the idealized trigonal bipyramidal arrangement is observed.

First author: Le Guennic, B, Density functional study of H-D coupling constants in heavy metal dihydrogen and dihydride complexes: The role of geometry, spin-orbit coupling, and gradient corrections in the exchange-correlation kernel,JOURNAL OF CHEMICAL THEORY AND COMPUTATION, 1, 601, (2005)
Abstract: The H-D nuclear spin-spin coupling constants J(H-D) of 14 heavy transition-metal clihydrogen and dihydride complexes were calculated with density functional theory using the “zeroth-order regular approximation” (ZORA) for the one-electron operators. The applied gradientcorrected density functional was able to achieve an average agreement with experimental data that is almost comparable to what has been obtained recently with hybrid functionals [J. Am. Chem. Soc. 2004, 126,14249]. However, a systematic overestimation of J(H-D) for complexes with short H-D distances was obtained, which could be traced back to problems of the gradient functional to describe the H-D coupling in free dihydrogen well. We implemented gradient corrections for the exchange-correlation (XC) kernel and employed a basis sets with highexponent 1s function for the coupled hydrogens. The gradient terms in the XC kernel turned out to be very important in order to achieve reasonable agreement with experimental coupling constants. On the other hand, our study reveals that spin-orbit relativistic corrections on the H-D coupling constants are comparatively small and need not to be considered at the accuracy level of currently available “standard” density functionals. The discussion of the results highlights the strong dependence of the coupling constants on the H-D distance and the possibility of large vibrational contributions to them. We also discuss the coupling constant for the hydrogen molecule in detail because of its relevance to the coupling in dihydrogen and dihydricle complexes.

First author: de Jong, GT, DFT benchmark study for the oxidative addition of CH4 to Pd. Performance of various density functionals, CHEMICAL PHYSICS, 313, 261, (2005)
Abstract: We have evaluated the performance of 24 popular density functionals for describing the potential energy surface (PES) of the archetypal oxidative addition reaction of the methane C-H bond to the palladium atom by comparing the results with our recent ab initio (CCSD(T)] benchmark study of this reaction. The density functionals examined cover the local density approximation (LDA), the generalized gradient approximation (GGA), meta-GGAs as well as hybrid density functional theory. Relativistic effects are accounted for through the zeroth-order regular approximation (ZORA). The basis-set dependence of the density-functional-theory (DFT) results is assessed for the Becke-Lee-Yang-Parr (BLYP) functional using a hierarchical series of Slater-type orbital (STO) basis sets ranging from unpolarized double-zeta (DZ) to quadruply polarized quadruple-zeta quality (QZ4P). Stationary points on the reaction surface have been optimized using various GGA functionals, all of which yield geometries that differ only marginally. Counterpoise-corrected relative energies of stationary points are converged to within a few tenths of a kcal/mol if one uses the doubly polarized triple-zeta (TZ2P) basis set and the basis-set superposition error (BSSE) drops to 0.0 kcal/mol for our largest basis set (QZ4P). Best overall agreement with the ab initio benchmark PES is achieved by functionals of the GGA, meta-GGA, and hybrid-DFT type, with mean absolute errors of 1.3-1.4 kcal/mol and errors in activation energies ranging from +0.8 to – 1.4 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with an only slightly larger mean absolute error of 2.5 kcal/mol and an underestimation by -1.9 kcal/mol of the overall barrier (i.e., the difference in energy between the TS and the separate reactants). For comparison, with B3LYP we arrive at a mean absolute error of 3.8 kcal/mol and an overestimation of the overall barrier by 4.5 kcal/mol.

First author: Sieger, M, Geometrical and electronic structures of dinuclear complex ions {(mu-bpym)[Cu(EAr3)(2)](2)}(2+) with intramolecular “organic sandwich” formation (E = P or As; Ar = Aryl; bpym=2,2 ‘-bipyrimidine), INORGANIC CHEMISTRY, 44, 4637, (2005)
Abstract: The compound {(mu-bpym)[Cu(AsPh3)(2)](2)}(BF4)(2) (1) has been prepared and studied in comparison with the triphenylphosphine analogue 2. Qualitatively, the structure of 1 with characteristically distorted copper(l) coordination caused by Ph/bpym/Ph sandwich interactions is similar to that of 2 and is approximately reproduced by DFT calculations for the model complex ions {(mu-bpym)[Cu(EMe2Ph)(2)](2)}(2+), E = P or As. In contrast, the dinuclear {(mu-bpym)[Cu(P(3-Me-C6H4)(3))(2)](2)}(BF4)(2) (3) displays a distinctly less distorted metal coordination geometry due to the steric requirements of the methyl groups in the meta-tolyl substituents. The electrochemical reduction of 1 is less reversible than for the phosphine analogues; the one-electron-reduced form 1.- exhibits a broad, unresolved EPR signal at g = 2.0023. Resonance Raman spectroscopy of 1 shows the typical vibrations of the bpym ligand in agreement with the MLCT assignment of the long-wavelength transitions below 500 nm. All three dinuclear complexes exhibit luminescence at room temperature in the solid and in solution.

First author: Karton, A, NLO properties of metallabenzene-based chromophores: A time-dependent density functional study, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 5454, (2005)
Abstract: The static and dynamic first hyperpolarizabilities for a series of substituted metallabenzene-based nonlinear optical (NLO) chromophores were determined by time-dependent density functional theory (TDDFT). The electronic excitation contributions to the first hyperpolarizability are rationalized in terms of the two-level model. The effects on the hyperpolarizabilities of (a) the metal center (Os, Ir, Pt); (b) the ligand environment (PH(3), CO, Cl); (c) various donor and acceptor substituents (NH(2), OH, Me, H, Cl, Br, I, COOMe, COOH, CN, NO(2)); and (d) the length of pi-conjugation were studied. Our calculations predict that metallabenzenes have significant second-order NLO susceptibilities, ranging from beta(0)(tot) = 10 x 10(-29) to 5.6 x 10(-28) esu and from u beta(0)(tot) = 3.0 x 10(-47) to 1.1 x 10(-44) esu, that can be tuned by changing the metal center and/or ligand environment.

First author: Stich, TA, Spectroscopic and computational studies of the ATP : Corrinoid adenosyltransferase (CobA) from Salmonella enterica: Insights into the mechanism of adenosylcobalamin biosynthesis, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 8710, (2005)
Abstract: CobA from Salmonella enterica is a member of an enzymatic system responsible for the de novo biosynthesis of adenosylcobalamin (AdoCbl), catalyzing the formation of the essential Co-C bond by transferring the adenosyl group from a molecule of ATP to a transient Co(1+)corrinoid species generated in the enzyme active site. A particularly fascinating aspect of this reaction is that the flavodoxin in vivo reducing agent that serves as the electron donor to CobA possesses a reduction potential that is considerably more positive than that of the CO2+/1+ couple of the corrinoid substrate. To explore how CobA may overcome this challenge, we have employed electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance (EPR) spectroscopies to probe the interaction between Co3+- and Co2+ corrinoids and the enzyme active site. Our data reveal that while Co(3+)corrinoids interact only weakly with CobA, Co(2+)corrinoids undergo partial conversion to a new paramagnetic species that can be obtained in nearly quantitative yield when CobA is preincubated with the co-substrate ATP. This “activated” species is characterized by a distinct set of ligand field transitions in the near-IR spectral region and EPR parameters that are unprecedented for Co(2+)corrinoids. Analysis of these data on the basis of qualitative spectral correlations and density functional theory computations reveals that this unique CO(2+)corrinoid species possesses an essentially square-planar Co2+ Center that lacks any significant axial bonding interactions. Possible implications of these findings for the mechanism of Co2+ -> Co1+ reduction employed by CobA and Co-C bond-forming enzymes in general are explored.

First author: Zhang, XY, Experimental and computational study of the [2+2] dissociation of rhenaoxetanes in the gas phase, ORGANOMETALLICS, 24, 3040, (2005)
Abstract: Energy-resolved collision-induced dissociation cross-section measurements are applied to the reaction by which a rhenium diolate complex loses an aldehyde to produce a rhenium carbene. The measured threshold as well as ab initio calculations indicate that the diolate has rearranged to a metallaoxetane, from which the [2 + 2] dissociation occurs. The measured activation energy is in good agreement with the computed values.

First author: Goumans, TPM, The asymmetric Schrock olefin metathesis catalyst. A computational study,ORGANOMETALLICS, 24, 3200, (2005)
Abstract: The mechanism of the transition metal catalyzed olefin metathesis reaction with the Schrock catalyst is investigated with pure (BP86) and hybrid (B3LYP) density functional theory. On the free-energy surface there is no adduct between ethylene and model catalyst (MeO)(2)Mo(CH2)NH but instead a single transition on a flat surface giving a metallacyclo-butane with a trigonal bipyramidal conformation that has a sizable barrier for conversion to the square pyramidal form. The model was expanded to the active asymmetric Schrock catalyst and investigated with QM/MM, using BP86 for the QM part and AMBER 95 for the MM part, for the experimentally known ring-closing metathesis of a symmetrical triene. The factors effecting chiral induction to the observed product are delineated.

First author: Zbiri, M, Theoretical investigation of the Cs*He n >= 3 exciplexes, PHYSICS LETTERS A, 341, 170, (2005)
Abstract: The X-2 Sigma(1/2), A(2)Pi(1/2), B-2 Sigma(1/2), and A(2)Pi(3/2) states of Cs*He-n exciplexes, with n = {3,…, 9}, are studied using first principle density functional theory (DFT) formalism combined with the zeroth order regular approximation (ZORA). The zero points energies (ZPEs) E-0 for Cs*He-n are calculated at a first step, in order to estimate the stability of the studied exciplexes. Relativistic calculations are carried out including spin-orbit effect (SO), and potential energy surfaces of Cs*He-6 are shown. The electronic distribution shapes of the studied molecules are also discussed, and their emission lines are evaluated and compared to a new discovered experimental emission spectra, which has been recently assigned, using a semi-empirical model, to Cs(A Pi(1/2))He-7. Our first principle calculation suggests that the discovered emission line is most probably due to a decay of Cs(A Pi(1/2))He-6.

First author: Neugebauer, J, Importance of vibronic effects on the circular dichroism spectrum of dimethyloxirane,JOURNAL OF CHEMICAL PHYSICS, 122, 170, (2005)
Abstract: We present a theoretical study on the vibrational structure of a circular dichroism (CD) spectrum using time-dependent density-functional theory in combination with a Franck-Condon-type approach. This method is applied to analyze the complex CD spectrum of dimethyloxirane, which involves delicate cancellations of positive and negative CD bands. Our approach reveals that these cancellations are strongly affected by the shapes of the CD bands, and that it is vital for an accurate simulation of the spectrum to take the different envelopes of these bands into account. One crucial point in some former theoretical studies on this compound, which were restricted to vertical excitations, was the appearance of a strong negative CD band in the energy range of 7.0-7.5 eV, which is not present in the experimental spectrum. We can explain the disappearance of this 2B band by a strong vibrational progression along normal modes with C-O stretching character, so that the band extends over an energy range of almost 1.1 eV. Thus, it overlaps with many other (mostly positive) CD bands, leading to a cancellation of its intensity. The dominant vibrational features in the experimental spectrum can be assigned to the 1B, 3B, and 5B bands, which show several clear vibrational peaks and a total bandwidth of only 0.3-0.5 eV. In order to obtain close agreement between the simulated and the experimental spectrum we have to apply small shifts to the vertical excitation energies that enter the calculation. These shifts account both for possible errors in the time-dependent density-functional theory calculations and for the neglect of differential zero-point energy between ground and excited states in our gradient-based vertical Franck-Condon approach.

First author: Jensen, L, Finite lifetime effects on the polarizability within time-dependent density-functional theory,JOURNAL OF CHEMICAL PHYSICS, 122, 170, (2005)
Abstract: We present an implementation for considering finite lifetime of the electronic excited states into linear-response theory within time-dependent density-functional theory. The lifetime of the excited states is introduced by a common phenomenological damping factor. The real and imaginary frequency-dependent polarizabilities can thus be calculated over a broad range of frequencies. This allows for the study of linear-response properties both in the resonance and nonresonance cases. The method is complementary to the standard approach of calculating the excitation energies from the poles of the polarizability. The real and imaginary polarizabilities can then be calculated in any specific energy range of interest, in contrast to the excitation energies which are usually solved only for the lowest electronic states. We have verified the method by investigating the photoabsorption properties of small alkali clusters. For these systems, we have calculated the real and imaginary polarizabilities in the energy range of 1-4 eV and compared these with excitation energy calculations. The results showed good agreement with both previous theoretical and experimental results.

First author: Zheng, SL, Syntheses, structures, photoluminescence and theoretical studies of xanthone in crystalline resorcinarene-based inclusion complexes, CHEMISTRY-A EUROPEAN JOURNAL, 11, 3583, (2005)
Abstract: Two new crystalline resorcinarene-based xanthone inclusion complexes, CECR(.)xanthone(.)MeOH (1), and HECR(.)2xanthone(.)6MeOH (2) (CECR = C-ethylcalix[4]rcsorcinarene, HECR = hexacthylresorcin[6]arene) have been prepared to study the relation between photophysical properties and solid-state structure. Compared with the neat crystals, the xanthone phosphorescence is severely quenched in both solids, but the lifetime is an order of magnitude larger in 2, in which xanthone occurs as a dimer, than in 1. in which it occurs as a monomer. The electronic transitions involved in the photoluminescent process, and the relation between the energy levels of host and guest and emission quenching of the guest in the supramolecular solid have been investigated by means of time-dependent density functional theory (TDDFT) calculations.

First author: Le Guennic, B, Density functional calculation of the electronic circular dichroism spectra of the transition metal complexes [M(phen)(3)](2+) (M = Fe, Ru,, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 4836, (2005)
Abstract: The circular dichroism spectra of the tris-bidentate metal complexes Lambda-[M(phen)(3)](2+), with M = Fe, Ru, Os and phen = 1,10-tris-phenanthroline, are investigated computationally, employing time-dependent density functional theory. Good agreement with experimental spectra is obtained for Ru and Os. The Lambda-[Os(phen)(3)](2+) spectrum is analyzed in detail. It is shown how relativistic effects red shift CD bands where the Os 5d-orbital participates to a large extent in the excitations. Further, the participation of the metal in the ligand pi -> pi* exciton CD is determined to be of the order of 10%. Though solvent effects can have a noticeable effect on individual transitions and rotatory strengths, they are demonstrated to have only a very small overall effect on the resulting simulated CD spectra. For Lambda-[Fe(phen)(3)](2+), the results are shown to be rather sensitive to the choice of the applied hybrid and nonhybrid density functionals, and the optimized geometries based thereupon. In particular, the sign pattern of the lower-energy part (up to 33 x 10(3) cm(-1)) of the Lambda-[Fe(phen)(3)](2+) CD spectrum is difficult to reproduce. Some combinations of functionals and geometries yield good agreement with experiment, but no “best” approach can be devised based on the available results. Possible sources of errors in the spectrum of Lambda-[Fe(phen)(3)](2+) due to deficiencies in the functionals and the exchange-correlation kernels are investigated.

First author: Yamashige, H, Local structure of nitrogen atoms in a porphine ring of meso-phenyl substituted porphyrin with an electron-withdrawing group using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, ANALYTICAL SCIENCES, 21, 635, (2005)
Abstract: This study investigated the protonation of nitrogen atoms in porphyrins with meso-phenyl p-substituted by an electron-withdrawing group using N Is X-ray photoelectron spectroscopy (XPS), the N K X-ray absorption near-edge structure (XANES), and the discrete variational (DV)-X alpha molecular orbital (MO) method. Both tetraphenylporphyrin (TPP) and tetrakis(p-sulfonatophenyl)porphyrin (TSPP) have a single structure: the former has two protonated and two nonprotonated N atoms in the porphine ring; the latter has four protonated N atoms in the porphine ring. In contrast, a combination of XPS, XANES, and DV-X alpha MO calculations shows that tetrakis(p-carboxyphenyl)porphyrin (TCPP) has a dual structure: one structure has two protonated and two non-protonated N atoms; the other has four protonated N atoms. Furthermore, this result was also considered based on the protonation constants of N atoms in the porphyrins. The correlation between the strength of electron-withdrawing groups and protonation to N atoms in porphyrins can be described using the spectral patterns of the N Is XPS and N K XANES spectra.

First author: Patchkovskii, S, Gauge invariance of the spin-other-orbit contribution to the g-tensors of electron paramagnetic resonance, JOURNAL OF CHEMICAL PHYSICS, 122, 635, (2005)
Abstract: The spin-other-orbit (SOO) contribution to the g-tensor (Delta g(SOO)) of electron paramagnetic resonance arises due to the interaction of electron-spin magnetic moment with the magnetic field produced by the orbital motion of other electrons. A similar mechanism is responsible for the leading term in nuclear magnetic-shielding tensors sigma. We demonstrate that analogous to sigma, paramagnetic Delta g(SOO) contribution exhibits a pronounced dependence on the choice of the magnetic-field gauge. The gauge corrections to Delta g(SOO) are similar in magnitude, and opposite in sign, to the paramagnetic SOO term. We calculate gauge-invariant Delta g(SOO) values using gauge-including atomic orbitals and density-functional theory. For organic radicals, complete gauge-invariant Delta g(SOO) values typically amount to less than 500 parts per million (ppm), and are small compared to other g-tensor contributions. For the first-row transition-metal compounds, Delta g(SOO) may contribute several thousand ppm to the g-tensor, but are negligible compared to the remaining deviations from experiment. With popular choices for the magnetic-field gauge, the individual gauge-variant contributions may be an order of magnitude higher, and do not provide a reliable estimation of Delta g(SOO).

First author: Chisholm, MH, Interconverting WW triple bonds and W-4 clusters: Structures of W-4(OPrn)(16) and [Li2W2(OPrn)(8)(DME)](2), JOURNAL OF CLUSTER SCIENCE, 16, 231, (2005)
Abstract: Alcoholysis Of W-2(NMe2)(6) with excess n-propanol in hexane yields the tetranuclear cluster, W-4(OPrn)(16), I. Reduction of I with two equivalents of Li2COT in THF gives a small yield of Li2W2(OPrn)(8). Single crystals were isolated by cooling the product mixture in DME and were shown to be [Li2W2(OPrn)(8)(DME)](2), II, which consists of a unique “dimer of dimers” structure. In this reaction sequence, W-4(16+) cluster formation is followed by four electron reduction to reform the (W = W)(6+) unit. Better yields of the lithium salt can be obtained by the addition of LiOPrn/HOPrn solutions to W-2(OBut)(6) in which case Li2W2(OPrn)(8) has been obtained as a 1: 1 adduct with LiOPr. This identity of this salt was confirmed by solution NMR spectroscopy. In the alternative reaction, the (W = W)(6+) center remains intact from reactant to product. No attempt has been made to separate the product from excess LiOPr. DFT (ADF 2004.01) molecular orbital calculations on the model cluster W-4(OH)(16) are used to help elucidate the disruption of the W-4 cluster upon four electron reduction. The molecular structures of compounds I and II are reported.

First author: Aspuru-Guzik, A, Software news and updates – Zori 1.0: A parallel quantum Monte Carlo electronic structure package, JOURNAL OF COMPUTATIONAL CHEMISTRY, 26, 856, (2005)
Abstract: The Zori 1.0 package for electronic structure computations is described. Zori performs variational and diffusion Monte Carlo computations as well as correlated wave function optimization. This article presents an overview of the implemented methods and code capabilities.

First author: Atanasov, M, Chemical bonding in molecules and complexes containing d-elements based on DFT,MONATSHEFTE FUR CHEMIE, 136, 925, (2005)
Abstract: Metal-ligand bonding in transition metal halide molecules and complexes with different central ions, oxidations states, and coordination numbers: Cr(III)X(6)(3-), Cr(IV)X(4), Cr(II)X(2) (X = F,Cl,Br,I), M(III)Cl(6)(3-) (M=Mo,W), M(III)(H(2)O)(6)(3+)(M = Cr,Co) and Re(2)Cl(8)(2-) has been studied in terms of the Extended Transition State (ETS) energy patitioning scheme within the DFT and electron density analysis (the Laplacian of the electron density and the electronic localization function). Bonding is found to be dominated by ionicity in all cases, especially so for complexes with higher coordination numbers. Covalent contributions to the metal-ligand bond are found to be mainly due to the nd-electrons and to lesser extent due to the metal (n + 1)s and (n + 1)p-orbitals, contributions from (n + 1)s increasing when going to lower coordination numbers. Metal-ligand bonding analysis have been Used in order to check some concepts emerging from ligand field theory when applied to the spectroscopy and magnetism of transition metal complexes. It is pointed out that for complexes of high symmetry (MX(6), O(h), MX(4), T(d), and MX(2), D(infinity h)) electron density analyses gain interpretative power when partitioned into contributions from occupied orbitals of different symmetry.

First author: Guerra, CF, Substituent effif ects on hydrogen bonding in Watson-Crick base pairs. A theoretical study,STRUCTURAL CHEMISTRY, 16, 211, (2005)
Abstract: We have theoretically analyzed Watson-Crick AT and GC base pairs in which purine C8 and/or pyrimidine C6 positions carry a substituent X = H, F, Cl or Br, using the generalized gradient approximation (GGA) of density functional theory at BP86/TZ2P. The purpose is to study the effects on structure and hydrogen bond strength if X = H is substituted by a halogen atom. Furthermore, we wish to explore the relative importance of electrostatic attraction versus orbital interaction in the above multiply hydrogen-bonded systems, using a quantitative bond energy decomposition scheme. We find that replacing X = H by a halogen atom has relatively small yet characteristic effects on hydrogen bond lengths, strengths and bonding mechanism. In general, it reduces the hydrogen-bond-accepting- and increases the hydrogen-bond-donating capabilities of a DNA base. The orbital interaction component in these hydrogen bonds is found for all substituents (X = H, F, Cl, and Br) to contribute about 41% of the attractive interactions and is thus of the same order of magnitude as the electrostatic component, which provides the remaining 59% of the attraction.

First author: Pietrzyk, P, Spectroscopy and computations of supported metal adducts. 1. DFT study of CO and NO adsorption and coadsorption on CU/SiO2, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 10291, (2005)
Abstract: Interactions of the CO and NO molecules with the Cu-II and Cu-I isolated sites on the amorphous silica surface are investigated by means of density functional theory (DFT) methods within the finite cluster model approach. The clusters of silica of increasing nT size (T = Si) are used, with n from 2 to 6. The Cull sites are characterized by calculated g-tensors and hyperfine coupling constants (HFCCs) and compared with experiment. On this basis, the three-coordinated complexes are the most plausible. Due to the charge transfer from the silica “ligand”, the metal charge shrinks and the spin density is distributed over silanol and siloxy groups up to 50%. The reduced sites are exclusively two-coordinated. Strong interaction of CO with Cu-I-nT sites (31-39 kcal/mol) gives rise to the formation of carbonyl adducts with planar coordination around copper. The population of the ligand pi* system shifts downward the stretching frequency in agreement with experiment. Reaction with a second CO molecule gives a geminal dicarbonyl of very uniform structure independent of the site. Carbonyl complexes with Cull are less stable and of tetrahedral coordination of the metal. Accumulation of the positive charge on the complex along with sigma overlap with d orbitals locates the calculated CO stretching frequency above free molecule value. NO molecule is preferably bound to the Cu-II-nT sites, forming a tetrahedral complex with tilted adsorbate and NO stretching frequency blue-shifted with respect to the free molecule value. The full set of electron paramagnetic resonance (EPR) parameters and vibrational frequencies for the copper(l) mononitrosyl, {CuNO}(11) , though not observed experimentally, are predicted and compared to the same magnetophore inside the ZSM-5 zeolite. The interaction energies show that in the CO/NO reaction mixture adsorption is selective and allows discrimination between Cu-I and Cu-II sites. However, for the Cu-I complex, formation of mixed-ligand structures of the {Cu(CO)(NO)}(11) type is possible.

First author: DeKock, RL, Possible side reactions due to water in emulsion polymerization by late transition metal complexes II: Deactivation of the catalyst by a Wacker-type reaction, ORGANOMETALLICS, 24, 2679, (2005)
Abstract: The transition metal catalyzed ethylene polymerization in water suspensions has been increasingly successful in the last couple of years. Water, however, adversely affects the polymerization process by deactivating the catalyst in a Wacker-type reaction. The cationic Pd(II)-diimine Brookhart catalyst 1 is studied here by a combination of quantum mechanics and molecular mechanics to determine the nature of the decomposition reaction. The study considers the decomposition process to take place in two stages. In the first stage, a coordinated olefin is attacked by a hydroxide group to ultimately produce a beta-hydroxy-ethyl complex 2. The second step represents a decomposition to acetaldehyde and Pd(O), the latter in the form of palladium black. For the attack of OH on the coordinated ethylene in 1, both an internal path involving transfer from a palladium-coordinated OH group produced from hydrolysis of a Pd-CH3 bond and an external path based on attack of H2O on coordinated ethylene were considered. Both paths are found to be feasible. For the second stage the most promising decomposition mode involves isomerization of 2 to the a-hydroxy-ethyl complex 5 followed by abstraction of a proton from the C-OH link to produce the Pd(O)-eta(2)-acetaldehyde complex 12. Finally, 12 releases acetaldehyde under the deposition of Pd(O) as palladium black.

First author: Wang, F, The calculation of excitation energies based on the relativistic two-component zeroth-order regular approximation and time-dependent density-functional with full use of symmetry, JOURNAL OF CHEMICAL PHYSICS,122, 2679, (2005)
Abstract: In the present work, we propose a relativistic time-dependent density-functional theory (TDDFT) based on the two-component zeroth-order regular approximation and a noncollinear exchange-correlation (XC) functional. This two-component TDDFT formalism has the correct nonrelativistic limit and affords the correct threefold degeneracy of triplet excitations. The relativistic TDDFT formalism is implemented into the AMSTERDAM DENSITY FUNCTIONAL program package for closed-shell systems with full use of double-group symmetry to reduce the computational effort and facilitate the assignments. The performance of the formalism is tested on some closed-shell atoms, ions, and a few diatomic molecules containing heavy elements. The results show that the fine structure of the excited states for most atoms and ions studied here can be accurately accounted for with a proper XC potential. For the excitation energies of the molecules studied here, the present formalism shows promise and the error encountered is comparable to that of nonrelativistic TDDFT calculations on light elements.

First author: Wei, HY, Magnetic exchange of trinuclear spin frustration system: CASPT2 and density functional theory study on hydroxo-bridged chromium complex [Cr-3(NH3)(10)(OH)(4)]center dot Br-5, CHEMICAL PHYSICS LETTERS,407, 147, (2005)
Abstract: The theoretical calculations on magnetic exchange of the title complex are carried out by using multiconfigurational second-order perturbation theory, CASPT2, and density functional theory combined with broken-symmetry approach, DFT-BS. The CASPT2 calculated quartet ground state, stabilized by 8.02 cm(-1) compared with the nearest excited doublet state, is close to 5 cm(-1) of the experimental measurement. The dependence of energy levels on the ratio J(23)/J(12,13) validates occurrence of the spin frustration I phenomenon. Both CASPT2 and DFT-BS calculations reproduce well experimentally coupling constants of J(12,13) = -9.35 cm(-1), = -3.95 cm(-1), for CASPT2 J(12,13) = -6 cm(-1), J(23) = -2 cm(-1) and for DFT-BS J(12,13) = -11.31 cm(-1), J(23) = -4.20 m(-1). The methodological comparison is briefly discussed.

First author: Huang, DM, Structure and stability of thiophene-hydrogen halide complexes: An ab initio molecular orbital study, CHEMICAL PHYSICS LETTERS, 407, 222, (2005)
Abstract: The structures of thiophene-hydrogen halide (HX) complexes are examined at the second-order Moller-Plesset (MP2) level to understand hydrogen bonding and intermolecular interactions of a heteroaromatic ring as the hydrogen bond acceptor. In contrast with the previous results on similar furan complexes, only the face-on geometry type is observed for the thiophene complexes. Variations in complex geometry can be accounted for by the differences in the electrostatic potential on the aromatic ring. A decomposition of interaction energy reveals that thiophene-hydrogen halide is bounded largely by orbital and electrostatic energy contributions.

First author: Dibrov, SM, Synthesis and structural characterization of some selenoruthenates and telluroruthenates,INORGANIC CHEMISTRY, 44, 3441, (2005)
Abstract: The reaction of solid [RuCICp(PPh3)(2)] with TeSe32- or Se-n(2-) in DMF leads to the formation of [RuCp(PPh3)(μ(2)-Se-2)](2) (1). In the structure of this compound the two bridging Se-2 groups lead to a six-membered Ru2Se4 ring in a chair conformation. Attached to each Ru center is a PPh3 ligand in an equatorial position and a Cp ring in an axial position. The compound is diamagnetic. The compound [Ru2Cp2(μ(3)-Se-2)(μ(3)-Se)](2)(2) is obtained under similar conditions in the presence of air. This structure comprises a centrosymmetric RU4Se6 dimer formed from the two bridging Se groups and the two bridging Se2 groups. Each Ru center is π-bonded to a Cp ring. The reaction of solid [RuCICp(PPh3)(2)] with a Te-n(2-) polytelluride solution in DMF leads to the diamagnetic compound [(RuCp-(PPh3))(2)(μ(2)-(1,4-η:3,6-η)Te-6)] (3). Here the Ru centers are bound to a bridging Te6 chain at the 1, 4, 3, and 6 positions, leading to a bicyclic Ru2Te6 ring. Each Ru atom is bound to a Cp ring and a PPh3 group. This dimer possesses a center of symmetry. The structure of 3 is the first example of a bicyclic complex where fusion occurs along a Te-Te bond. If the same reaction is carried out in DMF/CH2Cl2, rather than DMF, then [(RuCp(PPh3))(2)-(μ(2)-(1,4-η:3,6-η)Te-6)]-CH2CI2 (4) is obtained. In the solid state it possesses the same Ru2Te6 structural unit as does 3, but the unit lacks a crystallographically imposed center of symmetry. The electronic structures of 3 and 4 have been analyzed with the use of first principles density functional theory. Bond order analysis indicates that the Te-Te bond where fusion occurs has a shared bonding charge of about (2)/(3) of that found for Te-Te single bonds.

First author: Schuhly, W, Hyperolactone C: Determination of its absolute configuration by comparison of experimental and calculated CD spectra, CHIRALITY, 17, 250, (2005)
Abstract: A detailed conformational analysis of hyperolactone C diastereomers and enantiomers ((5R,9R),(5S,9S) and (5S,9R),(5R,9S)) was done with molecular mechanics and density functional theory methods. Time-dependent density functional theory (B3PW91/TZVP) was used to calculate electronic transition energies (U-V/vis spectra) and rotational strengths of the respective conformations. The effect of solvation (acetonitrile solution) on excitation energies and electronic circular dichroism was approximated by the polarizable continuum model. By comparison of the simulated CD spectrum with that measured for hyperolactone C isolated from Hypericum lloydii, its absolute configuration can be assigned as (5S,9S).

First author: Campbell, KA, Ab initio and microcalorimetric investigations of alkene adsorption on phosphotungstic acid,LANGMUIR, 21, 4738, (2005)
Abstract: The adsorption of ethene, propene, 1-butene, trans-2-butene, and isobutene on phosphotungstic acid has been characterized by density functional theory (DFT) calculations and microcalorimetric experiments. The DFT-calculated chemisorption energies to form the corresponding alkoxides for ethene, propene, 1-butene, trans-2-butene, and isobutene were -86.8, -90.3, -102.6, -79.9, and -91.4 M mol(-1), respectively (for their most-favorable binding modes). The relative chemisorption energies to form the alkoxides are dictated by the strength of interaction of the acidic proton with the carbon atom of the double bond that becomes protonated. The activation barrier for chemisorption was greatest for alkenes with primary (1&DEG;) carbenium-like transition states followed by secondary (2&DEG;) and tertiary (3&DEG;) transition states. The adsorption enthalpy established from microcalorimetric experiments with propene and isobutene was approximately – 100 M mol(-1), which is close to the DFT-calculated values. Chemisorption of ethene on phosphotungstic acid during microcalorimetric experiments was minimal, presumably because of the large activation barrier associated with a 1&DEG; carbenium-like transition state. The results from this study are compared with those in the literature for the adsorption of alkenes on zeolites, which have a similar adsorption mechanism. Our results suggest that alkene adsorption is stronger on phosphotungstic acid than on zeolites, as supported by the more exothermic chemisorption energies. Additionally, activation barriers for alkene adsorption are lower over phosphotungstic acid than over zeolites.

First author: Lein, M, Why do the heavy-atom analogues of acetylene E2H2 (E = Si-Pb) exhibit unusual structures?,JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 6290, (2005)
Abstract: DFT calculations at BP86/QZ4P have been carried out for different structures of E2H2 (E = C, Si, Ge, Sri, Pb) with the goal to explain the unusual equilibrium geometries of the heavier group 14 homologues where E = Si-Pb. The global energy minima of the latter molecules have a nonplanar doubly bridged structure A followed by the singly bridged planar form B, the vinylidene-type structure C, and the trans-bent isomer D1. The energetically high-lying trans-bent structure D2 possessing an electron sextet at E and the linear form HE equivalent to EH, which are not minima on the PES, have also been studied. The unusual structures of E2H2 (E = Si-Pb) are explained with the interactions between the EH moieties in the (X(2)Pi) electronic ground state which differ from C2H2, which is bound through interactions between CH in the a(4)Sigma(-) excited state. Bonding between two (X(2)Pi) fragments of the heavier EH hydrides is favored over the bonding in the a a(4)Sigma(-) excited state because the X(2)Pi -> a(4)Sigma(-) excitation energy of EH (E = Si-Pb) is significantly higher than for CH. The doubly bridged structure A of E2H2 has three bonding orbital contributions: one a bond and two E-H donor-acceptor bonds. The singly bridged isomer B also has three bonding orbital contributions: one pi bond, one E-H donor-acceptor bond, and one lone-pair donor-acceptor bond. The trans-bent form D1 has one pi bond and two lone-pair donor-acceptor bonds, while D2 has only one a bond. The strength of the stabilizing orbital contributions has been estimated with an energy decomposition analysis, which also gives the bonding contributions of the quasi-classical electrostatic interactions.

First author: Guerra, G, Molecular modeling of stereo- and regioselectivity of group 4 heterocenes in the polymerization of propene, MACROMOLECULES, 38, 3973, (2005)
Abstract: We report a theoretical analysis on the stereo- and regiochemical behavior of the S- and N-based group 4 heterocenes-developed by Ewen, Elder, and Jones-in the polymerization of propene to isotactic polypropylene. Somewhat surprisingly, the polymers produced with the N-based heterocene were shown to present a sensibly smaller amount of regiomistakes and a higher amount of stereomistakes relative to the S-based heterocene. To clarify the different mechanistic behavior of these structurally rather similar catalysts, we compared their stereo- and regioselectivity, and we extended our modeling to the corresponding all-carbon bis(indenyl)-based catalyst. Our analysis supports most of the ideas proposed by Ewen, Elder, and Jones to rationalize the experimental behavior.

First author: McDonough, JE, Solution calorimetric and stopped-flow kinetic studies of the reaction of center dot Cr(CO)(3)C5Me5 with PhSe-SePh and PhTe-TePh. Experimental and theoretical estimates of the Se-Se, Te-Te, H-Se, and H-Te bond strengths, INORGANIC CHEMISTRY, 44, 3127, (2005)
Abstract: The kinetics of the oxidative addition of PhSeSePh and PhTeTePh to the stable 17-electron complex center dot Cr(CO)(3)C5Me5 have been studied utilizing stopped-flow techniques. The rates of reaction are first-order in each reactant, and the enthalpy of activation decreases in going from Se (Delta H-double dagger = 7.0 ± 0.5 kcal/mol, Delta S-double dagger = -22 ± 3 eu) to Te (Delta H-double dagger = 4.0 ± 0.5 kcal/mol Delta S-double dagger = -26 ± 3 eu). The kinetics of the oxidative addition of PhSeH and center dot Cr(CO)(3)C5Me5 show a change in mechanism in going from low (overall third-order) to high (overall second-order) temperatures, The enthalpies of the oxidative addition of PhE-EPh to center dot Cr(CO)(3)C5Me5 in toluene solution have been measured and found to be -29.6, -30.8, and -28.9 kcal/mol for S, Se, and Te, respectively. These data are combined with enthalpies of activation from kinetic studies to yield estimates for the solution-phase PhE-EPh bond strengths of 46, 41, and 33 kcal/mol for E = S, Se, and Te, respectively. The corresponding Cr-EPh bond strengths are 38, 36, and 31 kcal/mol. Two methods have been used to determine the enthalpy of hydrogenation of PhSeSePh in toluene on the basis of reactions of HSPh and HSePh with either center dot Cr(CO)(3)C5Me5 or 2-pyrdine thione. These data lead to a thermochemical estimate of 72 kcal/mol for the PhSe-H bond strength in toluene solution, which is in good agreement with kinetic studies of H atom transfer from HSePh at higher temperatures, The reaction of H-Cr(CO)(3)C5Me5 with PhSe-SePh is accelerated by the addition of a Cr radical and occurs via a rapid radical chain reaction. In contrast, the reaction of PhTe-TePh and H-Cr(CO)(3)C5Me5 does not occur at any appreciable rate at room temperature, even in the presence of added Cr radicals. This is in keeping with a low PhTe-H bond strength blocking the chain and implies that H-TePh <= 63 kcal/mol. Structural data are reported for PhSe-Cr(CO)(3)C5Me5 and PhS-Cr(CO)(3)C5Me5. The two isostructural complexes do not show signs of an increase in steric strain in terms of metal-ligand bonds or angles as the Cr-EPh bond is shortened in going from Se to S. Bond strength estimates of the PhE-H and PhE-EPh derived from density functional theory calculations are in reasonable agreement with experimental data for E = Se but not for E = Te. The nature of the singly occupied molecular orbital of the center dot EPh radicals is calculated to show increasing localization on the chalcogenide atom in going from S to SD to Te.

First author: Cerofolini, GF, Theoretical search for atomic-like states of silicon at the surface of thermally grown SiO2 films on silicon surfaces, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 80, 1617, (2005)
Abstract: The existence of atomic-silicon cryptates in siloxanic networks has been studied theoretically via standard density functional theory calculations. Mimicking with model molecules the candidate sites to host atomic silicon, we found that stationary states are impossible in highly reticulated siloxanic networks; metastable adducts can only be formed at their external surfaces or in regions where the siloxanic network is subjected to weak steric constraints. This analysis suggests that the atomic silicon injected into the oxide during thermal oxidation of silicon by O-2 may be trapped as a metastable adduct at the oxide surface.

First author: Guha, S, Electronic structures and spectral properties of endohedral fullerenes, COORDINATION CHEMISTRY REVIEWS, 249, 1111, (2005)
Abstract: Endohedral fullerenes belong to a new class of compounds which are technologically and scientifically important owing to their unique structures and optoelectronic properties. This review focuses on theoretical calculations and spectroscopic (electronic, vibrational, and nuclear magnetic resonance (NMR)) studies of endohedral fullerenes thus far published. A theoretical background, with various computational methods used for determining energy-optimized electronic structure and calculation of vibrational spectra, is presented. Further, theoretical and spectroscopic investigations of individual endohedral fullerenes are discussed. Such studies provide structural information about the carbon cage, position of the encapsulated species, and the degree of charge transfer. In particular, C-13 NMR spectroscopy is indispensable for the determination of the cage symmetry. In some cases, NMR signals from Sc-45 encapsulated species yield information about dynamic behavior inside the cage. Vis-NIR absorption spectra determine the HOMO-LUMO band-gap energy. IR and Raman spectroscopy play an important role in elucidating the nature of interaction between the cage and encapsulated species. Novel vibrations resulting from these interactions appear in the low-frequency region, and the corresponding force constants serve as a measure of the strength of their interaction.

First author: Aspuru-Guzik, A, A sparse algorithm for the evaluation of the local energy in quantum Monte Carlo,JOURNAL OF COMPUTATIONAL CHEMISTRY, 26, 708, (2005)
Abstract: A new algorithm is presented for the sparse representation and evaluation of Slater determinants in the quantum Monte Carlo (QMC) method. The approach, combined with the use of localized orbitals in a Slater-type orbital basis set, significantly extends the size molecule that can be treated with the QMC method. Application of the algorithm to systems containing up to 390 electrons confirms that the cost of evaluating the Slater determinant scales linearly with system size.

First author: Garcia-Fernandez, P, Origin off warping in the E circle times e Jahn-Teller problem: Quadratic vibronic coupling versus anharmonicity and application to NaCl : Rh2+ and triangular molecules, PHYSICAL REVIEW B, 71, 708, (2005)
Abstract: A general method is suggested which allows one to separate the quadratic, cubic, and pseudo Jahn-Teller contributions to the warping of the adiabatic potential energy surface (APES) of the E circle times e problem employing ab initio calculations. Numerical results were obtained for NaCl:Rh2+ in a cluster approximation and triangular molecules (Na-3,K-3,Cu-3,Ag-3) using density functional theory (DFT) and multireference second-order perturbation theory (CASPT2) methods. A largely unexpected result is that the contribution of cubic anharmonicity to the energy barrier between the minima of the lower and upper branches of the APES is dominant in all the systems and amounts for not less than 60% of the total. Another feature is that the three different contributions mentioned above may have different signs, thus either enhancing or diminishing each other, affecting in different ways the lower and upper branches of the APES. Other details of the numerical results are also analyzed.

First author: Bon, RS, Multicomponent synthesis of 2-imidazolines, JOURNAL OF ORGANIC CHEMISTRY, 70, 3542, (2005)
Abstract: A multicomponent reaction (MCR) between amines, aldehydes, and isocyanides bearing an acidic alpha-proton gives easy access to a diverse range of highly substituted 2-imidazolines. The limitations of the methodology seem to be determined by the reactivity of the isocyanide and by the steric bulk on the in situ generated imine rather than by the presence of additional functional groups on the imine. Less reactive isocyanides, for example p-nitrobenzyl isocyanide 25a, react successfully with amines and aldehydes, using a catalytic amount of silver(I) acetate. Some of the resulting p-nitrophenyl-substituted 2-imidazolines undergo air oxidation to the corresponding imidazoles. Differences in reactivity of the employed isocyanides are explained with use of DFT calculations. Difficult reactions with ketones instead of aldehydes as the oxo-compound in this MCR are promoted by silver(I) acetate as well.

First author: Diefenbach, A, Activation of C-H, C-C and C-I bonds by Pd and cis-Pd(CO)(2)I-2 center dot catalyst-substrate adaptation, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 690, 2191, (2005)
Abstract: We have studied the oxidative addition reactions of methane and ethane C-H, ethane C-C and iodomethane C-I bonds to I’d and cis-Pd(CO)(2)I-2 at the ZORA-BP86/TZ(2)P level of relativistic density functional theory (I)FT). Our purpose, besides exploring these particular model reactions, is to understand how the mechanism of bond activation changes as the catalytically active species changes from a simple, uncoordinated metal atom to a metal-ligand coordination complex. For both I’d and cis-Pd(CO)(2)I-2, direct oxidative insertion (OxIn) is the lowest-barrier pathway whereas nucleophilic substitution (S(N)2) is highly endothermic, and therefore not competitive. Introducing the ligands, i.e., going from I’d to cis-Pd(CO)2I2, causes a significant increase of the activation and reaction enthalpies for oxidative insertion and takes away the intrinsic preference of I’d for C-I over C-H activation. Obviously, cis-Pd(CO)(2)I-2 is a poor catalyst in terms of activity as well as selectivity for one of the three bonds studied. However, its exploration sheds light on features in the process of catalytic bond activation associated with the increased structural and mechanistic complexity that arises if one goes from a monoatomic model catalysts to a more realistic transition-metal complex. First, in the transition state (TS) for oxidative insertion, the C-X bond to be activated can have, in principle, various different orientations with respect to the square-planar cis-Pd(CO)(2)I-2 complex, e.g., C-X or X-C along an I-Pd-CO axis, or in between two I-Pd-CO axes. Second, at variance to the uncoordinated metal atom, the metal complex may be deformed due to the interaction with the substrate. This leads to a process of mutual adjustment of catalyst and substrate that we designate catalyst-substrate adaptation. The latter can be monitored by the Activation Strain model in which activation energies Delta E-not equal are decomposed into the activation strain Delta E-strain(not equal) of and the stabilizing TS interaction Delta E-int(not equal) between the reactants in the activated complex: Delta E-not equal = Delta E-strain(not equal) + Delta E-int(not equal).

First author: Dorcier, A, Binding of organometallic ruthenium(II) and osmium(II) complexes to an oligonucleotide: A combined mass spectrometric and theoretical study, ORGANOMETALLICS, 24, 2114, (2005)
Abstract: A series of ruthenium(II) and osmium(II) p-cymene dichloride complexes with either a pta (1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane) or [pta-Me]Cl ligand which exhibit anticancer activity have been prepared and characterized by H-1 and P-31 NMR spectroscopy and mass spectrometry. Three of the complexes, viz. [OS(eta(6)-p-cymene)Cl-2(pta)] and [M(eta(6)-p- cymene)Cl-2(pta-Me)]Cl (M = Ru, Os), have also been characterized by single-crystal X-ray diffraction. The pta complexes are selective anticancer agents, whereas the pta-Me+ complexes are indiscriminate and damage both cancer and healthy cells but represent models for the protonated pta adduct which has been implicated in drug activity. To establish a link between their biological activity and the effect they have on DNA (a likely in vivo target), the reactivity of the complexes toward a 14-mer oligonucleotide (5′-ATACATGGTACATA-3′) was studied using electrospray ionization mass spectrometry. It was found that the complexes bind to the oligonucleotide with loss of chloride and in some cases loss of the arene. Loss of arene appears to be most facile with the ruthenium-pta complexes but also takes place with the ruthenium-pta-Me complexes, whereas arene loss is not observed for the osmium complexes. In addition, as pH is reduced, increased binding to the oligonucleotide is observed, as evidenced from mass spectrometric relative intensities. Binding energies between the metal centers and the surrounding ligands were calculated using density functional theory (DFT). The calculated energies rationalize the experimentally observed tendencies for arene loss and show that the pta ligands are relatively strongly bound. Exchange of metal center (ruthenium versus osmium), methylation or protonation of the pta ligand, or change of the arene (p-cymene versus benzene) results in significant differences in the metal-arene binding energies while leaving the metal-phosphine bond strength essentially unchanged. Significantly lower binding energies and reduced hapticity are predicted for the exchange of arene by nucleobases. The latter show higher binding energies for nitrogen sigma-bonding than for pi-bonding.

First author: Martin, N, Regioselective intramolecular Pauson-Khand reactions C-60: An electrochemical study and theoretical underpinning, CHEMISTRY-A EUROPEAN JOURNAL, 11, 2716, (2005)
Abstract: Suitably functionalized fulleropyrrolidines endowed with one or two propargyl groups at the C-2 position of the pyrrolidine ring (1,6-enynes) react efficiently and regioselectively with [Co-2(CO)(8)] to afford the respective Pauson-Khand products with an unprecedented three (5a-d, 7, and 24) or five (25) pentagonal rings, respectively, fused onto the fullerene sphere. Fulleropyrrolidines with 1,7-, 1,9-, 1,10-, or 1,11-enyne moieties do not undergo the PK reaction and, instead, the inter-mediate dicobalt complexes formed with the alkynyl group are isolated in quantitative yields. These differences in reactivity have been studied by DFT calculations with a generalized gradient approximation (GGA) functional and several important energy and structural differences were found for the intermediates formed by the interaction between the coordinatively unsaturated Co atom and the pi system Of C-60 in 1,6- and 1,7-enynes. The different lengths of the alkyne chains are responsible for the observed reactivities. Cyclic voltammetry reveals that the presence of the cyclopentenone’s carbonyl group connected directly to the C-60 core results in PK compounds with remarkable electron-accepting ability.

First author: Shao, JL, EPR, ENDOR, and DFT studies on (beta-octahalo-meso-tetraarylporphyrin)copper complexes: Characterization of the metal(d(x)(-y)(2)(2))-porphyrin(a(2u))orbital interaction, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 11, 1609, (2005)
Abstract: A series of planar (porphyrin)copper(ii) complexes and their beta-octahalogenated saddled derivatives have been studied by Electron Paramagnetic Resonance (EPR) spectroscopy, Electron Nuclear DOuble Resonance (ENDOR) spectroscopy, and Density Functional Theoretical (DFT) calculations. Both EPR/ENDOR spectroscopy and DFT calculations indicate a decrease in spin density on the central copper(ii) ion and on the nitrogen atoms in the saddled compounds relative to the planar complexes. The EPR/ENDOR measurements show that the hyperfine coupling decreases by 12% on the nitrogen atoms and 9% on the copper ion, in going from planar (5,10,15,20-tetraphenylporphyrin) copper (Cu[TPP]) to saddled (2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin) copper (Cu[Br8TPP]). Accordingly, saddling results in a decrease in the spin density on the copper ion and on the nitrogen atoms. DFT calculations on Cu[Br8TPP] yield spin populations of 42.4% on the copper ion, 9.9% on each nitrogen atom and 4.9% on each meso carbon atom, relative to DFT spin populations of 62, 10,2 and 0.3% on the copper ion, each nitrogen and each meso carbon atom, respectively, for porphinecopper (Cu[P]). These calculations further indicate that the decrease in spin density on the copper ion in the saddled complexes results from a saddling-induced Cu(d(x)(-y)(2)(2))-porphyrin(a(2u)) orbital overlap whereby some of the Cu spin density is delocalized onto the porphyrin ring. The decrease in nitrogen spin population with saddling appears to be a more subtle effect caused by a superposition of two opposing factors. Saddling decreases the overlap between the nitrogen tone pairs and the Cu d(x)(-y)(2)(2) orbital on one hand and enhances the overlap between the copper d(x)(-y)(2)(2) orbital and the porphyrin a(2u) HOMO on the other.

First author: Groenhof, AR, Electronic ground states of iron porphyrin and of the first species in the catalytic reaction cycle of cytochrome P450s, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 3411, (2005)
Abstract: Electronic structures of iron(II) and iron(III) porphyrins are studied with density functional theory (DFT) using the GGA exchange functional OPTX in combination with the correlation functional PBE (OPBE) and with the correlation functional Perdew (OPerdew) together with a triple zeta-type basis set. These functionals, known for accurately predicting the spin ground state of iron complexes, are evaluated against other functionals for their performance in calculating relative energies for the various electronic states of both the iron porphyrins. The calculated energy orderings are triplet < quintet < singlet for the iron(II) porphyrin and quartet < sextet < doublet for the iron(III) porphyrin cation. Complexation by a thiolate ion (SH-) changes the preferred ground state for both species to high spin. This thiolate complex is used as a mimic for the cytochrome P450s active site to model the first step of the catalytic cycle of this enzyme. This first step is believed to concern the removal of an axial oxygen donating ligand from the hexacoordinated aqua-thiolate-porphyrin-iron(III) resting state. The DFT results suggest that this is not a free water molecule, because of its repulsive nature, but that it has instead hydroxy anion character. These calculations are in line with the experimentally observed change in the spin state from low to high spin upon this removal of the axial hydroxo ligand by binding of the substrate in the heme pocket of cytochrome P450.

First author: Jensen, L, Refractive index and third-order nonlinear susceptibility of C-60 in the condensed phase calculated with the discrete solvent reaction field model, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 102, 612, (2005)
Abstract: We have calculated the frequency-dependent refractive index and the third-order nonlinear susceptibility for C-60 in the condensed phase, which is related to third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) experiments. This was done using the recently developed discrete solvent reaction field (DRF) model, which combines a time-dependent density functional theory (TD-DFT) description of the central C-60 molecule with a classical polarizable MM model for the rest of the fullerene cluster. Using this model, effective microscopic properties can be calculated that, combined with calculated local field factors, give macroscopic susceptibilities. The largest calculation was for a cluster of 63 C,, molecules in which the central molecule was treated with TD-DFT. For this molecule, the effective polarizability was increased with about 15% and the effective second hyperpolarizability with about 60% compared with the gas phase. The calculated refractive index was found to be in good agreement with experiments and other theoretical results. The agreement with THG experiments was within a factor of two, whereas for DFWM the agreement was less good due to the neglect of vibrational contributions in the calculations. It was found that it is more important to account for the dispersion in the third-order susceptibilities than in the corresponding second hyperpolarizability.

First author: Atanasov, M, Ground states, excited states, and metal-ligand bonding in rare earth hexachloro complexes: A DFT-based ligand field study, INORGANIC CHEMISTRY, 44, 2954, (2005)
Abstract: Metal (4f)-ligand (Cl 3p) bonding in LnCl(6)(3-) (Ln = Ce to Yb) complexes has been studied on the basis of 4f -> 4f and Cl,3p -> 4f charge-transfer spectra and on the analysis of these spectra within the valence bond configuration interaction model to show that mixing of Cl 3p into the Ln 4f ligand field orbitals does not exceed 1%. Contrary to this, Kohn-Sham formalism of density functional theory using currently available approximations to the exchange-correlation functional tends to strongly overestimate 4f-3p covalency, yielding, for YbCl63-, a much larger mixing of Cl 3p -> 4f charge transfer into the f(13) ionic ground-state wave function. Thus, ligand field density functional theory, which was recently developed and applied with success to complexes of 3d metals in our group, yields anomalously large ligand field splittings for Ln, the discrepancy with experiment increasing from left to the right of the Ln 4f series. It is shown that eliminating artificial ligand-to-metal charge transfer in Kohn-Sham calculations by a procedure described in this work leads to energies of 4f-4f transitions in good agreement with experiment. We recall an earlier concept of Ballhausen and Dahl which describes ligand field in terms of a pseudopotential and give a thorough analysis of the contributions to the ligand field from electrostatics (crystal field) and exchange (Pauli) repulsion. The close relation of the present results with those obtained using the first-principles based and electron density dependent effective embedding potential is pointed out along with implications for applications to other systems.

First author: De Angelis, F, Intramolecular coupling of eta(2)-iminoacyls on zirconium bis(aryloxides) and calix[4]arenes: Revised mechanism by DFT calculations and Car-Parrinello molecular dynamics simulations, ORGANOMETALLICS, 24, 1867, (2005)
Abstract: The intramolecular coupling of the two iminoacyl units in the bis(eta(2)-iminoacyl) complexes [calix[4](OMe)(2)(O)(2)-Zr(eta(2)-MeCNBUt)(2)] and [(2,6-(BU2C6H3O)-C-t)(2)-Zr(eta(2)-MeCNPh)(2)] to generate the corresponding enediamido species has been investigated by means of both static and dynamic density functional calculations. For both systems we have characterized the stationary points of the potential energy surfaces for the coupling reaction and evaluated the overall energy profile. Car-Parrinello molecular dynamics simulations have been performed to gain insight into the detailed mechanistic features of the iminoacyl coupling and surprisingly show that this reaction occurs through an asynchronous reaction mechanism in which a decoordinated iminoacyl attacks the residual coordinated iminoacyl C-N double bond. A transition state search, performed without any symmetry constraint, led to structures consistent with the proposed mechanism 23.8 and 21.2 kcal mol(-1) above the starting his(eta(2)-iminoacyl) reagents, in good agreement with experimental activation enthalpies. On the basis of the proposed reaction mechanism we are also able to reproduce the activation energy of the coupling reaction with electron-withdrawing substituents on the iminoacyl moiety. Inspection of the electronic structure changes along the proposed reaction pathway suggests that the iminoacyl coupling process can be described as an intramolecular attack of the decoordinated iminoacyl carbon lone-pair at the coordinated iminoacyl pi* orbital, thus reconciling the iminoacyl coupling reaction mechanism with the generally accepted pattern of acyl and iminoacyl reactivity.

First author: Hyman, MP, Theoretical study of the adsorption and dissociation of oxygen on Pt(111) in the presence of homogeneous electric fields, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 6304, (2005)
Abstract: The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/angstrom (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.

First author: Bachmann, J, Multielectron redox chemistry of iron porphyrinogens, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 4730, (2005)
Abstract: Iron octamethylporphyrinogens were prepared and structurally characterized in three different oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions. Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined. The iron center is easily oxidized by a single electron (E-1/2 = -0.57 V vs NHE in CH3CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E-P = +0.77 V vs NHE in CH3CN); one of the ligand-based electrons is intercepted for the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation equivalents in the macrocycle are stored in C-alpha-C-alpha bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Mossbauer measurements, and DFT computations of the redox states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin states, whereas the fully oxidized ligand possesses a weaker a-donor framework, giving rise to high-spin iron. Taken together, the results reported herein establish a metal -macrocycle cooperativity that engenders a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.

First author: van den Bosch, M, Calculation of the redox potential of the protein azurin and some mutants,CHEMBIOCHEM, 6, 738, (2005)
Abstract: Azurin from Pseudomonas aeruginosa is a small 128-residue, copper-containing protein. Its redox potential can be modified by mutating the protein. Free-energy calculations based on classical molecular-dynamics simulations of the protein and from mutants in aqueous solution at different pH values were used to compute relative redox potentials. The precision of the free-energy calculations with the coupling-parameter approach is evaluated as function of the number and sequence of lambda values, the sampling time and initial conditions. It is found that the precision is critically dependent on the relaxation of hydrogen-bonding networks when changing the atomic-charge distribution due to a change of redox state or pH value. The errors in the free energies range from 1 to 10 k(B)T, depending on the type of process. Only qualitative estimates of the change in redox potential by protein mutation can be obtained.

First author: Vaiana, AC, AFMM: A molecular mechanics force field vibrational parametrization program, COMPUTER PHYSICS COMMUNICATIONS, 167, 34, (2005)
Abstract: AFMM (Automated Frequency Matching Method) is a program package for molecular mechanics force field parametrization. The method used fits the molecular mechanics potential function to both vibrational frequencies and eigenvector projections derived from quantum chemical calculations. The program optimizes an initial parameter set (either pre-existing or using chemically-reasonable estimation) by iteratively changing them until the optimal fit with the reference set is obtained. By implementing a Monte Carlo-like algorithm to vary the parameters, the tedious task of manual parametrization is replaced by an efficient automated procedure. The program is best suited for optimization of small rigid molecules in a well-defined energy minimum, for which the harmonic approximation to the energy surface is appropriate for describing the intra-molecular degrees of freedom.

First author: Berger, JA, Analysis of the viscoelastic coefficients in the Vignale-Kohn functional: The cases of one- and three-dimensional polyacetylene, PHYSICAL REVIEW B, 71, 34, (2005)
Abstract: In this work we employ the Vignale-Kohn (VK) current functional in the calculation of the linear response properties of polyacetylene for both the one-dimensional infinite chain and the infinite three-dimensional crystal. We test the two existing parametrizations of the longitudinal and transverse exchange-correlation kernels of the homogeneous electron gas that enter the VK functional and show that they lead to very different results. We argue that this is mainly caused by the different values of these kernels in the zero-frequency limit in the two parametrizations. In this limit knowledge of the exchange correlation part of the shear modulus of the homogeneous electron gas becomes very important. It is exactly this quantity that is not known accurately. Furthermore, we show that our results are in good qualitative agreement with results obtained earlier using the Vignale-Kohn functional for polyacetylene oligomers.

First author: Romaniello, P, Time-dependent current-density-functional theory for the metallic response of solids,PHYSICAL REVIEW B, 71, 34, (2005)
Abstract: We extend the formulation of time-dependent current-density-functional theory for the linear response properties of dielectric and semi-metallic solids [Kootstra , J. Chem. Phys. 112, 6517 (2000)] to treat metals as well. To achieve this, the Kohn-Sham response functions have to include both interband and intraband transitions with an accurate treatment of the Fermi surface in the Brillouin-zone integrations. The intraband contributions in particular have to be evaluated using a wave-vector-dependent description. To test the method we calculate the optical properties of the two noble metals Cu and Ag. The dielectric and energy loss functions are compared with experiments and with the classical Drude theory. In general we find a good agreement with the experiments for the calculated results obtained within the adiabatic local density approximation. In order to describe the Drude-like absorption below the interband onset and the sharp plasma feature in silver exchange-correlation, effects beyond the adiabatic local density approximation are needed, which may be included in a natural way in the present current-density-functional approach.

First author: Breza, M, DFT studies of copper complexes with biphenyldiimino dithioether, JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 718, 175, (2005)
Abstract: Using DFT method, the optimized geometries of tetrahedral and square-planar complexes of Cu(I) and Cu(II) with tetrabenz[a,e,g,k]-15,18-dithia-9,24-diaza-cyclohexadeca-9,23-diene macrocyclic ligand (bite) have been investigated and compared with experimental data. The non-equivalence of Cu-S bonds in tetrahedral Cu(bite)(+) is caused by mechanical strain of the bite ligand and may be significantly influenced by crystal packing. The energy and electronic structure of Cu(bite)(+/2+) complexes indicates that the mechanical strain of the bite ligand should play the key role in electron transfer processes.

First author: Fratiloiu, S, Optical properties and delocalization of excess negative charges on oligo(phenylenevinylene)s: A quantum chemical study, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 5644, (2005)
Abstract: A quantum chemical study of the electronic structure of negatively charged phenylenevinylene (PV) oligomers and methoxy-substituted derivatives is presented. The geometries of the PV oligomers were optimized using density functional theory. The geometry deformations are found to be delocalized along the entire oligomer chain without indication of polaron formation. The optical absorption spectra of the negatively charged PVs were calculated using both time-dependent density functional theory (TDDFT) and the singly excited configuration interaction method with an intermediate neglect of differential overlap reference wave function (INDO/s-CIS). The available experimental optical absorption energies are reproduced by the calculations. Introduction of methoxy substituents reduces the transition energies, while this does not have a strong effect on the charge distribution along the chain. DFT calculations yield a more delocalized excess negative charge than that of INDO/s-CIS calculations.

First author: Kennepohl, P, Spectroscopy of non-heme iron thiolate complexes: Insight into the electronic structure of the low-spin active site of nitrile hydratase, INORGANIC CHEMISTRY, 44, 1826, (2005)
Abstract: Detailed spectroscopic and computational studies of the low-spin iron complexes [Fe-III((S2N3)-N-Me2(Pr,Pr))(N-3)] (1) and [Fe-III((S2N3)-N-Me2(Pr,Pr))](1+) (2) were performed to investigate the unique electronic features of these species and their relation to the low-spin ferric active sites of nitrile hydratases. Low-temperature UV/vis/NIR and MCD spectra of 1 and 2 reflect electronic structures that are dominated by antibonding interactions of the Fe 3d manifold and the equatorial thiolate S 3p orbitals. The six-coordinate complex 1 exhibits a low-energy S pi -> Fe 3d(xy) (similar to 13000 cm(-1)) charge-transfer transition that results predominantly from the low energy of the singly occupied Fe 3d(xy) orbital, due to pure pi interactions between this acceptor orbital and both thiolate donor ligands in the equatorial plane. The 3d(pi) -> 3d(sigma) ligand-field transitions in this species occur at higher energies (> 15000 cm(-1)), reflecting its nearoctahedral symmetry. The Fe 3d(xz,yz) -> Fe 3d(xy) (d pi -> d pi) transition occurs in the near-IR and probes the Fe-III-S pi-donor bond; this transition reveals vibronic structure that reflects the strength of this bond (v, 340 cm-1). In contrast, the ligand-field transitions of the five-coordinate complex 2 are generally at low energy, and the S pi -> Fe charge-transfer transitions occur at much higher energies relative to those in 1. This reflects changes in thiolate bonding in the equatorial plane involving the Fe 3d(xy) and Fe 3d(x)(2)-(2)(y) orbitals. The spectroscopic data lead to a simple bonding model that focuses on the a and T interactions between the ferric ion and the equatorial thiolate ligands, which depend on the S-Fe-S bond angle in each of the complexes. These electronic descriptions provide insight into the unusual S = (1)/(2) ground spin state of these complexes: the orientation of the thiolate ligands in these complexes restricts their pi-donor interactions to the equatorial plane and enforces a low-spin state. These anisotropic orbital considerations provide some intriguing insights into the possible electronic interactions at the active site of nitrile hydratases and form the foundation for further studies into these low-spin ferric enzymes.

First author: Cai, S, NMR and EPR studies of the bis(pyridine) and bis(tert-butyl isocyanide) complexes of iron(III) octaethylchlorin, INORGANIC CHEMISTRY, 44, 1890, (2005)
Abstract: The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)(2)](+) (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))(2)(d(xz)d(yz))(3) to (d(xz)d(yz))(4)(d(xy))(1). The NMR data from 303 to 183 K show. that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)(2)](+) complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large 7 spin density on the adjacent carbons which are pan of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Huckel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(H) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)(2)](+) looks somewhat like that of [(OEC)Fe(4-Me2NPY)(2)](+), while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)(2)](+). The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)(2)](+) shows that it has a mixed (d(xz)d(yz))(4)(d(xy))(1) and (d(xy))(2)(d(xz),d(yz))(3) electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the cc-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)(2)]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.

First author: Rautiainen, JM, A computational and experimental study of the structures and Raman and Se-77 NMR spectra of SeX3(+) and SeX2 (X = Cl, Br, I): FT-Raman spectrum of (Sel3)[AsF6], INORGANIC CHEMISTRY, 44, 1904, (2005)
Abstract: The ability of MP2, B3PW91 and PBE0 methods to produce reliable predictions in structural and spectroscopic properties of small selenium-halogen molecules and cations has been demonstrated by using 6-311G(d) and cc-pVTZ basis sets. Optimized structures and vibrational frequencies agree closely with the experimental information, where available. Raman intensities are also well reproduced at all levels of theory. Calculated GIAO isotropic shielding tensors yield a reasonable linear correlation with the experimental chemical shift data at each level of theory. The largest deviations between calculated and experimental chemical shifts are found for selenium-iodine species. The agreement between observed and calculated chemical shifts for selenium-iodine species can be improved by inclusion of relativistic effects using the ZORA method. The best results are achieved by adding spin-orbit correction terms from ZORA calculations to nonrelativistic GIAO isotropic shielding tensors. The calculated isotropic shielding tensors can be utilized in the spectroscopic assignment of the Se-77 chemical shifts of novel selenium-halogen molecules and cations. The experimental FT-Raman spectra of (Sel(3))[AsF6] in the solid state and in SO2(I) solution are also reported.

First author: Neugebauer, J, Combined theoretical and experimental deep-UV resonance Raman studies of substituted pyrenes, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 2100, (2005)
Abstract: The results of time-dependent density functional theory (TDDFT) calculations of resonance Raman intensities are combined with experimental deep-ultraviolet resonance Raman measurements at a single wavelength, i.e., 244 nm, in order to test the possibility to distinguish several very similar compounds. Pyrene and three of its substituted derivatives, in which a single hydrogen atom has been replaced by a halogen atom, are compared. The fixed 244 nm excitation wavelength overlapped with the same electronic transition of the four pyrenes. Ground-state calculations using the BP86 exchange-correlation functional were used to predict the Raman frequencies, whereas excited-state calculations have been carried out employing the “statistical averaging of (model) orbital potentials” (SAOP) potential within a linear-response TDDFT framework in combination with the short-time approximation of resonance Raman intensities. In view of the simplistic theoretical approach, we find a surprisingly good agreement between the simulated and measured resonance Raman spectra of pyrene and its substituted analogues in terms of frequencies and intensities, which shows that the calculations can be used reliably to interpret the experimental spectra. With this combined information, it is possible to find criteria to distinguish the compounds under investigation, although many features of their vibrational spectra are similar.

First author: Scott, NM, Interaction of a bulky N-heterocyclic carbene ligand with Rh(I) and Ir(I). Double C-H activation and isolation of bare 14-electron Rh(III) and Ir(III) complexes, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 3516, (2005)
Abstract: Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-heterocyclic carbene (NHC) ligands are presented. These systems are capable of intramolecular C-H bond activation and lead to coordinatively unsaturated 16-electron complexes. The resulting complexes can be further unsaturated by simple halide abstraction, leading to 14-electron species bearing an all-carbon environment. Saturation of the vacant sites in the 16- and 14-electron complexes with carbon monoxide permits a structural comparison. DFT calculations show that these electrophilic metal centers are stabilized by pi-donation of the NHC ligands.

First author: Basumallick, L, Spectroscopic and density functional studies of the red copper site in nitrosocyanin: Role of the protein in determining active site geometric and electronic structure, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 3531, (2005)
Abstract: The electronic structure of the red copper site in nitrosocyanin is defined relative to that of the well understood blue copper site of plastocyanin by using low-temperature absorption, circular dichroism, magnetic circular dichroism, resonance Raman, EPR and X-ray absorption spectroscopies, combined with DFT calculations. These studies indicate that the principal electronic structure change in the red copper site is the a rather than the pi donor interaction of the cysteine sulfur with the Cu 3d(x2-y2) redox active molecular orbital (RAMO). Further, MCD data show that there is an increase in ligand field strength due to an increase in coordination number, whereas resonance Raman spectra indicate a weaker Cu-S bond. The latter is supported by the S K-edge data, which demonstrate a less covalent thiolate interaction with the RAMO of nitrosocyanin at 20% relative to plastocyanin at 38%. EXAFS results give a longer Cu-S(Cys) bond distance in nitrosocyanin (2.28 angstrom) compared to plastocyanin (2.08 angstrom) and also show a large change in structure with reduction of the red copper site. The red copper site is the only presently known blue copper-related site with an exogenous water coordinated to the copper. Density functional calculations reproduce the experimental properties and are used to determine the specific protein structure contributions to exogenous ligand binding in red copper. The relative orientation of the CuNNS and the CuSCbeta planes (determined by the protein sequence) is found to be key in generating an exchangeable coordination position at the red copper active site. The exogenous water ligation at the red copper active site greatly increases the reorganization energy (by similar to 1.0 eV) relative to that of the blue copper protein site, making the red site unfavorable for fast outer-sphere electron transfer, while providing an exchangeable coordination position for inner-sphere electron transfer.

First author: Atanasov, M, Calculation of spin-orbit coupling within the LFDFT: Applications to [NiX4](2-) (X=F-, Cl-, Br-, I-),INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 102, 119, (2005)
Abstract: Spin-orbit coupling has been introduced into our newly developed ligand field density functional theory (LFDFT), using the zero-order regular approximation as implemented into the Amsterdam density functional (ADF) code. Application of the formalism to a series of NiX42- (X=F-, Cl-, Br-, I-) compounds shows the increasing importance of intra-ligand spin-orbit coupling across the F, Cl, Br, I series, to lead to sign reversal (in the case of Brand I-) of the spin-orbit splitting within the t(2)-orbitals manifold of Ni2+. Symmetry lowering from T-d to D-2d, due to the Jahn-Teller coupling for the e(4)t(2)(4) configuration of NiX42-, is used to manifest further the effect of bonding changes on the sign and magnitude of the spin-orbit constant. Ligand field and spin-orbit coupling matrices are found to be correlated, with the higher erxtent of antibonding being accompanied by lower values of the spin-orbit coupling constant. In cases of little or no symmetry, this leads to situations in which ligand field and spin-orbit coupling cannot be neatly separated in the mathematical description. Using these results, the electronic energy levels of this series of compounds are predicted to be in good agreement with available spectral and magnetic data from literature.

First author: Yuan, DW, Geometric, electronic, and bonding properties of AuNM (N=1-7, M=Ni, Pd, Pt) clusters, JOURNAL OF CHEMICAL PHYSICS, 122, 119, (2005)
Abstract: Employing first-principles methods, based on density functional theory, we report the ground state geometric and electronic structures of gold clusters doped with platinum group atoms, AuNM (N = 1-7, M = Ni, Pd, Pt). The stability and electronic properties of Ni-doped gold clusters are similar to that of pure gold clusters with an enhancement of bond strength. Due to the strong d-d or s-d inter play between impurities and gold atoms originating in the relativistic effects and unique properties of dopant delocalized s-electrons in Pd- and Pt-doped gold clusters, the dopant atoms markedly change the geometric and electronic properties of gold clusters, and stronger bond energies are. found in Pt-doped clusters.. The Mulliken populations analysis of impurities and detailed decompositions of bond energies as well as a variety of density of states of the, most stable dopant gold clusters are given to understand the different effects of individual dopant atom on bonding and electronic properties of dopant gold clusters. From the electronic properties of, dopant gold clusters, the different chemical reactivity toward O-2, CO, or NO molecule is predicted in transition metal-doped gold clusters compared to pure gold clusters.

First author: Cozzolino, AF, The nature of the supramolecular association of 1,2,5-chalcogenadiazoles, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 3184, (2005)
Abstract: Organochalcogen-nitrogen heterocycles such as the 1,2,5-chalcogenadiazoles have a distinct tendency to establish intermolecular links in the solid state through secondary bonding interactions (EN)-N-… (E = S, Se, Te). The association of these molecules was examined in detail using relativistic density functional theory. Although there is an important electrostatic component, the interaction between these molecules is dominated by contributions arising from orbital mixing, which can be interpreted as the donation of a nitrogen lone pair into the chalcogen-centered antilbonding orbitals. Because of its more polar character and lower-lying antibonding orbitals, the tellurium derivatives possess the strongest association energies; these are so large that the binding strength is comparable to that of some hydrogen bonds. In the absence of steric constraints, telluradiazoles associate in a coplanar fashion forming ribbon polymers. However, bulky susbstituents could be used to direct the formation of either helical chains or discrete dimers. In addition to its strength, the coplanar dimer is characterized by being rigid, yet no activation barrier is expected for the association/dissociation process. These attributes strongly indicate that tellurium-nitrogen heterocycles have great potential as building blocks in supramolecular architecture.

First author: Belanzoni, P, Atomic silicon in siloxanic networks: The nature of the oxo-oxygen-silicon bond, INORGANIC CHEMISTRY, 44, 1381, (2005)
Abstract: The existence of atomic silicon cryptates in siloxanic networks has been studied theoretically via density functional calculations. By modeling with model molecules the candidate sites to host atomic silicon, we found that metastable adducts can be formed only in regions where the siloxanic network is not subjected to steric constraints; stationary states are instead unstable in highly reticulated siloxanic networks. The nature of the oxo-oxygen-silicon bond at the SiO2 surface is analyzed in detail. It is concluded that silicon is kept at the surface in atomic-like configuration by (i) sigma charge donation from oxo-oxygen atoms into the empty silicon p(sigma) orbital; (ii) pi charge back-donation from singly occupied silicon 3p(pi) orbitals into empty sigma(*) model molecule orbitals. Surprisingly, these results attribute to atomic silicon the character of bifunctional Lewis acid.

First author: Miguirditchian, M, Thermodynamic study of the complexation of trivalent actinide and lanthanide cations by ADPTZ, a tridentate N-donor ligand, INORGANIC CHEMISTRY, 44, 1404, (2005)
Abstract: To better understand the bonding in complexes of f-elements by polydentate N-donor ligands, the complexation of americium(III) and lanthanide(III) cations by 2-amino-4,6-di-(pyridin-2-yl)-1,3,5-triazine (ADPTZ) was studied using a thermodynamic approach. The stability constants of the 1:1 complexes in a methanol/water mixture (75/25 vol %) were determined by UV-visible spectrophotometry for every lanthanide(III) ion (except promethium), and yttrium(III) and americium(III) cations. The thermodynamic parameters (DeltaHdegrees, DeltaSdegrees) of complexation were determined from the temperature dependence of the stability constants and by microcalorimetry. The trends of the variations of DeltaGdegrees, DeltaHdegrees, and DeltaSdegrees across the lanthanide series are compared with published results for other tridentate ligands and confirm strongly ionic bonding in the lanthanide-ADPTZ complexes. Comparison of the thermodynamic properties between the Am- and Ln-ADPTZ complexes highlights an increase in stability of the complexes by a factor of 20 in favor of the americium cation. This difference arises from a more exothermic reaction enthalpy in the case of Am, which is correlated with a greater degree of covalency in the americium-nitrogen bonds. Quantum chemistry calculations performed on a series of trivalent actinicle and lanthanide-ADPTZ complexes support the experimental results, showing a slightly greater covalence in the actinicle-ligand bonds that originates from a charge transfer from the ligand sigma orbitals to the 5f and 6d orbitals of the actinide ion.

First author: Yanagisawa, S, Theoretical investigation of adsorption of organic molecules onto Fe(110) surface, JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 716, 45, (2005)
Abstract: The adsorption of typical organic molecules onto a clean Fe(110) surface was investigated using density functional theory (DFT). DFT calculations were carried out with the BOP exchange-correlation functional in the DMol(3) program. The Fe(110) surface was modeled by using dilayer Fe-18 and Fe-32 slabs under the periodic boundary condition. The surface relaxation effect was taken into account in calculations using an Fe32 slab. The results show that the order of the molecular adsorption energies is methylamine > methylacetamide > methylacetate, while no bound molecular adsorption structure was obtained for acetic acid, dimethyl ether, or methanol. Accordingly, we conclude that the adhesiveness of organic molecules onto an iron surface may be closely related to the charge transferability of the molecules. The experimental results of X-ray photoelectron spectroscopy in vacuo (methanol on Fe(110) easily dissociates into methoxy and hydrogen fragments even at 100 K and sublimes at 140 K following the adsorption) may correspond to the result of the present calculation, where the adsorbed state of methanol was found to be unstable, in contrast to a previous DFT study, where the adsorption energy of methanol was estimated to be 21.03 kcal/mol.

First author: Kovacs, A, The nature of the chemical bond revisited: An energy-partitioning analysis of nonpolar bonds,CHEMISTRY-A EUROPEAN JOURNAL, 11, 1813, (2005)
Abstract: The nature of the chemical bond in nonpolar molecules has been investigated by energy-partitioning analysis (EPA) of the ADF program using DFT calculations. The EPA divides the bonding interactions into three major components, that is, the repulsive Pauli term, quasiclassical electrostatic interactions, and orbital interactions. The electrostatic and orbital terms are used to define the nature of the chemical bond. It is shown that nonpolar bonds between main-group elements of the first and higher octal rows of the periodic system, which are prototypical covalent bonds, have large attractive contributions from classical electrostatic interactions, which may even be stronger than the attractive orbital interactions. Fragments of molecules with totally symmetrical electrondensity distributions, like the nitrogen atoms in N-2, may strongly attract each other through classical electrostatic forces, which constitute 30.0% of the total attractive interactions. The electrostatic attraction can be enhanced by anisotropic charge distribution of the valence electrons of the atoms that have local areas of (negative) charge concentration. It is shown that the use of atomic partial charges in the analysis of the nature of the interatomic interactions may be misleading because they do not reveal the topography of the electronic charge distribution. Besides dinitrogen, four groups of molecules have been studied. The attractive binding interactions in HnE-EHn (E= Li to F; n=0-3) have between 20.7 (E=F) and 58.4% (E=Be) electrostatic character. The substitution of hydrogen by fluorine does not lead to significant changes in the nature of the binding interactions in FnE-EFn (E= Be to 0). The electrostatic contributions to the attractive interactions in FnE-EFn are between 29.8 (E=O) and 55.3% (E=Be). The fluorine substituents have a significant effect on the Pauli repulsion in the nitrogen and oxygen compounds. This explains why F2N-NF2 has a much weaker bond than H2N-NH2, whereas the interaction energy in FO-OF is much stronger than in HO-OH. The orbital interactions make larger contributions to the double bonds in HB=BH, H2C=CH2, and HN=NH (between 59.9% in B2H2 and 65.4 % in N2H2) than to the corresponding single bonds in HnE-EHn. The orbital term Delta E-orb (72.4%) makes an even greater contribution to the HC equivalent to CH triple bond. The contribution of Delta E-orb to the HnE=EHn bond increases and the relative contribution of the pi bonding decreases as E becomes more electronegative. The pi-bonding interactions in HC equivalent to CH amount to 44.4% of the total orbital interactions. The interaction energy in H,H3E-EH3 (E=C to Pb) decreases monotonically as the element E becomes heavier. The electrostatic contributions to the E-E bond increases from E=C (41.4%) to E=Sn (55.1%) but then decreases when E=Pb (51.7%). A true understanding of the strength and trends of the chemical bonds can only be achieved when the Pauli repulsion is considered. In an absolute sense the repulsive Delta E-Pauli term is in most cases the largest term in the EPA.

First author: Dorta, R, Steric and electronic properties of N-heterocyclic carbenes (NHC): A detailed study on their interaction with Ni(CO)(4), JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 2485, (2005)
Abstract: N-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and lCy (5) react with Ni(CO)(4) to give the saturated tricarbonyl complexes Ni(CO)(3)(IMes) (8), Ni(CO)(3)(SIMes) (9), Ni(CO)(3)(IPr) (10), Ni(CO)(3)(SIPr) (11), and Ni(CO)(3)(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)(3)(PR3) by recording their v(CO) stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1-5 are surprisingly small. Novel, unsaturated Ni(CO)(2)(IAd) (13) and Ni(CO)(2)((IBu)-Bu-t) (14) compounds are obtained from the reaction of Ni(CO)(4) with IAd (6) and (IBu)-Bu-t (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni-C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni(0) and Ni(II) complexes. Computational studies on compounds 8-14 are in line with the experimental findings and show that IAd (6) and (IBu)-Bu-t (7) are more bulky than IMes (1), SIMes (2), IPr (3), SIPr (4), and lCy (5). Furthermore, a method based on %V-bur values has been developed for the direct comparison of steric requirements of NHCs and tertiary phosphines. Complexes 8-14, as well as NiCl(C3H5)((IBu)-Bu-t) (16) and NiBr(C3H5)((IBu)-Bu-t) (17), have been characterized by X-ray crystallography.

First author: Coppens, P, The structure of short-lived excited states of molecular complexes by time-resolved X-ray diffraction, ACTA CRYSTALLOGRAPHICA SECTION A, 61, 162, (2005)
Abstract: Experimental and computational methods for time-resolved (TR) diffraction now allow the determination of geometry changes on molecular excitation. The first results indicate significant changes in the interatomic distances and molecular shape on photo-excitation, but also a dependence of the induced changes on the molecular environment. Though the use of high-brightness synchrotron sources is essential, it limits the time resolution to the width of the synchrotron pulse which is currently 70 – 100 ps. The experiments discussed fall into two categories: ( i) picosecond powder diffraction experiments on the molecular excitation to a singlet state, and ( ii) microsecond experiments on the excited states of inorganic complexes. Both involve reversible processes for which a stroboscopic technique can be applied.

First author: Yamashige, H, Electronic structure analysis of iron(III)-porphyrin complexes by X-ray absorption spectra at the C, N and FeK-edges, ANALYTICAL SCIENCES, 21, 309, (2005)
Abstract: X-ray absorption near edge structure (XANES) measurements at the C, N, and Fe K absorption edges were performed for iron(Ill)-tetraphenylporphyrin (FeTPP), iron(Ill)-tetrakis(p-carboxyphenyl)porphyrin (FeTCPP), and iron(Ill)-tetrakis(p-sulfonatophenyl)porphyfin (FeTSPP). The spectral shapes differ in the Fe K XANES, but not in C and N K XANES among FeTPP, FeTCPP, and FeTSPP. Crosschecks of XANES data for C, N, and Fe K absorption edges in combination with discrete variational (DV)-X alpha molecular orbital (MO) calculations indicate that each p-electron-withdrawing group on four meso-phenyl substitutes in an Fe(Ill)-porphyrin complex brings about a unique electron state through the complex because of the electron-withdrawal strength, itself. Consequently, they affect the positive charge of the center Fe(HI) ion.

First author: Neugebauer, J, The merits of the frozen-density embedding scheme to model solvatochromic shifts,JOURNAL OF CHEMICAL PHYSICS, 122, 309, (2005)
Abstract: We investigate the usefulness of a frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] for the calculation of solvatochromic shifts. The frozen-density calculations, particularly of excitation energies have two clear advantages over the standard supermolecule calculations: (i) calculations for much larger systems are feasible, since the time-consuming time-dependent density functional theory (TDDFT) part is carried out in a limited molecular orbital space, while the effect of the surroundings is still included at a quantum mechanical level. This allows a large number of solvent molecules to be included and thus affords both specific and nonspecific solvent effects to be modeled. (ii) Only excitations of the system of interest, i.e., the selected embedded system, are calculated. This allows an easy analysis and interpretation of the results. In TDDFT calculations, it avoids unphysical results introduced by spurious mixings with the artificially too low charge-transfer excitations which are an artifact of the adiabatic local-density approximation or generalized gradient approximation exchange-correlation kernels currently used. The performance of the frozen-density embedding method is tested for the well-studied solvatochromic properties of the n–>pi(*) excitation of acetone. Further enhancement of the efficiency is studied by constructing approximate solvent densities, e.g., from a superposition of densities of individual solvent molecules. This is demonstrated for systems with up to 802 atoms. To obtain a realistic modeling of the absorption spectra of solvated molecules, including the effect of the solvent motions, we combine the embedding scheme with classical molecular dynamics (MD) and Car-Parrinello MD simulations to obtain snapshots of the solute and its solvent environment, for which then excitation energies are calculated. The frozen-density embedding yields estimated solvent shifts in the range of 0.20-0.26 eV, in good agreement with experimental values of between 0.19 and 0.21 eV.

First author: Seth, M, Ab initio calculation of the C/D ratio of magnetic circular dichroism, JOURNAL OF CHEMICAL PHYSICS, 122, 309, (2005)
Abstract: A procedure for calculating the magnetic circular dichroism C/D ratio from density functional theory calculations is discussed. The method is simplified considerably through the application of group theory and the irreducible-tensor method and only requires integrals of the magnetic dipole moment operator over a few orbitals and published tables of symmetry factors. The implementation of the method is tested through application to several small and medium-sized molecules.

First author: Diefenbach, A, Activation of H-H, C-H, C-C and C-Cl bonds by Pd and PdCl-. Understanding anion assistance in C-X bond activation, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, 1, 286, (2005)
Abstract: To understand the mechanism of anion assistance in palladium-catalyzed H-H, C-H, C-C and C-Cl bond activation, several mechanistic pathways for oxidative addition of Pd and PdCl- to H-2 (H-H), CH4 (C-H), C2H6 (C-C and C-H) and CH3Cl (C-Cl) were studied uniformly at the ZORA-BP86/TZ(2)P level of relativistic nonlocal density functional theory (DFT). Oxidative addition of the neutral, uncoordinated Pd atom proceeds, as reported earlier, via direct oxidative insertion (Delta H-298(double dagger) is -22 to 10 kcal/mol), whereas straight S(N)2 substitution (yielding, e.g., PdCH3+ + X-) is highly endothermic (144-237 kcal/mol) and thus not competitive. Anion assistance (i.e., going from Pd to PdCl-) lowers all activation barriers and increases the exothermicity of all model reactions studied. The effect is however selective: it favors the highly endothermic S(N)2 mechanism over direct oxidative insertion (Oxln). Activation enthalpies Delta H-298(double dagger) for oxidative insertion of PdCl- increase along C-H (-14.0 and -13.5 kcal/mol for CH4 and C2H6) approximate to C-Cl (-11.2 kcal/mol) < C-C (6.4 kcal/mol), i.e., essentially in the same order as for neutral Pd. Interestingly, in case of PdCl- + CH3Cl, the two-step mechanism of S(N)2 substitution followed by leaving-group rearrangement becomes the preferred mechanism for oxidative addition. The highest overall barrier of this pathway (-20.2 kcal/mol) drops below the barrier for direct oxidative insertion (-11.2 kcal/mol). The effect of anion assistance is analyzed using the Activation Strain model in which activation energies AE(double dagger) are decomposed into the activation strain Delta E(double dagger)strain of and the stabilizing transition state (TS) interaction Delta E(double dagger)int between the reactants in the activated complex: Delta E-double dagger = Delta E(double dagger)strain + Delta E(double dagger)int. For each type of activated bond and reaction mechanism, the activation strain Delta E(double dagger)strain adopts characteristic values which differ only moderately, within a relatively narrow range, between corresponding reactions of Pd and PdCl-. The lowering of activation barriers through anion assistance is caused by the TS interaction DE int becoming more stabilizing.

First author: Belanzoni, P, The uranyl ion revisited: the electric field gradient at U as a probe of environmental effects,MOLECULAR PHYSICS, 103, 775, (2005)
Abstract: The experimental electric field gradient (EFG) at the U nucleus in uranyl is positive. It has been pointed out by Pyykko that this Could be a signature of a hole in the 6p shell induced by the strong bonding to the axial 0 atoms. We have revisited this issue with the help of relativistic density functional calculations, including accurate ZORA-4 calculations of the EFG. We confirm the existence of a 6p hole, with a positive contribution to the EFG, but We still find the EEG in the free uranyl ion to be negative due to the non-spherical electron distribution in the valence 5f shell caused by the bonding to the oxygens. A positive EFG only results in our calculations from the effect of the crystal environment of the uranyl ion, i.e. the coordination of three nitrate groups in the equatorial plane. Again the extended nature of 6p plays a key role, with an important positive contribution to the EEG coining from 6p tails in the high-lying electron pair orbitals of the closed shell nitrate ligands due to the orthogonality requirement. A further contribution comes from electron donation by the nitrate groups into the U 5f(phi), and 6d(delta) orbitals which both have their lobes in the equatorial plane. Our findings highlight the sensitivity of the EFG to the environment, through effects on the upper valence electronic structure.

First author: Diefenbach, A, Fragment-oriented design of catalysts based on the activation strain model, MOLECULAR PHYSICS, 103, 995, (2005)
Abstract: It is shown how a fragment approach toward analysing transition states (in terms of strained reactants that interact with each other: the Activation Strain model) provides a way of understanding the relative heights of reaction barriers and how this can be applied to achieving a more rational, fragment-oriented design of catalysts (FDC). The predictive value of the FDC approach is demonstrated in a computational application to Pd-0-catalyzed C-X bond activation. It is shown how and why a model catalyst’s stereochemical selectivity can be tuned towards retention or inversion of configuration of the carbon atom in the activated bond simply by omitting or introducing, respectively, anion assistance.

First author: Yazyev, OV, Core spin-polarization correction in pseudopotential-based electronic structure calculations,PHYSICAL REVIEW B, 71, 995, (2005)
Abstract: A simple scheme for the evaluation of the core spin-polarization contribution within pseudopotential electronic structure methods is proposed. The method uses a reconstruction of the all-electron wave functions and the frozen valence spin-density approximation to solve the Kohn-Sham equations for core electrons only. The core contribution to the spin-density at the point of the nucleus corrects for the leading error in the Fermi contact hyperfine coupling constants within pseudopotential-based electronic structure calculations. The correction is implemented in the framework of pseudopotential plane-wave density functional theory. Comparison with all-electron Slater-type orbital calculations on a number of molecular radicals containing first-row elements proves the accuracy of this approach.

First author: Venkatesan, K, mu-Carbon-carbon bonds of dinuclear manganese half-sandwich complexes as electron reservoirs, ORGANOMETALLICS, 24, 920, (2005)
Abstract: The mononuclear vinylidene complexes of the type Mn(C5H4R’)(R”2PCH2CH2PR”(2))(=C= C(R’)(SnMe3)) were obtained by the reaction Of Mn(C5H4R’)(eta(6)-cycloheptatriene) (R’ = H, 1a; CH3, 1b) with 1 equiv of R-1-C drop C-SnMe3 (R-1 = SnMe3, C6H5, C4H3S, C6H4CH3) and R”2PCH2CH2PR”(2) (R” = CH3 (dmpe), C2H5 (depe)) in toluene at 50degreesC for 3 h. The reactions of these tin-substituted complexes with 1 equiv of 1.0 M TBAF yielded the corresponding parent vinylidene species Mn(C5H4R’)(R”2PCH2CH2PR”(2))(=C=C(R-1)(H)). Treatment of some of these vinylidene species with 1 equiv of [Cp2Fe][PF6] led to the oxidative coupling product [(C5H4R’)(R”2PCH2CH2PR”(2))MndropC-CHR1-CHR1-CdropMn(R”2PCH2CH2PR”(2))(C 5H4R’)]-[PF6](2) (R’ = CH3, R” = CH3, R-1 = H; R’= CH3, R” = CH3, R-1 = C6H5; R’ = CH3, R” = CH3, R-1 = C4H3S; R’ = H, R” = C2H5, R-1 = H; R’ = H, R” = C2H5, R-1 = C6H5; R’ = H, R” C2H5, R-1 = C4H3S). In some cases these products of oxidative coupling, [(C5H4R’)(R”2PCH2CH2-PR”(2))Mn drop C-CHR1-CHR1-CdropMn(R”2PCH2CH2PR”(2))(C5H4R’)][PF6](2), were accompanied by formation of dinuclear complexes of the type [(C5H4R’)(R”2PCH2CH2PR”(2))MndropC-CR1=CR1-CdropMn(R”2PCH2CH2PR”(2))(C5H 4R’)][PF6](2) and of the cationic carbyne complexes [(C5H4R’)(R”2PCH2CH2PR”(2))MndropC-CH2R1] [PF6] obtained by proton transfer. Reduction of these dinuclear complexes with CP2*Co yielded back the corresponding mononuclear precursor complexes involving a reductive decoupling process. Both the reductive coupling and the oxidative coupling are fully reversible, which is supported by DFT calculations. The mononuclear and the dinuclear compounds were characterized by NMR, IR, and cyclic voltammetric studies. X-ray diffraction studies have been performed on complexes 3a, 11, 13a, 15a, and 22b.

First author: Claiser, N, Combined charge and spin density experimental study of the yttrium(III) semiquinonato complex Y(HBPz(3))(2)(DTBSQ) and DFT calculations, JOURNAL OF PHYSICAL CHEMISTRY B, 109, 2723, (2005)
Abstract: High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz(3))(2)(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1)/(2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz(3))(2)(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O-1 (0.12(1) mu(B)), O-2(0.11(1) mu(B))] and the carbon atoms [C-21 (0.24(1) mu(B)), C-22(0.20(1) muB), C-24(0-16(1) muB), C-25(0.12(1) mu(B))] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu(B) is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring.

First author: Machonkin, TE, Paramagnetic NMR spectroscopy and density functional calculations in the analysis of the geometric and electronic structures of iron-sulfur proteins, INORGANIC CHEMISTRY, 44, 779, (2005)
Abstract: Paramagnetic NMR spectroscopy has been underutilized in the study of metalloproteins. One difficulty of the technique is that paramagnetic relaxation broadens signals from nuclei near paramagnetic centers. In systems with low electronic relaxation rates, this makes such signals difficult to observe or impossible to assign by traditional methods. We show how the challenges of detecting and assigning signals from nuclei near the metal center can be overcome through the combination of uniform and selective H-2, C-13, and N-15 isotopic labeling with NMR experiments that utilize direct one-dimensional (H-2, C-13, and N-15) and two-dimensional (C-13-X) detection. We have developed methods for calculating NMR chemical shifts and relaxation rates by density functional theory (DFT) approaches. We use the correspondence between experimental NMR parameters and those calculated from structural models of iron-sulfur clusters derived from X-ray crystallography to validate the computational approach and to investigate how structural differences are manifested in these values. We have applied this strategy to three iron-sulfur proteins: Clostridium pasteurianum rubredoxin, Anabaena [2Fe-2S] ferredoxin, and human [2Fe-2S] ferredoxin. Provided that an accurate structural model of the iron-sulfur cluster and surrounding residues is available from diffraction data, our results show that DFT calculations can return NMR observables with excellent accuracy. This suggests that it might be possible to use calculations to refine structures or to generate structural models of active sites when crystal structures are unavailable. The approach has yielded insights into the electronic structures of these iron-sulfur proteins. In rubredoxin, the results show that substantial unpaired electron spin is delocalized across NH…S hydrogen bonds and that the reduction potential can be changed by 77 mV simply by altering the strength of one of these hydrogen bonds. In reduced [2Fe-2S] ferredoxins, hyperfine shift data have provided quantitative information on the degree of valence trapping. The approach described here for iron-sulfur proteins offers new avenues for detailed studies of these and other metalloprotein systems.

First author: Daul, C, DFT study of mixed-valent Mn(II/III) hexacyanide clusters, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 101, 753, (2005)
Abstract: The cubic Prussian blue analogue Mn-3[Mn(CN)(6)](2) (.) 15 H2O, which has the advantage of being transparent and magnetic (T-N = 35 K) at the same time, has been investigated by density functional theory (DFT) calculations. The three-dimensional structure is built of Mn-II ions linked to Mn-III ions by mu-bridging cyanides, to form a crystal structure, which is related to the NaCl type. In a first step, the relative stabilities of the mononuclear complexes [Mn(CN)(6)](z-) (z = 2 to 4) have been studied as a function of the oxidation state, spin configuration, and the linkage isomerism of the cyanide ligand. The results we have obtained by this investigation are in good agreement with our chemical expertise. In addition, the calculations have been extended to the dinuclear [Mn-2(CN)(11)](z-) (z = 5 and 6) clusters. Furthermore, we used DFT to model the magnetic properties as well as the T-3(1) –> T-1(2) transition, which has been observed by single-crystal near-IR spectra of Mn-3[Mn(CN)(6)](2) (.) 15 H2O.

First author: Vieito, I, Molecular impurities in crystals: (ReO4)(2-) Jahn-Teller ion in KCl, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 101, 802, (2005)
Abstract: First density functional calculations on ReO42- molecules embedded in KCl are reported. The results on the isolated ReO42- molecule confirm the existence of a strong Jahn-Teller effect induced by the e vibrational mode and favoring a squashed D-2d tetrahedron as stable geometry, where the unpaired electron is placed in an similar to3z(2)-r(2) orbital. Calculations carried out on an 81 atom cluster reveal that the Jahn-Teller distortion in KCl is essentially the same as that for the free ReO42- molecule, which is thus in agreement with the geometry inferred from EPR data on KCI: ReO42-. Despite this fact, the Cl- –> ReO42- replacement is shown to produce outwards or inwards relaxations on close ions in the KCI lattice, whose origin is briefly discussed.

First author: Poleshchuk, OK, Energy analysis of the chemical bond in group IV and V complexes: A Density Functional Theory study, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 101, 869, (2005)
Abstract: We analyzed the chemical bond between antimony, tin, titanium, and niobium chlorides with several Lewis bases using an energy partitioning analysis (EPA) and Weinhold’s NBO method. The optimized geometries are in good agreement with the experimental results. The NQR-Cl-35 frequencies appear to be fairly close to the experimental values. Theoretically obtained nuclear quadrupole coupling constants of antimony and niobium atoms are in good agreement with the experimental values. The bonding energies and experimentally known heats of formation are well reproduced by the employed level of theory. All investigated bonds in the donor-acceptor complexes are highly polar, so that the Townes-Dailey theory cannot be applied. The NBO approach shows that the donor-acceptor interactions of main-group elements such as Sb and Sn can basically be described in terms of sp hybridization, just as these interactions of the transition metal elements Ti and Nb can be described by sd hybridization.

First author: Le Guennic, B, The “Invisible” C-13 NMR chemical shift of the central carbon atom in [(Ph3PAu)(6)C](2+): A theoretical investigation, CHEMISTRY-A EUROPEAN JOURNAL, 11, 1677, (2005)
Abstract: The experimental C-13 NMR chemical shift of the central carbon atom in the octahedral [(Ph3PAu)(6)C](2+) cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. However, upon comparison with the C-13 NMR properties of the related complex [C{Au[P(C6H5)(2)(p-C6H4NMe2)]}(6)](2+), Schmidbaur et al. have assigned a signal at delta=135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.

First author: Neugebauer, J, Vibronic structure of the permanganate absorption spectrum from time-dependent density functional calculations, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 1168, (2005)
Abstract: The UV absorption spectrum of the permanganate anion is a prototype transition-metal complex spectrum. Despite this being a simple d(0) T-d system, for which a beautiful spectrum with detailed vibrational structure has been available since 1967. the assignment of the second and third bands is still very controversial. The issue can be resolved only by an elucidation of the intricate vibronic structure of the spectrum. We investigate the vibronic coupling by means of linear-response time-dependent density functional calculations. By means of a diabatizing scheme that employs the transition densities obtained in the TDDFT calculations in many geometries around R,, we construct a Taylor series expansion in the normal coordinates of adiabatic potential energy matrix, coupling 24 excited states. The simulated vibronic structure is in good agreement with the experimental absorption spectrum after the adjustment of some of the calculated vertical excitation energies. The peculiar blurred vibronic structure of the second band, which is a very distinctive feature of the experimental spectrum, is fully reproduced in the calculations. It is caused by the double-well shape of the adiabatic energy surface along the Jahn-Teller active e mode of the allowed E-1 state arising from the second IT, state, which exhibits a Jahn-Teller splitting into 1132 and E-1 states. We trace the double-well shape to an avoided crossing between two diabatic states with different orbital-excitation character. The crossing can be explained at the molecular orbital level from the Jahn-Teller splitting of the set of 7t(2){3d(xy), 3d(xz), 3d(yz)} orbitals (the LUMO + 1), to which the excitations characterizing the diabatic states take place. In contrast to its character in the two well regions. at R, the 2(1)T(2) state is not predominantly an excitation to the LUMO + 1, but has more HOMO – 1 –> LUMO (2e = {3dx(2)-x(2), 3d(z)(2)) character. The changing character of the 2(1)T(2) – E-1 state along the e mode implies that the assignment of the experimental bands to single orbital transitions is too simplistic intrinsically. This spectrum. and notably the blurring of the vibronic structure in the second band, can be understood only from the extensive configurational mixing and vibronic coupling between the excited states. This solves the long-standing assignment problem of these bands.

First author: Saeys, M, Ab initio reaction path analysis of benzene hydrogenation to cyclohexane on Pt(111), JOURNAL OF PHYSICAL CHEMISTRY B, 109, 2064, (2005)
Abstract: First-principles density functional theory calculations were performed to obtain detailed insight into the mechanism of benzene hydrogenation over Pt(111). The results indicate that benzene hydrogenation follows a Horiuti-Polanyi scheme which involves the consecutive addition of hydrogen adatoms. A first-principles-based reaction path analysis indicates the presence of a dominant reaction path. Hydrogenation occurs preferentially in the meta position of a methylene group. Cyclohexadiene and cyclohexene are expected to be at best minor products, since they are not formed along the dominant reaction path. The only product that can desorb is cyclohexane. Along the dominant reaction path, two categories of activation energies are found: lower barriers at similar to75 kJ/mol for the first three hydrogenation steps, and higher barriers of similar to88 kJ/mol for steps four and six, where hydrogen can only add in the ortho position of two methylene groups. The highest barrier at 104 kJ/mol is calculated for the fifth hydrogenation step, which may potentially be the rate-determining step. The high barrier for this step is likely the result of a rather strong C-(HPt)-Pt-… interaction in the adsorbed reactant state (1,2,3,5-tetrahydrobenzene*) which increases the barrier by similar to15 kJ/mol. Benzene and hydrogen are thought to be the most-abundant reaction intermediates.

First author: Wang, F, The performance of time-dependent density functional theory based on a noncollinear exchange-correlation potential in the calculations of excitation energies, JOURNAL OF CHEMICAL PHYSICS, 122, 2064, (2005)
Abstract: In the present work we have studied the accuracy of excitation energies calculated from spin-flip transitions with a formulation of time-dependent density functional theory based on a noncollinear exchange-correlation potential proposed in a previous study. We compared the doublet-doublet excitation energies from spin-flip transitions and ordinary transitions, calculated the multiplets splitting of some atoms, the singlet-triplet gaps of some diradicals, the energies of excited quartet states with a doublet ground state. In addition, we attempted to calculate transition energies with excited states as reference. We compared the triplet excitation energies and singlet-triplet separations of the excited state from spin-flip and ordinary transitions. As an application, we show that using excited quartet state as reference can help us fully resolve excited states spin multiplets. In total the obtained excitation energies calculated from spin-flip transitions agree quite well with other theoretical results or experimental data.

First author: Swart, M, Substrate binding in the active site of cytochrome P450cam, CHEMICAL PHYSICS LETTERS,403, 35, (2005)
Abstract: We have studied the binding of camphor in the active site of cytochrome P450cam with density functional theory (DFT) calculations. A strong hydrogen bond (>6 kcal/mol) to a tyrosine residue (Tyr96) is observed, that may account for the high specificity of the reaction taking place. The DFT interaction energy is well reproduced by QM/MM calculations, which allows for application of QM/MM to the catalytic cycle of cytochrome P450s. The substrate is distorted considerably due to the presence of the protein environment, which however does not have a large impact on the strong hydrogen bonding interactions.

First author: Jackson, TA, Probing the geometric and electronic structures of the low-temperature azide adduct and the product-inhibited form of oxidized manganese superoxide dismutase, BIOCHEMISTRY, 44, 1504, (2005)
Abstract: The geometric and electronic structures of the six-coordinate azide adduct of oxidized manganese superoxide dismutase (Mn3+SOD) that is formed at low temperatures, LT N-3-Mn3+SOD, has been examined in detail through a combined spectroscopic/computational approach. Electronic absorption, circular dichroism (CD), magnetic CID (MCD) and variable-temperature, variable-field (VTVH) MCD spectroscopies were used to determine electronic transition energies and to obtain an estimate of zero-field splitting parameters for LT N-3-Mn3+SOD. These experimental data were utilized in conjunction with semiempirical intermediate neglect of differential overlap/spectroscopic parametrization-configuration interaction (INDO/S-CI) and time-dependent density functional theory (TD-DFT) computations to evaluate hypothetical active-site models of LT N3-Mn3+SOD generated by constrained DFT geometry optimizations. Collectively, our spectroscopic/computational results indicate that N-3(-) binding to Mn3+SOD at low temperatures promotes neither protonation of the axial solvent ligand nor reorientation of the redoxactive molecular orbital, both of which had been previously suggested. Using the same experimentally validated computational approach, models of the product-inhibited form of MnSOD were also developed and evaluated by their relative energies and TD-DFT-computed absorption spectra. On the basis of our computational results as well as previously published kinetic data, we propose that the product-inhibited form of MnSOD is best described as a side-on peroxo-Mn3+ adduct possessing an axial (HO)-O-2 ligand. Notably, attempts to generate a stable hydroperoxo-Mn3+SOD species by protonation of the proximal 0 atom of the hydroperoxo ligand resulted in dissociation of HOO- and eventual H+ transfer from Tyr34 to HOO-, generating deprotonated Tyr34 and H2O2. The implications of these results with respect to the mechanism of O-2(.-) dismutation by MnSOD are discussed.

First author: Shen, YH, Density functional theory study of the Jahn-Teller effect and spin-orbit coupling for copper and gold trimers, JOURNAL OF PHYSICAL CHEMISTRY A, 109, 512, (2005)
Abstract: The Born-Oppenheimer potential energy hypersurfaces of copper and gold trimers were calculated using density functional theory with an analytic potential. The calculated Jahn-Teller distortion energies, pseudorotation barriers, dissociation, and isomerization energies for the two trimers are discussed. Global minima from the surfaces were optimized using the density functional theory method as well as the coupled cluster-singles-doubles-with-triples energies technique. The agreement of the optimized structures with the analytic potential was very good. The Mulliken population analysis compared favorably with the experimental electron spin resonance results. Spin-orbit coupling was subsequently included and the effect was significant for gold, but negligible for copper. The spin-orbit effect suppressed the Jahn-Teller distortion of the gold trimer, and the potential surface with the spin-orbit effect included was also obtained. The spin-orbit splitting splitting for the D-3h geometry of the gold trimer was in excellent agreement with the most recent infrared spectroscopic results.

First author: Jorge, FE, On the origin of optical activity in tris-diamine complexes of Co(III) and Rh(III): A simple model based on time-dependent density function theory, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 975, (2005)
Abstract: Time-dependent density functional theory (TD-DFT) is applied to the CID spectra of (Co(S-pn)(n)(en)(3-n)](3+) (n = 1, 2, 3) and A(deltadeltadelta)-(+)-[Co(en)(3)](3+) as well as the stereoisomers Delta-((delta)(n)(lambda)(3-n))-Co(S-pn)(n)(en)(3-n)](3+) (n = 1, 2, 3) and A(deltadeltadelta)-(-)-[Co(en)(3)](3+). Theory is able to reproduce the major differences in the CD spectra of the species with a A-configuration and their isomers with a A-configuration in both the d-d and ligand-to-metal CT region. It is further possible to rationalize the trend in terms of a larger azimuthal distortion away from the octahedral geometry in the A-conformation compared to the Delta-configuration. Considerations were also given to the CD spectra of the /e/(3)-isomer, Delta( lambdalambdalambda)-(-)-(Rh- (R-pm)(3)](3+) and the ob-isomer, Delta(lambdalambdalambda)-(+)-[Rh(S-pn)(3)](3+).

First author: George, SD, Metal and ligand K-Edge XAS of organotitanium complexes: Metal 4p and 3d contributions to pre-edge intensity and their contributions to bonding, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 127, 667, (2005)
Abstract: Titanium cyclopentadienyl (Cp) complexes play important roles as homogeneous polymerization catalysts and have recently received attention as potential anticancer agents. To systematically probe the contribution of the Cp to bonding in organotitanium complexes, Ti K-edge XAS has been applied to TiCl2 and then to the mono- and bis-Cp complexes, TiCpCl3 and TiCp2Cl2. Ti K-edge XAS is used as a direct probe of metal 3d-4p mixing and provides insight into the contribution of the Cp to bonding. These data are complimented by Cl K-edge XAS data, which provide a direct probe of the effect of the Cp on the bonding to the spectator chloride ligand. The experimental results are correlated to DFT Calculations. A model for metal 3d-4p mixing is proposed, which is based on covalent interactions with the ligands and demonstrates that metal K-pre-edge intensities may be used as a measure of ligand-metal covalency in molecular Ti(IV) systems in noncentrosymmetric environments.

First author: Jensen, L, Microscopic and macroscopic polarization within a combined quantum mechanics and molecular mechanics model, JOURNAL OF CHEMICAL PHYSICS, 122, 667, (2005)
Abstract: A polarizable quantum mechanics and molecular mechanics model has been extended to account for the difference between the macroscopic electric field and the actual electric field felt by the solute molecule. This enables the calculation of effective microscopic properties which can be related to macroscopic susceptibilities directly comparable with experimental results. By seperating the discrete local field into two distinct contribution we define two different microscopic properties, the so-called solute and effective properties. The solute properties account for the pure solvent effects, i.e.. effects even when the macroscopic electric field is zero, and the effective properties account for both the pure solvent effects and the effect from the induced dipoles in the solvent due to the macroscopic electric field. We present results for the linear and nonlinear polarizabilities of water and acetonitrile both in the gas phase and in the liquid phase. For all the properties we find that the pure solvent effect increases the properties whereas the induced electric field decreases the properties. Furthermore, we present results for the refractive index, third-harmonic generation (THG), and electric field induced second-harmonic generation (EFISH) for liquid water and acetonitrile. We find in general good agreement between the calculated and experimental results for the refractive index and the THG susceptibility. For the EFISH susceptibility, however, the difference between experiment and theory is larger since the orientational effect arising from the static electric field is not accurately described.

First author: Guan, W, Electronic properties and stability of dititanium(IV) substituted alpha-keggin polyoxotungstate with heteroatom phosphorus by DFT, INORGANIC CHEMISTRY, 44, 100, (2005)
Abstract: The electronic properties, redox properties, protonation, and stability of five [alpha-PTi2W10O40](7-) isomers have been investigated employing density-functional theory (DFT) method. The results reveal that the stability of [alpha-1,2PTi(2)W(10)O(40)](7-) and [alpha-1,6-PTi2W10O40](7-) is weaker and the redox ability is higher among five isomers, while for [alpha-1,5-PTi2W10O40](7-), [alpha-1,4-PTi2W10O40](7-), and [alpha-1,11-PTi2W10O40](7-) the stability is higher, but the redox ability is weaker. At the same time, Ti-substituted systems are preferentially reduced in the tungsten centers. A simple analysis of molecular electrostatic potential maps of [alpha-PTi2W10O40](7-) isomers suggests that the preferred protonation site corresponds to bridging oxygens (OTi2 and OTiW) and terminal oxygens (OTi), especially bridging oxygens bonded to titaniums (OTi2) in [alpha-1,2-PTi2W10O40](7-) and [alpha-PTi2W10O40](7-). It is proposed that the most stable structure is formed preferentially after protonation of the Ti-O site from the study of the protonated species, [alpha-1,4-HPTi2W10O40](6-). By means of total bonding energies of [alpha-PTi2W10O40](7-) isomers, the relative sequence of stability has then been shown to be [alpha-1,4-PTi2W10O40](7-) > [alpha-1,5-PTi2W10O40](7-) > [alpha-1,11-PTi2W10O40](7-) > [alpha-1,2PTi(2)W(10)O(40)](7-) > [alpha-1,6-PTi2W10O40](7-). In addition, the one-electron-reduced species of [alpha-PTi2W10O40](7-) are also discussed.

First author: Zanker, PP, Distance measurements between paramagnetic centers and a planar object by matrix Mims electron nuclear double resonance, JOURNAL OF CHEMICAL PHYSICS, 122, 100, (2005)
Abstract: Frequency-domain electron nuclear double resonance (ENDOR), two time-domain electron nuclear double resonance techniques, and electron spin echo envelope modulation spectroscopy are compared with respect to their merit in measurements of small hyperfine couplings to nuclei with intermediate gyromagnetic ratio such as P-31. The frequency-domain Mims ENDOR experiment is found to provide the most faithful line shapes. In the limit of long electron-nuclear distances of more than 0.5 nm, sensitivity of this experiment is optimized by matching the first interpulse delay to the transverse relaxation time of the electron spins. In the same limit, Mims ENDOR efficiency scales inversely with the sixth power of distance. Hyperfine splittings as small as 33 kHz can be detected, corresponding to an electron-P-31 distance of 1 nm. In systems, where a certain kind of nuclei is distributed in a plane, measurements of intermolecular hyperfine couplings can be analyzed in terms of a distance of closest approach of a paramagnetic center to that plane. By applying this technique to spin-labeled lipids in a fully hydrated lipid bilayer it is found that for a fraction of lipids, chain tilt angles can be 25degrees larger than the mean tilt angle of the lipid chains. This model of all-trans hydrocarbon chains with a broad distribution of tilt angles is also consistent with orientation selection effects in high-field ENDOR spectra.

First author: Zhang, JP, Metallophilicity versus pi-pi interactions: Ligand-unsupported argentophilicy/cuprophilicity in oligomers-of-dimers [M2L2](n) (M = Cu-I or Ag-I, L = tridentate ligand), CHEMISTRY-A EUROPEAN JOURNAL, 11, 552, (2005)
Abstract: To verify whether attractive metallophilic interactions exist in the dimer-of-dimers [Cu-2(ophen)(2)](2) (Hophen = 1H-[1,10]phenanthrolin-2-one) (1), we designed and synthesized a series of such [M2L2](2) structures by varying the d(10) metal and/or the ligand (M=Cu-I or Ag-I, L=ophen or obpy; Hobpy = 1H-[2,2′]bipyridinyl-6-one), and have successfully obtained three dimers-of-dimers: [Ag-2(ophen)(2)](2)(.)6H(2)O (2). [Cu-2(obpy)(2)](2) (3), and [Ag-2(obpy)(2)](2)(.)4.5H(2)O(.)0.5 DMF (4). X-ray analyses of these structures show that interdimer M-M separations in [Ag-2(ophen)(2)](2) (3.199 Angstrom) are remarkably shorter than those in [Cu-2(ophen)(2)](2) (3.595 Angstrom). Shorter interdimer M-M separations are found in the structures of [M-2(obPY)(2)](2) (2.986 and 2.993 Angstrom in [Cu-2(obpy)(2)](2), 3.037 to 3.093 Angstrom in [Ag-2(obpy)(2)](2)), in which the pi systems are smaller than in the complexes with the ophen ligand. Detailed structural comparison of these dimers-of-dimers indicates that the interdimer, face-to-face pi-pi interactions repulse rather than support the interdimer metal-metal attractive interactions. This study also yields qualitative comparison of the strengths between argentophilic, cuprophilic, and face-to-face T-a interactions. DFT calculations on the four dimers-of-dimers further support the above deduction. The structure of a trimer-of-dimers [Ag-2(obpy)(2)](3) (Ag-Ag 3.171 to 3.274 Angstrom) is further evidence that the oligomerization of the [M,L,] molecules is favored by stronger metallophilic and weaker face-to-face pi-pi interactions.

First author: Scheibitz, M, C5H4-BR2 bending in ferrocenylboranes: A delocalized through-space interaction between iron and boron, CHEMISTRY-A EUROPEAN JOURNAL, 11, 584, (2005)
Abstract: A comparison of the molecular structures of mono-, di- and tetra-borylated ferrocenes [Fc{B(R-1)(R-2)}] (R-1/R-2 =Br/Br, Br/Fc, Br/Me, Me/Me, Me/OH, OMe/OMe), 1,1′-[fc{B(R-1)-(R-2)}(2)] (R-1/R-2-Br/Br, Br/Me, OMe/OMe), and 1,1′,3,3′-[Fe{C5H3(BMe2)(2)}(2)] revealed the boryl substituent(s) to be bent out of the Cp ring plane towards the iron center. The corresponding dip angle alpha* decreases with decreasing Lewis acidity of the boron atom and with increasing degree of borylation at the ferrocene core. This trend is well reproduced by DFT calculations (including [FcBH(2)], not yet accessible experimentally). A Bader analysis of the electron density topology of [FcBH(2)] (alpha* = 26.5degrees; BP86/TZVP) clearly showed that there is no direct iron-boron bonding in this compound. Instead, strongly delocalized orbital interactions have been identified that involve the boron p orbital, C-ipso of the adjacent Cp ring, d orbitals at iron, and a through-space interaction with the second Cp ring. A second important factor is attractive electrostatic interactions, which are enhanced upon ligand bending. Cyclic voltammetric measurements on the series [FcBMe(2)], 1,1′-[fc(BMe2)(2)], and 1,1′,3,3′-[Fc{C5H3(BMe2)(2)}(2)] indicate a substantial anodic shift in the oxidation potential of the central iron atom upon introduction of BMe2 substituents. Addition of 4-dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the Fe-II/Fe-III. redox transition far beyond the half-wave potential of parent ferrocene. In the Mossbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe(2)] to 1,1′-[fc(BMe2)(2)], and to 1,1′,3,3′-[Fe{C5H3(BMe2)(2)}(2)]. In contrast, no significant differences are found between the QS values of ferrocene, [Fc(BMe2-DMAP)], and 1,1′-[fc(BMe2-DMAP)(2)] 1,1′,3,3′-[Fe{C5H3(BMe2)(2)}(2)] indicate a substantial anodic shift in the oxidation potential of the central iron atom upon introduction of BMe2 substituents. Addition of 4-dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the Fe-II/Fe-III. redox transition far beyond the half-wave potential of parent ferrocene. In the Mossbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe(2)b to 1,1′-[fc(BMe2)(2)], and to 1,1′,3,3′-[Fe{C5H3(BMe2)(2)}(2)]. In contrast, no significant differences are found between the QS values of ferrocene, [Fc(BMe2-DMAP)], and 1,1′-[fc(BMe2-DMAP)(2)].

First author: Wang, XJ, A theoretical investigation of the photo-induced intramolecular charge transfer excitation of cuprous (I) bis-phenanthroline by density functional theory, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 690, 187, (2005)
Abstract: This work reported an investigation on the excited state and electronic transfer excitation of cuprous (1) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.

First author: Dikarev, EV, Heterobimetallic main-group-transition-metal paddle-wheel carboxylates, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 44, 1721, (2005)
Abstract: This work reported an investigation on the excited state and electronic transfer excitation of cuprous (1) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.

First author: Bagno, A, Vicinal tungsten-tungsten coupling constants in polyoxotungstates: DFT calculations challenge an empirical rule, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 44, 2023, (2005)
Abstract: This work reported an investigation on the excited state and electronic transfer excitation of cuprous (1) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.

First author: Cappel, D, Direct estimate of the conjugative and hyperconjugative stabilization in diynes, dienes, and related compounds, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 44, 3617, (2005)
Abstract: This work reported an investigation on the excited state and electronic transfer excitation of cuprous (1) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.

First author: Blackmore, IJ, Experimental evidence for a Jahn-Teller distortion in AuCl3, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 44, 6746, (2005)
Abstract: This work reported an investigation on the excited state and electronic transfer excitation of cuprous (1) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.

First author: Mironov, YV, [Re12CS17(CN)(6)](n-) (n=6, 8): A sulfido-cyanide rhenium cluster with an interstitial carbon atom, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 44, 6867, (2005)
Abstract: This work reported an investigation on the excited state and electronic transfer excitation of cuprous (1) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.

First author: Friesner, RA, Ab initio quantum chemical and mixed quantum mechanics/molecular mechanics (QM/MM) methods for studying enzymatic catalysis, ANNUAL REVIEW OF PHYSICAL CHEMISTRY, 56, 389, (2005)
Abstract: We describe large scale ab initio quantum chemical and mixed quantum mechanics/molecular mechanics (QM/MM) methods for studying enzymatic reactions. First, technical aspects of the methodology are reviewed, including the hybrid density functional theory (DFT) methods that are typically employed for the QM aspect of the calculations, and various approaches to defining the interface between the QM and MM regions in QM/MM approaches. The modeling of the enzymatic catalytic cycle for three examples-methane monooxygenase, cytochrome P450, and triose phosphate isomerase-are discussed in some depth, followed by a brief summary of other systems that have been investigated by ab initio methods over the past several years. Finally, a discussion of the qualitative and quantitative conclusions concerning enzymatic catalysis that are available from modem ab initio approaches is presented, followed by a conclusion briefly summarizing future prospects.

First author: Schmidt, A, Coupled photocatalytic electron-transfers with 4,4 ‘-bipyridinium derivatives of a betaine alkaloid from Punica granatum, ARKIVOC, 56, 150, (2005)
Abstract: Bis- hydroquinone substituted 4,4′-bipyridinium is the formal dimer of an alkaloid isolated from Punica granatum and can exist as dication, cross-conjugated as well as pseudo-cross-conjugated mesomeric betaine, and dianion. It can moreover form persistent radicals in the solid state as well as in solution due to redox reactions of the reducing and oxidizing partial structures. A reversible coupled photocatalytic process with proflavine as the sensitizer and EDTA as the sacrificial donor is presented. We performed ESR and ENDOR spectroscopy as well as DFT calculations to gain additional knowledge about the radical species.

First author: Aguado, JE, Unprecedented eta(3)-M-3 coordination mode in a terpyridine ligand, CHEMICAL COMMUNICATIONS, 56, 3355, (2005)
Abstract: The gold(I) complex [Au-3(C6F5)(3)(eta(3)-Fcterpy)] (Fcterpy = 4′-ferrocenyl-2,2′:6′,2″-terpyridine) represents the first example of a terpyridine ligand bonded to three different metals. The aurophilic interactions present in the molecule may contribute to the overall stability of the system, as was shown by DFT calculations. The positive Mayer indices (0.152 and 0.138), as well as the magnitude of the binding interaction between terpy and the Au(I)L fragments, support this interpretation.

First author: Mendiratta, A, Synthesis of a four-coordinate titanium(IV) oxoanion via deprotonation and decarbonylation of complexed formate, CHEMICAL COMMUNICATIONS, 56, 3403, (2005)
Abstract: Deprotonation of the titanium formate complex [Ar(t-Bu)N](3)TiOCO) H with LiN(i- Pr)(2) resulted in the release of free CO and the formation of a titanium(IV) oxoanion complex, isolated as its lithium salt.

First author: Locatelli, D, The unexpected similar second-order NLO response for nearly planar and largely twisted push-pull stilbazole chromophores: EFISH and theoretical TD-DFT evidence, CHEMICAL COMMUNICATIONS, 56, 5405, (2005)
Abstract: The molecular second-order nonlinear optical properties of planar (E)-4-[2-(4-(N-methyl-N-hexadecylaminophenyl)ethenyl]pyridine (L-1) and [cis-Ir(CO)(2)ClL1] and of the significantly twisted new chromophores (E)-4-[(5,6,7,8-tetrahydro-5-isoquinolylidene) methyl]-N-methyl-N-hexadecylaniline (L-2) and [cis-Ir(CO)(2)ClL2] are reported, evidencing for the first time that planarity of a conventional (donor)(pi-bridge)(acceptor) structure is not compulsory to reach a large NLO response.

First author: Atanasov, M, Theoretical studies on the electronic properties and the chemical bonding of transition metal complexes using DFT and ligand field theory, CHIMIA, 59, 504, (2005)
Abstract: The research activity within our laboratory of computational chemistry at the University of Fribourg is presented. In this review, a brief outline of a recently proposed Ligand Field Density Functional Theory (LFDFT) model for single nuclear and its extension to dimer transition metal complexes is given. Applications of the model to dinuclear complexes are illustrated for the interpretation of exchange coupling in the bis-mu-hydroxo-bridged dimer of Cu(II) and to the description of the quadruple metal-metal bond in Re2Cl82-. The analysis of the chemical bonding is compared with results obtained using other approaches, i.e. the Extended Transition State model and the Electron Localization Function. It is shown that the DFT supported Ligand Field Theory provides consistent description of the ground and excited state properties of transition metal complexes.

First author: Beni, Z, Experimental and theoretical study of intramolecular exchange in Ir2Rh2(CO)(12) and Ir-4(CO)(11)(mu-SO2), DALTON TRANSACTIONS, 59, 310, (2005)
Abstract: As observed by variable-temperature and -pressure C-13 NMR, intramolecular carbonyl scrambling in Ir2Rh2(CO)(12) and Ir-4(CO)(11)(mu-SO2) proceeds via a ‘change of basal face’ mechanism. In both cases the site exchange process has a positive activation volume suggesting that the transition states contain longer M-M distances compared to ground states of C-s symmetry. Transition state structures have been located by density functional calculations including relativistic effects. These structures contain a new symmetry plane which interchanges the indistinguishable starting and final geometries. Both transition state structures contain one significantly elongated M-M distance, bearing the bridging ligand unaffected by the site exchange. Differences in molecular volumes of ground and transition state geometries as calculated from Connolly surfaces and electron densities confirm the volume expansion in both cases. The sign of the activation volume is therefore a good criterion for distinguishing between the two main site exchange processes occurring in tetrahedral d(9) carbonyl clusters, i.e. the change of basal face process and the ‘merry-go-round process, as the latter presents a negative activation volume.

First author: Beni, Z, Oxidative addition of iodo-acetonitrile and of elemental halogens to [Pt-3(mu-CO)(3)(PCy3)(3)],DALTON TRANSACTIONS, 59, 315, (2005)
Abstract: The reaction of [Pt-3(mu-CO)(3)(PCy3)(3)] (1) with one mole-equivalent of iodo-acetonitrile was quantitative at -70degreesC giving the oxidative addition product [Pt-3(mu-CO)(3)(PCy3)(3)(I)(CH2CN)] (2). Fragmentation of 2 was observed in solution giving [Pt2I(CH2CN)( CO)(2)(PCy3)(2)] (3) which is the major product at room temperature if the starting cluster/reactant ratio is equal to or less than 1 to 1.5. Dimer 3 decomposes slowly in solution giving [Pt2I2(CO)(2)(PCy3)(2)] (4a) and succinonitrile. Monomer [PtI(CH2CN)(CO)(PCy3)] was the final product of the reaction when using excess of iodo-acetonitrile. The reactions of 1 with one mole-equivalent of halogens X-2 gave the new 44-electron clusters [Pt3X(mu-CO)(2)(mu-X)(PCy3)(3)] (X = I-2 (7a) or Br-2 (7b)) by oxidative addition followed by substitution of CO by X-. Fragmentation of 7a and 7b took place in solution when using one and a half mole-equivalents of X-2 giving dimers 4a and [Pt2Br2(CO)(2)(PCy3)(2)] (4b) as well as [Pt2X2(mu-X)(2)(CO)(2)(PCy3)(2)]. Monomers cis-[PtX2(CO)(PCy3)] were the final products of the reaction of 1 with excess of halogens. Insertion of SnCl2 was observed into the Pt-Pt bond but not into the Pt-X bond, when equimolar amounts of SnCl2.2H(2)O were added to a solution of 4a or its chloro-analogue giving [Pt2X2(mu-SnCl2)(CO)(2)(PCy3)(2)]. The Pt(I) dimers have unusually small J(Pt-Pt) values as observed by Pt-195 NMR and calculated by DFT. These values showed periodic changes comparing 4a and its analogues with other halides and mixed halide dimers.

First author: Christian, G, Ligand rotation in [Ar(R)N](3)M-N-2-M ‘[N(R)Ar](3) (M, M ‘ = Mo-III, Nb-III; R = Pr-i and Bu-t) dimers, DALTON TRANSACTIONS, 59, 962, (2005)
Abstract: Earlier calculations on the model N-2-bridged dimer (mu-N-2)-{Mo[NH2]3}2 revealed that ligand rotation away from a trigonal arrangement around the metal centres was energetically favourable resulting in a reversal of the singlet and triplet energies such that the singlet state was stabilized 13 kJ mol 1 below the D-3d triplet structure. These calculations, however, ignored the steric bulk of the amide ligands N(R)Ar (R = Pr-i and Bu-t, Ar = 3,5-C6H3Me2) which may prevent or limit the extent of ligand rotation. In order to investigate the consequences of steric crowding, density functional calculations using QM/MM techniques have been performed on the (MoMoIII)-Mo-III and (MoNbIII)-Nb-III intermediate dimer complexes (mu-N-2)-{Mo[N(R)Ar](3)}(2) and [Ar(R)N](3)Mo-(mu-N-2)-Nb[N(R)Ar](3) formed when three-coordinate Mo[N(R)Ar](3) and Nb[N(R)Ar](3) react with dinitrogen. The calculations indicate that ligand rotation away from a trigonal arrangement is energetically favourable for all of the ligands investigated and that the distortion is largely electronic in origin. However, the steric constraints of the bulky amide groups do play a role in determining the final orientation of the ligands, in particular, whether the ligands are rotated at one or both metal centres of the dimer. Analogous to the model system, QM/MM calculations predict a singlet ground state for the (mu-N-2)-{Mo[N(R)Ar](3)}(2) dimers, a result which is seemingly at odds with the experimental triplet ground state found for the related (mu-N-2)-{Mo[N(Bu-t)Ph](3)}(2) system. However, QM/MM calculations on the (mu-N-2)-{Mo[N(Bu-t)Ph](3)}(2) dimer reveal that the singlet triplet gap is nearly 20 kJ mol(-1) smaller and therefore this complex is expected to exhibit very different magnetic behaviour to the (mu-N-2)-{Mo[N(R)Ar](3)}(2) system.

First author: Bowmaker, GA, Structural and spectroscopic studies on mercury(II) tribenzylphosphine complexes, DALTON TRANSACTIONS, 59, 1602, (2005)
Abstract: The tribenzylphosphine (PBz(3)) complexes of mercury(II), [Hg(PBz(3))(2)](BF4)(2), [Hg(PBz(3))(2)(NO3)(2)] and [HgX(NO3)(PBz(3))] (X = Cl, Br, I and SCN), have been synthesised and their structures determined by single-crystal X-ray crystallography. [Hg(PBz(3))(2)](BF4)(2) contains [Hg(PBz(3))(2)](2+) cations with linear P-Hg-P coordination, the first example of a truly two-coordinate [Hg(PR3)(2)](2+) complex. The mercury coordination in [Hg(PBz(3))(2)(NO3)(2)] can be described as distorted tetrahedral, with a significant deviation of the P-Hg-P angle from linearity as a result of coordination of the nitrate groups. Nitrate coordination is also observed in [HgX(NO3)(PBz(3))] (X = Cl, Br, I), resulting in significantly non-linear P-Hg-X coordination. The thiocyanate complex is a centrosymmetric thiocyanate-bridged dimer with distorted trigonal-pyramidal mercury coordination to the P atom of PBz(3), to the S and N atoms of two bridging thiocyanate groups, and to the O atom of one nitrate group. For all the nitrato complexes, secondary mercury-nitrate interactions (Hg-O 2.7-3.1 angstrom) effectively raise the coordination number of the Hg(II) centres to six. High-resolution P-31 solid-state NMR spectra of the six tribenzylphosphine mercury(II)-complexes, obtained by combining magic-angle spinning, proton dipolar decoupling and proton-phosphorus cross-polarization (CP-MAS), have been recorded. The spectra of [Hg(PBz(3))(2)](BF4)(2) and [HgX(NO3)(PBz(3))] (X = Cl, Br, I and SCN) exhibit a single line, due to species that contain non-magnetic isotopes of mercury, and satellite lines, due to (1)J(P-31-Hg-199) coupling. The asymmetric unit of [Hg(PBz(3))(2)(NO3)(2)] contains two molecules with four phosphorus environments, resulting in two AB spectra with (2)J(P-31-P-31) coupling, due to species that contain non-magnetic isotopes of mercury, and satellite lines consisting of two ABX spectra, due to (1)J(P-31-Hg-199) coupling. These spectra have been analysed to yield all of the chemical shifts and coupling constants involved. A remarkable increase in (1)J(P-31-Hg-199) is observed from [Hg(PBz(3))(2)](BF4)(2) to [Hg(PBz(3))(2)(NO3)(2)] as a consequence of the incorporation of the nitrate group into the Hg coordination sphere in the latter case. Several of the spectra also exhibit broader satellites due to the presence of scalar spin-spin coupling between P-31 and the quadrupolar Hg-201 nucleus. Slow-spinning methods have been used to analyze the spinning-sideband intensities of the NMR spectra, in order to obtain the P-31 shielding anisotropy and asymmetry parameters Delta sigma and eta. The Hg-199 and P-31 NMR shielding tensors of PMe3 models of the above six compounds have been calculated using relativistic density functional theory. The P-31 results are in good agreement with experiment and assist in the assignment of some of the signals.

First author: Deeth, RJ, Molecular modelling for coordination compounds: Cu(II)-amine complexes, DALTON TRANSACTIONS, 59, 3638, (2005)
Abstract: The Ligand Field Molecular Mechanics (LFMM) method has been applied to 85 Cu(II)-amine complexes, eighteen of which were selected to form a training set. A single set of parameters yields Cu-N bond lengths for four-, five- and six-coordinate systems generally within 0.04 angstrom of their X-ray crystallographic values. Larger deviations are indicative of counterion effects and/or crystallographic artefacts arising from Jahn-Teller averaging. The LFMM treatment is flexible and unbiased and for simple ligands, automatically gives planar CuN4 and tetragonally elongated CuN6 complexes. In agreement with experiment, square-pyramidal coordination is marginally favoured over trigonal bipyramidal coordination for CuN5 species. However, if the ligand requirements dictate, the LFMM accommodates trigonal bipyramidal coordination for CuN5 species, tetrahedral distortions of CuN4 species and cis-elongated CuN6 species.

First author: Schreckenbach, G, Density functional calculations of F-19 and U-235 NMR chemical shifts in uranium (VI) chloride fluorides UF6-nCln: Influence of the relativistic approximation and role of the exchange-correlation functional,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 101, 372, (2005)
Abstract: Relativistic density functional theory (DFT) has been applied to the calculation of the F-19 nuclear magnetic resonance (NMR) chemical shifts of the title compounds. It is shown that, while large-core effective core potentials (ECP) fail completely for the calculation of ligand NMR chemical shifts in uranium compounds, small-core ECPs are a valid relativistic method for this purpose. In an earlier study of the same systems, certain differences between theory and experiment had been observed, for instance, in the relative chemical shift of the A(4) and X sites in UF5Cl. The reason for these deviations has been investigated further in the current paper. By comparing different relativistic methods, it is shown that the relativistic approximation is not responsible for these deviations. The role of the approximation to the exchange-correlation (XC) functional of DFT has been probed, and generalized gradient approximations (GGA) as well as hybrid DFT methods have been investigated. None of these methods corrects the mentioned errors. It is argued that the neglect of environmental factors (solvent effects) remains as a possible error source, although the approximate XC functional appears to be the more likely cause of the problem. U-235 NMR shieldings and chemical shifts have been calculated, and the trends predicted earlier have been confirmed.

First author: Cybulski, SM, Critical examination of the supermolecule density functional theory calculations of intermolecular interactions, JOURNAL OF CHEMICAL PHYSICS, 122, 372, (2005)
Abstract: The results of calculations employing twelve different combinations of exchange and correlation functionals are compared with results of ab initio calculations for two different configurations of the water dimer and three different configurations of the thymine-adenine complex. None of the density functional theory (DFT) treatments could properly reproduce the results of coupled-cluster calculations for all configurations examined. The DFT approaches perform well when the interaction energy is dominated by the electrostatic component and the dispersion energy is less important. Two mechanisms that compensate for the missing dispersion component were identified. The first one is the decrease of the magnitude of the intermolecular exchange-repulsion and the second one is the increase of the magnitude of the attractive deformation energy. For some functionals both effects are observed together, but for some other ones only the second effect occurs. The three correlation functionals that were examined were found to make only very small contributions to the deformation energy. The examination of angular and distance dependence of the interactions shows that the currently available DFT approaches are not suitable for developing intermolecular potential energy surfaces. They could however be used to find global minima on potential energy surfaces governed by intermolecular electrostatic interactions. Additional single point ab initio calculations are recommended as the means of validating optimized structures.

First author: Wu, KC, Large hyperpolarizabilities of trinuclear transition metal clusters [MAg2X4(C5H5NS)(PPh3)(2)]center dot CH2Cl2 (M = Mo, W; X = S, Se): a DFT study, NEW JOURNAL OF CHEMISTRY, 29, 362, (2005)
Abstract: Static and dynamic first hyperpolarizabilities have been studied by a DFT approach for a series of trinuclear heterometallic transition metal molecular clusters, [MAg2X4(C5H5NS)(PPh3)(2)].CH2Cl2 (M = Mo, W; X = S, Se) with incomplete cubane-like configurations. Their nonlinear optical nature has been analyzed by using a two-level model. The large hyperpolarizabilities of about 100 x 10(-30) esu of these metal clusters are generated by intermolecular charge transfers from the metal core to the pyridine-2-thiol ligand as well as by intramolecular charge transfers within the metal core. The conjoint effects of stereo pi (3D) conjugation of the metal core and planar pi (2D) conjugation of the C5H5NS ligand enhance these hyperpolarizabilities, while the presence of CH2Cl2 has a negative effect by lowering them. Since these complexes are all crystallized in noncentrosymmetric configurations, they are promising candidates as IR second-order nonlinear optical transition metal coordinated materials.

First author: Mobius, K, High-field EPR spectroscopy applied to biological systems: characterization of molecular switches for electron and ion transfer, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 7, 19, (2005)
Abstract: The last decade witnessed a tremendous growth in combined efforts of biologists, chemists and physicists to understand the dominant factors determining the specificity and directionality of transmembrane transfer processes in proteins. A large variety of experimental techniques is being used including X-ray and neutron diffraction, but also time-resolved optical, infrared and magnetic resonance spectroscopy. This is done in conjunction with genetic engineering strategies to construct site-specific mutants for controlled modification of the proteins. As a general perception of these efforts, the substantial influence of weak interactions within the protein and its membrane interfaces is recognized. The weak interactions are subject to subtle changes during the reaction cycle owing to the inherent flexibility of the protein-membrane complex. Specific conformational changes accomplish molecular-switch functions for the transfer process to proceed with optimum efficiency. Characteristic examples of time varying non-bonded interactions are specific H-patterns and/or polarity effects of the microenvironment. The present perception has emerged from the coupling of newly developed spectroscopic techniques – and advanced EPR certainly deserves credit in this respect – with newly developed computational strategies to interpret the experimental data in terms of protein structure and dynamics. By now, the partners of this coupling, particularly high-field EPR spectroscopy and DFT-based quantum theory, have reached a level of sophistication that applications to large biocomplexes are within reach. In this review, a few large paradigm biosystems are surveyed which were explored lately in our laboratory. Taking advantage of the improved spectral and temporal resolution of high-frequency/high-field EPR at 95 GHz/3.4 T and 360 GHz/12.9 T, as compared to conventional X-band EPR (9.5 GHz/0.34 T), three biosystems are characterized with respect to structure and dynamics: (1) Light-induced electron-transfer intermediates in wild-type and mutant reaction-centre proteins from the photosynthetic bacterium Rhodobacter sphaeroides, (2) light-driven proton-transfer intermediates of site-specifically nitroxide spin-labelled mutants of bacteriorhodopsin proteins from Halobacterium salinarium, (3) refolding intermediates of site-specifically nitroxide spin-labelled mutants of the channel-forming protein domain of Colicin A bacterial toxin produced in Escherichia coli. The detailed information obtained is complementary to that of protein crystallography, solid-state NMR, infrared and optical spectroscopy techniques. A unique strength of high-field EPR is particularly noteworthy: it can provide highly desired detailed information on transient intermediates of proteins in biological action. They can be observed and characterized while staying in their working states on biologically relevant time scales. The review introduces the audience to origins and basic experiments of EPR in relation to NMR, describes the underlying strategies for extending conventional EPR to high-field/high-frequency EPR, and highlights those details of molecular information that are obtained from high-field EPR in conjunction with genetic engineering and that are not accessible by “classical” spectroscopy. The importance of quantum-chemical interpretation of the experimental data by DFT and advanced semiempirical molecular-orbital theory is emphasized. A short description of the laboratory-built 95 GHz and 360 GHz EPR/ENDOR spectrometers at FU Berlin is also presented. The review concludes with an outlook to future opportunities and challenges of advanced bio-EPR in interdisciplinary research.

First author: Hazra, A, Comparison of various Franck-Condon and vibronic coupling approaches for simulating electronic spectra: The case of the lowest photoelectron band of ethylene, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 7, 1759, (2005)
Abstract: Various time independent approaches for simulating electronic absorption spectra are discussed and applied to the lowest band (10.2 – 11.4 eV) of the photoelectron spectrum of ethylene. The electronic structure calculations for the ionized states of ethylene are performed using the Equation of Motion Coupled Cluster method for Ionization Potentials (IP-EOM-CCSD) in a TZ2P basis set. Various Franck – Condon (FC) and vibronic coupling model Hamiltonian approaches are used to simulate the spectrum and a comparison is made to the experimental data. The potential energy surface is highly anharmonic and it is necessary to use more sophisticated FC approaches than the traditional harmonic approach to obtain a qualitatively correct simulation of the spectrum. Duschinsky rotation is found to play an important role in determining the detailed intensity pattern of the spectrum, while non-adiabatic effects are found not to be important. The spectrum is found to be very sensitive to the precise values of some of the parameters used in the vibronic model, that determine the details of the adiabatic potential energy surface.

First author: Dovesi, R, Ab initio quantum simulation in solid mate chemistry, REVIEWS IN COMPUTATIONAL CHEMISTRY, VOL 21, 21, 1, (2005)
Abstract: Various time independent approaches for simulating electronic absorption spectra are discussed and applied to the lowest band (10.2 – 11.4 eV) of the photoelectron spectrum of ethylene. The electronic structure calculations for the ionized states of ethylene are performed using the Equation of Motion Coupled Cluster method for Ionization Potentials (IP-EOM-CCSD) in a TZ2P basis set. Various Franck – Condon (FC) and vibronic coupling model Hamiltonian approaches are used to simulate the spectrum and a comparison is made to the experimental data. The potential energy surface is highly anharmonic and it is necessary to use more sophisticated FC approaches than the traditional harmonic approach to obtain a qualitatively correct simulation of the spectrum. Duschinsky rotation is found to play an important role in determining the detailed intensity pattern of the spectrum, while non-adiabatic effects are found not to be important. The spectrum is found to be very sensitive to the precise values of some of the parameters used in the vibronic model, that determine the details of the adiabatic potential energy surface.

First author: Zwanziger, JW, The NMR response of boroxol rings: a density functional theory study, SOLID STATE NUCLEAR MAGNETIC RESONANCE, 27, 5, (2005)
Abstract: Cluster models of boron oxide glasses are studied computationally using density functional theory. It is shown that the isotropic chemical shielding of boron in boroxol rings is about 5 ppm less than for boron in non-ring BO3/2 units, and that the quadrupole coupling in ring sites is about 0.1 MHz larger than in non-ring sites, confirming assignments made in glasses and crystalline model compounds. The chemical shielding anisotropy of these sites is computed and shown to be in agreement with recent experimental measurements. Furthermore, it is shown that the reason for the different responses is not the co-planarity of BO3/2 groups bound in rings, but rather the contraction in the B-O-B bond angle from about 134degrees in relaxed structures to 120degrees as found in rings.

First author: Kaim, W, Complexes of 1,3-bis(diphenylphosphano)propane (dppp) with dichloroplatinum(II) and bis(chlorogold(I)): Intramolecular versus intermolecular Au-I-Au-I association of [(mu-dppp)(AuCl)(2)] in catena,ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, 631, 1355, (2005)
Abstract: The compound [(mu-dppp)(AuCl)(2)], previously reported to associate intermolecularly in a chain (catena) structure through Au-I-Au-I interactions (3.316 angstrom), was obtained from gold(III) precursors in a cyclo form with shortened intramolecular Au-I-Au-I contacts at 3.237 angstrom and a puckered AuPCCCPAu seven-membered ring. DFT calculations using a large relativistic basis to account for the d(10)-d(10) interaction reproduce the observed molecular structure in the crystal of this “linkage isomer”, including the conspicuous distortion at one of the gold atoms. The chelate complex [(dppp)PtCl2] was crystallized and structurally characterized as the dichloromethane solvate.

First author: Konkol, M, Trichloropalladate(II) complexes with pyridine ligands – Molecular structure and conformational analysis of [K(18C6)][PdCl3(py)], ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, 631, 1456, (2005)
Abstract: [K(18C6)](2)[Pd2Cl6] (1) (18C6 = 18-crown-6) was found to react with pyridines in a strictly stoichiometric ratio I : 2 in methylene chloride or nitromethane to yield trichloropalladate(II) complexes [K(18C6)][PdCl3(py*)] (py* = py, 2a: 4-Bnpy, 2b 4-tBupy, 2c: Bn = benzyl; tBu = tert-butyl). The reaction of I with pyrimidine (pyrm) in a I : I ratio led to the formation of the pyrimidine-bridged bis(trichloropalladate) complex [K(18C6)](2)[(PdCl3)(2)-(mu-pyrm)] (3). The identities of the complexes were confirmed by means of NMR spectroscopy (H-1, C-13) and microanalysis. The Xray structure analysis of 2a reveals square-planar coordination of the Pd atom in the [PdCl3(py)](-) anion. The pyridine plane forms with the complex plane an angle of 55.8(2)degrees. In the [K(18C6)](+) cation the K+ lies outside the mean plane of the crown ether (defined by the 6 0 atoms) by 0.816(l) A. There are tight K-Cl contacts between the cation and the anion (K center dot center dot center dot Cl 3,340(2) angstrom, K center dot center dot center dot Cl2 3.166(2) angstrom). To gain all insight into the conformation or the [PdCl3(py)] anion. DFT calculations., were performed showing that the equilibrium structure 6eq) has an angle between the pyridine ligand and the complex plane of 35.3 degrees. Rotation of the pyridine ligand around the Pd-N vector exhibited two transition states where the p ridine ligand fies either in the complex plane (6TS(pla), 0.87 kcal/mol above 6eq) or is perpendicular to it (6TS(per) 3.76 kcal/mol above 6eq). Based on an energy decomposition analysis the conformation of the anion is discussed in terms of repulsive steric interactions and of stabilizing sigma and pi orbital interactions between the PdCl3 moiety and the pyridine ligand.

First author: Szabo, A, Theoretical studies of inorganic compounds. 34 Energy decomposition analysis of E-E bonding in planar and perpendicular X2E-EX2 (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I), ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, 631, 1803, (2005)
Abstract: The nature of E-E bonding in group 13 compounds X2E-EX2 (E = B Al, Ga, In, T1; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory The calculated equilibrium geometries of all molecules B2X4-T2X4 have a perpendicular (D-2d) geometry The largest energy barriers for rotation about the E-E bond are predicted for the hydrogen species B2H4-Tl2H4. The EDA shows that the rotational barriers of B2X4-Tl2X4 may not be used for an estimate of the hyperconjugative strength in the D2d structures except for the tetrahydrides. The values for the planar (D-2h) transition states reveals that pi conjugation of the halogen lone-pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B2H4 is stronger than in B2H4 although the latter compound has a higher rotational barrier than the former. In B2F4, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al2X4-Tl2X4 have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al2H4-Tl2H4 compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (Delta E-elstat) and molecular orbital (Delta E-orb) components of the E-E bonding are similar in magnitude. The calculated B-B bond dissociation energies of B2X4 (D-e = 93.0 – 108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al-2-X-4-Tl2X4 have weaker bonds (D, 16.6-61.7 kcal/mol). In general, the X2E-EX2 bond dissociation energies follow the trend for atoms E: B >> Al > Ga > In > Tl and for atoms X: H > F > Cl > I.

First author: Klein, A, Organoplatinum complexes of the N,N-diisopropyl-diazabutadiene ligand: A structural and spectroscopic study, ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, 631, 2669, (2005)
Abstract: Organoplatinum complexes [Pt(R)(2)(iPr-DAB)], [PtCl(R)(iPr-DAB)] or [PtCl2(iPr-DAB)] (R = methyl or 2,4,6-trimethylphenyl = mesityl, iPr-DAB = N,N’-diisopropyl-1,4-diazabutadiene) have been investigated by a combination of multiple spectroscopy (NMR, UV/VIS), single crystal XRD and quantum chemical calculations. Excellent agreement of calculated and experimental structural data from XRD or NMR provide a good basis for the quantum mechanical calculations. DFT calculations reveal the contributions of the DAB ligand and the co-ligands to the electronic ground state. Methyl acts more or less purely as a alpha-donor, Cl or mesityl provide strong contributions (p or pi) to the highest occupied molecular orbital (HOMO) whereas the lowest unoccupied molecular orbital (LUMO) is mainly centred to the DAB ligand. The TD-DFT calculated electronic transitions were in excellent agreement with the experimental ones and allow reliable assignment of the absorption bands.

First author: Nachtigall, P, Applications of quantum chemical methods in zeolite science, ZEOLITES AND ORDERED MESOPOROUS MATERIALS: PROGRESS AND PROSPECTS, 157, 243, (2005)
Abstract: Organoplatinum complexes [Pt(R)(2)(iPr-DAB)], [PtCl(R)(iPr-DAB)] or [PtCl2(iPr-DAB)] (R = methyl or 2,4,6-trimethylphenyl = mesityl, iPr-DAB = N,N’-diisopropyl-1,4-diazabutadiene) have been investigated by a combination of multiple spectroscopy (NMR, UV/VIS), single crystal XRD and quantum chemical calculations. Excellent agreement of calculated and experimental structural data from XRD or NMR provide a good basis for the quantum mechanical calculations. DFT calculations reveal the contributions of the DAB ligand and the co-ligands to the electronic ground state. Methyl acts more or less purely as a alpha-donor, Cl or mesityl provide strong contributions (p or pi) to the highest occupied molecular orbital (HOMO) whereas the lowest unoccupied molecular orbital (LUMO) is mainly centred to the DAB ligand. The TD-DFT calculated electronic transitions were in excellent agreement with the experimental ones and allow reliable assignment of the absorption bands.