2003 publications citing ADF
First author: Janik, MJ, A computational and experimental study of anhydrous phosphotungstic acid and its interaction with water molecules, APPLIED CATALYSIS A-GENERAL, 256, 51, (2003)
Abstract: The hydration state of phosphotungstic acid (H3PW12O40) was explored through the use of in situ Fourier transform infrared (FT-IR) spectroscopy, water sorption microcalorimetry, and density functional theory (DFT) quantum chemical calculations. In addition. the proton affinities of the various sites on the Keggin unit were evaluated. The small differences (< 10 U mol(-1)) between the proton affinities of bridge and terminal oxygen atoms indicate that protons likely reside on both types of sites on the Keggin unit. Vibrational spectra calculated by DFT and measured experimentally by in situ FT-IR spectroscopy compare very,cry well. The adsorption energy of a water molecule on an anhydrous Keggin unit was calculated to be approximately -55 to -70 U mol(-1) for the formation of H3O+, independent of protonic site. The range compares well to the heat of water sorption (-65 U mol(-1)) determined from microcalorimetry on a sample dehydrated at 573 K. The uptake of gaseous water molecules was found to be five times higher on a sample pretreated at 473 K than on one pretreated at 573 K, indicating that the hydrated secondary structure does not reform after the high temperature pretreatment. In situ FT-IR spectroscopy confirms the lack of reversibility during dehydration/rehydration treatment for samples heated to 573 K.
First author: Mendiratta, A, Molybdenum chalcogenobenzimidato complexes: Radical synthesis and nitrile extrusion via beta-EPh (E = S, Se, and Te) elimination, INORGANIC CHEMISTRY, 42, 8621, (2003)
Abstract: Molybdenum chalcogenobenzimidates of formula (Ph[PhE]C=N)-Mo(N[t-Bu]Ar)(3) (Ar = 3,5-C6H3Me2) have been obtained by treatment of Mo(N[t-Bu]Ar)(3) sequentially with benzonitrile and 0.5 equiv of PhEEPh (E = S, Se, and Te). Molecular structure determinations have been carried out for the S and Se variants. The Te variant extrudes PhCN forming structurally characterized (PhTe)Mo(N[t-Bu]Ar)(3) with facility assessed via stopped-flow kinetic measurements, while the Se and S analogues exhibit increasing stability. Quantum chemical calculations and solution calorimetry have been employed as an aid to interpretation of the PhCN extrusion reaction.
First author: Kovalevsky, AY, Solid-state structure dependence of the molecular distortion and spectroscopic properties of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion, INORGANIC CHEMISTRY, 42, 8794, (2003)
Abstract: The relation between the geometry and spectroscopic properties of a series of salts of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion, (Cu-(I)(dmp)(2))(+), is explored. The distortions from the idealized D-2d geometry, which include flattening, rocking of the dmp ligands, and displacement of the Cu atoms out of the dmp planes, show considerable variation, indicating the importance of packing forces in the crystalline environment. The change in the absorption spectra upon flattening of the complex, expressed as the variation of the angle between the dmp planes, which varies from 88degrees in the BF4 and tosylate salts to 73degrees in the picrate, agrees qualitatively with parallel DFT calculations. No correlation is found between ground state geometry and luminescence lifetimes, recorded both at room temperature and at 16 K. The low temperature lifetimes vary by a factor of 8 among the (Cu-(I)(dmp)(2))(+) salts examined, the longest lifetime (2.4 mus at 16 K) being observed for the tosylate salt.
First author: Jorge, FE, On the origin of the optical activity in the d-d transition region of tris-bidentate Co(III) and Rh(III) complexes, INORGANIC CHEMISTRY, 42, 8902, (2003)
Abstract: Time-dependent density functional theory (TD-DFT) has been employed to calculate the rotatory strengths in the d-d transition region for various tris-bidentate Co(III) and Rh(III) complexes. Optimized structural parameters are also reported. Our results confirm a previously proposed relationship between the azimuthal distortion of a complex containing saturated tris(diamine) and its optical activity. Formally d-d transitions are forbidden and should not exhibit optical activity. However, it is shown here that the intensity of these bands originates from a coupling of even ligand combination (participating in the e(g) type LUMO) and an odd ligand combination (participating in the t(2g) HOMO). For complexes containing planar unsaturated ligands, the signs of the d-d bands observed from the single-crystal circular and linear dichroisms are in accordance with the TD-DFT predictions. It is shown that by using hypothetical Co(NH3)(6)(3+) complexes it is possible to estimate the contribution from the azimuthal distortion to the total rotatory strengths of the saturated tris(diamine) complexes. A discussion is also provided of previous theoretical studies and the way in which these investigations rationalized the optical activity.
First author: Jensen, L, A discrete solvent reaction field model for calculating frequency-dependent hyperpolarizabilities of molecules in solution, JOURNAL OF CHEMICAL PHYSICS, 119, 12998, (2003)
Abstract: We present a discrete solvent reaction field (DRF) model for the calculation of frequency-dependent hyperpolarizabilities of molecules in solution. In this model the solute is described using density functional theory (DFT) and the discrete solvent molecules are described with a classical polarizable model. The first hyperpolarizability is obtained in an efficient way using time-dependent DFT and the (2n+1) rule. The method was tested for liquid water using a model in which a water molecule is embedded in a cluster of 127 classical water molecules. The frequency-dependent first and second hyperpolarizabilities related to the electric field induced second harmonic generation (EFISH) experiment, were calculated both in the gas phase and in the liquid phase. For water in the gas phase, results are obtained in good agreement with correlated wave function methods and experiments by using the so-called shape-corrected exchange correlation (xc)-potentials. In the liquid phase the effect of using asymptotically correct functionals is discussed. The model reproduced the experimentally observed sign change in the first hyperpolarizaibility when going from the gas phase to the liquid phase. Furthermore, it is shown that the first hyperpolarizability is more sensitive to damping of the solvent-solute interactions at short range than the second hyperpolarizability.
First author: Fernandez, FJ, Generation and coupling of [Mn(dmpe)(2)(C CR)(C C)](center dot) radicals producing redox-active C-4-bridged rigid-rod complexes, CHEMISTRY-A EUROPEAN JOURNAL, 9, 6192, (2003)
Abstract: The symmetric d(5) trans-bis-alkynyl complexes [Mn(dmpe)(2)(C= CSiR3)(2)] (R = Me, 1a; Et, 1b; Ph, 1c) (dmpe = 1,2-bis(dimethylphosphino)ethane) have been prepared by the reaction of [Mn(dmpe)(2)Br-2] With two equivalents of the corresponding acetylide LiC=CSiR3. The reactions of species 1 with [CP2Fe][PF6] yield the corresponding d(4) complexes [Mn(dmpe)(2)(C=CSiR3)(2)][PF6] (R = Me, 2a; Et, 2b; Ph, 2c). These complexes react with NBu4F (TBAF) at -10degreesC to give the desilylated parent acetylide compound [Mn(dmpe)(2)(C= CH)21[PF6] (6), which is stable only in solution at below 0degreesC. The asymmetrically substituted trans-bis-alkynyl complexes [Mn(dmpe)2(C=CSiR3)(C=CH)] [PF6] (R = Me, 7a; Et, 7b) related to 6 have been prepared by the reaction of the vinylidene compounds [Mn(dmpe)(2)(C=CSiR3)(C=CH2)] (R = Me, 5a; Et, 5b) with two equivalents Of [CP2Fe][PF6] and one equivalent of quinuclidine. The conversion of [Mn(C5H4Me)(dmpe)I] with Me(3)SiCequivalent toCSnMe(3) and dmpe afforded the transiodide-alkynyl d(5) complex [Mn(dmpe)(2)(Cequivalent toCSiMe(3))I] (9). Com plex 9 proved to be unstable with regard to ligand disproportionation reactions and could therefore not be oxidized to a unique Mn-III product, which prevented its further use in acetylide coupling reactions. Compounds 2 react at room temperature with one equivalent of TBAF to form the mixed-valent species [{Mn(dmpe)(2)(Cequivalent toCH))(2)(mu-C-4)] [PF6] (11) by C-C coupling of [Mn(dmpe)(2)(Cequivalent toCH)(Cequivalent toC)] radicals generated by deprotonation of 6. In a similar way, the mixed-valent complex [[Mn(dmpe)(2)(Cequivalent toCSiMe(3))}(2)(mu-C-4)][PF6] [12](+) is obtained by the reaction of 7 a with one equivalent of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene). The relatively long-lived radical intermediate [Mn(dmpe),(Cequivalent toCH)(Cequivalent toC)]. could be trapped as the Mn-I complex [Mn(dmpe)(2)(Cequivalent toCH)(equivalent toC-CO2)] (14) by addition of an excess of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) to the reaction mixtures of species 2 and TBAF. The neutral dinuclear Mn-II/ Mn-II compounds [[Mn(dmpe)2(C= CR3)}(2)(mu-C-4)] (R = H, 11; R = SiMe3, 12) are produced by the reduction of [11](+) and [12](+), respectively, with [FeCp(C6Me6)], [11](+) and [12](+) can also be oxidized with [Cp2Fe][PF6] to produce the dicationic Mn-III/Mn-III species [{Mn(dmpe)(2)(Cequivalent toCR(3))}(2)(mu-C-4)] [PF6](2) (R = H, [11](2+); R = SiMe3, [1212,). Both redox processes are fully reversible. The dinuclear compounds have been characterized by NMR, IR, UV/Vis, and Raman spectroscopies, CV, and magnetic susceptibilities, as well as elemental analyses. X-ray diffraction studies have been performed on complexes 4b, 7b, 9, [12](+), [12](2+), and 14.
First author: Gutta, P, Unusual geometries and questions of oxidation state in potential Sn(III) chemistry, INORGANIC CHEMISTRY, 42, 8161, (2003)
Abstract: A mixed-valence compound (Sn2I3(NPPh3)(3)) with nonequivalent Sri atoms in characteristic 2+ and 4+ Sn geometries, raised the idea of an average Sn3+ structure. The extended structures of Sr4Sn2Se9 and Sr4Sn2Se10 contain an unusual Sn2Se6 subunit, which has two equal Se-Sn-Se angles close to 160degrees. This was suggestive of a Sn3+/ Sn3+ compound, similar to the putative transition state for the valence state interchange in the molecular compound. These interesting geometrical features of two quite different molecules prompt a series of computations, a detective story of geometries and oxidation states, which concludes tentatively that the Sn with the abnormal angle in the extended structure is still likely to be formally Sn4+.
First author: Chang, CJ, The Pacman effect: A supramolecular strategy for controlling the excited-state dynamics of pillared cofacial bisporphyrins, INORGANIC CHEMISTRY, 42, 8262, (2003)
Abstract: The molecular recognition properties of dizinc(II) bisporphyrin anchored by dibenzofuran (DPD), Zn-2(DPD) (1), were evaluated as a strategy for utilizing the Pacman effect to control the excited-state properties of cofacial bisporphyrin motifs. Crystallographic studies establish that DPD furnishes a cofacial system with vertical flexibility and horizontal preorganization. The structure determination of a substrate-bound DPD species, Zn-2(DPD)(2-aminopyrimidine) (2), completes a set of structurally homologous zinc(II) porphyrin host and host-guest complexes, which offer a direct structural comparison for the Pacman effect upon substrate complexation. Binding studies reveal that pyrimidine encapsulation by the DPD framework is accompanied by a markedly reduced entropic penalty (similar to60 J mol(-1)K(-1)) with respect to traditional face-to-face bisporphyrin systems, giving rise to a smaller conformational energy cost upon substrate binding. Transient absorption spectroscopy reveals that substrate encapsulation within the DPD cleft dramatically affects excited-state dynamics of cofacial bisporphyrins. The emission lifetime of host-guest complex 2 increases by more than an order of magnitude compared to free host 1. In the absence of the guest, the excited-state dynamics are governed by torsional motion of the porphyrin rings about the aryl ring of the DPD pillar. Host-guest binding attenuates this conformational flexibility, thereby removing efficient nonradiative decay pathways. Taken together, these findings support the exceptional ability of the DPD system to structurally accommodate reaction intermediates during catalytic turnover and provide a novel supramolecular approach toward developing a reaction chemistry derived directly from the excited states of Pacman constructs.
First author: Patchkovskii, S, Thermodynamic stability of hydrogen clathrates, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 100, 14645, (2003)
Abstract: The stability of the recently characterized type 11 hydrogen clathrate [Mao, W. L., Mao, H.-K., Goncharov, A. F., Struzhkin, V. V., Guo, Q., et al (2002) Science 297, 2247-2249] with respect to hydrogen occupancy is examined with a statistical mechanical model in conjunction with first-principles quantum chemistry calculations. It is found that the stability of the clathrate is mainly caused by dispersive interactions between H-2 molecules and the water forming the cage walls. Theoretical analysis shows that both individual hydrogen molecules and nH(2) guest clusters undergo essentially free rotations inside the clathrate cages. Calculations at the experimental conditions -2,000. bar (1 bar = 100 kPa) and 250 K confirm multiple occupancy of the clathrate cages with average occupations of 2.00 and 3.96 H-2 molecules per D-5(12) (small) and H-5(12)6(4) (large) cage, respectively. The H-2-H2O interactions also are responsible for the experimentally observed softening of the H-H stretching modes. The clathrate is found to be thermodynamically stable at 25 bar and 150 K.
First author: Filatov, M, Representation of the exact relativistic electronic Hamiltonian within the regular approximation,JOURNAL OF CHEMICAL PHYSICS, 119, 11526, (2003)
Abstract: The exact relativistic Hamiltonian for electronic states is expanded in terms of energy-independent linear operators within the regular approximation. An effective relativistic Hamiltonian has been obtained, which yields in lowest order directly the infinite-order regular approximation (IORA) rather than the zeroth-order regular approximation method. Further perturbational expansion of the exact relativistic electronic energy utilizing the effective Hamiltonian leads to new methods based on ordinary (IORAn) or double [IORAn(2)] perturbation theory (n: order of expansion), which provide improved energies in atomic calculations. Energies calculated with IORA4 and IORA3(2) are accurate up to c(-20). Furthermore, IORA is improved by using the IORA wave function to calculate the Rayleigh quotient, which, if minimized, leads to the exact relativistic energy. The outstanding performance of this new IORA method coined scaled IORA is documented in atomic and molecular calculations.
First author: Heuft, JM, Density functional theory based molecular-dynamics study of aqueous chloride solvation,JOURNAL OF CHEMICAL PHYSICS, 119, 11788, (2003)
Abstract: The aqueous solvation shell of chloride is studied using density functional theory based molecular-dynamics simulations. This method enables us to obtain a detailed understanding of the structural, dynamic and electronic properties of the system. Special attention is paid to the dynamic properties of the first solvation shell. The results obtained here are in good agreement both with experiments and other simulations. Our results suggest that the surrounding shell of water molecules is quite rigid. We observe no influence of the chloride ion on the electronic and structural properties of the coordinating water molecules.
First author: Duran, J, Stereodiscrimination in phosphanylthiolato nickel(II) complexes, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 119, 4147, (2003)
Abstract: The presence of a stereogenic carbon centre (R or S) in the racemic ligand 1-(diphenylphosphanyl)propane-2-thiol induces a conformational preference (lambda or 6) in the five-membered chelate ring of its 2:1 and 2:2 coordination compounds with Ni-II: the mononuclear trans-[Ni(SCH(CH3)CH2PPh2P,S)(2)] (1) and the binuclear trans-[Ni{mu-SCH(CH3)CH2PPh2P,S)(Cl)](2) (2). Both complexes exist as mixtures of two diastereomers: racemic-trans (R-lambda, R-lambda, and S-delta, S-delta) and meso-trans (R-lambda, S-delta), in an equilibrium displaced towards the more stable isomer, the meso-trans for 1 (calculated DeltaE = < -0.1 kcal-mol(-1)) and the racemic-trans for 2 (calculated DeltaE = -5.1 kcal-mol(-1)). This phenomenon is especially evident for complex 2, in which an efficient chiral recognition between the two enantiomeric forms of the racemic ligand was observed. (
First author: Petrie, S, Chemistry in stringland: One-dimensional complexes of main-group metal ions with the ligands NC2nX (X = N, CH; n=0, 1, 2, 3), JOURNAL OF PHYSICAL CHEMISTRY A, 107, 10441, (2003)
Abstract: Complexation by M+ (M = Li, Be, Na, Mg, Al, K, Ca) of the highly unsaturated linear molecules NC2nX (X = N, CH; n = 0, 1, 2, 3) occurs exclusively by a coordination to the terminal N atom, yielding linear molecular cations that are here characterized using high-level, counterpoise-corrected ab initio calculations. We argue that these complexes, with a total absence of steric hindrance through nonbonding interactions, form an excellent “test set” for the purpose of investigating, in detail, the nature of the metal ion/ligand interaction. We analyze the influence of ionic, covalent, and repulsive energy contributions to the M+/ligand interaction for these species, using two different energy-decomposition schemes, and present also a complementary interpretation using the atoms-in-molecules (AIM) approach. Differences between the M+-NC2nX bond dissociation energies (BDEs) of the highly polar cyanopolyynes HC2n+1N versus the analogous nonpolar dicyanopolyynes NC2nN din-finish as the intervening carbon chain length increases, indicating that the local bond polarity of the coordinating CN group dominates over the ligand’s overall polarity (or lack thereof) as an influence of M+/ ligand bond strength. The cyanopolyyne and dicyanopolyyne adducts of alkaline earth ions Be+, Mg+, and Ca+ and of All, whereas largely ionic in character, also possess significant overlying covalent tendencies. In contrast, the adducts of alkali metal ions Li+, Na+, and K+ can indeed be treated as essentially purely ionic. Among the results reported here, one striking observation (for which an underlying physical basis remains elusive) is that the M+-NC2nX series featuring an alkali metal ion exhibits a remarkably close adherence to the r(-12) dependence of the empirically assigned repulsive potential energy term in the (12, 6) Lennard-Jones potential.
First author: Dunlap, BI, Analytic and variational X alpha in the Slater-Roothaan method, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 10082, (2003)
Abstract: An analytic and mathematically variational linear-combination-of-atomic-orbitals density-functional method that allows arbitrary scaling of the Slater-Gasper-Kohn-Sham exchange-correlation potential about each atom is described. The method can be made exact in the separated-atom limit. It is based on robust and variational fitting, which is reviewed and extended to fast-multipole methods. The Slater-Roothaan method requires four basis sets and delivers a total energy that is independent of all fitting errors through first order. A database of atomic Gaussian basis sets is used to construct inputs for a standard set of 56 molecules. That database contains our basis sets as well as the DGauss DZVP2 double-zeta and 6-311G triple-zeta polarized Gaussian basis sets for fitting molecular orbitals. Another two subdatabases contain the s- and non-s-type basis sets for fitting the charge density and the exchange-correlation potentials, which are related to the cube root of a partitioned density and its square. A bond-centered basis function can be added to all fitting bases via software. Eight different fitting basis sets are studied. Using the Hartree-Fock values of alpha, these molecules are overbound on average, but using a uniform alpha = 0.7, these molecules are underbound on average, independent of fitting basis. Mixing exact exchange will not obviously improve the method.
First author: Miller, JE, Spectroscopy and reactivity of a photogenerated tryptophan radical in a structurally defined protein environment, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 14220, (2003)
Abstract: An analytic and mathematically variational linear-combination-of-atomic-orbitals density-functional method that allows arbitrary scaling of the Slater-Gasper-Kohn-Sham exchange-correlation potential about each atom is described. The method can be made exact in the separated-atom limit. It is based on robust and variational fitting, which is reviewed and extended to fast-multipole methods. The Slater-Roothaan method requires four basis sets and delivers a total energy that is independent of all fitting errors through first order. A database of atomic Gaussian basis sets is used to construct inputs for a standard set of 56 molecules. That database contains our basis sets as well as the DGauss DZVP2 double-zeta and 6-311G triple-zeta polarized Gaussian basis sets for fitting molecular orbitals. Another two subdatabases contain the s- and non-s-type basis sets for fitting the charge density and the exchange-correlation potentials, which are related to the cube root of a partitioned density and its square. A bond-centered basis function can be added to all fitting bases via software. Eight different fitting basis sets are studied. Using the Hartree-Fock values of alpha, these molecules are overbound on average, but using a uniform alpha = 0.7, these molecules are underbound on average, independent of fitting basis. Mixing exact exchange will not obviously improve the method.
First author: Bagno, A, Predicting C-13 NMR spectra by DFT calculations, JOURNAL OF PHYSICAL CHEMISTRY A,107, 9964, (2003)
Abstract: C-13 chemical shifts and (n)J(CH) coupling constants have been determined both experimentally (by means of J-resolved NMR spectroscopy) and theoretically (by DFT calculations) for a series of organic molecules. With the exception of halogen-bonded carbon nuclei, a good correlation is observed between experimental and calculated data. The magnitude of the most important contributions to the spin-spin coupling constant (Fermi-contact, diamagnetic, and paramagnetic spin-orbit contributions) has been determined. The spin-orbit terms are negligible or cancel Out ((1)J(CH) and (3)J(CH)), thus leaving the contact term as the only relevant contribution, but become important for (2)J(CH) in aromatic (but not in aliphatic) compounds. Relativistic effects on the C-13 chemical shift of carbon bonded to a fairly heavy atom (bromine) have also been investigated. Finally, conformational effects on the long-range (n)J(CH) coupling constants has been investigated in a model alkane derivative (n-butyl chloride). The implications to structure prediction and determination by NMR are discussed.
First author: Baik, MH, Theoretical study of cisplatin binding to purine bases: Why does cisplatin prefer guanine over adenine?, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 14082, (2003)
Abstract: The thermodynamics and kinetics for the monofunctional binding of the antitumor drug cisplatin, cis-diamminedichloroplatinum(II), to a purine base site of DNA were studied computationally using guanine and adenine as model reactants. A dominating preference for initial attack at the N7-position of guanine is established experimentally, which is a crucial first step for the formation of a 1,2-intrastrand cross-link of adjacent guanine bases that leads to bending and unwinding of DNA. These structural distortions are proposed ultimately to be responsible for the anticancer activity of cisplatin. Utilizing density functional theory in combination with a continuum solvation model, we developed a concept for the initial Pt-N7 bond formation to atomic detail. In good agreement with experiments that suggested DeltaG(double dagger) = similar to23 kcal/mol for the monofunctional platination of guanine, our model gives DeltaG(double dagger) = 24.6 kcal/mol for guanine, whereas 30.2 kcal/mol is computed when adenine is used. This result predicts that guanine is 3-4 orders of magnitude more reactive toward cisplatin than adenine. A detailed energy decomposition and molecular orbital analysis was conducted to explain the different barrier heights. Two effects are equally important to give the preference for guanine over adenine: First, the transition state is characterized by a strong hydrogen bond between the ammine-hydrogen of cisplatin and the O=C6 moiety of guanine in addition to a stronger electrostatic interaction between the two reacting fragments. When adenine binds, only a weak hydrogen bond forms between the chloride ligand of cisplatin and the H2N-C6 group of adenine. Second, a significantly stronger molecular orbital interaction is identified for guanine compared to adenine. A detailed MO analysis is presented to provide an intuitive view into the different electronic features governing the character of the Pt-N7 bond in platinated purine bases.
First author: Senthilkumar, K, Mapping the sites for selective oxidation of guanines in DNA, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 13658, (2003)
Abstract: The thermodynamics and kinetics for the monofunctional binding of the antitumor drug cisplatin, cis-diamminedichloroplatinum(II), to a purine base site of DNA were studied computationally using guanine and adenine as model reactants. A dominating preference for initial attack at the N7-position of guanine is established experimentally, which is a crucial first step for the formation of a 1,2-intrastrand cross-link of adjacent guanine bases that leads to bending and unwinding of DNA. These structural distortions are proposed ultimately to be responsible for the anticancer activity of cisplatin. Utilizing density functional theory in combination with a continuum solvation model, we developed a concept for the initial Pt-N7 bond formation to atomic detail. In good agreement with experiments that suggested DeltaG(double dagger) = similar to23 kcal/mol for the monofunctional platination of guanine, our model gives DeltaG(double dagger) = 24.6 kcal/mol for guanine, whereas 30.2 kcal/mol is computed when adenine is used. This result predicts that guanine is 3-4 orders of magnitude more reactive toward cisplatin than adenine. A detailed energy decomposition and molecular orbital analysis was conducted to explain the different barrier heights. Two effects are equally important to give the preference for guanine over adenine: First, the transition state is characterized by a strong hydrogen bond between the ammine-hydrogen of cisplatin and the O=C6 moiety of guanine in addition to a stronger electrostatic interaction between the two reacting fragments. When adenine binds, only a weak hydrogen bond forms between the chloride ligand of cisplatin and the H2N-C6 group of adenine. Second, a significantly stronger molecular orbital interaction is identified for guanine compared to adenine. A detailed MO analysis is presented to provide an intuitive view into the different electronic features governing the character of the Pt-N7 bond in platinated purine bases.
First author: Senthilkumar, K, Charge transport in columnar stacked triphenylenes: Effects of conformational fluctuations on charge transfer integrals and site energies, JOURNAL OF CHEMICAL PHYSICS, 119, 9809, (2003)
Abstract: Values of charge transfer integrals, spatial overlap integrals and site energies involved in transport of positive charges along columnar stacked triphenylene derivatives are provided. These parameters were calculated directly as the matrix elements of the Kohn-Sham Hamiltonian, defined in terms of the molecular orbitals on individual triphenylene molecules. This was realized by exploiting the unique feature of the Amsterdam density functional theory program that allows one to use molecular orbitals on individual molecules as a basis set in calculations on a system composed of two or more molecules. The charge transfer integrals obtained in this way differ significantly from values estimated from the energy splitting between the highest occupied molecular orbitals in a dimer. The difference is due to the nonzero spatial overlap between the molecular orbitals on adjacent molecules. Calculations were performed on unsubstituted and methoxy- or methylthio-substituted triphenylenes. Charge transfer integrals and site energies were computed as a function of the twist angle, stacking distance and lateral slide distance between adjacent molecules. The variation of the charge transfer integrals and site energies with these conformational degrees of freedom provide a qualitative explanation of the similarities and differences between the experimental charge carrier mobilities in different phases of alkoxy- and alkylthio-substituted triphenylenes. The data obtained from the present work can be used as input in quantitative studies of charge transport in columnar stacked triphenylene derivatives.
First author: Autschbach, J, A theoretical study of the NMR spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-) (n=0-3) and [(NC)(5)Pt-Tl-Pt(CN)(5)](3-): A lesson on environmental effects, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 13585, (2003)
Abstract: The molecular geometries and the nuclear spin-spin coupling constants of the complexes [(NC)(5)Pt-TI(CN)(n)](n-), n = 0-3, and the related system [(NC)(5)Pt-TI-Pt(CN)(5)](3-) are studied. These complexes have received considerable interest since the first charactetization of the n = 1 system by Glaser and co-workers in 1995 [J. Am. Chem. Soc. 1995, 117, 7550-7551]. For instance, these systems exhibit outstanding NMR properties, such as extremely large Pt-TI spin-spin coupling constants. For the present work, all nuclear spin-spin coupling constants J(Pt-TI), J(Pt-C), and J(TI-C) have been computed by means of a two-component relativistic density functional approach. It is demonstrated by the application of increasingly accurate computational models that both the huge JPt-TI for the complex (NC)(5)Pt-TI and the whole experimental trend among the series are entirely due to solvent effects. An approximate inclusion of the bulk solvent effects by means of a continuum model, in addition to the direct coordination, proves to be crucial. Similarly drastic effects are reported for the coupling constants between the heavy atoms and the carbon nuclei. A computational model employing the statistical average of orbital-dependent model potentials (SAOP) in addition to the solvent effects allows to accurately reproduce the experimental coupling constants within reasonable limits.
First author: Diez, RP, Metastable states in AU(2)(2+): a density functional study, JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 639, 203, (2003)
Abstract: The presence of metastable states in the doubly ionized gold dimer is studied using gradient-corrected scalar-relativistic density functional theory. Twenty metastable states are found within an energy range of less than 3.5 eV. The ground state of Au-2(2+), however, is found to be unbound. A relationship between dissociation energies with respect to the top of the barrier, metastable equilibrium bond lengths, molecular orbitals occupancies, and Mulliken overlap populations is found. A bond energy analysis indicates that the strong stabilization undergone by the sigma(g) molecular orbitals is correlated to the magnitude of Au-2(2+) the dissociation energies with respect to the top of the barrier. Finally, the bonding pattern in the set of metastable states of Au-2(2+) is found to be dominated by sigma(g) molecular orbitals, the 4sigma(g) one in particular, with little influence of pi(u) molecular orbitals. ne results agree well with ab initio CASSCF and CASPT2 calculations, proving the ability of the density functional theory to treat molecules and clusters containing heavy transition metals.
First author: Sherman, DM, Surface complexation of arsenie(V) to iron(III) (hydr)oxides: Structural mechanism from ab initio molecular geometries and EXAFS spectroscopy, GEOCHIMICA ET COSMOCHIMICA ACTA, 67, 4223, (2003)
Abstract: Arsenic(V), as the arsenate (AsO4)(3-) ion and its conjugate acids, is strongly sorbed to iron(III) oxides (alpha-Fe2O3), oxide hydroxides (alpha-,gamma-FeOOH) and poorly crystalline ferrihydrite (hydrous ferric oxide). The mechanism by which arsenate complexes with iron oxide hydroxide surfaces is not fully understood. There is clear evidence for inner sphere complexation but the nature of the surface complexes is controversial. Possible surface complexes between AsO4 tetrahedra and surface FeO6 polyhedra include bidentate corner-sharing (C-2), bidentate edge-sharing (E-2) and monodentate corner-sharing (V-1). We predicted the relative energies and geometries of AsO4-FeOOH surface complexes using density functional theory calculations on analogue Fe-2(OH)(2)(H2O)(n)AsO2(OH)(2)(3+) and Fe-2(OH)(2)(H2O)(n)AsO4+ clusters. The bidentate corner-sharing complex is predicted to be substantially (55 kJ/mole) more favored energetically over the hypothetical edge-sharing bidentate complex. The monodentate corner-sharing (V-1) complex is very unstable. We measured EXAFS spectra of 0.3 wt. % (AsO4)(3-) sorbed to hematite (alpha-Fe2O3), goethite(alpha-FeOOH), lepidocrocite(gamma-FeOOH) and ferrihydrite and fit the EXAFS directly with multiple scattering. The phase-shift-corrected Fourier transforms of the EXAFS spectra show peaks near 2.85 and 3.26 Angstrom that have been attributed by previous investigators to result from E-2 and C-2 complexes. However, we show that the peak near 2.85 Angstrom appears to result from As-O-O-As multiple scattering and not from As-Fe backscatter. The observed 3.26 Angstrom As-Fe distance agrees with that predicted for the bidentate corner-sharing surface (C-2) complex. We find no evidence for monodentate (V-1) complexes; this agrees with the predicted high energies of such complexes.
First author: Tapilin, VM, Numerically integrable confined basis functions for band structure calculations, JOURNAL OF STRUCTURAL CHEMISTRY, 44, 911, (2003)
Abstract: Numerical atomic and Slater-type radial functions are matched with polynomials which vanish at a predetermined radius. Outside this radius the functions are, identically equal to zero. Such functions with a. and matching radii were used for band structure different number of derivatives continuous at the cutoff calculations of copper. The utilization of Gauss quadratures, constructed without accounting for discontinuity of the basis functions, was shown to ensure the required accuracy of numerical integration for matrix element calculations with the basis functions that have three lowest derivatives discontinuous at the cutoff and matching radii. In the case of matching radius variations over a wide range, the quality of the basis set is virtually independent of this value. Increasing the cutoff radius improves the basis quality. Moreover, a basis limit is reached even at the cutoff radius smaller than a two-fold distance between the nearest neighbors. The resulting basis set is not inferior in quality to conventional bases of atomic or Slater-type functions, and in contrast to conventional functions, requires no special techniques of numerical integration. The proposed basis set enables a reduction of computational time by a factor of 2 approximately.
First author: Taylor, S, Catalytic asymmetric heterogeneous aziridination using CuHY/bis(oxazoline): effect of reaction conditions on enantioselectivity, TOPICS IN CATALYSIS, 25, 81, (2003)
Abstract: The copper-catalyzed aziridination of styrene with copper-exchanged zeolite HY(CuHY) and copper(II) triflate (trifluoromethanesulfonate) (Cu(OTf)(2)) as catalysts is described using N-(p-tolylsulfonyl) imino] phenyliodinane (PhI=NTs) as the nitrene donor. The effects on the ee and yield of the aziridine when the catalyst is modified by the presence of a chiral bis(oxazoline) are investigated in detail. The heterogeneously catalyzed reaction under these conditions shows a slight, but significant, enhancement in ee with increasing conversion at 25degreesC. This is not observed in the more rapid homogeneously catalyzed reaction under identical reaction conditions using PhINTs as the nitrene donor. The enhancement in ee is proposed to result from the preferential reaction of the (S)-aziridine with the Cu2+:bis(oxazoline) complex in the presence of PhI=NTs, leading to an enhancement of the (R)-aziridine in the remaining aziridine product.
First author: Wasinger, EC, L-edge X-ray absorption spectroscopy of non-heme iron sites: Experimental determination of differential orbital covalency, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 12894, (2003)
Abstract: X-ray absorption spectroscopy has been utilized to obtain the L-edge multiplet spectra for a series of non-heme ferric and ferrous complexes. Using these data, a methodology for determining the total covalency and the differential orbital covalency (DOC), that is, differences in covalency in the different symmetry sets of the d orbitals, has been developed. The integrated L-edge intensity is proportional to the number of one-electron transition pathways to the unoccupied molecular orbitals as well as to the covalency of the iron site, which reduces the total L-edge intensity and redistributes intensity, producing shake-up satellites. Furthermore, differential orbital covalency leads to differences in intensity for the different symmetry sets of orbitals and, thus, further modifies the experimental spectra. The ligand field multiplet model commonly used to simulate L-edge spectra does not adequately reproduce the spectral features, especially the charge transfer satellites. The inclusion of charge transfer states with differences in covalency gives excellent fits to the data and experimental estimates of the different contributions of charge transfer shake-up pathways to the t(2g) and e(g) symmetry orbitals. The resulting experimentally determined DOC is compared to values calculated from density functional theory and used to understand chemical trends in high- and low-spin ferrous and ferric complexes with different covalent environments. The utility of this method toward problems in bioinorganic chemistry is discussed.
First author: Han, WG, A theoretical study of the UV/visible absorption and emission solvatochromic properties of solvent-sensitive dyes, CHEMPHYSCHEM, 4, 1084, (2003)
Abstract: Using the density-functional vertical self-consistent reaction field (VSCRF) solvation model, incorporated with the conductor-like screening model (COSMO) and the self-consistent reaction field (SCRF) methods, we have studied the solvatochromic shifts of both the absorption and emission bands of four solvent sensitive dyes in different solutions. The dye molecules studies here are: S-TBA merocyanine, Abdel-Halim’s merocyanine, the rigidified amino-coumarin C153, and Nile red. These dyes were selected because they exemplify different structural features likely to impact the solvent-sensitive fluorescence of “push-pull”, or merocyanine, fluorophores. All trends of the blue or red shifts were correctly predicted, comparing with the experimental observations. Explicit h-bonding interactions were also considered in several protic solutions like H2O, methanol and ethanol, showing that including explicit H-bonding solvent molecule(s) in the calculations is important to obtain the correct order of the excitation and emission energies. The geometries, electronic structures, dipole moments, and intra- and intermolecular charge transfers of the dyes in different solvents are also discussed.
First author: Poater, J, Electron pairing analysis of the Fischer-type chromium-carbene complexes (CO)(5)Cr=C(X)R (X = H, OH, OCH3, NH2, NHCH3 and R = H, CH3,CH=CH2,Ph, C CH), CHEMICAL PHYSICS, 294, 129, (2003)
Abstract: The electron-pair density distributions of a series of 25 Fischer carbene complexes of the type (CO)(5)Cr=C(X)R (X = H, OH, OCH3, NH2, NHCH3 and R = H, CH3, CH = CH2, Ph, C equivalent to CH) are analyzed using the Atoms in Molecules theory. Localization and delocalization indices are used to characterize the electron pairing taking place in the Cr = C-X moiety in these complexes. Electron delocalization between the Cr and C atoms and between the C atom and the X group are related to the pi-donor strength of the X group and the degree of back-donation between the chromium pentacarbonyl and the carbene fragments. The results obtained with the Atoms in Molecules theory complement those obtained in a previous study by means of energy and charge decomposition analyses. Electron delocalization between the Cr atom and the X group is consistent with the hypothesis of a weak 3-center 4-electron bonding interaction in the Cr = C-X group of atoms. Except for X = H, delta(Cr,X) increases with the decrease of the pi-donor character of the X group.
First author: Coat, F, Chemistry of the 1,3,5,7-octatetraynediyl carbon rod end-capped by two electron-rich (eta(5)-C5Me5)(eta(2)-dppe)Fe groups, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 683, 368, (2003)
Abstract: The synthesis of the organoiron complex [(eta(5)-C5Me5)(eta(2)-dppe)Fe-Cequivalent toC-Cequivalent toC-Cequivalent toC-Cequivalent toC-Fe(eta(2)-dppe)(eta(5)-C5Me5)] (2, dppe = 1,2-bis(diphenylphosphino)ethane) is reported with its full spectroscopic characterizations (H-1-, P-31-, and C-13-NMR, IR, Raman, UV-vis and Fe-57 Mossbauer). The X-ray analysis of 2 shows that the molecule adopts a geometry very close to the anti conformation in the solid state. The shortening of the Fe-C bond distance associated with the increase in the number of carbon atoms suggests some cumulenic contribution to the description of the electronic structure of the all-carbon bridge. The C-13-NMR, Fe-57 Mossbauer data and theoretical calculations confirm this trend and indicate that the cumulenic contribution is significant in the vicinity of the metal center but vanishes in the middle of the carbon rod. Vibrational spectroscopy carried out on the single crystals of [(eta(5)-C5Me5)(eta(2)-dppe)Fe-Cequivalent toC-Cequivalent toC-Fe(eta(2)-dppe)(eta(5)-C5Me5)] (1) and 2 and on solutions of these compounds indicates that the Cequivalent toC bond stretching mode is not very sensitive to the relative orientation of the terminal endgroups. The electronic structure of the titled compound has been investigated using density functional theory. The geometrical changes occurring upon elongation of the carbon chain were nicely reproduced and interpreted. Time-dependant density functional theory calculations have been performed to rationalize the optical spectra.
First author: Hillier, AC, A combined experimental and theoretical study examining the binding of N-heterocyclic carbenes (NHC) to the Cp*RuCl (Cp* = eta(5)-C5Me5) moiety: Insight into stereoelectronic differences between unsaturated and saturated NHC ligands, ORGANOMETALLICS, 22, 4322, (2003)
Abstract: Combined solution calorimetric and quantum mechanics studies of reactions involving saturated and unsaturated N-heterocyclic carbene (NHC) ligands show that the difference in their relative bond dissociation energies is very small (1 kcal.mol(-1)). Structural and computational studies reveal small metric parameter differences. These observations in conjunction with relative reactivity profiles of NHC-modified ruthenium-based olefin metathesis catalysts suggest that very small changes in the donor properties of the NHC ligands can translate into significant differences in catalytic properties.
First author: Daoudi, A, Electronic structure and molecular spectroscopic constants of ScN and ScP investigated by several quantum chemistry methods, MOLECULAR PHYSICS, 101, 2929, (2003)
Abstract: The electronic structure of the ScN and ScP molecules is a subject of controversy and turns out to be a challenging problem in quantum chemistry. We show that the ground-state electronic structure for both molecules depends critically on the choice of methods used which incorporate different ways of accounting for electron correlation. A parallel ab initio, DFT and TD-DFT study is performed for this purpose and uses sufficiently flexible basis sets able to reproduce accurate electronic structures, as well as correct spectroscopic constants.In the ab initio methodology, results have been obtained with methods such as Hartree-Fock (HF), Moller-Plesset perturbation theory (MPn), direct configuration interaction (CI), quadratic configuration interaction (QC), coupled cluster configuration interaction (CC), complete active space self-consistent field (CASSCF) and multireference configuration interaction (CIPSI) methods. In the DFT methodology, various ‘pure’ and ‘hybrid’ density functionals are used and the corresponding results are compared to sophisticated ab initio methods and to available experimental data.All the methods used show that the ground state of both molecules is (1)Sigma(+), but two electronic structure natures, (1)Sigma(+) open-shell or (1)Sigma(+) closed-shell, are competitive and depend on the method employed. All the ab initio methods based on a single determinant wavefunction suffer seriously in predicting clearly the exact nature: of the ground state or its correct structural and spectroscopic parameters. However, the ab initio methods based on a multiconfigurational wavefunction appear to be successful in describing correctly, within one shot, the electronic structure and the molecular spectroscopic constants. The ground state, particularly for the ScN molecule, presents an unusual electronic structure: the presence of degenerate determinants, quasidegeneracy with other states and one avoided crossing in the region around the equilibrium distances. The bonding of the ground state is a two open-shell (1)Sigma(+) state described as a pi double bond and a sigma dative bond; the real triple bond (1)Sigma(+) state, i.e. closed-shell state, is found to lie higher in energy. The potential energy curves of the low-lying electronic states, the derived electronic structures and various molecular spectroscopic constants are presented and discussed for each method employed.
First author: Gutta, P, Propensity of different AgBr surfaces for photoinduced silver cluster formation: A molecular orbital analysis, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 8184, (2003)
Abstract: Ag cluster formation on AgBr is probed in three ways. First, qualitative orbital arguments, largely based on symmetry considerations, are introduced to determine which AgBr surfaces are more likely to lead to silver cluster formation induced by photoelectrons. The analysis indicates that silver atoms on the (111) surface and the troughs and wedges of a twin plane are likely to form clusters. Silver atoms on (100) and (110) surfaces do not appear to possess the electronic features favoring cluster formation. Second, DFT electronic structure calculations performed on cluster models produced trends similar to those emerging from the group theoretical analysis. Third, we looked at electron density shifts in models specifically constructed to create contrasting bonding environments on AgBr surfaces. An increase in electron density on the essential silver atoms is observed in a triplet state model for photoinduced cluster formation.
First author: Hu, XL, Copper complexes of nitrogen-anchored tripodal N-heterocyclic carbene ligands, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 12237, (2003)
Abstract: Incorporation of a nitrogen functionality into a tripodal N-heterocyclic carbene ligand system affords the first N-anchored tetradentate tris-carbene ligands TIMENR (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment of the methyl derivatized [H3TIMENMe](PF6)(3) imidazolium salt (H(3)5a) with silver oxide yields the silver complex [(TIMENMe)(2)Ag-3](PF6)(3) (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the trinuclear copper(I) complex [(TIMENMe)(2)Cu-3](PF6)(3) (10). Deprotonation of the tert-butyl and benzyl derivatives [H3TIMENt-Bu](PF6)(3) and [H3TIMENBz](PF6)(3) yields the free tris-carbenes TIMENt-Bu (5b) and TIMENBz (5c), which react readily with copper(I) salts to give mononuclear complexes [(TIMENt-Bu)Cu](PF6) (11b) and [(TIMENBz)Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were determined by single-crystal X-ray diffraction. While the TIMENMe ligand yields trinuclear complex 10, with both T-shaped three-coordinate and linear two-coordinate copper(I) centers, the TIMENt-Bu and TIMENBz ligands induce mononuclear complexes 11b and 11c, rendering the cuprous ion in a trigonal planar ligand environment of three carbenoid carbon centers and an additional, weak axial nitrogen interaction. Complexes 11b and 11c exhibit reversible one-electron redox events at half-wave potentials of 110 and -100 mV vs Fc/Fc(+), respectively, indicating sufficient electronic and structural flexibility of both TIMENR ligands (R = t-Bu, Bz) to stabilize copper(I) and copper(II) oxidation states. Accordingly, a copper(II) NHC complex, [(TIMENBz)Cu](OTf)(2) (12), was synthesized. Paramagnetic complex 12 was characterized by elemental analysis, EPR spectroscopy, and SQUID magnetization measurements.
First author: Mawhinney, RC, A theoretical analysis of the conformational behaviour of substituted methylenecyclohexanes, CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE, 81, 1101, (2003)
Abstract: The use of the PBE0 hybrid density functional theory method in conjunction with the COSMO solvation model allowed us to reproduce, both qualitatively and quantitatively, the experimentally observed conformational compositions of 2-substituted and 2,7-disubstituted methylenecyclohexanes. An analysis revealed several different interactions that influence the overall equilibrium. It was found that the endo (general) anomeric effect plays a significant role in the equilibrium and that the “unsaturation effect” possibly comprises two effects.
First author: Apra, E, NWChem for materials science, COMPUTATIONAL MATERIALS SCIENCE, 28, 209, (2003)
Abstract: This paper focuses on describing the computational chemistry software, NWChem, and its use in materials science research. The current functionalities and capabilities are outlined, as well as future features. Specific computational examples are given to show the flexibility and usefulness of NWChern to answering materials science problems.
First author: Molchanov, S, Nuclear spin relaxation study of the solution reorientation of 3,5-dichlorophenyl- and phenylethynyl-mercury cyanide molecules and shielding tensors of nuclei forming the -HgCN and -CCHgCN groups: estimation of the heavy atom effect of mercury, MAGNETIC RESONANCE IN CHEMISTRY, 41, 788, (2003)
Abstract: 3,5-Dichlorophenylmercury cyanide (1) and phenylethynylmercury cyanide (2) dissolved in DMSO-d(6) have been investigated using C-13, N-15 and Hg-199 NMR and density functional theory (DFT) calculations. The longitudinal relaxation times of C-13 and Hg-199 nuclei of compounds 1 and 2 and of the N-15 nucleus of 1 have been measured at two magnetic fields. On the basis of these data the rotational diffusion constants describing the reorientation rates of the investigated molecules in solution and the shielding anisotropy parameters for nuclei of atoms forming the -HgCN group have been determined. The available NMR characteristics of several other cyanomercury compounds and those containing the ethynylmercury fragment have also been collected. The experimental values of carbon and nitrogen shielding parameters for the cyanomercury and ethynylmercury groups have been compared with those calculated theoretically using the DFT GIAO method. This comparison has allowed an estimation of the size of the deshielding heavy atom effect experienced by the sp-hybridized mercury-bonded carbons atoms.
First author: Kirillov, E, [(Cp-CMe2-Flu)(2)Ln](-)[Li(ether)(n)](+) (Ln = Y, La): Complexes with unusual coordination modes of the fluorenyl ligand and the first examples of bis-ansa lanthanidocenes, ORGANOMETALLICS, 22, 4038, (2003)
Abstract: Salt metathesis reactions between LnCl(3)(THF)(n) (Ln = Y, La) and 1 equiv of the dilithium salt of the isopropylidene-bridged ligand [Flu-CMe2-Cp]Li-2 (Flu = fluorenyl), in diethyl ether solution, led to the isolation of new ionic metallocene complexes, [(Cp-CMe2-Flu)(2)Ln](-) [Li(ether)(n)](+) (ether = Et2O, THF; Ln = Y, n = 4, 2; Ln = La, n = 2, 3), which contain two chelating ligand units per metal center. The ionic complex 2 presumably originates from ligand redistribution in the primary formed heteroleptic ate complex [(Cp-CMe2-Flu)YCl2](-)-[Li(ether)(4)](+) (1) upon crystallization. Complex 2 was selectively prepared on using 2 equiv of [Cp-CMe2-Flu]Li-2 vs YCl3(THF)(3.5). The solid-state structures of 2 and 3 were established by X-ray diffraction studies. Three polymorphic varieties of 2 were identified and all shown to correspond to a fully dissociated ion pair with the formula [(eta(3):eta(5)-Flu-CMe2-Cp)(eta(1):eta(5)-Flu-CMe2-Cp)Y](-) [Li(Et2O)(THF)(3)](+) (2). The fluorenyl ligands in 2 show an unprecedented eta(1) bonding mode and a rare eta(3) bonding mode involving, respectively, a carbon atom of a phenyl ring and the bridgehead carbon atom of the central ring and the two adjacent carbon atoms of one six-membered ring. DFT computations carried out on the anionic fragment of 2 corroborated the nature of these bonding modes. Only the last exocyclic eta(3)-bonding mode is observed for the fluorenyl moieties in complex 3, which features an associated ion-pair structure with the formula [(eta(3):eta(5)-Flu-CMe2-Cp)(2)La](-)[Li(OEt2)(2)](+). For comparison purposes, the isopropylidene-bridged bis(indenyl) complex [(Ind-CMe2-Ind)(2)Y](-) [Li(THF)(4)](+) (4) was synthesized by a salt metathesis procedure and characterized by X-ray diffraction. In contrast to fluorenyl-containing complexes 2 and 3, only the cyclopentadienyl rings of the indenyl moieties coordinate to yttrium. in 4. Ionic complexes 2-4 constitute the first structurally characterized examples of bis-ansa lanthanidocenes.
First author: Narr, E, Structure and dynamics of copper complexes with 2,2 ‘: 6 ‘,2 ”-terpyridines in glassy matrices,PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 5, 3959, (2003)
Abstract: Continuous-wave and pulse electron paramagnetic resonance (EPR) as well as electron-nuclear double resonance (ENDOR) techniques are applied for determination of the electronic and geometric structure of copper(II) complexes with terpyridine and related ligands that are relevant in the context of supramolecular chemistry. The results are analysed in conjunction with density functional theory Computations and are compared to the crystal structure of the bis(terpyridyl) copper(II) complex in the limit of static Jahn-Teller distortion (R. Allmann, W. Henke and D. Reinen, Inorg. Chem. 1978, 17, 378, ref. 6). The static structure in disordered environments is subject to some strain in both the g-values and copper-ligand distances, but otherwise is rather similar to the structure in crystals. The formation of coordination oligomers is indicated by broadening of the lineshape and a decrease in the transverse relaxation time at low fields which are both related to exchange coupling between copper centres. At high fields of approximately 3 T and a temperature of 15 K, the transverse relaxation rate is governed by modulation of the g-values induced by small-amplitude libration along the Jahn-Teller active mode. A study of the dynamics in a temperature range from below the glass transition temperature to above the melting point of ethanol by CW EPR reveals that the complex is a sensitive probe for matrix dynamics, which detects dynamic heterogeneities and the transition from the structural glass to the crystalline phase. Jahn-Teller dynamics is completely unfrozen only on melting of the matrix.
First author: Chelikowsky, JR, Time-dependent density-functional calculations for the optical spectra of molecules, clusters, and nanocrystals, JOURNAL OF PHYSICS-CONDENSED MATTER, 15, R1517, (2003)
Abstract: In this review, we will describe calculations using time-dependent density-functional theory (DFT) combined with pseudopotentials to determine excited state properties of matter. While a computational framework for ground state properties of condensed matter is well established, calculations for excited state properties are at a more formative stage. Time-dependent DFT represents an important advance by providing an explicit treatment of relevant correlation effects for electronic excitations. As such, it offers an ab initio formalism for excited states that avoids many of the drawbacks associated with empirical or semi-empirical methods. We will illustrate applications of time-dependent DFT to a variety of systems ranging from molecules and atomic clusters to quantum dots, which contain several hundred atoms.
First author: Mananes, A, Analysis of the bonding and reactivity of H and the Al-13 cluster using density functional concepts, JOURNAL OF CHEMICAL PHYSICS, 119, 5128, (2003)
Abstract: The bonding of hydrogen in the Al13H aggregate is analyzed in the framework of density functional theory using the local density approximation. The interaction between the H-1s orbital and only certain molecular orbitals of Al-13 is responsible for the binding. Different measures of the charge transfer give consistent results and predict the stabilization of a sizable amount of electronic charge, about two electrons, around the proton site. The state of the H atom can be described as a negatively charged impurity screened by the surrounding electron gas, similarly to a H impurity embedded in a vacancy in metallic aluminum. Friedel-type oscillations can be appreciated in the screening charge. Local Fukui functions and condensed Fukui indexes associated to the ground state of the cluster Al-13 are used as indicators of molecular reactivity. Those indices allow to predict and understand the equilibrium location of H found in the total energy calculations for Al13H.
First author: Luo, Y, Theoretical investigation of mixed-ligand lanthanocene complexes, (eta(5)-C5H5)(2)LnX center dot OC4H8 (Ln = La, Gd, Lu; X = F, Cl, Br, I), INORGANIC CHEMISTRY COMMUNICATIONS, 6, 1243, (2003)
Abstract: For the first time, density functional theory (DFT) calculation was performed on the titled complexes and the calculated geometries are in good agreement with the available experimental data. The decrease in the bond distances (Ln-X) as well as the increase in the Hirshfeld charges and the chemical hardness from La to Lu indicates an increase in ionic character on going from La to Lu. The lanthanide contraction and ionic character of metal-ligand bond are found to follow the order: Ln-X > Ln-Cp > Ln-O.
First author: Borovkov, VY, Theoretical and experimental studies of the nature of the catalytic activity of VOx/TiO2 systems, KINETICS AND CATALYSIS, 44, 710, (2003)
Abstract: The states of supported vanadium and the nature of activation of ammonia adsorbed on vanadium sites of VOx/TiO2 catalysts are studied by V-51 NMR spectroscopy and diffuse-reflectance IR Fourier-transform (DRIFT) spectroscopy using cluster quantum chemical calculations of NH3 adsorption. We employ the VOx/TiO2 catalyst of two types: the monolayer catalyst in which vanadium is located on the surface of well-crystallized anatase and the catalyst in which vanadium embedded in the anatase lattice at a rather great depth. It is shown that ammonia is predominantly adsorbed on Lewis acid sites of the monolayer catalyst, whereas most of NH3 adsorbed on the catalyst containing bulk vanadium is in the form of ammonium ions. Analysis of experimental and calculated data suggests that in the monolayer catalyst, NH3 Molecules in the selective reduction of nitrogen oxides are activated on Lewis acid sites. Ammonia activation involves the dissociation of the N-H bond in a coordinated molecule, which results in the formation of the amide V-NH2 group and a water molecule coordinated by a V5+ ion. It is likely that, in the case of the catalyst containing bulk vanadium, this reaction occurs with the predominant participation of ammonium ions.
First author: Bridgeman, AJ, A comparative investigation of structure and bonding in Mo and W[TeM6O24](6-) and [PM12O40](3-) heteropolyanions, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 6613, (2003)
Abstract: The structure and bonding in [TeM6O24](6-) (Anderson) and [PM12O40](3-) (alpha-Keggin) heteropolyanions have been investigated by density-functional methods. Various molecular-orbital and population approaches have been employed in the analysis of the structural and electronic properties. Good agreement between computational and experimental or bond-valence results for bond parameters has been obtained. The M-O interactions have been found to be largely of Md-Op character, whereas the orbital nature of Te-O and P-O bonding can be predominantly characterized as Tesp or Psp and Osp. The interactions between the [TeO6] or [PO4] fragments and the [MpOq] cages have been explored by a decomposition approach to the polyanion structure. The results obtained have suggested that the molecular-orbital structure of the Te-O and P-O bonds is not significantly different in the presence or absence of the metal-oxygen cages but that there is nonetheless some covalent character in the bonding of the [TeO6] and [PO4] fragments to the surrounding clusters.
First author: Seth, M, Polymerization properties of a heterogeneous Ziegler-Natta catalyst modified by a base: A theoretical study, MACROMOLECULES, 36, 6613, (2003)
Abstract: The effects of an external base (tetrahydrofuran, THF) on the ethylene polymerization properties of the TiCl4/MgCl2 heterogeneous Ziegler-Natta catalyst are examined using density functional methods. THF is found to bind to all of the proposed active sites and most strongly to those formed from TiCl3. THF is also found to bind to the aluminum-alkyl compounds that may be present, and an analysis of equilibrium concentration distributions shows that the most common place for a THF to bind is to an aluminum-alkyl monomer or a TiCl3-based site. The polymerization and termination mechanisms of the remaining sites are examined. It is found that the binding of an ethylene molecule to the site is weakened, and the barrier to insertion of that ethylene molecule into the Ti-C bond is increased by the presence of a THF molecule. However, the polymer produced by each site is expected to have a higher molecular weight than in the absence of THF because the barrier to termination is increased even more than that of insertion by the addition of the base.
First author: Liu, TQ, DFT calculations of isomer shifts and quadrupole splitting parameters in synthetic iron-oxo complexes: Applications to methane monooxygenase and ribonucleotide reductase, INORGANIC CHEMISTRY, 42, 5244, (2003)
Abstract: To predict isomer shifts and quadrupole splitting parameters of Fe atoms in the protein active sites of methane monooxygenase and ribonucleoticle reductase, a correlation between experimental isomer shifts ranging 0.1-1.5 mm s(-1) for Fe atoms in a training set with the corresponding density functional theory (DFT) calculated electron densities at the Fe nuclei in those complexes is established. The geometries of the species in the training set, consisting of synthetic polar monomeric and dimeric iron complexes, are taken from the Cambridge structural database. A comparison of calculated Mossbauer parameters for Fe atoms from complexes in the training set with their corresponding experimental values shows very good agreement (standard deviation of 0.11 mm/s, correlation coefficient of -0.94). However, for the Fe atoms in the active sites of the structurally characterized proteins of methane monooxygenase and ribonucleoticle reductase, the calculated Mossbauer parameters deviate more from their experimentally measured values. The high correlation that exists between calculated and observed quadrupole splitting and isomer shift parameters for the synthetic complexes leads us to conclude that the main source of the error arising for the protein active sites is due to the differing degrees of atomic-level resolution for the protein structural data, compared to the synthetic complexes in the training set. Much lower X-ray resolutions associated with the former introduce uncertainty in the accuracy of several bond lengths. This is ultimately reflected in the calculated isomer shifts and quadrupole splitting parameters of the Fe sites in the proteins. For the proteins, the closest correspondence between predicted and observed Mossbauer isomer shifts follows the order MMOH(red), RNR(red), MMOH(ox), and RNR(ox), with average deviations from experiment of 0.17, 0.17, 0.17-0.20, and 0.32 mm/s, but this requires DFT geometry optimization of the iron-oxo dimer complexes.
First author: Cavigliasso, G, Influence of the ligand on the coupling between the metal-based electrons in face-shared [M2X9](3-) (M = Mo, W; X = F, Cl, Br, I) systems, INORGANIC CHEMISTRY, 42, 5252, (2003)
Abstract: Orbital overlap and spin polarization effects in Mo and W[M2X9](3-) halide and in [M2X’X-3″(6)](3-) mixed-halide systems have been investigated by means of density-functional calculations performed on the S = 0, S = 3, and reference states of these species. For the regular [M2X9](3-) systems, a strong linear correlation between the two factors has been obtained, and decreasing trends in both the overlap energy and the spin polarization energy upon descending the halide group have been observed. These trends can be related to the changes in the size and covalency of the ligands and in the nature of the metal-bridge interaction. For the mixed-ligand [M2X’X-3″(6)](3-) systems, important deviations (from the behavior of the regular systems), which are apparently the result of particular structural and energetic characteristics, have been observed.
First author: Guerra, CF, Orbital interactions and charge redistribution in weak hydrogen bonds: The Watson-Crick AT mimic adenine-2,4-difluorotoluene, JOURNAL OF CHEMICAL PHYSICS, 119, 4262, (2003)
Abstract: The discovery by Kool and co-workers that 2,4-difluorotoluene (F) mimics thymine (T) in DNA replication has led to a controversy about the question if this mimic has the capability of forming hydrogen bonds with adenine (A). In the present study, we address not only the question about the strengths of the hydrogen bonds in AF as compared to those in AT but we focus in particular on the nature of these interactions. Thus, we have analyzed AF and AT at the BP86/TZ2P level of density functional theory (DFT). In line with previous experience, this approach is shown to achieve close agreement with the available data from ab initio computations and experiment: the complexation energy of AF (-3.2 kcal/mol) is confirmed to be much weaker indeed than that of AT (-13.0 kcal/mol). Interestingly, the weak hydrogen bonds in AF still possess a significant orbital interaction component that resembles the situation for the more strongly bound AT, as follows from (1) an analysis of the orbital electronic structure of AF and AT, (2) a quantitative decomposition of the A-F and A-T bond energies, as well as (3) a quantitative decomposition of the charge redistribution associated with the A-F and A-T interactions based on the Voronoi deformation density (VDD) method. The VDD method has been further developed such that the charge redistribution DeltaQ per atom can be decomposed into a component associated with the Pauli repulsive orbital interactions and a component associated with the bonding orbital interactions: DeltaQ=DeltaQ(Pauli)+DeltaQ(oi). Implications of our findings for the mechanism of DNA replication are discussed.
First author: Heine, T, Analysis of aromatic delocalization: Individual molecular orbital contributions to nucleus-independent chemical shifts, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 6470, (2003)
Abstract: Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributions provide interpretive insights that complement and extend those given by the localized MO (“dissected NICS”, LMO-NICS) method. Applications to (4n + 2) pi-electron systems, ranging from [n] annulenes to D-nh S-3, S-5, and N6H62+ rings as well as to D-2h cyclobutadiene, show the extent to which their diatropic character results from the sigma framework and from the pi orbitals. The diatropicity of both these contributions decreases with the number of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic. This is generally the case with the sigma orbitals, but is found only for “electron-rich” pi systems such as sulfur rings. MO-NICS contributions, which can be interpreted using London-Huckel theory, correlate with inverse ring size.
First author: Salsbury, FR, Temperature-dependent behavior of protein-chromophore interactions: A theoretical study of a blue fluorescent antibody, CHEMPHYSCHEM, 4, 848, (2003)
Abstract: The unusual temperature-dependent excited-state dynamics of a stilbene-antibody complex reported by Simeonov et al. are explored using theoretical methods. The anomalous temperature-dependent fluorescent emission and lifetime are shown to be the result of interplay among temperature-modulated protein flexibility, the excited-state potential surface for the stilbene central twist, and changes in the stilbene charge distribution upon excitation. Stilbene is found to possess a similar geometry and orientation within the antibody binding pocket at all temperatures in the ground state and at low temperatures (approximate to 260 K), the excited-state conformation twists around the central double bond and adopts an alternative orientation within the binding pocket. These changes result from protein side chain and loop motions that are frozen out at a lower temperatures and account for the observed red shift of the fluorescence emission spectrum (a calculated shift of 3.8 kcal mol(-1) compares favorably with the approximate to 5 kcal.mol(-1) observed experimentally). Approximately 3.0 kcal mol(-1) of this stabilization is global in nature and not attributable to specific local interactions. Local interactions between stilbene and Tyr-B39 contribute approximate to 0.8 kcal mol(-1) to the fluorescence shift. The primary structural change in the simulations of the high temperature excited state involves a decrease in the stilbene-tyrosine distance and a relative change in orientation of the aromatic rings. We identify several nearby charged residues that contribute to the fluorescence emission shift and provide targets for mutagenesis to probe the temperature-dependent dynamics of protein chromophore inter-actions.
First author: Jensen, L, A discrete solvent reaction field model for calculating molecular linear response properties in solution, JOURNAL OF CHEMICAL PHYSICS, 119, 3800, (2003)
Abstract: A discrete solvent reaction field model for calculating frequency-dependent molecular linear response properties of molecules in solution is presented. The model combines a time-dependent density functional theory (QM) description of the solute molecule with a classical (MM) description of the discrete solvent molecules. The classical solvent molecules are represented using distributed atomic charges and polarizabilities. All the atomic parameters have been chosen so as to describe molecular gas phase properties of the solvent molecule, i.e., the atomic charges reproduce the molecular dipole moment and the atomic polarizabilities reproduce the molecular polarizability tensor using a modified dipole interaction model. The QM/MM interactions are introduced into the Kohn-Sham equations and all interactions are solved self-consistently, thereby allowing for the solute to be polarized by the solvent. Furthermore, the inclusion of polarizabilities in the MM part allows for the solvent molecules to be polarized by the solute and by interactions with other solvent molecules. Initial applications of the model to calculate the vertical electronic excitation energies and frequency-dependent molecular polarizability of a water molecule in a cluster of 127 classical water molecules are presented. The effect of using different exchange correlation (xc)-potentials is investigated and the results are compared with results from wave function methods combined with a similar solvent model both at the correlated and uncorrelated level of theory. It is shown that accurate results in agreement with correlated wave function results can be obtained using xc-potentials with the correct asymptotic behavior.
First author: Dietz, O, Molecular structures, bond energies, and bonding analysis of group 11 cyanides TM(CN) and isocyanides TM(NC) (TM = Cu, Ag, Au), INORGANIC CHEMISTRY, 42, 4977, (2003)
Abstract: We report on quantum chemical calculations at the DFT (BP86/TZP) and ab initio (CCSD(T)/III+) levels of the title compounds. The geometries, vibrational spectra, heats of formation, and homolytic and heterolytic bond dissociation energies are given. The calculated bond length of Cu-CN is in reasonable agreement with experiment. The theoretical geometries for CuNC and the other group 11 cyanides and isocyanicles which have not been measured as isolated species provide a good estimate for the exact values. The theoretical bond dissociation energies and heats of formation should be accurate with an error limit of +/-5 kcal/mol. The calculation of the vibrational spectra shows that the C-N stretching mode of the cyanides, which lies between 2170 and 2180 cm(-1), is IR inactive. The omega(1)(C-N) vibrations of the isocyanicles are shifted by similar to100 cm(-1) to lower wavenumbers. They are predicted to have a very large IR intensity. The nature of the metal-ligand interactions was investigated with the help of an energy partitioning analysis in two different ways using the charged fragments TM+ + CN- (TM = transition metal) and the neutral fragments TM. + CN. as bonding partners. The calculations suggest that covalent interactions are the driving force for the formation of the TM-CN and TM-NC bonds, but the finally formed bonds are better described in terms of interactions between TM+ and CN-, which have between 73% and 80% electrostatic character. The contribution of the pi bonding is rather small. The lower energy of the metal cyanides than that of the isocyanicles comes from the stronger electrostatic interaction between the more diffuse electron density at the carbon atom of the cyano ligand and the positively charged nucleus of the metal.
First author: Stephens, FH, Molybdenum-phosphorus triple bond stabilization by ancillary alkoxide ligation: Synthesis and structure of a terminal phosphide tris-1-methylcyclohexanoxide complex, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 9264, (2003)
Abstract: We report on quantum chemical calculations at the DFT (BP86/TZP) and ab initio (CCSD(T)/III+) levels of the title compounds. The geometries, vibrational spectra, heats of formation, and homolytic and heterolytic bond dissociation energies are given. The calculated bond length of Cu-CN is in reasonable agreement with experiment. The theoretical geometries for CuNC and the other group 11 cyanides and isocyanicles which have not been measured as isolated species provide a good estimate for the exact values. The theoretical bond dissociation energies and heats of formation should be accurate with an error limit of +/-5 kcal/mol. The calculation of the vibrational spectra shows that the C-N stretching mode of the cyanides, which lies between 2170 and 2180 cm(-1), is IR inactive. The omega(1)(C-N) vibrations of the isocyanicles are shifted by similar to100 cm(-1) to lower wavenumbers. They are predicted to have a very large IR intensity. The nature of the metal-ligand interactions was investigated with the help of an energy partitioning analysis in two different ways using the charged fragments TM+ + CN- (TM = transition metal) and the neutral fragments TM. + CN. as bonding partners. The calculations suggest that covalent interactions are the driving force for the formation of the TM-CN and TM-NC bonds, but the finally formed bonds are better described in terms of interactions between TM+ and CN-, which have between 73% and 80% electrostatic character. The contribution of the pi bonding is rather small. The lower energy of the metal cyanides than that of the isocyanicles comes from the stronger electrostatic interaction between the more diffuse electron density at the carbon atom of the cyano ligand and the positively charged nucleus of the metal.
First author: Esterhuysen, C, Comparison of side-on and end-on coordination of E-2 ligands in complexes [W(CO)(5)E-2] (E = N, P, As, Sb, Bi, Si-, Ge-, Sn-, Pb-), CHEMISTRY-A EUROPEAN JOURNAL, 9, 3518, (2003)
Abstract: Complexes Of W(CO)5 with neutral diatomic pnictogen ligands N-2, P-2, AS(2), Sb-2,Sb- and Bi-2 and anionic Group 14 ligands Si-2(2-), Ge-2(2-), Sn-2(2-), and Pb-2(2-) coordinated in both side-on and end-on fashion have been optimized by using density functional theory at the BP86 level with valence sets of TZP quality. The calculated bond energies have been used to compare the preferential binding modes of each respective ligand. The results were interpreted by analyzing the nature of the interaction between the ligands and the metal fragment using an energy partitioning method. This yields quantitative information regarding the strength of covalent and electrostatic interactions between the metal and ligand, as well as the contributions by orbitals of different symmetry to the covalent bonding. Results show that all the ligands studied bind preferentially in a side-on coordination mode, with the exception of N-2 which prefers to coordinate in an end-on mode. The preference of the heavier homologues P-2-Bi-2 for binding in a side-on mode over the end-on mode in the neutral complexes R(CO)(5)WE2] comes mainly from the much stronger electrostatic attraction in the former species. The energy difference between the side-on and end-on isomers of the negatively charged complexes with the ligands Si-2(2-), Ge-2(2-), Sn-2(2-), and Pb-2(2-) is much less and it cannot be ascribed to a particular bonding component.
First author: Termaten, AT, N-Heterocyclic carbene functionalized iridium phosphinidene complex [Cp*(NHC)Ir=PMes*]: Comparison of phosphinidene, imido, and carbene complexes, CHEMISTRY-A EUROPEAN JOURNAL, 9, 3577, (2003)
Abstract: The novel phosphinidene complex [Cp*(NHC)Ir = PMes*] (3; NHC = 1,3-diisoprop yl-4,5-dimethylimidazol-2-ylidene) was prepared in high yield from [Cp*(NHC)IrCl2 (2) and [LiPHMes*] . 3 THF. It represents the first example of an NHC ligated transition metal phosphinidene complex. The X-ray crystal structure for 3 is also reported. DFT calculations on the N-heterocyclic carbene containing parent complexes [Cp(NHC)Ir=E] (E =PH, NH, CH2) show that the NHC ligand acts as good sigma-donor/weak pi-acceptor ligand and forms strong Ir-CNHC single bonds. The Ir=E double bonds result from strong triplet-triplet interactions between [Cp(NHC)Ir] and E.
First author: Rayon, VM, Bis(benzene)chromium is a delta-bonded molecule and ferrocene is a pi-bonded molecule,ORGANOMETALLICS, 22, 3304, (2003)
Abstract: The nature of the metal-ligand bonding in ferrocene and bis(benzene)chromium has been analyzed with the help of an energy partitioning scheme using the results of DFT calculations. The bonding analysis suggests that the Cr-Bz(2) bond is 37.9% electrostatic and 62.1% covalent. The binding interactions in ferrocene are predicted to be 51.1% electrostatic and 48.9% covalent if the charged species Fe2+ and (Cp-)(2) are used as interacting fragments, while they are 45.0% electrostatic and 55.0% covalent if neutral Fe and Cp-2 in the triplet states are used. The largest contributions to the orbital interactions in bis(benzene)chromium come from the Cr–>Bz(2) delta-back-donation, while the most important orbital contribution in ferrocene comes from the Fe<–Cp-2 pi-donation. The larger contributions of the e(1g)(pi) orbitals in ferrocene are caused by better energy matching rather than better overlapping of the interacting orbitals.
First author: Luo, Y, Density functional study of lanthanide complexes (eta(5)-C5H5)(2)LnX center dot OC4H8 (Ln = La-Lu; X F, Cl, Br and I), JOURNAL OF ORGANOMETALLIC CHEMISTRY, 679, 84, (2003)
Abstract: Density functional calculations were performed on a series of mixed-ligand organolanthanide. complexes, (eta(5)-C5H5)(2)LnX . OC4H8 (eta(5)-C5H5 = Cp; Ln = La-Lu; X = F, Cl, Br and I; OC4H8 = THF). The calculated geometrical parameters are in reasonable agreement with the experimental data. The distances between Ln and ligands follow linearity along the ionic radius of lanthanide metal, as the same as that observed in experiment. In the mixed-ligand complexes, Ln-Cp and Ln-THF bonds are more covalent compared to Ln-X. The lanthanide contraction of various bond and the metal-ligand interaction energy followed the order of Ln-X > Ln-Cp > Ln-OC4H8. The orbital population and dipole moment were also discussed.
First author: Swart, M, Accuracy of geometries: influence of basis set, exchange-correlation potential, inclusion of core electrons, and relativistic corrections, THEORETICAL CHEMISTRY ACCOUNTS, 110, 34, (2003)
Abstract: The geometries of a set of small molecules were optimized using eight different exchange-correlation (xc) potentials in a few different basis sets of Slater-type orbitals, ranging from a minimal basis (I) to a triple-zeta valence basis plus double polarization functions (VII). This enables a comparison of the accuracy of the xc potentials in a certain basis set, which can be related to the accuracies of wavefunction-based methods such as Hartree-Fock and coupled cluster. Four different checks are done on the accuracy by looking at the mean error, standard deviation, mean absolute error and maximum error. It is shown that the mean absolute error decreases with increasing basis set size, and reaches a basis set limit at basis VI. With this basis set, the mean absolute errors of the xc potentials are of the order of 0.7-1.3 pm. This is comparable to the accuracy obtained with CCSD and MP2/MP3 methods, but is still larger than the accuracy of the CCSD(T) method (0.2 pm). The best performing xc potentials are found to be Becke-Perdew, PBE and PW91, which perform as well as the hybrid B3LYP potential. In the second part of this paper, we report the optimization of the geometries of five metallocenes with the same potentials and basis sets, either in a nonrelativistic or a scalar relativistic calculation using the zeroth-order regular approximation approach. For the first-row transition-metal complexes, the relativistic corrections have a negligible effect on the optimized structures, but for ruthenocene they improve the optimized Ru-ring distance by some 1.4-2.2 pm. In the largest basis set used, the absolute mean error is again of the order of 1.0 pm. As the wavefunction-based methods either give a poor performance for metallocenes (Hartree-Fock, MP2), or the size of the system makes a treatment with accurate methods such as CCSD(T) in a reasonable basis set cumbersome, the good performance of density functional theory calculations for these molecules is very promising; even more so as density functional theory is an efficient method that can be used without problems on systems of this size, or larger.
First author: Senegas, JM, Connecting terminal carboxylate groups in nine-coordinate lanthanide podates: Consequences on the thermodynamic, structural, electronic, and photophysical properties, INORGANIC CHEMISTRY, 42, 4680, (2003)
Abstract: The hydrolysis of terminal (t)butyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 – 3H](3-) reacts with Ln(ClO4)(3) to give the poorly soluble and stable podates [Ln(L13 – 3H)] (log(beta(110)) = 6.7-7.0, Ln = La-Lu). The isolated complexes [Ln(L13 – 3H)](H2O)(7) (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(Ill) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln-O(carboxamide) bonds are only marginally longer than the Ln-O(carboxylate) bonds in [Ln(L13 – 3H)], thus producing a regular triple helix around Ln(Ill) which reverses its screw direction within the covalent Me-TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 – 3H)] (Phi(Eu)(tot) = 1.8 x 10(-3)) and [Tb(L13 – Eu 3H)] (Phi(Eu)(tot) = 8.9 X 10(-3)) in water remain modest and strongly contrast with that obtained for the lanthanicle luminescence step (Phi(Eu) = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet ((1)pipi*) and triplet ((3)pipi*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the “carboxylate effect” in lanthanide complexes are evaluated for programming predetermined properties in functional devices.
First author: Saladino, AC, Relativistic DFT calculations of copper hyperfine coupling constants: Effect of spin-orbit coupling, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 5583, (2003)
Abstract: Relativistic density functional theory (DFT) calculations of transition metal hyperfine interaction (A) tensors have been completed for a series of Cu2+ complexes including Cu(Quin)(2), Cu(Acac)(2), Cu((L)-AlaO)(2), and [Cu(Ox)(2)](2-). The A tensors were calculated with the zero order regular approximation (ZORA) for relativistic effects as implemented in the Amsterdam Density Functional (ADF) program. For the isotropic hyperfine coupling constant, the agreement between the calculated and experimental values was quite good, but the good agreement was determined to be a result of a cancellation of errors due to the neglect of spin-orbit coupling and an underestimation of core spin polarization. The anisotropic components of the hyperfine coupling constant calculated with the scalar-relativistic spin-restricted open-shell Kohn Sham (SO + SR ROKS) method provided the best agreement with experimental values.
First author: Zhang, Y, An investigation of the unusual Fe-57 Mossbauer quadrupole splittings and isomer shifts in 2 and 3-coordinate Fe(II) complexes, JOURNAL OF PHYSICAL CHEMISTRY B, 107, 7180, (2003)
Abstract: The Fe-57 Mossbauer quadrupole splittings (DeltaE(Q)) and isomer shifts (delta(Fe)) in 3-coordinate high-spin Fe(II) complexes are unusually small, and previous attempts to reproduce their DeltaE(Q) values have been unsuccessful. We show here that, by using large structural models and basis sets, both DeltaE(Q) and delta(Fe) values can be quite accurately predicted by using density functional theory. Four systems were investigated: the three 3-coordinate species [LFeX](0) (L = beta-diketiminate; X = Cl-, CH3-) and [Fe(SC6H2-2,4,6-tBu(3))(3)](-), in addition to an uncommon 2-coordinate high-spin ferrous thiolate, [Fe(SC6H3-2,6-mes(2))(2)] (mes = mesityl = 214,6-Me3C6H2)Both Gaussian-type-orbital and Slater-type-orbital basis sets were investigated, and both yielded DeltaE(Q) and delta(Fe) values in good accord with experiment. There were no improvements in these property predictions when (approximate) relativistic effects were included in the calculations. An MO analysis provided a detailed picture of the origin of the small DeltaE(Q) values seen in the 3-coordinate complexes. These results extend the scope of DFT/Mossbauer investigations beyond the 4-6-coordinate systems described previously to 2- and 3-coordinate systems, which should open the way to using these parameters in structure refinement, especially in large systems, such as proteins.
First author: Filatov, M, On the physical meaning of the ZORA Hamiltonian, MOLECULAR PHYSICS, 101, 2295, (2003)
Abstract: By expanding the Foldy-Wouthuysen representation of the Dirac equation near the free-particle solution it is shown that the Hamiltonian of the zeroth-order regular approximation (ZORA) leads to an infinite summation of the leading relativistic corrections to the free-particle, non-relativistic energy. The analysis of the perturbation expansion of the ZORA Hamiltonian reveals that the ZORA Hamiltonian recovers all terms of the Breit-Pauli theory to second order. This result is general and applies not only to hydrogen-like atomic ions (as was demonstrated before) but also to a wide variety of physical problems. ZORA is analogous to the random phase approximation in many-body theory in the sense that both methods include an infinite-order summation of the asymptotically non-vanishing terms. This highlights the difference between ZORA and the Douglas-Kroll method, with the latter being analogous to finite-order many-body perturbation theory. On the basis of this analysis the performance of ZORA when calculating various molecular properties is discussed.
First author: Hunt, P, Thermal versus electronic broadening in the density of states of liquid water, CHEMICAL PHYSICS LETTERS, 376, 68, (2003)
Abstract: The one-electron density of states of liquid water computed from an ab initio molecular dynamics trajectory is analyzed in terms of interactions between effective molecular orbitals localized on single molecules. These orbitals are constructed from the occupied extended (Kohn-Sham) orbitals using the maximally localized Wannier function method. Band positions are related to average orbital energies. The width of a band is resolved into contributions from thermal fluctuations in the orbital energies and the electronic broadening due to intermolecular coupling. It is found that the thermal and electronic broadening are of comparable magnitude with electronic broadening being the leading effect.
First author: Filatov, M, Calculation of electric properties using regular approximations to relativistic effects: The polarizabilities of RuO4, OsO4, and HsO(4) (Z=108), JOURNAL OF CHEMICAL PHYSICS, 119, 1412, (2003)
Abstract: Analytic expressions for the derivatives of the total molecular energy with respect to external electric field are derived within the regular approximation to the full four-component relativistic Hamiltonian and presented in matrix form suitable for implementation in standard quantum-chemical codes. Results of benchmark calculations using the infinite-order regular approximation with modified metric method are presented and discussed. The static electric dipole polarizabilities of group VIII metal tetroxides MO4 for M=Ru, Os, Hs (Z=108) are studied with the help of second-order Moller-Plesset perturbation theory using the infinite-order regular approximation with modified metric Hamiltonian. The polarizabilities obtained vary in the sequence RuO4>OsO4>HsO(4), which is different from those obtained in other studies. However, it is in line with calculated T-1(2)<–(1)A(1) excitation energies of the group VIII tetroxides, which provide a measure for the magnitude of their polarizabilities.
First author: Van Lenthe, E, Optimized slater-type basis sets for the elements 1-118, JOURNAL OF COMPUTATIONAL CHEMISTRY, 24, 1142, (2003)
Abstract: Seven different types of Slater type basis sets for the elements H (Z = 1) up to E 118 (Z = 118), ranging from a double zeta valence quality up to a quadruple zeta valence quality, are tested in their performance in neutral atomic and diatomic oxide calculations. The exponents of the Slater type functions are optimized for the use in (scalar relativistic) zeroth-order regular approximated (ZORA) equations. Atomic tests reveal that, on average, the absolute basis set error of 0.03 kcal/mol in the density functional calculation of the valence spinor energies of the neutral atoms with the largest all electron basis set of quadruple zeta quality is lower than the average absolute difference of 0.16 kcal/mol in these valence spinor energies if one compares the results of ZORA equation with those of the fully relativistic Dirac equation. This average absolute basis set error increases to about 1 kcal/mol for the all electron basis sets of triple zeta valence quality, and to approximately 4 kcal/mol for the all electron basis sets of double zeta quality. The molecular tests reveal that, on average, the calculated atomization energies of 118 neutral diatomic oxides MO, where-the nuclear charge Z of M ranges from Z = 1-118, with the all electron basis sets of triple zeta quality with two polarization functions added are within 1-2 kcal/mol of the benchmark results with the much larger all electron basis sets, which are of quadruple zeta valence quality with four polarization functions added. The accuracy is reduced to about 4-5 kcal/mol if only one polarization function is used in the triple zeta basis sets, and further reduced to approximately 20 kcal/mol if the all electron basis sets of double zeta quality are used. The inclusion of g-type STOs to the large benchmark basis sets had an effect of less than I kcal/mol in the calculation of the atomization energies of the group 2 and group 14 diatomic oxides. The basis sets that are optimized for calculations using the frozen core approximation (frozen core basis sets) have a restricted basis set in the core region compared to the all electron basis sets. On average, the use of these frozen core basis sets give atomic basis set errors that are approximately twice as large as the corresponding all electron basis set errors and molecular atomization energies that are close to the corresponding all electron results., Only if spin-orbit coupling is included in the frozen core calculations larger errors are found, especially for the heavier elements, due to the additional approximation that is made that the basis functions are orthogonalized on scalar relativistic core orbitals.
First author: Petrie, S, Cl3V(mu-S(CH3)(2))(3)VCl32-: A first-row, face-shared bioctahedral complex with multiple metal-metal bonding, INORGANIC CHEMISTRY, 42, 4417, (2003)
Abstract: Density functional theory calculations have been used to investigate the structure and bonding of the d(3)d(3) bioctahedral complexes X3V(u-S(CH3)(2))(3)VX32- (X = F-, Cl-, OH-, SH-, NH2-). According to geometry optimizations using the broken-symmetry approach and the VWN+B-LYP combination of density functionals, the halide-terminated complexes have a V-V bond order of approximately 2, while complexes featuring OH-, SH-, or NH2- as terminal ligands exhibit full triple bonding between the vanadium atoms. The tendency toward triple bonding in the latter complexes is consistent with an increased covalency of the vanadium-ligand bonds, and the influence of bond covalency is apparent also in the tendency for V-V bond elongation in the complexes with OH- and NH2- terminal ligands. Detailed examination of the composition of molecular orbitals in all of the thioether-bridged V-II complexes substantiates the conclusion that the strong antiferromagnetic coupling which we have determined for these complexes (-J > 250 cm(-1)) is due to direct bonding between metal atoms rather than superexchange through the bridging ligands. As such, these V-II complexes comprise the first apparent examples of multiple metal-metal bonding in first-transition-row, face-shared dinuclear complexes and are therefore of considerable structural and synthetic interest.
First author: Costas, M, Dinuclear copper(I) complexes with hexaaza macrocyclic dinucleating ligands: Structure and dynamic properties, INORGANIC CHEMISTRY, 42, 4456, (2003)
Abstract: The synthesis and structural and spectroscopic characterization of a family of copper(I) complexes, containing a dinucleating hexaaza macrocyclic ligand, of general formula [Cu-2(L)(X)(2)](2+) (L = Me2p, Me2m, Me3p, or Me3m; X = MeCN, n-PrCN, CO, t-BuNC, or PPh3) is described. This family of complexes contains ligands that differ from one another in the number of methylenic units linking the tertiary amines and in the meta or para substitution of their aromatic rings. The structural characterization in the solid-state includes a single-crystal X-ray diffraction analysis of [Cu-2(Me(2)p)(CO)(2)](2+) and of [Cu-2(Me2m)(t-BuNC)(2)](2+). In solution, those complexes are structurally characterized through NMR spectroscopy that also allows us to put forward and establish their fluxional behavior. Theoretical calculations at the DFT level have also been performed in order to further analyze the relative energy of the different potential isomers as well as to gain insight into their chemical properties. Finally, the influence of the hexaaza ligands over different structural aspects as well as on its potential chemical reactivity is discussed.
First author: Xie, RH, Structural, electronic, and magnetic properties of heterofullerene C48B12, CHEMICAL PHYSICS LETTERS, 375, 445, (2003)
Abstract: Bonding, electric (hyper)polarizability, vibrational, and magnetic properties of heterofullerene C48B12 are studied by first-principles calculations. Infrared- and Raman-active vibrational frequencies Of C48B12 are assigned. Eight C-13 and two B-11 nuclear magnetic resonance (NMR) spectral signals Of C48B12 are characterized. The average second hyper-polarizability of C48B12 is about 180% larger than that of C-60. Our results suggest that C48B12 is a candidate for photonic and optical limiting applications because of the enhanced third-order optical non-linearities.
First author: Thompson, MJ, Photoisomerization and proton transfer in photoactive yellow protein, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 8186, (2003)
Abstract: The photoactive yellow protein (PYP) is a bacterial photosensor containing a para-coumaryl thioester chromophore that absorbs blue light, initiating a photocycle involving a series of conformational changes. Here, we present computational studies to resolve uncertainties and controversies concerning the correspondence between atomic structures and spectroscopic measurements on early photocycle intermediates. The initial nanoseconds of the PYP photocycle are examined using time-dependent density functional theory (TDDFT) to calculate the energy profiles for chromophore photoisomerization and proton transfer, and to calculate excitation energies to identify photocycle intermediates. The calculated potential energy surface for photoisomerization matches key, experimentally determined, spectral parameters. The calculated excitation energy of the photocycle intermediate cryogenically trapped in a crystal structure by Genick et al. [Genick, U. K.; Soltis, S. M.; Kuhn, P.; Canestrelli, 1. L.; Getzoff, E. D. Nature 1998, 392, 206-209] supports its assignment to the PYPB (1(0)) intermediate. Differences between the time-resolved room temperature (298 K) spectrum of the PYPB intermediate and its low temperature (77 K) absorbance are attributed to a predominantly deprotonated chromophore in the former and protonated chromophore in the latter. This contrasts with the widely held belief that chromophore protonation does not occur until after the PYPL (1(1) or pR) intermediate. The structure of the chromophore in the PYPL intermediate is determined computationally and shown to be deprotonated, in agreement with experiment. Calculations based on our PYPB and PYPL models lead to insights concerning the PYPBL intermediate, observed only at low temperature. The results suggest that the proton is more mobile between Glu46 and the chromophore than previously realized. The findings presented here provide an example of the insights that theoretical studies can contribute to a unified analysis of experimental structures and spectra.
First author: Tsai, YC, Reactions of organic nitriles with a three-coordinate molybdenum(III) complex and with a related molybdaziridine-hydride, ORGANOMETALLICS, 22, 2902, (2003)
Abstract: Reactions of nitriles RCN with the sterically encumbered Mo(N[t-Bu]Ar)(3) (1, Ar = 3,5-C6H3Me2) or the somewhat less hindered Mo(H)(eta(2)-Me2CNAr)(N[i-Pr]Ar)(2) (2) have been investigated. Where R = Me or Ph, reaction with 1 results in reductive nitrile coupling and the formation of a diiminato product [mu-NC(R)C(R)N][1](2). In contrast, reaction of 1 with Me2NCN surprisingly results in a stable, albeit highly congested, eta(2) adduct of the nitrile. When the less sterically hindered 2 is used, reaction with PhCN gives the diiminato product analogous to the one mentioned for the tert-butyl system, [mu-NC(Ph)C(Pb)N][Mo(N[i-Pr]Ar)(3)](2), where molybdaziridine-hydride 2 has provided access to the three-coordinate Mo(N[i-Pr]Ar)(3) (3) moiety. Use of a more bulky nitrile such as MesCN (Mes = 2,4,6-C6H2Me3) results in formation of a bis-eta(1) compound, (eta(1)-MesCN)(2)[3]. Use of 9-anthracenylcarbonitrile results in head-to-tail C-C coupling of two monomers via the anthracenyl moiety. Detailed variable-temperature EPR and H-2 NMR data are included for both molybdenum-containing starting materials and selected reaction intermediates and products.
First author: Kubo, M, Combinatorial computational chemistry approach to the high-throughput screening of metal sulfide catalysts for CO hydrogenation process, ENERGY & FUELS, 17, 857, (2003)
Abstract: We have already proposed that a “Combinatorial Computational Chemistry” approach is very effective for performing the theoretical high-throughput screening of new catalysts, and its validity was strongly confirmed in various catalyst systems. In the present study, we applied our combinatorial computational chemistry approach to the design of new metal sulfide catalysts for the CO hydrogenation process and proposed new guidance for designing the highly selective catalysts for methanol synthesis. We investigated H-2 and CO adsorption on a large number of metal and metal sulfide catalysts by first-principles calculations, and succeeded in clarifying the relationship between the metal species in the metal and metal sulfide catalysts and the products of the CO hydrogenation processes. Our results indicated that Co, Mo, Ru, Rh, Ir, and Pd sulfide catalysts selectively produce methanol, while Re and Os sulfide catalysts selectively produce hydrocarbons. The above results are in good agreement with the experimental results of Koizumi and co-workers. Moreover, we proposed that the Pd sulfide catalyst has the highest selectivity for methanol from the CO hydrogenation process. This result strongly supports the experimental results by Koizumi and co-workers. Moreover, we propose that the metal sulfide catalysts, which realize the bridge-site adsorption of the CO molecule on both the metal and sulfur atoms, have high selectivity for methanol. This proposed guidance for designing the highly selective metal sulfide catalysts for methanol may be useful for the experiments.
First author: Tapilin, VM, Quantum-chemical calculation of matrix elements in a basis of functions with polynomial tails,JOURNAL OF STRUCTURAL CHEMISTRY, 44, 531, (2003)
Abstract: A numerical integration method is suggested for calculating Hamiltonian matrix elements in a basis of Junctions with polynomial tails with allowance for discontinuities of higher-order derivatives of the basis function within the domain of integration. The method is tested by calculating matrix elements for a copper crystal. The results for the overlap matrix elements are presented demonstrating efficiency of the method.
First author: Straka, M, Why are hexavalent uranium cyanides rare while U-F and U-O bonds are common and short?,THEORETICAL CHEMISTRY ACCOUNTS, 109, 332, (2003)
Abstract: Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91-PW91 studies are reported for the series UF(4)X(2) (X = H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF(6) is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these sigma and pi orbital interactions, the electrostatic attraction is important. Evidence for pi bonding in the other systems studied was also found. The triatomic pseudohalides as well as fluorine and chlorine are in this sense better ligands than cyanide. The -CN group is a sigma donor and pi acceptor, as uranium itself, and hence is unfit to bond to U(VI). The sigma-bonded UH(6) is octahedral.
First author: Mosey, NJ, Finite temperature structure and dynamics of zinc dialkyldithiophosphate wear inhibitors: A density functional theory and ab initio molecular dynamics study, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 5058, (2003)
Abstract: The thermal decomposition of several zinc dialkyldithiophosphate (ZDDP) antiwear additives has been explored with both finite temperature gas-phase ab initio molecular dynamics (MD) simulations and static quantum chemical calculations at the density functional (DFT) level of theory. Calculations have been performed on the ZDDP monomers (Zn(S2P(OR)(2))(2)), ZDDP dimers, and the corresponding linkage isomers (LI-ZDDPs) with a variety of substituents (R = H, Me, Et, Pr-i, Bu-t, Ph). The results show that the monomeric form of ZDDP likely dominates at finite temperatures for all substituents considered and that the LI-ZDDP isomer is nearly thermoneutral with respect to the parent ZDDP monomer. Optimized geometries of the ZDDP monomer give 4-coordinate Zn structures that are consistent with previously reported theoretical calculations. However, ab initio molecular dynamics simulations of the ZDDP monomers at elevated temperatures show that 2- and 3-coordinate complexes are instead favored and that the decrease in coordination number has a significant effect on the electronic structure of the molecule that may affect the reactivity of ZDDPs with other chemical species present in engine oils. The ab initio MD simulations also provide insight into several decomposition pathways of the various ZDDP species that include the loss of either alkyl or alkoxy radicals as well as the elimination of olefins and sulfides from the ZDDP molecule. The results are discussed in terms of how the observed processes will affect the overall abilities of the antiwear film.
First author: Michalak, A, A comparison of Ni- and Pd-diimine complexes as catalysts for ethylene/methyl acrylate copolymerization. A static and dynamic density functional theory study, ORGANOMETALLICS, 22, 2660, (2003)
Abstract: Gradient-corrected density functional theory (DFT) has been used to study the elementary reactions for the mechanism of ethylene/methyl acrylate copolymerization catalyzed by Pd-and Ni-diimine complexes, Nboolean ANDN-M-(n-C3H7)(+); Nboolean ANDN = -N(Ar)-C(R)-C(R)-N(Ar)-. The main goal was to understand the differences between the Pd- (active copolymerization catalyst) and Ni-systems (inactive under the same conditions) and, thus, the factors that determine the catalyst activity in these processes. The acrylate insertion into the metal-alkyl bond, the stability of the insertion products, and the complexation of the next monomer (ethylene and acrylate) have been studied by static calculation, and the molecular dynamics approach has been applied to study the ethylene insertion following the acrylate insertion. To account for the steric influence on the acrylate insertion barriers and the stability of isomeric chelate-ethylene complexes, calculations have been carried out for both the model [Ar = H, R = HI and the real catalyst [Ar = 2,6-C6H3(i-Pr)(2); R = CH3] – It has been found that acrylate insertion follows the same mechanism for the Ni- and Pd-complexes. The 2,1-insertion is a preferred pathway for the acrylate incorporation. For both catalysts the acrylate 2,1-insertion barriers (12.4 and 13.5 for Pd and Ni with the real catalyst, respectively) are lower than the barriers for the insertion of ethylene (16.8 and 14.2 for Pd and Ni). The chelates formed after the acrylate insertion are slightly more stable for Ni than for Pd, with the five-membered system having the lowest energy. The MD results show that after the acrylate incorporation the activation barriers for the ethylene insertions starting from the isomers without the chelating bond are substantially lower than those starting from the chelated complexes. The ethylene insertion barriers are lower for Ni than for Pd in any case. The barriers for the opening of the chelate prior to insertion have been found to be lower for Pd (AG(double dagger) = 11.3 kcal/mol) than for Ni (14.4 kcal/mol) for the generic catalyst. The opening of the chelates is facilitated by the presence of the steric bulk on the catalyst; the effect is stronger for Ni than for Pd. Therefore, the present results suggest that the most important difference in the mechanisms of the copolymerization of the methyl acrylate with ethylene between the Pd- and Ni-diimine complexes is an initial poisoning of the catalyst by the O-binding mode in the latter case.
First author: Massera, C, Energy partitioning analysis of the bonding in L2TM-C2H2 and L2TM-C2H4 (TM = Ni, Pd, Pt; L-2 = (PH3)(2), (PMe3)(2), H2PCH2PH2, H2P(CH2)(2)PH2, ORGANOMETALLICS, 22, 2758, (2003)
Abstract: The equilibrium geometries and bond dissociation energies of the complexes L2TM-C2H2 and L2TM-C2H4 (TM = Ni, Pd, Pt) with the monodentate ligands L-2 = (PH3)(2), (PMe3)(2) and the bidentate ligands L-2 = eta(2)-diphosphinomethane (dpm), eta(2)-diphosphinoethane (dpe) have been calculated using gradient-corrected DFT methods. The nature of the bonding interactions between the metal and the 7 ligands ethene and ethyne was investigated with an energy partitioning analysis (EPA). The ethene and ethyne ligands are more strongly bonded to the metal when L-2 = dpm, dpe. The EPA results reveal that the reason for the stronger bonds of (dpm)TM-C2Hx and (dpe)TM-C2Hx is the smaller preparation energy of (dpm)TM and (dpe)TM that is necessary to deform the metal fragments from the equilibrium geometry to the geometry in the complex. The L2TM-C2Hx interaction energies between the fragments with a frozen geometry do not significantly vary when L-2 consists of a bidentate or two monodentate ligands. The EPA shows also that the nature of the L2TM-C2Hx bonding does not change a lot when L2 = (PH3)(2), (PMe3)(2) or when L-2 = dpm, dpe. The metal-carbon bonds always have a higher electrostatic (54.1-62.3%) than covalent (37.7-45.9%) character. The covalent bonding in the ethyne and ethene complexes comes mainly from the TM–>C2Hx in-plane pi back-donation, while the relative contribution of the TM <– C2Hx sigma donation is much less. The contributions of the out-of-plane a(2)(delta) and b(1)(pi(perpendicular to)) orbital interactions are very small even for the ethyne complexes. The bonding analysis suggests that the ethyne ligand in the complexes (PH3)(2)TM-C2H2 and (PMe3)(2)TM-C2H2 should be considered as a two-electron donor and not a four-electron donor.
First author: Autschbach, J, Relativistic density-functional computations of the chemical shift of Xe-129 in Xe@C-60,JOURNAL OF PHYSICAL CHEMISTRY A, 107, 4967, (2003)
Abstract: Relativistic contributions to the chemical shift of Xe-129 in Xe@C-60 have been computed by means of a two-component relativistic density functional approach. It is demonstrated that in all-electron calculations the scalar relativistic effects are moderate, of the order of 20 ppm depending on the basis set used, and tend to increase the chemical shift. The spin-orbit coupling induced contributions are small (a few ppm) and negative in all but two cases. The basis set dependence of the results is pronounced. In contrast to nonrelativistic Hartree-Fock studies of Xe@C-60, which strongly underestimate the experimental value of 179.24 ppm with respect to Xe gas, [61-72 ppm, Chem. Phys. Lett. 1997, 275, 14; 103 ppm, present work], our DFT results systematically overestimate the xenon shift (relativistically, greater than or equal to246 ppm). The large difference between the present nonrelativistic DFT results and the Hartree-Fock data can serve as an estimate of the important contributions due to electron correlation. Compared to the chemical shift range of Xe-129 (7000 ppm) the chemical shift in Xe@C-60 is not large, and sizable relative errors can therefore be expected from currently feasible computations. Many comparatively small effects, with opposite signs, contribute to the final result and must therefore not be neglected.
First author: Hsieh, CH, Nickel complexes of o-amidochalcogenophenolate(2-)/o-iminochalcogenobenzosemiquinonate(1-) pi-radical: Synthesis, structures, electron spin resonance, and x-ray absorption spectroscopic evidence, INORGANIC CHEMISTRY, 42, 3925, (2003)
Abstract: The preparation of complexes trans-[Ni(-SeC6H4-o-NH-)(2)](-) (1), cis-[Ni(-TeC6H4-o-NH-)(2)](-) (2), trans-[Ni(-SC6H4-o-NH-)2](-) (3), and [Ni(-SC6H4-o-S-)(2)]- (4) by oxidative addition of 2-aminophenyl dichalcogenides to anionic [Ni(CO)(SePh)(3)](-) proves to be a successful approach in this direction. The cis arrangement of the two tellurium atoms in complex 2 is attributed to the intramolecular (TeTe)-Te-… contact interaction ((TeTe)-Te-… contact distance of 3.455 Angstrom). The UV-vis electronic spectra of complexes 1 and 2 exhibit an intense absorption at 936 and 942 nm, respectively, with extinction coefficient epsilon > 10000 L mol(-1) cm(-1). The observed small g anisotropy, the principal g values at g(1) = 2.036, g(2) = 2.062, and g(3) = 2.120 for 1 and g(1) = 2.021, g(2) = 2.119, and g(3) = 2.250 for 2, respectively, indicates the ligand radical character accompanied by the contribution of the singly occupied d orbital of Ni(III). The X-ray absorption spectra of all four complexes show L-III peaks at similar to854.5 and similar to853.5 eV. This may indicate a variation of contribution of the Ni(II)-Ni(III) valence state. According to the DFT calculation, the unpaired electron of complex 1 and 2 is mainly distributed on the 3d(xz) orbital of the nickel ion and on the 4p(z) orbital of kselenium (tellurium, 5p(z)) as well as the 2p(z) orbital of nitrogen of the ligand. On the basis of X-ray structural data, UV-vis absorption, electron spin resonance, magnetic properties, DFT computation, and X-ray absorption (K- and L-edge) spectroscopy, the monoanionic trans-[Ni(-SeC6H4-o-NH-)(2)](-) and cis-[Ni(-TeC6H4-o-NH-)(2)](-) complexes are appositely described as a resonance hybrid form of Ni(III)-bis(o-amidochalcogenophenolato(2-)) and Ni(II)-(o-amidochalcogenophenolato(2-))-(o-iminochalcogenobenzosemiquino nato(1-) pi-radical; i.e., complexes 1 and 2 contain delocalized oxidation levels of the nickel ion and ligands.
First author: Yang, XZ, Time-dependent density functional theory study of the electronic states of BI, JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 36, 2283, (2003)
Abstract: By using time-dependent density functional theory, the vertical excitation energies of the boron monoiodide molecule are calculated. The Vosko-Wilk-Nusair functional with Stoll correction is used as the. local density approximation, while the RPBE functional is used as the general gradient approximation for the calculations of vertical excitation energies.,The potential energy curves of the ground state, the 12 valence states and the eight Rydberg excited states are plotted. The spectroscopic constants, including the equilibrium internuclear distance, r(e), the harmonic vibrational wavenumber, omega(e), and the relative electronic energy referred to the ground state, T-e, of bound states, are obtained. theoretically for the first time. The transition properties of the (1)Pi-X (1)Sigma(+) transition, including the transition dipole moments, the radiative lifetimes and the Franck-Condon factors, are predicted.
First author: Saladino, AC, Density functional theory calculations of nitrogen hyperfine and quadrupole coupling constants in Oxovanadium(IV) complexes, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 4735, (2003)
Abstract: Relativistic density functional theory (DFT) calculations of nitrogen hyperfine and quadrupole coupling constants were conducted for a series of oxovanadium complexes with axial and equatorial nitrogen ligands. The computational results qualitatively reproduced the observed experimental trends in nitrogen hyperfine coupling constants with ligand type (amine, imine, and isothiocyanate) and coordination (axial vs equatorial). The best quantitative agreement between calculated and experimental nitrogen coupling constants was obtained using the scalar-relativistic, spin-unrestricted, open-shell Kohn-Sham (SR UKS) method. These results have important implications for the interpretation of high-resolution electron paramagnetic resonance (EPR) spectra of oxovanadium complexes with nitrogen ligands.
First author: Chen, LX, MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 7022, (2003)
Abstract: The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu-I(dmP)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The Cooper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 Angstrom shorter than that of the ground state [Cu-I(dMP)(2)](+). The results confirm the previously proposed “exciplex” structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the groundstate copper(I) and copper(II) complexes.
First author: Mitani, M, Unprecedented living olefin polymerization derived from an attractive interaction between a ligand and a growing polymer chain, CHEMISTRY-A EUROPEAN JOURNAL, 9, 2397, (2003)
Abstract: Ti complexes incorporating fluorine-containing phenoxy-imine chelate ligands (fluorinated Ti-FI catalysts) have been demonstrated to induce an unprecedented living polymerization effect with both ethylene and propylene, through an attractive interaction between the fluorine atom in the ligand and a P-hydrogen atom on the growing polymer chain. With the aid of this attractive interaction, highly controlled living ethylene polymerization, highly-syndiospecific living propylene polymerization, the synthesis of unique block copolymers from ethylene and propylene, and the catalytic production of monodisperse polyethylene and Zn-terminated polyethylene have been realized. The attractive interaction provides a conceptually new strategy for the achievement of controlled living olefin polymerization.
First author: Bridgeman, AJ, Computational analysis of Mo and W oxoanions through bond order and bonding energy approaches, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 4568, (2003)
Abstract: The nature of the metal-oxygen interactions and the bonding properties of oxygen sites have been explored by combining population analysis, including bond and valency indexes, with information based on the composition of molecular orbitals and the calculation of bonding energetics. Particular attention has been focused on the effects of basis sets and functionals on the correlations between the various approaches. The results obtained from population analysis have been found to be qualitatively consistent with those provided by bonding-energy approaches for basis sets of triple-zeta quality and all functionals tested. Use of smaller basis sets has had only a relatively minor effect on the bonding-energy results but has led to some significant discrepancies in the population analysis.
First author: Fernandez, PG, Optical transitions of a D-4h complex with one electron in a(1g) (approximate to 3z(2)-r(2)): study of RhCl64-, CHEMICAL PHYSICS LETTERS, 374, 151, (2003)
Abstract: Optical transitions of the elongated RhCl64- complex in NaCl have been calculated in the DFT framework together with the oscillator strength of allowed gamma –> a(1g) charge transfer (CT) transitions. The experimental peak at 3.66 eV is shown to arise from a jump from a(2u)(ax) mainly built from axial ligands and thus the onset of CT transitions for RhCl64- appears at a lower energy than that for RhCl63-. This fact is partially related to the Jahn-Teller effect in RhCl64- The present calculations on RhCl64- also explain the crystal field spectrum and predict the existence of additional CT transitions in the 4-5 eV range.
First author: Jemmis, ED, Condensed polyhedral boranes and analogous organometallic clusters: a molecular orbital and density functional theory study on the cap-cap interactions, APPLIED ORGANOMETALLIC CHEMISTRY, 17, 480, (2003)
Abstract: The interactions between the non-bonded atoms on adjacent units were assumed to be one of the major factors that hinder the exploration and advancement of macropolyhedral borane chemistry. In sandwich complexes involving boron as the bridging atom, the interaction between non-bonded atoms tends to be antibonding, but a closer analysis of various condensed systems shows that this. cannot be generalized. The overlap populations (OPs) calculated for structures optimized at the B3LYP/6-31g* level [B21H181- (5), B20H16 (6)f [Al(C2B4H6)(2)](1-) (7), B12H102- (8), B10H82- (9 and 14), B11H13 (10), B10H14 (11), C2B8H12 (13) and B20H182- (15)] indicate bonding interactions between the caps, except for 7 and 13. This is substantiated by a detailed extended Huckel-based. molecular orbital (MO) study using B10H14 as a model system to represent macropolyhedral boranes with higher fusions. An isolobal equivalent structure, [C8H6 (Ru(CO)(2)Me)(2)] (17), studied at the B3LYP/LANL2DZ level has weak Ru-Ru interactions. An analysis-of the nature of the MOs in B10H14 (11) shows that there is no direct head on overlap of the cap orbitals that are antibonding; this is in contradiction to sandwiched molecules (7), where there are two occupied MOs with antibonding interactions. The m + n + o electron pair count (m is the number of cages involved in condensation, n is the number of vertices and o is the number of single vertex condensations) of sandwich complexes requires the filling of these two MOs. The negative OP between the carbon atoms in 13 is attributed to the greater electronegativity of carbon and is substantiated by a fragment MO analysis.
First author: Luo, Y, Ring opening of methylenecyclopropane over lanthanocene catalyst: A quantum-chemical molecular dynamics simulation study, ORGANOMETALLICS, 22, 2181, (2003)
Abstract: The ring opening of methylenecyclopropane (MCP) catalyzed by lanthanocene (Cp2LaH) was simulated using a novel tight-binding quantum-chemical molecular dynamics method under reaction temperature. The results indicate the formation of the Cp2LaH-MCP complex by 1,2-insertion with a subsequent hydrogen transfer followed by the ring opening of the proximal bond.
First author: Shen, YH, Ag-3 Born-Oppenheimer potential hypersurfaces, JOURNAL OF CHEMICAL PHYSICS, 118, 9241, (2003)
Abstract: The full Born-Oppenheimer potential energy hypersurfaces of the Ag-3 system have been explored by the local spin-density scheme using an analytic potential. Our calculated physical properties, such as dissociation energies and barriers to isomerization, compare well with the available high-level configuration interaction calculations and experiments at the equilibrium geometry. Despite its simplicity, the analytic potential provides an excellent description of the neutral trimer system for silver and the other transition metals.
First author: Grozema, FC, Tuning of the excited state properties of phenylenevinylene oligomers: A time-dependent density functional theory study, JOURNAL OF CHEMICAL PHYSICS, 118, 9441, (2003)
Abstract: This paper discusses a time-dependent density functional theory study of the effect of molecular structure on the excited state polarizability of conjugated molecules. A short phenylenevinylene oligomer containing three phenyl rings (PV2, distyryl benzene) is taken as a model system. Introduction of methyl substituents is shown to have only a small influence on the increase in polarizability upon excitation (the excess polarizability, Delta(alpha) over bar). Methoxy groups have a much larger effect but in this case Delta(alpha) over bar depends strongly on the dihedral angle between the side chain and the backbone of the molecule. If the central phenyl ring of PV2 has a meta-configuration rather than para, both the optical absorption spectrum and the excess polarizability change considerably.
First author: Wang, YB, Supramolecular interactions between fullerenes and porphyrins, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 6072, (2003)
Abstract: This paper discusses a time-dependent density functional theory study of the effect of molecular structure on the excited state polarizability of conjugated molecules. A short phenylenevinylene oligomer containing three phenyl rings (PV2, distyryl benzene) is taken as a model system. Introduction of methyl substituents is shown to have only a small influence on the increase in polarizability upon excitation (the excess polarizability, Delta(alpha) over bar). Methoxy groups have a much larger effect but in this case Delta(alpha) over bar depends strongly on the dihedral angle between the side chain and the backbone of the molecule. If the central phenyl ring of PV2 has a meta-configuration rather than para, both the optical absorption spectrum and the excess polarizability change considerably.
First author: Xie, RH, First-principles calculations of structural, electronic, vibrational, and magnetic properties of C-60 and C48N12: A comparative study, JOURNAL OF CHEMICAL PHYSICS, 118, 8621, (2003)
Abstract: The structural, electronic, vibrational, and magnetic properties of the C48N12 azafullerene and C-60 are comparatively studied from the first-principles calculations. Full geometrical optimization and Mulliken charge analysis are performed. Electronic structure calculations of C48N12 show that the highest occupied molecular orbital (HOMO) is a doubly degenerate level of a(g) symmetry and the lowest unoccupied molecular orbital (LUMO) is a nondegenerate level of a(u) symmetry. The calculated binding energy per atom and HOMO-LUMO energy gap of C48N12 are about 1 eV smaller than those of C-60. Because of electron correlations, the HOMO-LUMO gap decreases about 5 eV and the binding energy per atom increases about 2 eV. The average second-order hyperpolarizability of C48N12 is about 55% larger than that of C-60. Our vibrational frequency analysis predicts that C48N12 has 58 infrared-active and 58 Raman-active vibrational modes. Two different methods for calculating nuclear magnetic shielding tensors of C-60 and C48N12 are compared, and we find that C48N12 exhibits eight C-13 and two N-15 NMR spectral signals. Our best-calculated results for C-60 are in excellent agreement with experiment. Our results suggest that C48N12 has potential applications as semiconductor components, nonlinear optical materials, and possible building blocks for molecular electronics and photonic devices.
First author: van Erp, TS, Ab initio molecular dynamics study of aqueous solvation of ethanol and ethylene, JOURNAL OF CHEMICAL PHYSICS, 118, 8831, (2003)
Abstract: The structure and dynamics of aqueous solvation of ethanol and ethylene are studied by density functional theory based Car-Parrinello molecular dynamics. We did not find an enhancement of the structure of the hydrogen bonded network of hydrating water molecules. Both ethanol and ethylene can easily be accommodated in the hydrogen-bonded network of water molecules without altering its structure. This supports the conclusion from recent neutron diffraction experiments that there is no hydrophobic hydration around small hydrophobic groups. Analysis of the electronic charge distribution using Wannier functions shows that the dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while the apolar ethylene molecule attains an average dipole moment of 0.5 D. For ethylene, we identified configurations with pi-H bonded water molecules, that have rare fourfold hydrogen-bonded water coordination, yielding instantaneous dipole moments of ethylene of up to 1 D. The results provide valuable information for the improvement of empirical force fields, and point out that for an accurate description of the aqueous solvation of ethanol, and even of the apoler ethylene, polarizable force fields are required.
First author: Michalak, A, Polymerization of ethylene catalyzed by a nickel(+2) anilinotropone-based catalyst: DFT and stochastic studies on the elementary reactions and the mechanism of polyethylene branching, ORGANOMETALLICS, 22, 2069, (2003)
Abstract: The results of combined DFT/stochastic studies on the mechanism of ethylene polymerization catalyzed by a neutral Ni-anilinotropone complex are presented. The generation of active species by phosphine dissociation and chain propagation and chain isomerization reactions have been investigated. The alternative methyl acrylate binding modes have also been studied. Further, the DFT-calculated energetics of the elementary reactions have been used to model the influence of the reaction conditions (T, P) on the branching/microstructure of polyethylenes produced in this process. The model and real catalysts (N<^>O-Ni; N<^>O = -N(Ar)-(C7H5)-O-,with Ar = H and Ar =C6H3(i-Pr)(2), respectively) have been considered to account for the electronic and steric effects. The presence of two cis/trans isomers for all the reactions has been considered. The results indicate that for the real anilinotropone catalyst the phosphine dissocciation is less endothermic (22 kcal/mol) than for the corresponding salicylaldiminato system (29 kcal/mol). The more branched alkyl agostic complexes are found to be more stable than the less branched and the linear isomers, while the stability order for the olefin-alkyl complexes is opposite. Thus, the stability of the alkyl complexes shifts the equilibrium toward formation of the branched species, while the stability of ethylene complexes favors formation of linear structures. The DFT results show that the energetically preferred pathways for the chain propagation and isomerization reactions start from the higher energy cis/trans isomers (with the alkyl positioned trans to the N atom on the catalyst). The preferred isomerization reactions have very low barriers (2.4-4.5 kcal/mol for different alkyl species). Along these pathways the unusually stable olefin-hydride complexes are formed, some of them being more stable (by 1.5-3 kcal/mol) than the alkyl agostic complexes. The results of the calculations for the methyl acrylate complexes confirm the high tolerance of the anilinotropone catalyst toward polar groups: the pi-complexes are more stable by 8-13 kcal/mol than the systems with the acrylate molecule bound by its carbonyl oxygen. This functional group tolerance is larger than for the Grubbs catalyst. Also, the acrylate pi-complexation energies are less affected by the steric bulk than in the case of salicylamidato catalysts and the diimine catalysts. Finally, the results of the stochastic simulations quantitatively reproduce the experimental trends in the temperature and pressure dependence of the average number of branches. In addition, the stochastic simulations provide detailed information about the variations in the topology of polyethylenes produced under different conditions.
First author: Ishii, S, Ethylene and ethylene/propylene polymerization behavior of bis(phenoxy-imine) Zr and Hf complexes with perfluorophenyl substituents, MACROMOLECULAR RAPID COMMUNICATIONS, 24, 452, (2003)
Abstract: The catalytic properties of bis(phenoxyimine) Zr and Hf complexes incorporating perfluorophenyl groups with methylaluminoxane were investigated. The fluorinated complexes produced far higher-molecular-weight polyethylenes and ethylene/propylene copolymers with increased activities compared with the non-fluorinated congeners. Moreover, the fluorinated complexes displayed a higher incorporation ability for propylene.
First author: Autschbach, J, Density functional calculations on electronic circular dichroism spectra of chiral transition metal complexes, INORGANIC CHEMISTRY, 42, 2867, (2003)
Abstract: Time-dependent density functional theory (TD-DFT) has for the first time been applied to the computation of circular dichroism (CD) spectra of transition metal complexes, and a detailed comparison with experimental spectra has been made. Absorption spectra are also reported. Various Co-III complexes as well as [Rh(en)(3)](3+) are studied in this work. The resulting simulated CD spectra are generally in good agreement with experimental spectra after corrections for systematic errors in a few of the lowest excitation energies are applied. This allows for an interpretation and assignment of the spectra for the whole experimentally accessible energy range (UV/vis). Solvent effects on the excitations are estimated via inclusion of a continuum solvent model. This significantly improves the computed excitation energies for charge-transfer bands for complexes of charge +3, but has only a small effect on those for neutral or singly charged complexes. The energies of the weak d-to-d transitions of the Co complexes are systematically overestimated due to deficiencies of the density functionals. These errors are much smaller for the 4d metal complex. Taking these systematic errors and the effect of a solvent into consideration, TD-DFT computations are demonstrated to be a reliable tool in order to assist with the assignment and interpretation of CD spectra of chiral transition metal complexes.
First author: Spuhler, P, Theoretical studies of inorganic compounds. 27. Quantum chemical investigations of transition metal nitrido complexes with a TM-N-E linkage (E equals main group element), ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, 629, 803, (2003)
Abstract: It is reported about quantum chemical DFT calculations of various transition metal (TM) nitrido complexes which contain a TM-N-E linkage. The goal is to elucidate the nature of the TM-N-E bonding situation with modern quantum chemical tools. Five comparative investigations have been carried out. (a) Comparison of the N-donor ability in the nitrido complexes Cl3W-N-ECln where ECln = NaCl, MgCl2, AlCl3- (b) Comparative analysis of the bonding situation in Cl4W-N-X where X = Na, MgCl, AlCl2, SiCl3, PCl2, SCl, Cl. (c) Comparison of the structure and bonding in Cl5W-NPH3, Cl5W-OPH3+, Cl4W-(NPH3)(OPH3)(+). (d) Comparative analysis of the bonding situation in Cl5Ta-OPH3, Cl5W-NPH3, Cl5Re-CPH3. (e) Energy decomposition analysis of the bonding of the isolobal ligands NPH3 and Cp with WCl5.
First author: Termaten, AT, Terminal phosphinidene complexes Cp-R(L)M=PAr of the group 9 transition metals cobalt, rhodium, and iridium. Synthesis, structures, and properties, ORGANOMETALLICS, 22, 1827, (2003)
Abstract: Novel terminal rhodium- and cobalt-complexed phosphinidenes, Cp*(PR3)Rh=PAr (3-5) and Cp(PPh3)Co=PAr (8), were obtained by dehydrohalogenation of the primary phosphine complexes Cp*RhCl2(PH2Ar) (2) and CPCoI2(PH2Ar) (7) in the presence of a phosphine. X-ray crystal structures are reported for Cp*(PPh3)Rh=PMes* (3) and Cp(PPh3)Co=PMes* (8). A comparative reactivity study and a computational survey were performed on the Co-, Rh-, and Ir-containing phosphinidene complexes. All react with organic dihalides to form phosphaalkenes, with the rhodium congener being far more reactive than the iridium and cobalt complexes. Density functional theory calculations give geometrical parameters and P-31 NMR chemical shifts in good agreement with experimental data. The rhodium congeners exhibit the most pronounced charge separation of the Rh=P bond, which may explain its higher reactivity. The M-L bond is strong in all Cp(L)M=PH (M = Co, Rh, Ir) complexes and inhibits reactivity at the metal center. Comparisons with the Zr-containing complex Cp-2(PH3)Zr-PH are made.
First author: Zhai, HJ, Electronic and structural evolution of monoiron sulfur clusters, FeSn- and FeSn (n=1-6), from anion photoelectron spectroscopy, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 2821, (2003)
Abstract: We report a photoelectron spectroscopic investigation of a series of monoiron-sulfur clusters FeSn- (n = 1-6) at various photon energies. Vibrationally resolved spectra were measured for FeS- and FeS3-. A wealth of electronic structure information was obtained for FeS and were tentatively assigned, yielding a (5)Delta ground state for FeS and a (7)Sigma(+) and a (5)Delta excited state at 0.675 and 1.106 eV above the ground state, respectively. Franck-Condon factor simulations were performed for the vibrationally resolved (5)Delta ground state and the (5)Delta excited state, yielding an Fe-S bond length of 2.18 and 2.29 Angstrom for the anion ground state and the (5)Delta excited state, respectively, as well as a vibrational temperature of 180 K for the anion. The electron affinities (EA’s) of FeSn were measured to be 1.725 +/- 0.10, 3.222 +/- 0.009, 2.898 +/- 0.008, 3.129 +/- 0.008, 3.262 +/- 0.010, and 3.52 +/- 0.02 eV for n = 1-6, respectively. A significant EA increase was only observed from FeS to FeS2, whereas all larger species FeSn (n = 3-6) possess EA’s similar to that of FeS2 within +/-0.3 eV. By comparing the trend of EA in FeSn to that of FeOn, we proposed that all the FeSn- (n > 1) species take (S-m(2-))Fe3+(S-n-m(2-)) type structures, in which Fe assumes its favorite +3 formal oxidation state. Preliminary density functional calculations were carried out and the obtained structures support the proposed structural evolution of the FeSn clusters.
First author: Saeys, M, Density functional theory analysis of benzene (De)hydrogenation on Pt(111): Addition and removal of the first two H-atoms, JOURNAL OF PHYSICAL CHEMISTRY B, 107, 3844, (2003)
Abstract: The hydrogenation and dehydrogenation of benzene on Pt(1 1 1) is examined from first principles using DFT-GGA cluster calculations. The reactive benzene species is adsorbed at the hollow site. The addition of the first H-atom has a barrier of 74 kJ/mol and is +11 kJ/mol endothermic. There are five different pathways available for the addition of the second hydrogen atom. The dominant path is the one that forms the 1,3-dihydrobenzene intermediate. This reaction has a barrier of 72 kJ/mol and is +34 kJ/mol endothermic. The hydrogenation of the C6H7* intermediate can also form 1,3-cyclohexadiene, which has a barrier of 91 kJ/mol and is +38 kJ/mol endothermic, or 1,4-cyclohexadiene, which has a barrier of 115 kJ/mol and is +36 kJ/mol endothermic. Two types of hydrogenation mechanisms were distinguished. The “three-centered” mechanism was found to be more favorable than the “slip” mechanism. The dehydrogenation of benzene to phenyl is +76 kJ/mol endothermic. Therefore benzene dehydrogenation is neither thermodynamically nor kinetically a favorable reaction path. Dehydrogenation to o-benzyne is +14 kJ/mol endothermic relative to benzene. The calculated barriers are in qualitative and quantitative agreement with experimental data.
First author: Autschbach, J, A theoretical study of the large Hg-Hg spin-spin coupling constants in Hg(2)(2+), Hg(3)(2+), and Hg(2)(2+)-crown ether complexes, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 4937, (2003)
Abstract: Nuclear spin-spin coupling constants (1)J(Hg-Hg) in the systems Hg(2)(2+) and Hg(3)(2+) represent the largest coupling constants so far observed in NMR experiments. We have performed a computational study on these ions, on Hg(2)(2+) complexes with 18-crown-6 and 15-crown-5, and on Hg(3)(2+) with solvent molecules and counterions. The results obtained with our recently developed program for the density functional computation of heavy nucleus spin-spin coupling constants are in good agreement with experiments. The data reveal that the bare ions Hg(2)(2+) and Hg(3)(2+) Would afford much larger coupling constants than those experimentally observed, with an upper limit of approximately 0.9 MHz for Hg(2)(2+). This limit is much larger than that previously estimated by Huckel theory. It is demonstrated that in solution or due to complexation the experimentally determined values are much smaller than the free ion’s coupling constants. With the help of intuitive MO arguments, it is illustrated how the environment strongly reduces the coupling constants in Hg(2)(2+) and Hg(3)(2+). The two-bond coupling constant (2)J(Hg-Hg) in Hg(3)(2+) is also examined.
First author: Lein, M, Structures and bonding of the sandwich complexes [Ti(eta(5)-E-5)(2)](2-) (E = CH, N, P, As, Sb): A theoretical study, INORGANIC CHEMISTRY, 42, 2504, (2003)
Abstract: Quantum chemical calculations using gradient-corrected DFT at the BP86/TZ2P level of the compounds [Ti(eta(5)-E-5)(2)](2-) (E = CH, N, P, As, Sb) are reported. The nature of the metal-ligand bonding has been analyzed with an energy decomposition method, and the results are compared with [Fe(eta(5)-E-5)(2)]. The bonding in both series of complexes is more covalent than electrostatic. The energy decomposition analysis shows that the dominant orbital interactions in the negatively charged titanium species come from the (e(2)’) Ti –> [(eta(5)-E-5)(2)](2) back-donation (delta bonding) while the covalent bonding in the iron complexes come mainly from (e(1)’) (Cp)(2) –> Fe2+ donation (pi bonding). The nature of the metal-ligand interactions does not change very much for different ligands cyc-E-5 within the two series of compounds. The calculated bond dissociation energies for breaking one metal-ligand bond of the molecules [Ti(eta(5)-E-5)(2)](2-) shows for E the order P > As > Sb much greater than N much greater than CH. The central message of this work is that the complexes [Ti(eta(5)-E-5)(2)](2-) are delta bonded molecules.
First author: Persson, P, The smallest possible nanocrystals of semiionic oxides, JOURNAL OF PHYSICAL CHEMISTRY B, 107, 3336, (2003)
Abstract: General bonding principles are used to predict the structure of individual nanocrystals in nanocrystalline materials with semiionic bonding. The relationship between the general principles and actual nanocrystal structures is demonstrated using titanium dioxide in the anatase form. The proposed nanocrystals simultaneously fulfill strict criteria of stoichiometry, high coordination, and balanced charge distribution. The smallest such nanocrystals are remarkably simple, e.g., consisting of less than 100 atoms in anatase. According to computer simulations, these nanocrystals show strong quantum size effects, while other clusters of similar size instead show typical defect characteristics.
First author: Castro-Rodriguez, I, Uranium tris-aryloxide derivatives supported by triazacyclononane: Engendering a reactive uranium(III) center with a single pocket for reactivity, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 4565, (2003)
Abstract: The synthesis and spectroscopic characterization of the mononuclear uranium complex [((ArO)(3)tacn)U-III(NCCH3)] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)(3)tacn)U-IV(N-3)] and [((ArO)(3)tacn)U-V(NSi(CH3)(3))]. Single-crystal X-ray diffraction, spectroscopic, and computational studies of this analogous series of uranium tris-aryloxide complexes supported by triazacyclononane are described. The hexadentate, tris-anionic ligand coordinates to the large uranium ion in unprecedented fashion, engendering coordinatively unsaturated and highly reactive uranium centers. The macrocyclic triazacyclononane tris-aryloxide derivative occupies six coordination sites, with the three aryloxide pendant arms forming a trigonal plane at the metal center. DFT quantum mechanic methods were applied to rationalize the reactivity and to elucidate the electronic structure of the newly synthesized compounds. It is shown that the deeply colored uranium(III) and uranium(V) species are stabilized via pi-bonding interaction, involving uranium f-orbitals and the axial acetonitrile and imido ligand, respectively. In contrast, the bonding in the colorless uranium(IV) azido complex is purely ionic in nature. The magnetism of the series of complexes with an [N3O3-N-ax] core structure and oxidation states +III, +IV, and +V is discussed in context of the electronic structures.
First author: Filatov, M, Analytic energy derivatives for regular approximations of relativistic effects applicable to methods with and without correlation corrections, JOURNAL OF CHEMICAL PHYSICS, 118, 6741, (2003)
Abstract: Analytic expressions are derived for the evaluation of derivatives of the total molecular energy with respect to external parameters (nuclear coordinates, external electric fields, etc.) within the relativistic regular approximation. The presented formalism employs the spectral resolution of the identity avoiding, however, the explicit use of an auxiliary basis set in the calculation of the matrix elements of the regular relativistic Hamiltonian. The final formulas for the total energy and energy derivatives are presented in matrix form suitable for implementation into standard quantum chemical packages. Results of benchmark calculations for gold containing diatomic molecules and for xenone hexafluoride performed at the Hartree-Fock and various correlation corrected levels of theory are presented and discussed.
First author: Bagno, A, Relativistic DFT calculations of polyoxotungstate W-183 NMR spectra: Insight into their solution structure, CHEMPHYSCHEM, 4, 517, (2003)
Abstract: Analytic expressions are derived for the evaluation of derivatives of the total molecular energy with respect to external parameters (nuclear coordinates, external electric fields, etc.) within the relativistic regular approximation. The presented formalism employs the spectral resolution of the identity avoiding, however, the explicit use of an auxiliary basis set in the calculation of the matrix elements of the regular relativistic Hamiltonian. The final formulas for the total energy and energy derivatives are presented in matrix form suitable for implementation into standard quantum chemical packages. Results of benchmark calculations for gold containing diatomic molecules and for xenone hexafluoride performed at the Hartree-Fock and various correlation corrected levels of theory are presented and discussed.
First author: Lichtenberger, DL, Electron distribution, bonding, and J(Si-H) NMR coupling constant in (eta(5)-C5H5)(CO)(2)MnHSiCl3: The molecular orbital view, ORGANOMETALLICS, 22, 1599, (2003)
Abstract: Analytic expressions are derived for the evaluation of derivatives of the total molecular energy with respect to external parameters (nuclear coordinates, external electric fields, etc.) within the relativistic regular approximation. The presented formalism employs the spectral resolution of the identity avoiding, however, the explicit use of an auxiliary basis set in the calculation of the matrix elements of the regular relativistic Hamiltonian. The final formulas for the total energy and energy derivatives are presented in matrix form suitable for implementation into standard quantum chemical packages. Results of benchmark calculations for gold containing diatomic molecules and for xenone hexafluoride performed at the Hartree-Fock and various correlation corrected levels of theory are presented and discussed.
First author: Kovacs, A, Thermodynamic properties of PaCl5 and PaF5 based on quantum chemical calculations,JOURNAL OF ALLOYS AND COMPOUNDS, 353, 128, (2003)
Abstract: Quantum chemical calculations using the Becke-Perdew (BP86) and Becke3-Lee-Yang-Parr (B3LYP) exchange-correlation density functionals in conjunction with basis sets of polarised valence triple-zeta quality have been performed for PaCl5 and PaF5. The two methods consistently gave the trigonal bipyramid (D-3h) structure as the global minimum on the potential energy surface, being 4-6 kJ mol(-1) more stable than the tetragonal pyramid (C-4v) structure. The geometrical parameters and vibrational frequencies of the D-3h structures computed by the two methods are in good agreement. The results have been used to calculate the thermodynamic functions of the molecules. In combination with literature data the enthalpies of formation have been evaluated.
First author: Bagno, A, DFT study of the NMR properties of xenon in covalent compounds and van der Waals complexes-implications for the use of Xe-129 as a molecular probe, CHEMISTRY-A EUROPEAN JOURNAL, 9, 1486, (2003)
Abstract: The NMR properties (chemical shift and spin-spin coupling constants) of Xe-129 in covalent compounds and weakly bound complexes have been investigated by DFT methods including relativistic effects. For covalent species, a good agreement between experimental and calculated results is achieved without scalar relativistic effects, but their inclusion (with a triple-zeta, doublepolarization basis set) leads to some improvement in the quality of the correlation. The spin-orbit coupling term has a significant effect on the shielding constant, but makes a small contribution to the chemical shift. Coupling constants contain substantial contributions from the Fermi contact and paramagnetic spin-orbit terms; unlike light nuclei the spin-dipole term is also large, whereas the diamagnetic spin-orbit term is negligible. For van der Waals dimers, the dependence of the xenon chemical shift and anisotropy is calculated as a function of the distance. Small (< 1 Hz) but non-negligible throughspace coupling constants between Xe-129 and C-13 or H-1 are predicted. Much larger couplings, of the order of few Hz, are calculated between xenon and O-17 in a model silicate residue.
First author: Joshi, HK, Investigation of metal-dithiolate fold angle effects: Implications for molybdenum and tungsten enzymes, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA,100, 3719, (2003)
Abstract: Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur pi-orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin Mo/W enzymes. The compounds (Tp*)Moo(bdt) (compound 1), Cp2Mo(bdt) (compound 2), and Cp2Ti(bdt) (compound 3) [where Tp* is,hydrotris(3,5-dimethyl-1-pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is eta(5)-cyclopentadienyl] provide access to three different electronic configurations of the metal; formally d(1), d(2), and d(0), respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A “dithiolate-folding-effect” involving an interaction of the metal in-plane and sulfur-pi orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes.
First author: Saladino, AC, Density functional theory calculations of the electron paramagnetic resonance parameters for VO2+ complexes, JOURNAL OF PHYSICAL CHEMISTRY A, 107, 1872, (2003)
Abstract: Density functional theory calculations of electron paramagnetic resonance (EPR) parameters, such as electronic g tensors and metal hyperfine interaction (A) tensors, have been completed for a series of VO2+ complexes. g tensors were calculated with the zeroth-order regular approximation (ZORA) for relativistic effects as incorporated into the Amsterdam Density Functional (ADF) program. The A tensors were calculated by relativistic and nonrelativistic methods as implemented in ADF and Gaussian98 programs, respectively. The best overall agreement with experimental A values was obtained with the nonrelativistic method and the half-and-half hybrid functionals, such as BHPW91, BHP86, and B14LYP. The isotropic A values (A(iso)) calculated nonrelativistically with the BHPW91 functional deviated by about 10% from the experimental A(iso) values. The A(iso) values calculated with the relativistic effects and pure generalized gradient correction (GGA) functionals, such as BP86, deviated systematically by approximately 40% compared to the experimental A(iso) values. The difference in performance of the two methods for these complexes is attributed to the improved performance of hybrid functionals for treating core shell spin polarization. The calculation of the anisotropic or dipolar hyperfine interactions, A(D), was less sensitive to the choice of functional, and therefore, the relativistic and nonrelativistic calculations of A(D) exhibited comparable accuracy.
First author: Andrews, L, Noble gas-actinide complexes of the CUO molecule with multiple Ar, Kr, and Xe atoms in noble-gas matrices, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 3126, (2003)
Abstract: Laser-ablated U atoms react with CO in excess argon to produce CUO, which is trapped in a triplet state in solid argon at 7 K, based on agreement between observed and relativistic density functional theory (DFT) calculated isotopic frequencies ((CO)-C-12-O-16, (CO)-C-13-O-16, (CO)-C-12-O-18). This observation contrasts a recent neon matrix investigation, which trapped CUO in a linear singlet state calculated to be about 1 kcal/mol lower in energy. Experiments with krypton and xenon give results analogous to those with argon. Similar work with dilute Kr and Xe in argon finds small frequency shifts in new four-band progressions for CUO in the same triplet states trapped in solid argon and provides evidence for four distinct CUO(Ar)(4-n)(Ng)(n) (Ng = Kr, Xe, n = 1, 2, 3, 4) complexes for each Ng. DFT calculations show that successively higher Ng complexes are responsible for the observed frequency progressions. This work provides the first evidence for noble gas-actinide complexes, and the first example of neutral complexes with four noble gas atoms bonded to one metal center.
First author: Frenking, G, Towards a rigorously defined quantum chemical analysis of the chemical bond in donor-acceptor complexes, COORDINATION CHEMISTRY REVIEWS, 238, 55, (2003)
Abstract: The results of an energy decomposition analysis of various classes of donor-acceptor complexes of transition metals and main-group elements are discussed. It is shown that the nature of the chemical bond can be quantitatively identified in terms of Pauli repulsion. electrostatic attraction and covalent bonding. The covalent and electrostatic contributions to the interatomic attraction can be precisely given by using a well defined partitioning method in conjunction with accurate quantum chemical calculations of the geometries and bond energies. This is shown for six classes of donor-acceptor complexes: (a) transition metal carbonyl complexes; (b) transition metal complexes with Group-13 diyl ligands ER (E B-Tl); (c) transition metal complexes with phosphane ligands (CO)(5)TMPX3 (TM Cr, Mo, W; X = H, Me, F, Cl); (d) main group complexes with phosphane ligands X3B-PY3 and X3Al-PY3 (X = K F Cl; Y F, Cl, Me, CN); (e) transition metal metallocene complexes Fe(eta(5)-E-5)(2) and FeCp(eta(5) -E-5) (E = CH. N, P, As, Sb); (f) main group metallocenes ECp2 (E = Be-Ba, Zn, Si-Pb) and ECp (E = Li-Cs, B-Tl).
First author: Daniel, C, Electronic spectroscopy and photoreactivity in transition metal complexes, COORDINATION CHEMISTRY REVIEWS, 238, 143, (2003)
Abstract: The recent developments in quantum chemistry providing the theoretical tools to determine the properties of transition metal complexes in their electronic excited states are presented. The contrast between the impressive fast evolution of the computational strategies adapted to the treatment of ground state molecular properties and the gradual improvement of the methods that are more specifically turned towards the study of electronic excited states is discussed. Recent applications in transition metal coordination chemistry are selected to outline the degree of methodological maturity in electronic spectroscopy and photo-induced reactivity, illustrating the necessity for a strong interplay between theory and experiment.
First author: Ramos, J, Computational studies of the Brookhart’s type catalysts for ethylene polymerisation. Part 2: ethylene insertion and chain transfer mechanisms, POLYMER, 44, 2169, (2003)
Abstract: This report describes complete density functional theory studies performed to elucidate ethylene polymerisation mechanisms of the Brookhart-type catalysts N,N’-(2,6-dimethylphenyl)ediylenediimine nickel (II) and N-(2,6-dimethylphenyl)pyridine-2-carboxaldiimine nickel (II). Both catalysts showed conformations that blocked the active site and thus hindered ethylene coordination. These conformations were related to the rotational capacity of the ancillary aryl group attached to the ligands, which generates agostic interactions between nickel and hydrogen atoms. It was calculated that these conformations were 8.3 and 10.7 kcal/mol (respectively for each catalyst) more stable than non-blocking cationic conformations. From a design perspective, catalytic systems would need to include bulky constituents to avoid these types of blocking conformation. The two possible mechanisms already proposed for the chain initiation step evoke approaches for ethylene inside and out of the equatorial plane of the catalyst. Herein, the in-plane mechanism was found to be the most favourable. However, for chain propagation, the out-plane approach for ethylene proved to be the most feasible, due to nickel agostic interactions with the growing alkyl chain. The energy barriers calculated for the propagation step were 9.3 and 10.6 kcal/mol for the two catalysts, respectively. These findings indicate the ethylenediimine catalyst is slightly more active than the pyridine catalyst. Three possible chain transfer mechanisms were also considered for the two catalysts: beta-hydride transfer, beta-hydride transfer to monomer and an association mechanism; the last two being the mostly likely mechanisms in thermodynamics and kinetic terms. Chain transfer processes for the pyridine-2-carboxaldiimine based catalyst were found to be more favourable than those of the ethylenediimine based catalyst. This finding is in agreement with experimental results which indicate that the former catalyst gives rise to lower molecular weight polymers.
First author: Elanany, M, A quantum molecular dynamics simulation study of the initial hydrolysis step in sol-gel process,JOURNAL OF PHYSICAL CHEMISTRY B, 107, 1518, (2003)
Abstract: The dynamic behavior of the hydrolysis reaction of Si(OCH3)(4) under neutral, basic, and acidic conditions was investigated, for the first time, at the atomic level with short time intervals using a novel tight-binding quantum chemical molecular dynamics program “Colors”. The initial parameters required for the computation were determined completely on the basis of the first principles density functional calculations using Amsterdam density functional program. The simulation results of this study clearly indicate that a flank-side attack mechanism is favored, in all the three cases, for the hydrolysis process, and pentacoordinate silicon intermediates are easy pathways for the displacement of -OCH3 by -OH on silicon. Moreover, the presence of the acid or the base as catalyst promotes the hydrolysis by rapid formation of Si-OH bond in comparison to the hydrolysis under neutral condition. Furthermore, in the case of the latter condition, it was observed that the proton oscillates between -OH and -OCH3 before it migrates to the latter group.
First author: Harrington, LE, Spectroscopic and computational assessment of the rotational barrier of a ferrocenyl-stabilized cyclopentadienyl cation: Evidence for the first hydroxyfulvalene ligand, ORGANOMETALLICS, 22, 663, (2003)
Abstract: Protonation of 3-ferrocenyl-2,4,5-triphenylcyclopentadienone with trifluoroacetic acid results in the formation of the first hydroxyfulvalene complex. The dramatic deshielding and decoaleseence of the H-1 and C-13 NMR signals in the ferrocenyl region offer evidence for the participation of the metal in the stabilization of the positive charge, which results in restricted rotation of the ferrocenyl group. As determined by NMR spectroscopy, the maximum barrier to rotation in the neutral species is ca. 9 kcal mol(-1), whereas the minimum barrier in the cation considerably exceeds 13 kcal mol(-1). The difference in rotational barriers for the neutral and protonated species is rationalized by using DFT calculations.
First author: Kennepohl, P, Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 1. Photoelectron spectroscopic determination of electronic relaxation, INORGANIC CHEMISTRY, 42, 679, (2003)
Abstract: Electronic relaxation, the change in molecular electronic structure as a response to oxidation, is investigated in [FeX4](2-),(1-) (X = Cl, SR) model complexes. Photoelectron spectroscopy, in conjunction with density functional methods, is used to define and evaluate the core and valence electronic relaxation upon ionization of [FeX4]2-. The presence of intense yet formally forbidden charge-transfer satellite peaks in the PES data is a direct reflection of electronic relaxation. The phenomenon is evaluated as a function of charge redistribution at the metal center (Deltaq(rlx)) resulting from changes in the electronic structure. This charge redistribution is calculated from experimental core and valence PES data using a valence bond configuration interaction (VBCI) model. It is found that electronic relaxation is very large for both core (Fe 2p) and valence (Fe 3d) ionization processes and that it is greater in [Fe(SR)(4)](2-) than in [FeCl4](2-). Similar results are obtained from DFT calculations. The results suggest that, although the lowest-energy valence ionization (from the redox-active molecular orbital) is metal-based, electronic relaxation causes a dramatic redistribution of electron density (similar to0.7 (e) over bar) from the ligands to the metal center corresponding to a generalized increase in covalency over all M-L bonds. The more covalent tetrathiolate achieves a larger Deltaq(rlx) because the LMCT states responsible for relaxation are significantly lower in energy than those in the tetrachloride. The large observed electronic relaxation can make significant contributions to the thermodynamics and kinetics of electron transfer in inorganic systems.
First author: Kennepohl, P, Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 2. Reduction potentials, INORGANIC CHEMISTRY, 42, 689, (2003)
Abstract: This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl4](2-,1-) and [Fe(SR)(4)](2-,1-) (DeltaE(0) approximate to 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (Delta/(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to Delta/(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to Delta/(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I-vert relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge (Z(eff)(Fe)) of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe) (3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases Z(eff)(Fe) and significantly lowers its redox potential.
First author: Kennepohl, P, Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 3. Kinetics of electron transfer, INORGANIC CHEMISTRY, 42, 696, (2003)
Abstract: The kinetics of electron transfer for rubredoxins are examined using density functional methods to determine the electronic structure characteristics that influence and allow for fast electron self-exchange in these electron-transport proteins. Potential energy surfaces for [FeX4](2-,1-) models confirm that the inner-sphere reorganization energy is inherently small for tetrathiolates (lambda(i)(ese) approximate to 0.1 eV), as evidenced by the only small changes in the equilibrium Fe-S bond distance during redox (Deltar(redox) approximate to 0.05 Angstrom). It is concluded that electronic relaxation and covalency in the reduced state allow for this small lambda(i)(ese) in this case relative to other redox couples, such as the tetrachloride. Using a large computational model to include the protein medium surrounding the [Fe(SCys)(4)](2-),(1-) active site in Desulfovibrio vulgaris Rubredoxin, the electronic coupling matrix element for electron self-exchange is defined for direct active-site contact (H-DA(0)). The simple Beratan-Onuchic model is used to extend coupling over the complete surface of the protein to provide an understanding of probable electron-transfer pathways. Regions of similar coupling properties are grouped together to define a surface coupling map, which reveals that very efficient self-exchange occurs only within 4 sigma-bonds of the active site. Longer-range electron transfer cannot support the fast rates of electron self-exchange observed experimentally. Pathways directly through the two surface cysteinate ligands dominate k(ese)(infinity) but surface-accessible amides hydrogen-bonded to the cysteinates also contribute significantly to the rate of electron self-exchange.
First author: Bridgeman, AJ, Density functional study of the vibrational frequencies of alpha-Keggin heteropolyanions,CHEMICAL PHYSICS, 287, 55, (2003)
Abstract: The structures and vibrational spectra of the alpha-Keggin heteropolyanions [PMo12O40](3-), [PW12O40](3-), [AsMo12O40](3-), [SiMo12O40](4-), [GeMo12O40](4-), [AlMo12O40](5-) and [GaMo12O40](5-) have been calculated using density functional theory. The calculations represent the first non-empirical study of the vibrational frequencies of this important class of polyoxometalates. The agreement between the previously reported vibrational spectra and the calculated values is, in general, good. A number of previously reported assignments have been confirmed or clarified. The calculations are extremely computationally demanding requiring precise energies and geometries and cannot presently be considered a standard task for the study of these large, heavy element cluster anions. Characteristic group frequencies for the type I polyoxometalates with isopolyanions with the Lindqvist structure and heteropolyanions with the alpha-Keggin structure have been identified and the effect of the heteroatom on these frequencies studied. The vibrational analyses confirm the high symmetry for these anions suggested by previous geometry calculations.
First author: Autschbach, J, Theoretical investigation of the apparently irregular behavior of Pt-Pt nuclear spin-spin coupling constants, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 1028, (2003)
Abstract: One-bond Pt-Pt nuclear spin-spin coupling constants J(Pt-Pt) for closely related dinuclear Pt complexes can differ by an order of magnitude without any obvious correlation with Pt-Pt distances. As representative examples, the spin-spin couplings of the dinuclear Pt-1 complexes [Pt-2(CO)(6)](2+) (1) and [Pt-2(CO)(2)Cl-4](2-) (2) have been computationally studied with a recently developed relativistic density functional method. The experimental values are (1)J(Pt-195-Pt-195) = 5250 Hz for 2 but 551 Hz for 1. Many other examples are known in the literature. The experimental trends are well reproduced by the computations and can be explained based on the nature of the ligands that are coordinated to the Pt-Pt fragment. The difference for J(Pt-Pt) of an order of magnitude is caused by a sensitive interplay between the influence of different ligands on the Pt-Pt bond, and relativistic effects on metal-metal and metal-ligand bonds as well as on “atomic orbital contributions” to the nuclear spin-spin coupling constants. The results can be intuitively rationalized with the help of a simple qualitative molecular orbital diagram.
First author: Swart, M, AddRemove: A new link model for use in QM/MM studies, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 91, 177, (2003)
Abstract: The division of a system under study in a quantum mechanical (QM) and a classical system in QM/MM molecular mechanical calculations is sometimes very natural, but a problem arises in the case of bonds crossing the QM/MM boundary. A new link model that uses a capping (link) atom to satisfy the valences of the quantum chemical system is presented, with the position of the capping atom depending on the positions of the real atoms involved in the link bond. Using this method no degrees of freedom for the capping atom are introduced. Moreover, the introduction of this artificial atom is corrected for by subtracting the classical MM interactions with the real QM system it would have if it were a classical atom. That is, the capping atoms are added and removed. The new model has been tested on three amino acid residues and shows a clear improvement over other link models (as represented by the Integrated Molecular Orbital and Molecular Model (IMOMM)/ADF implementation). The average absolute deviation for the C-alpha-C-beta bond distance, as obtained when comparing the full QM and QM/MM results, is around 0.75 pm. The IMOMM model predicts distances for the C-alpha-C-backbone and C-alpha-N-backbone bonds, with an average absolute deviation of 2.3-2.8 and 5.3-5.5 pm, respectively; this is an increase by a factor of 3.1-4.0 and 7.1-7.3 compared with the C-alpha-C-beta bond. For the new AddRemove model, the average absolute deviations are 1.0-1.2 and 0.6-0.9 pm, respectively, for the C-alpha-C-backbone and C-alpha-N-backbone bonds; compared with the C.-C. bond, this indicates only a slight change, with a factor of 1.3-1.6 and 0.8-1.2, respectively. The new AddRemove model therefore performs much better and is shown to be a substantial improvement over the IMOMM model.
First author: Jensen, L, A discrete solvent reaction field model within density functional theory, JOURNAL OF CHEMICAL PHYSICS, 118, 514, (2003)
Abstract: In this work we present theory and implementation for a discrete reaction field model within Density Functional Theory (DFT) for studying solvent effects on molecules. The model combines a quantum mechanical (QM) description of the solute and a classical description of the solvent molecules (MM). The solvent molecules are modeled by point charges representing the permanent electronic charge distribution, and distributed polarizabilities for describing the solvent polarization arising from many-body interactions. The QM/MM interactions are introduced into the Kohn-Sham equations, thereby allowing for the solute to be polarized by the solvent and vice versa. Here we present some initial results for water in aqueous solution. It is found that the inclusion of solvent polarization is essential for an accurate description of dipole and quadrupole moments in the liquid phase. We find a very good agreement between the liquid phase dipole and quadrupole moments obtained using the Local Density Approximation and results obtained with a similar model at the Coupled Cluster Singles and Doubles level of theory using the same water cluster structure. The influence of basis set and exchange correlation functional on the liquid phase properties was investigated and indicates that for an accurate description of the liquid phase properties using DFT a good description of the gas phase dipole moment and molecular polarizability are also needed.
First author: Foerster, S, Single crystal EPR studies of the reduced active site of [NiFe] hydrogenase from Desulfiovibrio vulgaris Miyazaki F, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 125, 83, (2003)
Abstract: In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the “substrate hydrogen”. In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio, vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H-) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(l) oxidation state is assumed for this species with a proton (HI) removed from the bridge between the nickel and the iron atom.
First author: Aramburu, JA, Jahn-Teller ions in cubic crystals explored through DFT calculations, ADVANCES IN QUANTUM CHEMISTRY, VOL 44: MANIFESTATIONS OF VIBRONIC COUPLING IN CHEMISTRY AND PHYSICS, 44, 445, (2003)
Abstract: In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the “substrate hydrogen”. In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio, vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H-) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(l) oxidation state is assumed for this species with a proton (HI) removed from the bridge between the nickel and the iron atom.
First author: Lein, M, A novel class of aromatic compounds: metal-centered planar cations [Fe(Sb-5)](+) and [Fe(Bi-5)](+),ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 42, 1303, (2003)
Abstract: In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the “substrate hydrogen”. In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio, vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H-) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(l) oxidation state is assumed for this species with a proton (HI) removed from the bridge between the nickel and the iron atom.
First author: Weinhold, F, Rebuttal to the Bickelbaupt-Baerends case for steric repulsion causing the staggered conformation of ethane, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 42, 4188, (2003)
Abstract: In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the “substrate hydrogen”. In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio, vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H-) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(l) oxidation state is assumed for this species with a proton (HI) removed from the bridge between the nickel and the iron atom.
First author: Suenobu, T, Excited-state deprotonation and H/D exchange of an iridium hydride complex, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 42, 5492, (2003)
Abstract: In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the “substrate hydrogen”. In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio, vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H-) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(l) oxidation state is assumed for this species with a proton (HI) removed from the bridge between the nickel and the iron atom.
First author: Venkatesan, K, A facile and novel route to unprecedented manganese C-4 cumulenic complexes,CHEMICAL COMMUNICATIONS, 42, 2006, (2003)
Abstract: The theoretically characterized (DFT) C-4 cumulenic species Mn(C5H4R)(dmpe) {=C=C=C=C(SnPh3)(2)} was obtained by photolysis of the C(sp)2-Sn bond in the vinylidene complex Mn(C5H4R)(dmpe)[=C=C(SnPh3)-Cequivalent toCSnPh(3)], which in turn was prepared by a thermal reaction from MnC5H4R(dmpe)(C7H8) and Ph3Sn-C-4-SnPh3.
First author: Bursten, BE, The quantum chemistry of d- and f-element complexes: From an approximate existence to functional happiness, FARADAY DISCUSSIONS, 124, 1, (2003)
Abstract: The field of modern quantum inorganic chemistry is just over 50 years old, dating back to 1951, when quantitative LCAO molecular orbital theory was developed and ferrocene was discovered. This Lecture provides a survey of the development of the field through about 1980, which has led to its current state. The application of modern quantum chemical techniques are illustrated via two disparate examples from the authors’ research group. First, the recent discovery of uranium-noble gas bonds is discussed including the synergy between the theoretical and experimental investigations of this phenomenon. New theoretical results using coupled-cluster [CCSD(T)] methodology is contrasted to the original scalar-relativistic density functional theory results. Second, new applications of time-dependent density functional theory to the rich photochemistry of a dinuclear organometallic complex, (eta(5)-C5H5)(2)Fe-2(mu-CO)(2)(CO)(2), are discussed.
First author: Pyykko, P, On the nature of the short Pt-Tl bonds in model compounds [H5Pt-TlHn](n-), FARADAY DISCUSSIONS, 124, 41, (2003)
Abstract: RHF, DFT and MP2 calculations are reported for the compounds [H5Pt-TlHn](n-), n = 0-2. These serve as analogues for the experimentally known [(NC)(5)Pt-Tl(CN)(n)](n-) species. The very short bond between platinum and thallium is discussed.
First author: Zurek, E, A theoretical study of the insertion barrier of MAO (methylaluminoxane)-activated, Cp2ZrMe2-catalyzed ethylene polymerization: further evidence for the structural assignment of active and dormant species,FARADAY DISCUSSIONS, 124, 93, (2003)
Abstract: Density functional theory has been used to calculate the ethylene insertion barrier into the Zr-C bonds of the [Cp2ZrMe](+)[AlMe3Me(A-MAO)](-) and [Cp2ZrMe](+)[Me(A-MAO)](-) ion pairs where (AlOMe)(6) was used to model the active forms of MAO (A-MAO). The results support the proposal that the former is the active and the latter the dormant species in MAO-activated olefin polymerization. For the first insertion, with the active species the trans approach has a lower barrier to insertion than the cis approach in solution, due to the larger ion pair separation. It is likely that the separated ion pairs recombine between subsequent insertions. For the second insertion, we have considered frontside, backside and combined insertion mechanisms showing that the total barrier to insertion is approximately the same in all cases ranging from 13.8 kcal mol(-1)-14.8 kcal mol(-1). For the frontside and combined insertion this barrier is a result of the rotation of the propyl chain; for the backside mechanism it is stems from the ethylene insertion barrier. Comparison of the results for the ion pair and for the naked cation show that in the former case olefin complexation is an endothermic process whereas for the latter it is exothermic. In general, the large ion pair separation prevents the orientation of the anion from exerting much influence on the geometry or energy of the cation. However, when structures with alpha-agostic interactions are considered the ion pair separation is small enough so that the orientation has some influence on the reaction mechanism. For the second insertion, dissociated transition states were found to have lower barriers than associated ones.
First author: Bridgeman, AJ, Towards an understanding of the bonding in polyoxometalates through bond order and bond energy analysis, FARADAY DISCUSSIONS, 124, 239, (2003)
Abstract: The molecular and electronic structures of transition metal complexes, [MOCl5](n-) (n = 2 for M = V, Nb, Ta and n = 1 for Mo, W) and mixed-metal polyoxometalates, [M’M5O19](3-) (M’ = V, Nb, Ta, M = Mo, W) containing a single terminal oxo group on each metal, and of complexes of the uranyl ion [UO2](2+), [UO2(H2O)(5)](2+) and [UO2Cl4](2-), have been calculated using density functional methods. The calculated structures of the complexes are in good agreement with available experimental parameters. For the mixed-metal hexametalates, for which no crystallographic data is available, the calculations predict a small tetragonal compression of the clusters with only minor structural changes compared to the parent molybdate and tungstate. The metal-oxygen bonding in these anions has been probed using Mayer-Mulliken, bond energy and atoms in molecule analyses ( AIM). These methods provide a consistent description of the bonding in polyoxometalates. The terminal bonds between transition metal or uranium and oxygen atoms have large sigma and pi components with the p contributions exceeding the s bonding. The transition metals utilize their d orbitals almost exclusively to bond to oxygen whilst uranium uses both its 5f and 6d orbitals. Oxygen atom charges increase and covalency indexes decrease with coordination number, with a marked separation of these terms according to the oxygen atom type. The total valency and AIM energies of the oxygen atoms are predicted to be almost constant for all types of oxygen site. The constancy of the bonding power of the oxygen atoms appears to be an important factor in determining the gross structures and details of the bonding in polyoxometalates. The Mayer-Mulliken approach provides direct characterization of the bonding power of atoms and the extent of the interaction between pairs of atoms that is consistent with the results of the considerably more computationally demanding bond energy and AIM approaches.
First author: Lein, M, Theoretical studies of inorganic compounds. Part XXVIII. Energy decomposition analysis of the chemical bond in main group and transition metal compounds, FARADAY DISCUSSIONS, 124, 365, (2003)
Abstract: The nature of the chemical bond in the main group diborane(4) compounds X2B-BX2 (X = H, F, Cl, Br, I) and in the Fischer- and Schrock-type transition metal carbene and carbyne complexes and heavier homologues (CO)(5)W-CH2, (CO)(5)W-E(OH)(2), Cl4W-EH2, Cl(CO)(4)W-EH and Cl3W-EH (E = C, Si, Ge, Sn, Pb) have been investigated with an energy decomposition analysis (EDA). The results give a deep insight into the nature of the chemical interactions. The EDA results can be used as a bridge between the heuristic models of experimental chemists which have been proven as useful ordering schemes for experimental observations and the physical mechanism which leads to a chemical bond. At the time the data give a well defined qantitative answer to the questions about the strength of the covalent and electrostatic interactions and about the contributions of sigma and pi electrons to the covalent bond.
First author: Seuret, P, An experimental and theoretical study of [RhCl(PF3)(2)](2) fragmentation, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 5, 268, (2003)
Abstract: Experimental and theoretical techniques have been applied to study the decomposition of the [RhCl(PF3)(2)](2) molecule which is known as a precursor in electron beam induced deposition (EBID) of Rh. Mass spectrometry (MS) has been carried out to study the electron ionisation and fragmentation of isolated molecules. Auger electron spectroscopy has been used to characterize the EBID deposit. The MS data indicate the presence of free phosphorus and rhodium ions. This is in agreement with the analysis of the composition of the EBID deposit containing: 60% Rh, 12-25% P, 2-13% Cl, no F, 3-20% O and N. Theoretical calculations ( density functional theory) has been used to characterize the precursor molecule and to derive the enthalpies of several simple decomposition reactions. The calculated geometries are in a good agreement with the available X-ray crystallographic data. The [RhCl(PF3)(2)](2) appears not to be rigid: the PF3 groups can rotate with a relatively low barrier (0.6 kcal mol(-1)) whereas the barrier for the butterfly-like motion of (RhCl)(2) moiety is only 3.5 kcal mol(-1). According to the theoretical results, the lowest energy pathway of the decomposition corresponds to a consecutive loss of PF3 ligands, resulting in a (RhCl)(2) moiety (without phosphorus). The same conclusion is also valid for the ionised precursor. Experimental data combined with the theoretical results concerning the energetics of the considered various simple decomposition processes indicate that the electron induced dissociation of the precursor cannot be seen as a simple one-step decomposition process.
First author: Jokisaari, J, C-13-Se-77 and Se-77-Se-77 spin-spin coupling tensors in carbon diselenide: NMR experiments and ZORA DFT calculations, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 5, 4551, (2003)
Abstract: The isotropic average and anisotropy of the C-13- Se-77 and Se-77 – Se-77 spin-spin coupling tensors in carbon diselenide (CSe2) were calculated by applying relativistic density functional theory. Computed nuclear shielding anisotropies are also reported. The coupling tensor anisotropies were derived experimentally with the liquid crystal (LC) NMR method. Due to the fact that the number of adjustable parameters in the experimental analysis is greater than the number of anisotropic coupling constants, D-MN(exp), the ratio of the coupling tensor anisotropies, DeltaJ(CSe)/DeltaJ(SeSe), had to be fixed to the value obtained from the calculations. It appears that both tensor anisotropies are large, and in particular 1/2(SeSe)(Janiso) is a clearly dominating contribution in D-SeSe(exp). The remarkable 1/2(CSe)(Janiso) contribution in the D-CSe(exp) leads to a correction of the earlier reported shielding tensor anisotropies, Deltasigma(C) and Deltasigma(Se), determined by LCNMR.
First author: Aramburu, JA, Big off-center displacements of ions in insulators: The Jahn-Teller ion Ni+ in CaF2, PHYSICAL REVIEW B, 67, 4551, (2003)
Abstract: The big off-center displacement along <001> directions observed for Ni+ in CaF2 is explored through density functional theory (DFT) calculations on a NiF8Ca1217+ cluster, placing the Ni+ ion at different (0,0,z) points. For supporting the results additional calculations on clusters with a total of 51, 99, and 107 atoms have also been carried out. In a first step calculations have been performed for the (xy)(5/3)(xz)(5/3)(yz)(5/3) configuration where the three xy,xz, and yz antibonding crystal-field levels (degenerated in cubal geometry) are forced to have the same electronic occupancy. While the energy minimum for this configuration of NiF8Ca1217+ appears at z=0, it is found, however, at z(0)=1.07 Angstrom when the unpaired electron is fully located on the b(2) orbital transforming like xy. The z(0) values obtained for clusters with 51 (z(0)=1.13 Angstrom), 99 (z(0)=1.17 Angstrom) and 107 (z(0)=1.17 Angstrom) atoms outline the small dependence of the phenomenon on cluster size. This big off-center motion, which concurs with electron paramagnetic resonance data, is shown to arise mainly from a delicate balance between an electrostatic barrier, avoiding the impurity displacement and the energy gained through bonding on passing from eightfold to fourfold coordination. Following this analysis an increase of the lattice parameter and a diminution of ligand electronegativity both favor the off-center motion of d(9) ions in lattices with fluorite structure. First DFT calculations by means of a 21-atom cluster lead in fact to an off-center motion for Ag2+ and Cu2+ in SrCl2, while both ions would be on center in CaF2, these results being in agreement with experimental findings. At variance with the normal situation for on-center 3d impurities the energy minimum for the crystal-field excited state corresponding to the xy–>x(2)-y(2) transition is found at z=0, thus involving a huge displacement with respect to the ground state. To our knowledge this DFT study is the first one on off-center d(9) ions in CaF2-type lattices. The present results stress that the phenomenon though subtle can be understood with a modest computational effort.