2002 publications citing ADF
First author: Gosalvez, MA, Atomistic simulations of surface coverage effects in anisotropic wet chemical etching of crystalline silicon, APPLIED SURFACE SCIENCE, 202, 160, (2002)
Abstract: Atomistic simulations of anisotropic wet chemical etching of crystalline silicon have been performed in order to determine the dependence of the etch rates of different crystallographic orientations on Surface coverage and clustering of OH radicals. We show that the etch rate is a non-monotonic function of OH coverage and that there always exists a coverage value at which the etch rate reaches a maximum. The dependence of the anisotropy of the etching process on coverage. including the dependence of the fastest-etched plane orientation, is implicitly contained in the model and predictions of convex corner under-etching structures are made. We show that the whole etching process is controlled by only a few surface configurations involving a particular type of next-nearest neighbours. The relative value of the removal probabilities of these configurations determines the balance in the occurrence of step propagation and etch pitting for all surface orientations.
First author: Luo, Y, The adsorption of nitrogen oxides and water on rare-earth ion-exchanged ZSM-5: a density functional study, APPLIED SURFACE SCIENCE, 202, 283, (2002)
Abstract: In this study, we present the adsorption behavior of NO, NO2 and H2O on trivalent rare-earth ion-exchanged ZSM-5 (REZSM-5; RE = La, Ce. Nd, Sm, Gd and Dy) using density functional theory. The results show that Ce-ZSM-5 is more effective for the activation of NOx than La-ZSM-5 and Nd-ZSM-5, which is in good agreement with experimental results. The present investigation also suggests that Dy-ZSM-5 has a considerable ability for the activation of NO2 as compared to Sm-ZSM-5 and Gd-ZSM-5. Furthermore, the Ce-, Nd- and Dy-analogues posses a quite stronger affinity for NO,, and that the low affinity of H2O indicate the poisoning resistance ability of these catalysts. In addition, the relationship between the adsorption energy of NO and the contribution of NO-2pi(g)(1) molecular orbital was also investigated in the NO/RE-ZSM-5 adsorption complex.
First author: Steiner, RA, Anaerobic enzyme-substrate structures provide insight into the reaction mechanism of the copper-dependent quercetin 2,3-dioxygenase, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 99, 16625, (2002)
Abstract: Quercetin 2,3-dioxygenase (2,3QD) is the only firmly established copper dioxygenase known so far. Depending solely on a mononuclear Cu center, it catalyzes the breakage of the O-heterocycle of flavonols, producing more easily degradable phenolic carboxylic acid ester derivatives. In the enzymatic process, two C-C bonds are broken and concomitantly carbon monoxide is released, The x-ray structures of Aspergillus japonicus 2,3QD anaerobically complexed with the substrate kaempferol and the natural substrate quercetin have been determined at 1.90- and 1.75-Angstrom resolution, respectively. Flavonols coordinate to the copper ion as monodentate ligands through their 3OH group. They occupy a shallow and overall hydrophobic cavity proximal to the metal center. As a result of a van der Waals contact between the most outward flavonol A-ring and Pro(164), a flexible loop in front of the active site becomes partly ordered. Interestingly, flavonols bound to 2,3QD are bent at the C2 atom, which is pyramidalized. The increased local sp(3) character at this atom may stabilize a carbon-centered radical activated for dioxygen attack. Glu(73) coordinates the copper through its Oepsilon2 atom. The short distance of about 2.55 Angstrom between its Oepsilon2 atom and the flavonol O3 atom suggests that a hydrogen bond exists between the two atoms, indicating that Glu(73) can act as a base in flavonol deprotonation and that it retains the proton. Structure-based geometric considerations indicate O(2) binding to the flavonol C2 atom as the preferred route for flavonol dioxygenation.
First author: Nemcsok, DS, Nature of the transition-metal-eta(2)-H-2 bond in TM(CO)(3)X-2 (eta(2)-H-2) (TM = Cr, Mo, W; X = CO, PH3, PCl3, PMe3) complexes, ORGANOMETALLICS, 21, 5803, (2002)
Abstract: Quantum-chemical calculations at the gradient-corrected DFT level using BP86 of the structures and bond dissociation energies of the title compounds are reported. The nature of the metal-H-2 bonding is quantitatively analyzed with an energy decomposition method. It is found that the metal-H-2 bonds in TM(CO)(5)H-2 are similar to47% covalent and similar to53% electrostatic. Two-thirds of the covalent interactions come from TM <– sigma(H-2) sigma donation, while one-third is due to TM –>*(H-2) pi back-donation. Substitution of two CO ligands of the tungsten complex cis to H-2 by PR3 (R = H, Cl, Me) leads to stronger W-H-2 interactions. The electrostatic and covalent bonding both increase in the PR3-substituted species. Inspection of the covalent term shows that the W–>*(H-2) pi back-donation becomes stronger when CO is substituted by PR3. The change of the DeltaE(pi) values follows closely the change of the total interaction energy DeltaE(int). The changes in DeltaE(int) upon rotating the H-2 ligand in W(CO)(3)(PR3)(2)H-2 also correlate quantitatively with the strength of the W–>sigma*(H-2) pi back-donation. Thus, the changes in the metal-H-2 bonding situation which are invoked by different substituents and by rotation of the H-2 ligand can be explained with then contribution of the covalent bonding.
First author: Grozema, FC, Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: Effects of chain length and alkoxy substitution, JOURNAL OF CHEMICAL PHYSICS, 117, 11366, (2002)
Abstract: In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene-vinylene oligomers (PV’s) is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm (0.9 to 2.5 eV). The geometries of the PV’s studied were optimized using density functional theory (DFT) for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites.
First author: Saito, J, Propylene polymerization with bis(phenoxy-imine) group-4 catalysts using (Bu3Al)-Bu-i/Ph3CB(C6F5)(4) as a cocatalyst, MACROMOLECULAR RAPID COMMUNICATIONS, 23, 1118, (2002)
Abstract: The catalytic behaviour of three bis(phenoxy) group-4 transition-metal complexes (M=Ti, Zr, Hf), with (Bu3AlCB)-Bu-i(C6F5)(4) cocatalyst systems towards propylene polymerization was investigated under atmospheric pressure at 25degreesC. The Ti complex produced ultrahigh-molecular-weightatactic poly(propylene), whereas Zr and Hf complexes formed high-molecular-weight isotactic poly(propylene)s via a site-control mechanism. The isotactic poly(propylene) obtained with the Hf complex displayed a high melting temperature of 123.8degreesC.
First author: Bridgeman, AJ, The electronic structure and stability of the isomers of octamolybdate, JOURNAL OF PHYSICAL CHEMISTRY A, 106, 12151, (2002)
Abstract: The structure of the gamma-, delta-, is an element of-, and xi-isomers and the bonding in the alpha-, beta-, gamma-, delta-, is an element of-, and xi-isomers of [Mo8O26](4-) isopolyanions have been calculated using density functional theory. The optimized structures are in reasonably good agreement with those determined experimentally with the exception of the gamma-form. For this isomer, the optimization leads to a lengthening of an internal bond, and as a result, the topology of this isomer becomes identical to that of the xi-form. The electronic structure and relative stability have been probed using a bond order and valency analysis and through a decomposition of the bonding energy. The terminal Mo-O bonds possess fractional multiple bond character with similar values for those attached to four, five, and six coordinate metal atoms. The Mo-O bond order decreases as the coordination number of the oxygen increases, and a number of pseudoterminal oxygen sites have been located. The bond order analysis appears to confirm the coordination numbers of the metal atoms in the delta- and xi-isomers but suggests that additional contacts should be considered for one of the terminal oxygen atoms in the c-isomer, leading to a topology intermediate between that of the beta-isomers and of the previously predicted (beta-gamma) intermediate. Despite the range of coordination numbers, charges, and bond orders, the overall bonding capacity of the oxygen atoms, measured through the full valency index, appears similar. The alpha- and delta-isomers are predicted to be the most intrinsically stable while the beta-form is the least stable. The relative stability of the isomers is due to a balance between steric interactions and favorable atomic interactions, both of which correlate with the number of Mo-O bonds. Although the alpha- and delta-isomers possess relatively few bonds, their open structures lead to low steric crowding. The compact structure of the beta-form leads to highly unfavorable steric interactions.
First author: Cooper, J, A density functional study of S(N)2 substitution at square-planar platinum(II) complexes,INORGANIC CHEMISTRY, 41, 6614, (2002)
Abstract: The energetics and reaction path in a series of S(N)2 substitution reactions at square-planar Pt(II) complexes have been studied by the application of density functional theory (DFT). Calculated free energies show excellent correlation with their experimental counterparts, while the enthalpic and entropic contributions individually indicate the presence of weak intermolecular interactions not accounted for in the present model, The nature of the leaving ligand has been shown to be much more significant in determining the activation barrier than that of the entering ligand; it is inferred (and confirmed by analysis of individual bond energies) that the reaction is driven by the dissociation of the leaving ligand, with the entering ligand playing a more passive role. Analysis of the intrinsic reaction coordinate indicates, further, that the trans ligand plays an unexpectedly dynamic role in stabilizing the transition state due to competition between stabilization and the steric effects of the entering and leaving ligands, The cis ligands, by contrast, are shown to move only slightly through the course of the reaction.
First author: Loh, ZH, Excited-state dynamics of cofacial pacman porphyrins, JOURNAL OF PHYSICAL CHEMISTRY A,106, 11700, (2002)
Abstract: Cofacial palladium(II) bisporphyrins anchored by xanthene [Pd-2(DPX)] and dibenzofuran [Pd-2(DPD)] pillars were prepared, and the dynamics of their triplet excited states were investigated. The phosphorescence quantum yield [(Phip = 4.59(121) x 10(-3) in CH2Cl2] and lifetime [tau(p) = 18.2(2) mus in CH2Cl2] of Pd-2(DPD) are significantly attenuated compared to those of its Pd2(DPX) congener [(Phi(p) = 29.4(5) x 10(-3) and tau(p) 102(3) mus in CH2-Cl-2]. Electronic absorption and emission spectroscopy and electrochemical measurements establish that the porphyrin rings of the DPX cofacial analogue are closer to each other than the porphyrin rings of the DPD analogue in fluid solution. These observations are supported by X-ray crystallographic analyses, which show that the metal-metal distance for the dibenzofuran-bridged bisporphyrin in the solid state is almost 3 A greater than that for bisporphyrins linked by the xanthene spacer. The spectroscopic, photophysical, and structural results suggest that the decreased (Phi(p) and tau(p) of Pd-2(DPD) are not a consequence of simple interplanar interactions but rather arise from the increased conformational flexibility of the porphyrin rings about the aryl ring of the DPD pillar. In support of this contention, the photophysical properties of monomeric palladium etioporphyrin I [Pd(Etio)] and meso-phenyl-substituted palladium etioporphyrin II [Pd(PhEtio)] were examined and found to be similar to those of Pd-2(DPX) and Pd-2(DPD), respectively. Density functional theory calculations reveal that torsional motion about the C(meso)-C(aryl bridge) bond induces nonplanar distortion of the porphyrin framework, causing a substantial decrease in the T-1 -So energy gap. This structural perturbation is consistent with the observed results of enhanced deactivation of Pd-2(DPD) and Pd(PhEtio) triplet excited states.
First author: Thybaut, JW, Hydrogenation kinetics of toluene on Pt/ZSM-22, CHEMICAL ENGINEERING JOURNAL, 90, 117, (2002)
Abstract: Kinetic experiments on the hydrogenation of toluene were performed on 0.5 wt.% Pt/ZSM-22 at temperatures in the range 423-498 K, H(2) inlet partial pressures of 100-300 kPa and toluene inlet partial pressures of 10-60 kPa. Construction of a kinetic model was based on a critical evaluation of available literature data on the hydrogenation of aromatic components together with physicochemical studies on the interaction of aromatic components and related hydrogenated products with metal surfaces as well as on quantumchemical calculations. This lead to a general kinetic model, analogous to the Horiuti Polanyi mechanism for ethylene hydrogenation, with the first four H atom addition steps not in quasi-equilibrium. Chemisorption of H(2) and toluene was assumed to occur on identical sites. No dehydrogenated surface species was taken into account. The preexponential factors were calculated using transition state theory. A model with equal surface reaction rate coefficients for the H addition steps was selected as the best model. The estimated toluene and H(2) chemisorption enthalpies amounted to -70 and -42 kJ mol(-1). An activation energy in the range of 40-50 kJ mol(-1) was found. Under typical reaction conditions, 60% of the surface is covered by toluene and 20% by H atoms. The remaining 20% are free. Negligible amounts of partially hydrogenated species were found to be present on the catalyst surface.
First author: Zhang, Y, Mossbauer quadrupole splittings and electronic structure in heme proteins and model systems: A density functional theory investigation, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 13921, (2002)
Abstract: We report the results of a series of density functional theory (DFT) calculations aimed at predicting the Fe-57 Mossbauer electric field gradient (EFG) tensors (quadrupole splittings and asymmetry parameters) and their orientations in S = 0, 1/2, 1, 3/2, 2, and 5/2 metalloproteins and/or model systems. Excellent results were found by using a Wachter’s all electron basis set for iron, 6-311G* for other heavy atoms, and 6-31G* for hydrogen atoms, BPW91 and B3LYP exchange-correlation functionals, and spin-unrestricted methods for the paramagnetic systems. For the theory versus experiment correlation, we found R-2 = 0.975, slope = 0.99, intercept = -0.08 mm sec(-1), rmsd = 0.30 mm sec(-1) (N = 23 points) covering a DeltaE(Q) range of 5.63 mm s(-1) when using the BPW91 functional and R-2 = 0.978, slope = 1.12, intercept = -0.26 mm sec(-1), rmsd = 0.31 mm sec(-1) when using the B3LYP functional. DeltaE(Q) values in the following systems were successfully predicted: (1) ferric low-spin (S = 1/2) systems, including one iron porphyrin with the usual (d(xy))(2)(d(xy)d(yz))(3) electronic configuration and two iron porphyrins with the more unusual (d(xz)d(yz))(4)(d(xy))(1) electronic configuration; (2) ferrous NO-heme model compounds (S = 1/2); (3) ferrous intermediate spin (S = 1) tetraphenylporphinato iron(II); (4) a ferric intermediate spin (S = 3/2) iron porphyrin; (5) ferrous high-spin (S = 2) deoxymyoglobin and deoxyhemoglobin; and (6) ferric high spin (S = 5/2) metmyoglobin plus two five-coordinate and one six-coordinate iron porphyrins. In addition, seven diamagnetic (S = 0, d(6) and d(8)) systems studied previously were reinvestigated using the same functionals and basis set scheme as used for the paramagnetic systems. All computed asymmetry parameters were found to be in good agreement with the available experimental data as were the electric field gradient tensor orientations. In addition, we investigated the electronic structures of several systems, including the (dxy)(2) (d(xz),d(yz))(3) and (d(xz),d(yz))(4)(d(xy))(1) [Fe(III)/porphyrinate](+) cations as well as the NO adduct of Fe(II)(octaethylporphinate), where interesting information on the spin density distributions can be readily obtained from the computed wave functions.
First author: Arbuznikov, AV, Validation study of meta-GGA functionals and of a model exchange-correlation potential in density functional calculations of EPR parameters, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 4, 5467, (2002)
Abstract: We report the first implementation of the DFT calculation of electronic g-tensors and hyperfine coupling constants using meta-GGA exchange-correlation functionals (both Laplacian and kinetic-energy-density dependent) and statistical average of orbital model exchange-correlation potentials (SAOP). The g-tensors for a set of eleven small main group radicals and for a series of ten 3d and two 4d transition metal complexes have been compared using two meta-GGA functionals (FT98 and PKZB), SAOP, as well as the BP86 GGA, and B3PW91 and BHPW91 hybrid functionals. The same functionals have been compared in the calculation of hyperfine coupling constants for thirteen small main group radicals and twelve 3d transition metal compounds. The results for main group radicals are satisfactory, both with the newly validated and previously tested functionals. In contrast, the accurate evaluation of the EPR parameters for 3d transition metal complexes remains a challenge for “pure” density functional theory: the new functionals (potentials) do not improve the results compared to GGA functionals, whereas hybrid functionals tend to provide superior results. SAOP leads to large spin contamination for a number of transition metal complexes. This has been traced back to the orbital-energy dependence of the potential.
First author: Gosalvez, MA, Multiscale modeling of anisotropic wet chemical etching of crystalline silicon, EUROPHYSICS LETTERS, 60, 467, (2002)
Abstract: We combine ab initio and Monte Carlo simulations in multiscale modelling of anisotropic wet chemical etching of silicon. The anisotropy of the macroscopic etching patterns observed in the experiments is explained by two mechanisms at an atomistic scale: the weakening of backbonds following OH termination of surface sites and the existence of significant interaction between the surface-terminating species (H and OH). For the first time, we demonstrate that the H/OH and OH/OH interactions have an essential role, directly controlling the appearance of the fastest-etched planes in the macroscopic etching patterns.
First author: Hieringer, W, Response properties of furan homologues by time-dependent density functional theory,JOURNAL OF PHYSICAL CHEMISTRY A, 106, 10380, (2002)
Abstract: The electronic excitations and frequency-dependent electronic second hyperpolarizability gamma(-omega;omega,omega,-omega) of the five-ring heterocycles furan, thiophene, selenophene, and tellurophene have been reinvestigated using time-dependent density functional theory. Aspects of basis set saturation, the performance of exchange-correlation potentials, and relativistic effects are discussed. Increased hyperpolarizabilities for molecular dimer species suggest that intermolecular interactions may provide a simple explanation for the large deviations between recent ab initio calculations and experimental condensed-phase data.
First author: Rayon, VM, Structures, bond energies, heats of formation, and quantitative bonding analysis of main-group metallocenes [E(Cp)(2)] (E=Be-Ba, Zn, Si-Pb) and [E(Cp)] (E=Li-Cs, B-Tl), CHEMISTRY-A EUROPEAN JOURNAL, 8, 4693, (2002)
Abstract: The geometries, metal-ligand bond dissociation energies, and heats of formation of twenty sandwich and half-sandwich complexes of the main-group elements of Groups 1, 2, 13, and 14, and Zn have been calculated with quantum chemical methods. The geometries of the [E(Cp)] and [E(Cp)(2)] complexes were optimized using density functional theory at the BP86 level with valence basis sets, which have DZP and TZP quality. Improved energy values have been obtained by using coupled-cluster theory at the CCSD(T) level. The nature of the metal – ligand bonding has been analyzed with an energy-partitioning method. The results give quantitative information about the strength of the covalent and electrostatic interactions between En+ and (Cp-)(n) (n = 1, 2). The contributions of the orbitals with different symmetry to the covalent bonding are also given.
First author: Deubel, DV, Orbital symmetry as a tool for understanding the bonding in Krossing’s cation, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 12312, (2002)
Abstract: The geometric and electronic structure of Krossing’s cation, Ag(eta(2)-P-4)(2)(+), which shows an unexpected planar coordination environment at the metal center and D-2h symmetry both in solution and in the solid state, have been investigated using density functional theory and orbital-symmetry-based energy decomposition. The analysis reveals that the contribution from electrostatic interactions to the bond energy is greater than that of orbital interactions. Partitioning of the latter term into the irreducible representations shows that, in addition to the 5s orbital, 5p orbitals of silver act as acceptor orbitals for electron donation from sigma(P-P) orbitals (a(1g), b(1u)) and n(P) orbitals (b(3u)). Back-donation from the 4d(10) closed shell of Ag into sigma* orbitals of the pnictogen cages (b(2g)) is also important. However, this contribution is shown not to determine the D-2h structure, contradicting conclusions from the pioneering study of the title cation (J. Am. Chem. Soc. 2001, 123, 4603). The contributions from the irreducible representations to the stabilizing orbital interactions in the D-2h structure and in its D-2sigma-symmetric conformer are analogous, indicating that the planar coordination environment at the metal center in Ag(eta(2)-P-4)(2)(+) is induced by intermolecular rather than by intramolecular interactions. Because ethylene coordination to a metal ion is an elementary reaction step in industrial processes, the bonding in Ag(C2H4)(2)(+) has been analyzed as well and compared to that in Krossing’s cation. Surprisingly, similar contributions to the bond energies and an involvement of metal 4d and 5p orbitals have been found, whereas a recent atoms in molecules analysis suggested that the metal-ligand interactions in silver(I) olefin complexes fundamentally differ from those in tetrahedro P-4 complexes. The only qualitative difference between the bonding patterns in Ag(eta(2)- P-4)(2)(+) and Ag(C2H4)(2)(+) is the negligible energy contribution from the b(3u) irreducible representation in the ethylene complex because a respective symmetry-adapted linear combination of ligand orbitals is not available.
First author: Deubel, DV, Unified view of the interaction of tetrakis(carboxylato)dirhodium(II) with axial donor-acceptor ligands, ORGANOMETALLICS, 21, 4303, (2002)
Abstract: Density functional calculations rationalize the bonding in [Rh-2(mu-O2CR)(4)L] complexes with strong and very weak axial donor-acceptor ligands L such as “Arduengo” carbenes and aromatic hydrocarbons.
First author: Deubel, DV, Challenge of the copolymerization of olefins with N-containing polar monomers. Systematic screening of nickel(II) and palladium(II) catalysts with Brookhart and Grubbs ligands. 2. Chain-propagation barriers, intrinsic regioselectivity, and Curtin-Hammett reactivity, ORGANOMETALLICS, 21, 4432, (2002)
Abstract: The second level of a computational screening of late-transition-metal catalysts and nitrogen-containing polar monomers toward an incorporation of amines and nitriles in the polymer chain of polyolefins is reported. The structures and energies of the transition states for the insertion of the C=C bond of ethylene, propylene, acrylonitrile, and vinylamine into the metal-carbon bond of generic models for N(II) and Pd(II) complexes with diimine (Brookhart) and salicylaldiminato (Grubbs) ligands have been calculated using density functional theory. The calculations reveal the general trend that the activation energies for the ethylene, propylene, and acrylonitrile insertion in the Brookhart systems are similar, whereas the activation energies for the vinylamine insertion are much higher. The nickel systems show lower insertion barriers than do their palladium counterparts. For the chain propagation with the Grubbs catalysts, a Curtin-Hammett-type energy profile involving cis-trans isomerization and subsequent C-C insertion is predicted. The regioselectivity of the propylene, acrylonitrile, and vinylamine insertion is rationalized by the analysis of the frontier orbitals of the free monomers.
First author: Reynisson, J, Photophysics of trioxatriangulenium ion. Electrophilic reactivity in the ground state and excited singlet state, PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES, 1, 763, (2002)
Abstract: Trioxatriangulenium (TOTA(+), 4,8,12-trioxa-4,8,12,]2c-tetrahydro-dibenzo[cd,mn]-pyrenylium) is a closed shell carbenium ion, which is stable in non-nucleophilic polar solvents at ambient temperatures. In alcohols, small quantities of the leuco ether are formed in a reversible reaction. The physical and chemical properties of the excited singlet state of the trioxatriangulenium (TOTA(+)) carbenium ion are investigated by experimental and Computational means. The degeneracy of the lowest excited states is counteracted by Jahn-Teller-type distortion, which leads to vibronic broadening of the long wavelength absorption band. A strong fluorescence is observed at 520 nm (tau(n) = 14.6 ns, phi(n) = 0.12 in deaerated acetonitrile). The fluorescence is quenched by 10 aromatic electron donors predominantly via a dynamic charge transfer mechanism, but ground state complexation is shown to contribute in varying degrees. Quenching is also observed in the presence of halide ions. Quenching rate constants are derived from lifetime measurements while charge transfer (CT) complex formation constants follow from the steady-state Stern-Volmer plots. CT-complex formation with three discogenic triphenylenes is studied separately. Phosphorescence spectra, triplet lifetimes, and triplet-triplet absorption spectra are provided. In the discussion, TOTA(+) is compared to the unsubstituted xanthenium ion and its 9-phenyl derivative with respect to the excited state properties.
First author: Cases, M, Molecular structure and bond characterization of the Fischer-type chromium-carbene complexes (CO)(5)Cr = C(X)R (X = H, OH, OCH3, NH2, NHCH(3)and R = H, CH3, CH = CH2, Ph, C CH), ORGANOMETALLICS, 21, 4182, (2002)
Abstract: In this work we have examined a series of 25 different Fischer carbene complexes of the type (CO)(5)Cr = (C)X)R with X = H, OH, OCH3, NH2, and NHCH3 and R = H, CH3, CH = CH2, Ph, and C equivalent to CH, to analyze the influence of each substituent on the molecular structure and bonding of these complexes. Energy and charge decomposition analysis of the interaction between the chromium pentacarbonyl and the carbene fragments reveal that for the metalcarbene bond donation is quantitatively more important than back-donation. However, it is the back-donation rather than donation that correlates with most geometrical and electronic parameters of the complexes studied. The reasons for the larger influence of back-donation on the molecular structure of Fischer carbene complexes are discussed.
First author: Seth, M, A density functional embedded cluster study of proposed active sites in heterogeneous Ziegler-Natta catalysts, MACROMOLECULES, 35, 7815, (2002)
Abstract: A number of proposed models of the active sites in TiCl4/MgCl2 heterogeneous Ziegler-Natta catalysts are examined using density functional methods. Using a number of different models for the surface, the sites formed when unreduced TiCl4 is adsorbed onto MgCl2 are predicted to be unstable. In contrast, TiCl3 and TiCl2,are found to bind strongly to MgCl2. Alternative models of the surface where one or all of the Mg atoms is replaced with Ti(II) give larger binding energies for TiCl4. Each step of the ethylene polymerization reaction is considered for each of the site models that are expected to be active. Two possible termination mechanisms, chain transfer to the monomer and beta-hydrogen elimination, are also examined. The sites formed from TiCl4 bind ethylene more weakly than those formed from TiCl3. No particular trends with respect to the type of site are found for the barrier to insertion, but the direction of approach of the ethylene molecule is important. The beta-hydrogen elimination termination mechanism is predicted to be much less important than chain transfer to the monomer. From a comparison of ethylene insertion and termination barriers, the TiCl3-based sites on the TiCl2 surface and the TiCl3-based Corradini site on MgCl2 are shown to be very poor catalysts while the others, in particular the TiCl3-based edge site on MgCl2, appear to be moire promising models of the actual active sites.
First author: Bridgeman, AJ, Bonding in [W4O16](8-) isopolyanions, POLYHEDRON, 21, 2201, (2002)
Abstract: The electronic structures of the T-d and C-2h forms of the [W4O16](8-) polyanion have been investigated using density-functional methods. The C-2h isomer has been found to be more stable than the T,1 isomer, primarily due to somewhat stronger W-O orbital interactions. Various analyses based on molecular-orbital and bonding-energy methods have indicated that these W-O interactions are predominantly of Wd-Op character. Delocalised bonds with a closed-loop structure involving bridging-oxygen atoms have been observed in both isomers.
First author: Atanasov, M, The unique spectroscopic behavior of the Fe(III)-nitroprusside: a DFT study of the vibronic coupling in the ground and in the lowest excited state, JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 592, 79, (2002)
Abstract: The adiabatic potential energy surface of the ground state (GS-APES) of the Fe-III-nitroprusside anion [Fe(CN)(5)NO](2-) (Fe-NTP) has been studied by means of density functional theory (DFT). A decomposition of the total bonding energy into orbital interaction, electrostatic bonding and exchange (Pauli) repulsion terms (extended transition state method, ETS) has been carried out and the chemical bonding at all stationary points of the ground state potential surface (minima and saddle points) has been characterized. On this basis, the unique topology of the electronic ground state with minima for linear Fe-N-O (ground state, GS) and Fe-O-N (metastable state, MS1) and side-on Fe-(NO) bonded (metastable state, MS2) has been rationalized. The potential energy surface of the lowest excited [3d –> pi*(NO)] electronic state (ES-APES) with minima at non-linear Fe-N-O and Fe-O-N arrangements has been calculated starting from higher symmetric (C-4v) structures (linear Fe-N-O and Fe-O-N bond) utilizing a E-1 Jahn-Teller coupling mechanism. We found that after geometrical relaxation, the ES-APES crosses the GS-APES in the vicinity of the transition states, thus affecting the photo-isomerisation process. In the light of these results a combined ground and excited state configurational energy diagram is presented and used to discuss the mechanism of photo-isomerisation.
First author: Wu, KC, Reinvestigation of hydrogen bond effects on the polarizability and hyperpolarizability of urea molecular clusters, JOURNAL OF PHYSICAL CHEMISTRY B, 106, 8954, (2002)
Abstract: The oriented-gas model based on the additivity hypothesis is widely used in predicting the macroscopic nonlinear optical susceptibility of a molecular crystal from molecular (hyper)polarizability calculations. Here, we argue that the intermolecular hydrogen bond interactions will break the additivity relationship for the first hyperpolarizability of urea hydrogen-bonded clusters on the basis of our high-level ab initio studies. The calculations at the HF level without electron correlation correction result in the underestimation of the hydrogen bond effect on the first hyperpolarizabilities to about 15% in the urea case. On the other hand, both HF and MP2 results show the additivity of linear polarizability of urea clusters. We hope that the study will be helpful to the areas of molecular design and simulation of novel nonlinear optical materials.
First author: Arieli, D, The process of Mn(II) incorporation into aluminophosphate zeotypes through high-field ENDOR spectroscopy and DFT calculations, JOURNAL OF PHYSICAL CHEMISTRY B, 106, 9086, (2002)
Abstract: High-field EPR and pulsed electron-nuclear double resonance (ENDOR) spectroscopies were used to investigate the formation of Mn-AlPO4-11, Mn-AlPO4-5, and Mn-SAPO-5. Samples recovered from reaction mixtures quenched at different times were subjected to EPR, ENDOR and X-ray diffraction (XRD) measurements, and the variations in the (31)p and H-1 hyperfine couplings, which are sensitive probes to the Mn-P interaction and the Mn(II) hydration, respectively, were followed. The intensity of the 1H ENDOR signal decreased with reaction time, showing that the amount of both water ligands and solvent water in the Mn(II) vicinity decreased. A relatively large isotropic P-31 hyperfine coupling (A(iso)((31)p) approximate to 7 MHz), confirming the formation of Mn(II) framework sites, was found in all final products, whereas a smaller A(iso)(P-31), 4-5 MHz, was detected in samples quenched at early stages of the reaction. The latter was assigned to Mn(II) incorporated into a network of disordered aluminophosphate precursors. These precursors are formed prior to the detection of an XRD pattern, and are gradually transformed to the final three-dimensional crystalline structures. The changes in A(iso)(P-31) were attributed to transformations occurring both in the bonding topology and in the coordination sphere of Mn(II), where water ligands are gradually replaced by -O-P linkages. This interpretation was supported by the decrease in the intensity of the H-1 ENDOR signals, and by a series of DFT cluster model optimizations on intermediates of the form [Mn(H2O)x(OP(OH)(3))y](2+), where x + y = 6, 5 or 4, followed by calculations of hyperfine coupling constants. Although the theoretical hyperfine values were overestimated with respect to the experimental ones, a satisfactory correlation was found between the trends within the calculated A(iso)(Mn-55,(31)p), and the experimental trends observed during the molecular sieves formation.
First author: Filatov, M, A variationally stable quasi-relativistic method: low-order approximation to the normalized elimination of the small component using an effective potential, THEORETICAL CHEMISTRY ACCOUNTS, 108, 168, (2002)
Abstract: A simple and variationally stable quasi-relativistic method based on a modified low-order (LO) approximation to the normalized elimination of the small component (NESC) method is presented. The modification of the original LO-NESC scheme implies the use of an energy-independent factor in the relativistic correction to the potential energy. This factor cuts off the potential energy at short distances from the nucleus and in this way restores the variational stability of LO-NESC. The new method, dubbed LO-NESC-effective potential (EP) was tested in calculations on one-, two- and many-electron atoms. The LO-NESC-EP can be easily implemented into the existing nonrelativistic quantum-chemical program codes because its Hamiltonian matrix can be expressed entirely in terms of the integrals appearing in a nonrelativistic calculation.
First author: Asthagiri, D, Density functional study of the mechanism of a tyrosine phosphatase: 1. Intermediate formation,JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 10225, (2002)
Abstract: The first step in the catalytic mechanism of a protein tyrosine phosphatase, the transfer of a phosphate group from the phosphotyrosine substrate to a cysteine side chain of the protein to form a phosphoenzyme intermediate, has been studied by combining density functional calculations of an active-site cluster with continuum electrostatic descriptions of the solvent and the remainder of the protein. This approach provides the high level of quantum chemical methodology needed to adequately model phosphotransfer reactions with a reasonable description of the environment around the active site. Energy barriers and geometries along a reaction pathway are calculated. In the literature, mechanisms assuming both a monoanionic and a dianionic substrate have been proposed; this disagreement is addressed by performing calculations for both possibilities. For the dianionic substrate, a dissociative reaction pathway with early proton transfer to the leaving group and a 9 kcal/mol energy barrier is predicted (the experimental estimate is ca. 14 kcal/mol), while for the monoanionic substrate, an associative pathway with late proton transfer and a 22 kcal/mol energy barrier is predicted. These results, together with a review of experimental evidence, support the dianionic-substrate/dissociative-pathway alternative. The relationship between a dianionic or monoanionic substrate and a dissociative or associative pathway, respectively, can be understood in terms of classical organic chemical reaction pathways.
First author: Frunzke, J, Structures, metal-ligand bond strenght, and bonding analysis of ferrocene derivatives with group-15 heteroligands Fe(eta(5)-E-5)(2) and FeCp(eta(5)-E-5) (E = N, P, As, Sb). A theoretical study, ORGANOMETALLICS,21, 3351, (2002)
Abstract: Quantum chemical DFT calculations using B3LYP and BP86 functionals have been carried out for the title compounds. The equilibrium geometries and bond dissociation energies are reported. The metal-ligan bonding was analyzed with an energy partitioning method. The strongest bonded homoleptic complex with a heterocyclic ligand is Fe(eta(5)-P-5)(2). The bond dissociation energy yielding the Fe atom and two cyclo-P-5 ligands (D-o = 128.3 kcal/mol) is nearly the same as for ferrocene (D-o = 131.3 kcal/mol). The nitrogen, arsenic, and antimony analogues of Fe(eta(5)-E-5)(2) have significantly weaker metal-ligand bonds, which, however should still be strong enough to make them isolable under appropriate conditions. The calculated heats of formation show also that the phosphorus complex is the most stable species of the heterocyclic Fe(eta(5)-E-5)(2) series. The Fe-(eta(5)-E-5) bonding in the mixed sandwich complexes. FeCp(eta(5)-E-5) is much stronger compared to the homoleptic molecules. The heterocyclic ligands cyclo-E-5 in the mixed complexes FeCp(eta(5)-E-5) bind as strongly or in case of phosphorus even stronger that one Cp ligand does in FeCp2 except for E=Sb. The metal fragments Fe(eta(5)-E-5)(+) have a pyramidal geometry except for E = Sb, which is predicted to be a planar ion with D-5h symmetry. The energy partitioning shows that the binding interactions between the closed shell cyclo-E-5(-) ligand and the Fe(eta(5)-E-5)(+) fragment do not change very much for the different ligand atoms E in the homoleptic and heteroleptic complexes. The bonding comes from 53%-58% electrostatic attraction; while 42%-47% come from covalent interactions. The latter contribution comes mainly from the donation of the occupied e(1)(pi) orbital of the ligand into empty orbital of the metal fragment.
First author: Saeys, M, Density functional study of benzene adsorption on Pt(111), JOURNAL OF PHYSICAL CHEMISTRY B, 106, 7489, (2002)
Abstract: The adsorption of benzene on Pt(111) was analyzed using first-principles density functional theoretical cluster and periodic slab calculations. The preferred adsorption site at low coverage is the bridge(30) site with an adsorption energy of 117 kJ/mol. At the bridge(30) site, two of the C p(z) orbitals are well aligned for overlap with the metal d(z)(2) and d(yz) orbitals, leading to a strong C-Pt bond and a strong adsorption energy. The molecule’s second important site is the hollow(0) site with anadsorption energy of 75 kJ/mol. Comparing calculated and experimental vibrational frequencies confirms the preference,for the bridge site at low coverage and also indicates that adsorption at the hollow(0) site becomes preferred at higher coverage. Adsorption at the hollow(30), the bridge(0) and at the atop sites was found to be unfavorable.
First author: Arieli, D, Isomorphous substitution of Mn(II) into aluminophosphate zeotypes: A combined high-field ENDOR and DFT study, JOURNAL OF PHYSICAL CHEMISTRY B, 106, 7509, (2002)
Abstract: The incorporation of low levels of Mn(II) (Mn/Alsimilar to0.001) into five aluminophosphate zeotypes was studied by high-field echo-detected EPR, and by P-31 and H-1 electron-nuclear double resonance (ENDOR) spectroscopies. The zeotype structures investigated-SOD, AEL; AFI, SBS, and SBT-cover. a variety of channel morphologies, and span a range of framework densities. The highly resolved EPR spectra could distinguish between two types of Mn with different 55 Mn hyperfine couplings in structures containing more than one T site. Mims and Davies P-31 ENDOR spectra, recorded at a field set to one of the \-1/2, m(l)> –> \+1/2, M-l> Mn-55 hyperfine components consist of a symmetric doublet, with a splitting in the range of 5-8 MHz. The large open structures showed smaller couplings than the denser morphologies. A similar IT hyperfine was also detected for Fe(III) incorporated into aluminophosphate zeotype with the SOD structure. Variations in the H-1 ENDOR spectra of the various Mn(II) substituted zeotypes, particularly in the relative intensity of the H-1 matrix line, were detected as well. These ENDOR results indicate a common mechanism of framework substitution in which Mn(II) and Fe(III) are replacing Al (or Mg). Moreover, the spectra serve as a probe for the differences in the local environment and bonding topology of these substituted framework sites. A qualitative interpretation of the P-31 ENDOR data is provided, based on, relevant crystallographic information, and the H-1 ENDOR signals are partially attributed to the interactions with the templates occluded in the zeotype cages. To further relate the isotropic P-31 hyperfine couplings to structural properties, DFT methods were employed for cluster model optimizations and hyperfine coupling constants calculations. Geometry optimizations of substituted rings, derived from the SOD and AEL framework structures, indicate considerable distortions of the coordination environment of framework Mn as compared to Al. A systematic study of the hyperfine interactions of a series of model structures containing tetrahedral and octahedral Mn(II) show that both Mn-O-bond lengths and Mn-O-P bond angles contribute significantly to the variation in the isotropic and anisotropic P-31 hyperfine coupling.
First author: Ishii, S, A bis(phenoxy-imine)Zr complex for ultrahigh-molecular-weight amorphous ethylene/propylene copolymer, MACROMOLECULAR RAPID COMMUNICATIONS, 23, 693, (2002)
Abstract: A new bis(phenoxy-imine)Zr complex has been developed. This complex in conjunction with (Bu3Al)-Bu-t/Ph3CB(C6F5)(4) at 70degreesC produces ultrahigh-molecular-weight amorphous ethylene/propylene copolymer with a weight-average molecular weight of 10200000 g/mol versus polystyrene standards, which represents the highest molecular weight known for linear, synthetic copolymers to date.
First author: Groenhof, G, Signal transduction in the photoactive yellow protein. I. Photon absorption and the isomerization of the chromophore, PROTEINS-STRUCTURE FUNCTION AND BIOINFORMATICS, 48, 202, (2002)
Abstract: Molecular dynamics simulation techniques together with time-dependent density functional theory calculations have been used to investigate the effect of photon absorption by a 4-hydroxy-cinnamic acid chromophore on the structural properties of the photoactive yellow protein (PYP) from Ectothiorodospira halophila. The calculations suggest that the protein not only modifies the absorption spectrum of the chromophore but also regulates the subsequent isomerization of the chromophore by stabilizing the isomerization transition state. Although signaling from PYP is thought to involve partial unfolding of the protein, the mechanical effects accompanying isomerization do not appear to directly destabilize the protein.
First author: Groenhof, G, Signal transduction in the photoactive yellow protein. II. Proton transfer initiates conformational changes, PROTEINS-STRUCTURE FUNCTION AND BIOINFORMATICS, 48, 212, (2002)
Abstract: Molecular dynamics simulation techniques, together with semiempirical PM3 calculations, have been used to investigate the effect of photoisomerization of the 4-hydroxy-cinnamic acid chromophore on the structural properties of the photoactive yellow protein (PYP) from Ectothiorodospira halophila. In this bacteria, exposure to blue light leads to a negative photoactic response. The calculations suggest that the isomerization does not directly destabilize the protein. However, because of the isomerization, a proton transfer from a glutamic acid residue (Glu(46)) to the phenolate oxygen atom of the chromophore becomes energetically favorable. The proton transfer initiates conformational. changes within the protein, which are in turn believed to lead to signaling.
First author: Cavallo, L, Mechanism of ruthenium-catalyzed olefin metathesis reactions from a theoretical perspective,JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 8965, (2002)
Abstract: This paper presents a density functional theory study of the ruthenium-catalyzed clef in metathesis reactions. The ligand binding energy has been calculated in the first generation of Grubbs-type (PCy3)(2)-Cl2Ru=CHPh (pre)catalyst, as well as in the heteroleptic (pre)catalytic systems in which a N-heterocyclic carbene, NHC, ligand substitutes a single phosphine. In agreement with experiments PCy3 coordinates more strongly to Ru in the heteroleptic (pre)catalysts than in the Grubbs-type (pre)catalyst. Moreover, ethene coordination and insertion into the Ru-alkylidene bond in the above-mentioned systems, as well as in the Hofmann type catalytic system with a cis-coordinated phosphane ligand, has been studied. The calculated insertion barrier for the NHC systems are lower than that of the (PCy3)(2)Cl2Ru=CHPh system. This is consistent with the higher activity experimentally observed for the NHC-based system.
First author: Saeys, M, Density functional study of the adsorption of 1,4-cyclohexadiene on Pt(111): origin of the C-H stretch red shift, SURFACE SCIENCE, 513, 315, (2002)
Abstract: The adsorption of 1,4-cyclohexadiene (CHD14) on Pt(I 1 1) was analysed using first-principle density functional theoretical calculations. CHD14 adsorbs at the bridge site via quadra-sigma-type bonding. The adsorption energy was calculated to be 146 kJ/mol. CHD14 can also adsorb at the hollow site via the formation of a di-sigma- and a pi-bond with an adsorption energy of 142 kJ/mol. Upon adsorption CHD14 adopts a boat conformation. Strong pi-electron donation to the platinum surface in combination with a hyperconjugative interaction of these pi-orbitals with the axial sigmaCH orbital helps to explain the observed red shift of the axial C-H stretching frequency and the corresponding C-H bond lengthening.
First author: Boeijenga, NH, Liquid xenon as an ideal probe for many-body effects in impulsive Raman scattering,JOURNAL OF CHEMICAL PHYSICS, 117, 1181, (2002)
Abstract: The collision induced effects in the third-order Raman response of liquid xenon have been studied both experimentally and theoretically. The effect of electron cloud overlap on the polarizability of xenon dimers was studied using accurate time-dependent density functional theory calculations. The dimer polarizabilities were used to fit parameters in a direct reaction field model that can be generalized to condensed phase systems. This model was employed in molecular dynamics simulations in order to calculate the impulsive Raman response of liquid xenon. Excellent agreement is found between the shape of the calculated and the measured anisotropic part of the response. The shape of this response is little affected by the electron overlap effects, but the intensity is strongly influenced by it. The shape of the isotropic response is predicted to be strongly dependent on electron overlap effects.
First author: Ryeng, H, Do nonplanar distortions of porphyrins bring about strongly red-shifted electronic spectra? Controversy, consensus, new developments, and relevance to chelatases, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 8099, (2002)
Abstract: TDDFT calculations confirm a controversial proposal by DiMagno and co-workers that ruffling distortions, by themselves, do not bring about sizable red shifts in the electronic absorption spectra of “simple” nontransition-metal porphyrins. We now report that the same also holds for saddling distortions. The situation is more complex for transition metal porphyrins. For example, ruffling does bring about strong red shifts in the electronic spectra of nickel porphyrins because of a specific metal(d)-porphyrin(pi) orbital interaction.
First author: Wasbotten, IH, Electronic absorption, resonance Raman, and electrochemical studies of planar and saddled copper(III) meso-triarylcorroles. Highly substituent-sensitive soret bands as a distinctive feature of high-valent transition metal corroles, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 8104, (2002)
Abstract: present here a first systematic study of substituent effects in metallocorroles, based on electronic absorption, resonance Raman (RR), and infrared (IR) spectroscopic studies and electrochemical measurements on 10 copper(III) meso-triarylcorroles, Cu-III[beta-Y8TArC], where the beta-substituent Y = H or Br and the meso-aryl group Ar = C6F5 or p-X-C6H4 and X = CF3, H, CH3, and OCH3. The results afford a number of significant inisights. (1) The FIR (and IR) results show that at least two and possibly more high-frequency bands in the 1400-1550 cm(-1) region exhibit significant frequency downshifts on beta-octabromination and, thus, qualify as structure-sensitive marker bands. DFT geometry optimizations indicate that the saddled conformation should be clearly preferred for the beta-octabromo-meso-triarylcorrole derivatives studied and that beta-octabromination results in expansion of a number of skeletal bond distances of the corrole macrocycle, consistent with observed frequency downshifts. (2) Electrochemical measurements on planar Cu-III[TArC] derivatives have shown that the para substituents on the meso-aryl groups exert a strong influence on the half-wave potentials for oxidation (rho(ox) = DeltaE(1/2ox)/Delta(3sigma) = 95 mV), suggesting that oxidation involves removal of an electron from the corrole “b(1)” HOMO, which has significant amplitudes at the meso postions and crudely resembles a porphyrin a(2u) HOMO in shape. In contrast, the Hammett pox is much lower for the nonplanar Cu-III[Br8TArC] derivatives and we suggest that this ultimately results from a b(1)-t(0)-a(2) HOMO reversal which in turn stems from a metal (d(x)(2)-(2)(y))-corrole (“b(1)”) orbital interaction that becomes symmetry-allowed under a saddle distortion of the corrole macrocycle. In contrast to what has been observed for metallotetraphenylporphyrins, beta-octabromination dramatically raises the half-wave potential for one-electron oxidation of the triarylcorrole derivatives studied. This appears to be due to the fact that both the “a(2)” and “b(1)” HOMOs of a corrole (in C-2v notation) have significantly higher amplitudes at the P positions, compared to a porphyrin a(2u) HOMO. Thus, although many metallocorroles are significantly more easily oxidizable than analogous metalloporphyrins, certain beta-octahalogeno-meso-triarylcorrole derivatives can indeed be extremely electron deficient and oxidation resistant and may, therefore, find use as rugged catalysts or reagents under highly oxidizing conditions. (3) Finally, the Soret absorption maxima of high-valent metallotriarylcorroles exhibit a uniquely sensitive dependence on the substituents on the meso-aryl groups. Thus, on going from Cu-III[T(p-CF3-P)C] (T(p-CF3-P)C = meso-tris((p-trifluoromethyl)phenyl)corrolato) to Cu-III[T(p-OM-P)C] (T(p-OM-P)C = meso-tris(p-methoxyphenyl)corrolato), the Soret maximum red shifts by 26 nm, from 407 to 433 rim. Similarly, on going from Cu-III[Br8T(p-CF3-P)C] (Br8T(p-CF3-P)C = beta-octabromomeso-tris((p-trifluoromethyl)phenyl)corrolato) to Cu-III[Br8T(p-OM-P)C] (Br8T(p-CF3-P)C = beta-octabromo-mesotris(p-methoxyphenyl)corrolato)the Soret maximum red shifts by 34 nm, from 434 to 468 nm.Time-dependent DFT calculations suggest that this substituent dependence reflects significant ligand-to-metal charge-transfer character of certain transitions in the Soret region. The optical spectra of free-base and non-high-valent transition metal tetrapyrroles, in general, do not exhibit a similar substituent dependence.
First author: Autschbach, J, Chiroptical properties from time-dependent density functional theory. II. Optical rotations of small to medium sized organic molecules, JOURNAL OF CHEMICAL PHYSICS, 117, 581, (2002)
Abstract: We report an implementation for the computation of optical rotations within the Amsterdam Density Functional program package. The code is based on time-dependent density functional response theory. Optical rotations have been calculated for a test set of 36 organic molecules with various density functionals, and employing basis sets of different quality. The results obtained in this work with nonhybrid functionals are comparable in quality to those recently reported by other authors for the B3LYP hybrid functional, but show a somewhat larger tendency to produce outlyers. The median error is approximately 20degrees/(dm g/cm(3)) for specific rotations [alpha](D) as compared to experimental data (approximately 30% median deviation from experimental values). Thereby it is demonstrated that density functional computations can be employed to assist with the solution of stereochemical problems in case the specific rotations of the species involved are not small and their structures are rigid. Recent newly developed functionals are investigated with respect to their applicability in computations of optical rotations.
First author: Frenking, G, Nature of the metal-ligand bond in M(CO)(5)PX3 complexes (M = Cr, Mo, W; X = H, Me, F, Cl): Synthesis, molecular structure, and quantum-chemical calculations, ORGANOMETALLICS, 21, 2921, (2002)
Abstract: The syntheses of the phosphane complexes M(CO)(5)PH3 (M = MO, W), W(CO)(5)PD3, and W(CO)(5)PF3 and the results of X-ray structure analyses of W(CO)(5)PH3 and Mo(CO)(5)PCl3 are reported. Quantum-chemical DFT calculations of the geometries and M-P bond dissociation energies of M(CO)(5)PX3 (M = Cr, Mo, W; X = H, Me, F, Cl) have been carried out. There is no correlation between the bond lengths and bond dissociation energies of the M-P bonds. The PMe3 ligand forms the strongest and the longest M-P bonds of the phosphane ligands. The analysis of M-PX3 bonds shows that PCl3 is a poorer sigma donor and a stronger pi(P) acceptor than the other phosphanes. The energy decomposition analysis indicates that the M-P bonds of the PH3 and PMe3 complexes have a higher electrostatic than covalent character. The electrostatic contribution is between 56 and 66% of the total attractive interactions. The orbital interactions in the M-PH3 and M-PMe3 bonds have more sigma character (65-75%) than T character (25-35%). The M-P bonds of the halophosphane complexes M(CO)(5)PF3 and M(CO)(5)PCl3 are nearly half covalent and half electrostatic. The pi bonding contributes similar to50% to the total orbital interaction.
First author: Bridgeman, AJ, Electronic structure of the alpha and beta isomers of [Mo8O26](4-), INORGANIC CHEMISTRY, 41, 3500, (2002)
Abstract: The structure and bonding in alpha and beta octamolybdate anions have been investigated using density functional methods. In general, good computational-experimental agreement for the geometrical parameters has been obtained. The electronic structure of the anions has been probed with molecular orbital and Mulliken-Mayer methods. All Mo-O interactions have been found to be predominantly d(Mo)-p(O) in character. Several multicentered molecular orbitals can be described as sigma or pi closed-loop structures, but the proposed connection with the stability of the polyanions is not completely supported by the calculations. Mayer indexes correspond to fractional multiple character for terminal bonds and approximately single or low-order character for bridging bonds, in accordance with structural and bond valence results. The valency analysis has yielded similar overall bonding capacity for the various oxygen atoms. A distribution of the negative charge over all types of oxygen sites and metal charges considerably smaller than the formal oxidation states have been obtained from the Mulliken analysis.
First author: Jensen, L, Time-dependent density functional study of the static second hyperpolarizability of BB-, NN- and BN-substituted C-60, CHEMICAL PHYSICS LETTERS, 359, 524, (2002)
Abstract: In this work we have investigated the effects of substituting carbon atoms with B and N on the second hyperpolarizability of C-60 using time-dependent density functional theory. We have calculated the second hyperpolarizability of the double substitute-doped fullerenes C58NN, C58BB and C58BN. For C-60 only small changes in the second hyperpolarizability were found when doping with either 2B or 2N. However, by doping C-60 with both B and N, creating an donor-acceptor system, an increase in the second hyperpolarizability with about 50% was found.
First author: Bridgeman, AJ, Structure and bonding in [W10O32](n-) isopolyanions, JOURNAL OF PHYSICAL CHEMISTRY A, 106, 6114, (2002)
Abstract: The structure and bonding in oxidized and reduced decatungstate anions have been investigated using density-functional methods. The computational-experimental agreement is good for the geometrical parameters of the oxidized species, The electronic structure of the anions has been probed with molecular-orbital, Mulliken-Mayer, and bonding-energy approaches, and the various analyses are in general accordance with spectroscopic evidence and theoretical models. The results have indicated that W-O interactions are largely W d-O p in character, and that sigma and pi bonds link the metal centers to terminal and bridging (O-2c) oxygen atoms. Some W-O-2, orbital interactions can be represented as [W4O4] or [W6O6] closed-loop structures, but these bonding modes have not been found to make a particularly outstanding contribution to the stability of the molecules. Mayer indexes correspond to (fractional) multiple, approximately single, and low-order character for terminal, bridging, and internal bonds, respectively, and the valency analysis has yielded similar bonding capacities for the different oxygen atoms. A distribution of the negative charge over all types of oxygen sites, and metal charges and orbital populations considerably different from the formal assignments have been obtained from the Mulliken analysis. Minor structural changes have been detected in reduced decatungstates, in accord with the general properties of the orbitals occupied by the added electrons.
First author: Matsugi, T, New titanium complexes bearing two indolide-imine chelate ligands for the polymerization of ethylene, MACROMOLECULES, 35, 4880, (2002)
Abstract: New titanium complexes 5-8 with two indolide-imine chelate ligands [7-(RN-CH)C8H5N](2)-TiCl2 (R: 5, phenyl; 6, 2,6-difluorophenyl; 7, 2,4,6-trifluorophenyl; 8, pentafluorophenyl) were synthesized and investigated as ethylene polymerization catalysts. On activation with methylalumoxane (NIAO), all of the complexes were active ethylene polymerization catalysts at 25 degreesC to produce linear polyethylenes. The catalytic activity (TOF) increased sharply with the number of fluorine atom in the ligand. In addition, complexes 5-8 potentially exhibit the characteristics of a living ethylene polymerization. Complexes 6 and 7 promoted room temperature living ethylene polymerization to produce polyethylenes having extremely narrow molecular weight distributions (6, M-w/M-n 1.09-1.14; 7, M-w/M-n 1.05-1.23). On the other hand, at -10 degreesC complex 8 afforded monodisperse polyethylenes (M-w/M-n 1.12-1.25), with exceptionally high activities for a living ethylene polymerization (TOF: maximum 1155 min(-1) atm(-1)). Using complex 7/MAO catalyst system, a polyethylene-b-poly(ethylene-co-propylene) block copolymer was successfully synthesized.
First author: Grozema, FC, Hole mobility in DNA: Effects of static and dynamic structural fluctuations, CHEMPHYSCHEM,3, 536, (2002)
Abstract: Charge migration in DNA has been a subject of considerable debate over the last decade. The interior of DNA consist of a stack of aromatic hydrogen-bonded base pairs that resembles a one-dimensional pi-conducting structure found in discotic materials (see graphic), but the structure of DNA is highly disordered and fluctuations occur on timescales of pico- to nanoseconds. Here, a theoretical study of the effect of static and dynamic structural fluctuations on the mobility of positive charges along a model B-form poly(G) – poly(C) double helix is presented.
First author: Atanasov, M, Predictive concept for lone-pair distortions – DFT and vibronic model studies of AX(n)(-(n-3)) molecules and complexes (A = N(III) to Bi(III); X=F(-1) to I(-1); n=3-6), JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 6693, (2002)
Abstract: The stereochemical and energetic consequences of the lone-pair effect in the title molecules and complexes have been studied by DFT calculations based on a vilbronic coupling concept. The anionic complexes were examined as bare entities and, more realistically, in a polarizable charge-compensating solvent continuum. The tendency for distortions of AX(3) compounds away from the high-symmetry parent geometry becomes more pronounced the larger the chemical hardness of a molecule and its constituents is; on the other hand, anionic complexes AX(n)-((n-3)) (n = 4-6) become softer and less susceptible to distortion as compared to the corresponding AX3 molecule, the larger the coordination number and the anionic charge are. Thus, while all AX(3) compounds adopt the distorted C(3v) structure, only very few AX(6)(3-) species are calculated to deviate from the parent Oh geometry. If a complex possesses a low stabilization energy due to an unfavorable central ion/ligand size ratio, vibronic coupling may even lead to complete dissociation of one (SbF(6)(3-) –> SbF(5)(2-) + F(-)) or more (PF(6)(3-) –> PF(4)(-) + 2F(-)) ligands. The derived hardness rule perfectly covers the reported structural findings. The calculations indicate that the lone-pair effect is an orbital overlap phenomenon. The interpair repulsion within the valence shell, keeping the average bond distances constant, does not stabilize the distorted with respect to the parent geometry, in disagreement with the VSEPR model.
First author: Bridgeman, AJ, Density functional study of the vibrational frequencies of Lindqvist polyanions, CHEMICAL PHYSICS, 279, 143, (2002)
Abstract: The structures and vibrational spectra of the Lindqvist polyanions [Mo6O19](2-), [W6O19](2-), [Mo6O19](3-), [VMo5O19](3-) and [VMo5O19](4-) have been calculated using density functional theory. The agreement between the previously reported vibrational spectra and the calculated values is extremely good. For [Mo6O19](2-), a detailed comparison between the performance of commonly used functionals and basis sets and between Hartree-Fock and density functional methods has been performed. Whilst all density functional methods perform adequately, the Hartree-Fock method overestimates the strength of the metal-oxygen bonding and this is reflected in poor reproduction of the stretching frequencies. The results suggest that the local density approximation with Slater type orbitals and relativistic corrections is best able to model both the structure and vibrational spectra of these polyanions when treated as pseudo-gas phase species. 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. The vibrational analyses confirm the high symmetry for these anions suggested by previous geometry calculations.
First author: Deubel, DV, On the competition of the purine bases, functionalities of peptide side chains, and protecting agents for the coordination sites of dicationic cisplatin derivatives, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY,124, 5834, (2002)
Abstract: The Pt-L bond energies of simple triammineplatinum(II) complexes, [Pt(NH3)(3)L](2+), with oxygen-, nitrogen-, and sulfur-containing donor ligands L have been predicted and rationalized using density functional theory. The ligands L have been chosen as models for functionalities of peptide side chains, for sulfur-containing protecting agents, and for adenine and guanine sites of the DNA as the ultimate target of platinum anticancer drugs. Calculation of the Pt-L bond energy in [Pt(NH3)(3)L](2+) reveals that the soft metal center of triammineplatinum(II) prefers N ligands over S ligands. This remarkable result has been discussed in light of several interpretations of the hard and soft acids and bases principle. The concept of orbital-symmetry-based energy decomposition has been employed for the determination of the contributions from c and T orbital interactions, electrostatics, and intramolecular hydrogen bonding to the Pt-L bond energy. The calculations show that considerable differences in the bond energies of the triammineplatinum(I I) complexes with N-heterocycles such as 1-methylimidazole, 9-methyladenine, and 9-methylguanine arise from electrostatics rather than from orbital interactions. Surprisingly, the net stabilization by hydrogen bonding between the (Pt)N-H group and the oxygen of 9-methylguanine is as weak as the intramolecular hydrogen bond in the aqua complex [Pt(NH3)(3)(H2O)](2+), challenging the common hypothesis that DNA-active anticancer drugs require carrier ligands with N-H functionalities because of their hydrogen-bonding ability. The influence of a polarizable environment on the stability of the complexes has been investigated systematically with the dependence of the dielectric constant epsilon. With increasing epsilon, the complexes with S-containing ligands are more strongly stabilized than the complexes of the N-containing heterocycles. At epsilon = 78.4, the dielectric constant of water, 9-methylguanine remains the only purine derivative investigated which is competitive to neutral sulfur ligands. These findings are particularly important for a rationalization of the results from recent experimental studies on the competition of biological donor ligands L for coordination with the metal center of [Pt(dien)L](2+) (dien = 1,5-diamino 3-azapentane).
First author: Lovell, T, A structural model for the high-valent intermediate Q of methane monooxygenase from broken-symmetry density functional and electrostatics calculations, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 5890, (2002)
Abstract: A combined broken-symmetry density functional and electrostatics approach has been used to construct a model for the high-valent diiron intermediate Q of methane monooxygenase. The presence of high-spin or intermediate spin iron centers gives rise to two structurally distinct spin-coupled states of the cluster for which calculated geometries, net spin populations, Heisenberg J values, Mossbauer isomer shifts, and quadrupole splittings are compared and contrasted with the available spectroscopic data.
First author: Patchkovskii, S, Improving “difficult” reaction barriers with self-interaction corrected density functional theory,JOURNAL OF CHEMICAL PHYSICS, 116, 7806, (2002)
Abstract: We examined 11 difficult reactions with self-interaction corrected density (SIC) functional theory. The data set includes dissociation of radicals into symmetric fragments (H-2(+)–>H+H+, He-2(+)–>He+He+), radical hydrogen abstraction (H+H-2–>H-2+H, H+HCl–>H-2+Cl, H+N2H2–>N2H+H-2, CH3+H-2–>CH4+H), proton transfer [HC(OH)CHC(O)H–>HC(O)CHC(OH)H], S(N)2 halogen exchange (X-+CH3X–>CH3X+X-, X=F,Cl,Br), and closed-shell unimolecular dissociation of tetrasine (C2N4H2–>N-2+2HCN). Calculated self-interaction energies cancel, almost identically, for the reaction energies (DeltaE(R)), so that SIC functionals do not lead to a systematic improvement in DeltaE(R). Self-interaction correction increases for reaction transition structures, leading to higher calculated activation barriers (DeltaE(not equal)). The average absolute deviation in DeltaE(not equal), from ab initio and experimental barriers, is reduced from 14 kcal/mol for Vosko-Wilk-Nusair (VWN) or 12 kcal/mol for revised Perdew-Burke-Ernzerhof (revPBE) functionals to 5.4 (SIC-VWN) or 3.4 (SIC-revPBE) kcal/mol. Reorganization of the electron density, due to removal of self-interaction, appears to be important. When SIC is included as a perturbation, using self-consistent densities of the parent functional, the average absolute deviations for the barriers increase to 7.5 (VWN+SIC) or 5.3 (revPBE+SIC) kcal/mol. Gradient-corrected functionals (revPBE, BP86) reduce the magnitude of the total self-interaction correction, by improving the description of the core orbitals. For the valence orbitals, both the magnitudes of the self-interaction corrections, and their change between reagents and transition structures, are similar for VWN local density approximation, and generalized gradient approximation functionals. Reducing the magnitude of the self-interaction energy for valence electrons thus appears to be a promising direction for the development of chemically accurate exchange-correlation functionals.
First author: van Faassen, M, Ultranonlocality in time-dependent current-density-functional theory: Application to conjugated polymers, PHYSICAL REVIEW LETTERS, 88, 7806, (2002)
Abstract: We solve the long-standing problem of the large overestimation of the static polarizability of conjugated polymers obtained using the local density approximation within density-functional theory. The local approximation is unable to describe the highly nonlocal exchange and correlation (xc) effects found in these quasi-one-dimensional systems. Time-dependent current-density-functional theory enables a local current description of ultranonlocal xc effects using the Vignale-Kohn functional [G. Vignale and W. Kohn, Phys. Rev. Lett. 77, 2037 (1996)]. Except for the model hydrogen chain, our results are in excellent agreement with the best available correlated methods.
First author: Guerra, CF, Orbital interactions in hydrogen bonds important for cohesion in molecular crystals and mismatched pairs of DNA bases, CRYSTAL GROWTH & DESIGN, 2, 239, (2002)
Abstract: Model systems (of up to 116 atoms) for molecular crystals and mismatched pairs of DNA bases have been studied using GGA density functional theory (DFT) at BP86/TZ2P. In line with previous studies, we find that our approach is adequate for accurately describing the present model systems all of which involve hydrogen bonding. For example, our DFT bond energies for 17 DNA base pairs involving adenine (A), thymine (T), guanine (G), and cytosine (C) agree excellently with ab initio MP2 results (root-mean-square deviation = 1.1 kcal/mol). Our main purpose is to clarify the relative importance of electrostatic attraction versus orbital interaction in the hydrogen bonds involved in our model systems, using a quantitative bond energy decomposition scheme. At variance with widespread belief, the orbital interaction component in these hydrogen bonds is found to contribute about two-fifths (36-43%) of the attractive interactions and is thus of the same order of magnitude as the electrostatic component. Interestingly, we find a similarly prominent role for orbital interaction in the hydrogen bonds that are responsible for the cohesion within a layer of the molecular crystal of Watson-Crick pairs of 9-ethylguanine and 1-methylcytosine.
First author: Autschbach, J, Chiroptical properties from time-dependent density functional theory. I. Circular dichroism spectra of organic molecules, JOURNAL OF CHEMICAL PHYSICS, 116, 6930, (2002)
Abstract: We report the implementation of the computation of rotatory strengths, based on time-dependent density functional theory, within the Amsterdam Density Functional program. The code is applied to the simulation of circular dichroism spectra of small and moderately sized organic molecules, such as oxiranes, aziridines, cyclohexanone derivatives, and helicenes. Results agree favorably with experimental data, and with theoretical results for molecules that have been previously investigated by other authors. The efficient algorithms allow for the simulation of CD spectra of rather large molecules at a reasonable accuracy based on first-principles theory. The choice of the Kohn-Sham potential is a critical issue. It is found that standard gradient corrected functionals often yield the correct shape of the spectrum, but the computed excitation energies are systematically underestimated for the samples being studied. The recently developed exchange-correlation potentials “GRAC” and “SAOP” often yield much better agreement here with experiments for the excitation energies. The rotatory strengths of individual transitions are usually improved by these potentials as well.
First author: Deubel, DV, DFT study of olefin versus nitrogen bonding in the coordination of nitrogen-containing polar monomers to diimine and salicylaldiminato nickel(II) and palladium(II) complexes. Implications for copolymerization of olefins with nitrogen-containing polar monomers, ORGANOMETALLICS, 21, 1603, (2002)
Abstract: An initial screening of late-transition-metal catalysts and nitrogen-containing polar monomers toward an incorporation of amines or nitriles in the polymer chain of polyolefins has been performed using density functional theory. Substrates of the type CH2=CH(CH2)(n)X (X = polar group) can bind either with the N-containing polar group or with the pi moiety to the metal center of the catalyst. Monomer-catalyst combinations favoring the pi complex over the N complex are promising, because the pi-binding mode can subsequently lead to polymer growth. The stabilization energies for the pi and N complexes of monomers of the type CH2=CH(CH2)(n)CN, CH2=CH(CH2)(n)NH2, and CH2=CH(CH2)(n)N(CH3)(2) with generic models for the recently reported nickel(II) and palladium(II) catalysts with diimine (“Brookhart”) and salicylaldiminato (“Grubbs”) ligands have been calculated. While the investigated polar monomers have been shown to form very strong metal-nitrogen bonds with the Brookhart nickel catalysts, the enamine prefers the pi binding mode in its complexes with all model catalysts. Promising results have also been obtained for the coordination of nitriles and amines with the Grubbs nickel catalysts. The palladium systems show an even larger preference for pi coordination than their nickel counterparts. An energy-decomposition scheme has been used to rationalize the relative strength of the catalyst-monomer bonds.
First author: Bridgeman, AJ, Structure and bonding in [M6O19](n-) isopolyanions, INORGANIC CHEMISTRY, 41, 1761, (2002)
Abstract: The structure and bonding in [M6O19](n-) isopolyanions of Nb, Ta, Mo, and W have been investigated using density-functional methods. The computatlonal-experimental agreement is good for the geometrical parameters of Mo and W species but less satisfactory for Nb and Ta. clusters. The electronic structure of the anions has been probed with molecular-orbital, Mulliken-Mayer, and bonding-energy approaches. The results have indicated that M-O interactions are largely M d-O p in character and that or and pi bonds link the metal centers to terminal and bridging (O-b) oxygen atoms. Some M-O-b bonds exhibit a [M4O4] Closed-loop structure, but this orbital-interaction mode has not been found to make a particularly outstanding contribution to the bonding stability of the molecules. Mayer indexes correspond to (fractional) multiple, approximately single, and low-order character for terminal, bridging, and internal bonds, respectively, and the valency analysis has yielded similar bonding capacities for the different oxygen atoms. A distribution of the negative charge over all types of oxygen sites and metal charges considerably smaller than the formal oxidation states have been obtained from the Mulliken analysis. Minimal structural changes have been detected on reduction of molybdates and tungstates, in accord with the general properties of the orbitals occupied by the added electrons.
First author: Kirillov, MA, Cluster models of Co2+ at the cation sites of zeolite ZSM-5, JOURNAL OF STRUCTURAL CHEMISTRY, 43, 225, (2002)
Abstract: The local structure of Co2+ at the alpha-, beta-, and gamma-cation sites of zeolite ZSM-5 was calculated in terms of density functional theory using the cluster approach. The local geometry of the oxygen environment of Co2+ is characterized; it is found that the ion stabilization energy increases in the series alpha –> beta –> gamma.
First author: Jansen, TLC, Collision effects in the nonlinear Raman response of liquid carbon disulfide, JOURNAL OF CHEMICAL PHYSICS, 116, 3277, (2002)
Abstract: A model of the polarizability of carbon disulfide dimers was constructed, using polarizabilities from accurate time-dependent density functional theory calculations as reference. This direct reaction field model takes dipole-induced dipole effects, induced multipole effects and effects due to the overlap of the electronic clouds into account in an approximate way. The importance of the induced multipole and the overlap effects is investigated. This polarizability model is subsequently used to calculate the third-order time-domain Raman response of liquid carbon disulfide. These results are compared to experimental data and earlier calculated response in which only dipole-induced dipole effects on the polarizability were included. The multipole effects are found to give a significant contribution to the subpico second part of the third-order Raman response.
First author: Tielens, F, An ab initio study of the interaction of SCN- with a silver electrode: The prediction of vibrational frequencies, JOURNAL OF PHYSICAL CHEMISTRY A, 106, 1450, (2002)
Abstract: Experimental results for the adsorption frequencies of SCN- on a silver electrode obtained with the surface-enhanced Raman spectrometry (SERS) method are presented and discussed. These results were confronted with a quantum chemical DFT-based study. Different clusters were used from the simple monoatomic case to a much more realistic representation of the silver electrode by means of a Ag-23 cluster simulating the (100) surface. The calculations were performed at the B3PW91/LanL2DZ and the BP86/TZP level. The interaction energies indicate the importance of the cluster size and calculation level. The cluster size was increased until the results were converged. Calculated vibrational frequencies for the different possible adsorption geometries on a (100) surface are compared with experiment. Together with the interaction energy results, they indicate that the preferred interaction via the sulfur atom on a hollow site of the silver electrode is in agreement with the hard and soft acids and bases principle.
First author: Bickelhaupt, FM, The short N-F bond in N2F+ and how Pauli repulsion influences bond lengths. Theoretical study of N2X+, NF3X+, and NH3X+ (X = F, H), JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124, 1500, (2002)
Abstract: Exceptionally short N-F bond distances of only 1.217 Angstrom (crystal) and 1.246 Angstrom (gas phase) have been reported for N2F+, making it the shortest N-F bond ever observed. To trace the origin of this structural phenomenon, we have analyzed the model systems N2X+, NF3X+, and NH3X+ (X = F, H) using generalized gradient approximation density functional theory at BP86/TZ2P. In good agreement with experiment, the computations yield an extremely short N-F bond for N2F+: we find N-F bond distances in N2F+, NF4+, and NH3F+ of 1.245, 1.339, and 1.375 Angstrom, respectively. The N-X bonding mechanisms are quantitatively analyzed in the framework of Kohn-Sham MO theory. At variance with the current hypothesis, reduced steric and other Pauli repulsion (of substituents or lone pairs at N with F) rather than the extent of s-p hybridization of N (i.e., sp versus sp(3)) are responsible for the much shorter N-F distance in N2F+ compared to NF4+. The results for our nitrogen compounds are furthermore discussed in the more general context of how bond lengths are determined by both bonding and repulsive orbital interactions.
First author: Han, WG, Density functional studies of the ground- and excited-state potential-energy curves of stilbene cis-trans isomerization, CHEMPHYSCHEM, 3, 167, (2002)
Abstract: Although it appears to be a simple reaction, cisreversible arrowtrans isomerization of compounds such as stilbene (see graphic) is still not fully understood. This Article sheds some light on the problem by reporting calculations of singlet and triplet ground-and excited-state potential-energy curves which support the proposed triplet mechanism for the sensitized photoisomerization and the nonadiabatic singlet mechanism.
First author: Patchkovskii, S, Phosphorus NMR chemical shifts with self-interaction free, gradient-corrected DFT,JOURNAL OF PHYSICAL CHEMISTRY A, 106, 1088, (2002)
Abstract: We examine the origin of the orbital localization requirement, commonly imposed on effective potential implementations of the Perdew-Zunger (PZ) self-interaction correction (SIC). We demonstrate that the condition arises because of the presence of irreducible off-diagonal Lagrangian multipliers in the coupled PZ eigenequations. Thus, this condition is essential for obtaining an energy-minimizing solution. Further, we report on an implementation of PZ SIC for the generalized gradient approximation (GGA) to the exchange-correlation energy within density functional theory (DFT). The implementation relies on the Krieger-Li-lafrate (KLI) approximation to the optimized effective potential (OEP), simplifying the evaluation of molecular properties, such as the NMR chemical shifts. We examine several approaches toward incorporating the frozen core orbitals within the SIC-KLI-OEP scheme. To achieve an accurate description of both the energetic and magnetic properties, core orbitals must be included in the KLI potential on an equal footing with the valence orbitals. Implementation of the frozen core orbitals enables incorporation of relativistic effects via the quasirelativistic Pauli Hamiltonian. As the first application of the SIC-GGA approach, we examine (31)P, NMR chemical shifts of IS representative small molecules, as well as the previously reported C, H, N, O, and F SIC-LDA (local density approximation) test set. For C, N, O, and F NMR chemical shifts, SIC-GGA performs similarly to SIC-LDA, whereas a significant improvement is observed for hydrogen. Almost identical results. both for the chemical shifts and absolute shieldings, are obtained with different parent GGAs. For the (31)P test set, SIC-revPBE (revised Perdew-Burke-Ernzerhof functional of Zhang and Yang) leads to an root-mean-square (R-MS) residual error of 23 ppm, compared to 54 ppm for its parent GGA, and 40 ppm for the SIC-VWN (Vosko-Wilk-Nusair) LIDA functional. In particular, SIC-revPBE correctly reproduces the experimental trends in the PF(3)-PCl(3)-PBr(3)-PI(3) series, which is described qualitatively incorrectly by VWN. revPBE, and SIC-VWN calculations. A similar improvement is observed for the (31)P shielding tensor components. The spurious self-interaction, in modern approximate DFT, appears to be a major, and so far largely overlooked, source of errors in calculations of the NMR shielding tensors.
First author: Bridgeman, AJ, A matrix isolation and DFT study of the generation and characterization of monomeric vapour phase platinum chlorides, CHEMICAL PHYSICS LETTERS, 351, 319, (2002)
Abstract: Molecular platinum monochloride (PtCl) and platinum dichloride (PtCl2) have been prepared from platinum atoms and chlorine doped argon in a hollow-cathode sputtering device, matrix isolated in solid argon and characterized using electronic, infrared and X-ray absorption spectroscopies together with high level DFT calculations.
First author: Piana, S, The mechanism of catalytic enantioselective fluorination: Computational and experimental studies,ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 41, 979, (2002)
Abstract: Molecular platinum monochloride (PtCl) and platinum dichloride (PtCl2) have been prepared from platinum atoms and chlorine doped argon in a hollow-cathode sputtering device, matrix isolated in solid argon and characterized using electronic, infrared and X-ray absorption spectroscopies together with high level DFT calculations.
First author: Guerra, CF, Orbital interactions in strong and weak hydrogen bonds are essential for DNA replication,ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 41, 2092, (2002)
Abstract: Molecular platinum monochloride (PtCl) and platinum dichloride (PtCl2) have been prepared from platinum atoms and chlorine doped argon in a hollow-cathode sputtering device, matrix isolated in solid argon and characterized using electronic, infrared and X-ray absorption spectroscopies together with high level DFT calculations.
First author: Adlhart, C, Ligand rotation distinguishes first- and second-generation ruthenium metathesis catalysts,ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 41, 4484, (2002)
Abstract: Molecular platinum monochloride (PtCl) and platinum dichloride (PtCl2) have been prepared from platinum atoms and chlorine doped argon in a hollow-cathode sputtering device, matrix isolated in solid argon and characterized using electronic, infrared and X-ray absorption spectroscopies together with high level DFT calculations.
First author: Li, X, Experimental observation and confirmation of icosahedral W@Au-12 and Mo@Au-12 molecules,ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 41, 4786, (2002)
Abstract: Molecular platinum monochloride (PtCl) and platinum dichloride (PtCl2) have been prepared from platinum atoms and chlorine doped argon in a hollow-cathode sputtering device, matrix isolated in solid argon and characterized using electronic, infrared and X-ray absorption spectroscopies together with high level DFT calculations.
First author: Yoshida, Y, Catalytic behavior of bis(pyrrolide-imine) and bis(phenoxy-imine) titanium complexes for the copolymerization of ethylene with propylene, 1-hexene, or norbornene, ISRAEL JOURNAL OF CHEMISTRY, 42, 353, (2002)
Abstract: This contribution reports the catalytic behavior of bis(pyrrolideimine)Ti complexes 1 and 2, [2-(RNCH)-C4H3N](2)TiCl2 (1, R = Ph; 2, R = cyclohexyl), and bis(phenoxy-imine)Ti complex 3, [2-(Ph-NCH)-3-tBu-C6H3O](2)TiCl2 for the copolymerization of ethylene with propylene, 1-hexene, or norbornene. An inspection of the X-ray structures of complexes 1-3 suggested that complexes 1 and 2 with pyrrolide-imine ligands would provide more space for olefin polymerization than complex 3 with phenoxy-imine ligands. In addition, DFT calculations also showed that active species derived from complexes 1 and 2 possess higher electrophilicity of the Ti center compared to that from complex 3. Complexes 1 and 2 on activation with methylalurnoxane (MAO) had higher affinity for propylene and 1-hexene and incorporated higher amounts of propylene (1; 30.5 mol%, 2; 23.4 mol%) and 1-hexene (1; 1.9 mol%, 2; 1.7 mol%) than complex 3 (propylene; 4.5 mol%, 1-hexene; 0.4 mol%). The incorporation levels of propylene and 1-hexene displayed by complexes 1 and 2 were lower than those for Cp2TiCl2 (propylene; 41.6 mol%, 1-hexene; 5.1 mol%) under identical conditions. In contrast, complexes 1 and 2 exhibited higher incorporation ability for norbornene and produced copolymers with much higher norbornene contents (1; 32.0 mol%, 2; 26.5 mol%) than Cp2TiCl2 (1.2 mol%) under the same conditions. Additionally, complex 3 also promoted higher norbornene incorporation (4.3 mol%) than Cp2TiCl2 and provided a copolymer with extremely narrow molecular weight distribution (M-w/M-n 1.14). A correlation exists between electrophilicity of the Ti center in active species and norbornene incorporation.
First author: Bridgeman, AJ, Electronic structure of Mo and W [M7O24](6-) isopolyanions, JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS, 42, 2244, (2002)
Abstract: The structure and bonding in [M7O24](6-) isopolyanions of Mo and W have been investigated using density-functional methods. Good computational-experimental agreement for the geometrical parameters has been obtained. The electronic structure of the anions has been probed with molecular-orbital and Mulliken-Mayer methods. All M-O interactions have been found to be largely Md-Op in character. Multicentred sigma and pi bonding involving different metal and bridging-oxygen atoms has been observed, and some of the corresponding molecular orbitals can be described in terms of [MnOn] closed loops. Mayer indexes correspond to fractional multiple character for terminal bonds, and approximately single or low-order character for bridging bonds, and are thus in satisfactory agreement with bond-valence results. The valency analysis has yielded similar overall bonding capacity for the various oxygen atoms. A distribution of the negative charge over all types of oxygen sites, and metal charges considerably smaller than the formal oxidation states have been obtained from the Mulliken analysis.
First author: Murphy, DM, Conformational changes of an oxovanadium complex probed by ENDOR spectroscopy and DFT calculations, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 4, 4937, (2002)
Abstract: An angular selective ENDOR study, in combination with DFT calculations, was used to investigate the structure and conformational changes of the bis(acetylacetonato)-oxovanadium(IV) complex ([V-IV=O(acac)(2)]) in frozen solution. In the presence of a non-coordinating solvent ( dichloromethane), both ENDOR analysis and DFT calculations confirm the expected square pyramidal structure of the complex. However, when a strongly coordinating solvent (d(5)-pyridine) was used, different structural isomers of the resulting ([V-IV=O(acac)(2) (C5D5N)]) adduct were found to co-exist in the frozen solution. ENDOR simulations indicate that 60% of the adduct is present as the cis-isomer, while 40% is present as the trans-isomer. The ligand coordinates of both isomers, as determined by ENDOR spectroscopy, were in excellent agreement with the optimised structure based on DFT calculations. In addition, the DFT calculations showed that the total energy of the two isomers for the pyridine case differ by only 3 kJ mol(-1), in agreement with the experimentally observed equilibrium mixture containing both isomers.
First author: Garcia-Fernandez, P, Study of silver atoms in anionic position in KCL, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 157, 829, (2002)
Abstract: The local geometry and electronic distribution of silver atoms trapped in anionic centres (Ag-a(0)) of KCl has been studied by means of Density Functional Theory (DFT) and cluster models involving up to 123 ions. This leads to an important outwards relaxation of the metal-ligand distance of about 13.1% with respect to the pure lattice (RK-Cl =3.14 Angstrom), which plays an important role for understanding experimental EPR data. The higher stability and more localised nature of this centre, compared to Ag-0 at a cationic site (Ag-c(0)), is shown to be related to the energy difference between the Ag(5s) level and the conduction band. The charge distribution in the a(1g) level for Ag-a(0) and Ag-c(0) are compared. These results allow one to understand the different hyperfine constants due to both centres in KCl.
First author: Garcia-Lastra, JM, Structural and electronic properties of Fe3+-O2- pairs in KMF3 lattices (M=Mg, Zn): Is the superposition model valid?, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 157, 931, (2002)
Abstract: The geometrical and electronic properties of the tetragonal FeF5O4- complex which is likely formed in KMgF3 and KZnF 3 fluoroperovskites have been investigated by means of Density Functional calculations. The properties of this centre are dependent on the Fe-O distance (R-ox) and also on the distances R-eq and R-ax between Fe3+ and the four equatorial F- ions and the apical F- ion respectively. The calculated equilibrium distance for the isolated Fe3+ -O2- pair is equal to R-ox =1.63 Angstrom while when it is introduced in KMF3 lattices (M =Mg, Zn) R-ox similar to1.82 Angstrom. Compared to FeF63- in KMgF3 (R-eq = R-ax =1.94 Angstrom) the obtained equilibrium distances R-ax =2.05 Angstrom and R-eq =2.02 Angstrom mean an outwards relaxation of 6% and 4% respectively induced by the F- –>O2- substitution. The ratio of the associated superhyperfine (shf) constants of equatorial (A(eq)) and axial (A(ax)) F- ligands is A(eq) / A(ax) approximate to2 and thus account for an experimental fingerprint of this centre. The huge difference between A(s) (eq) and A(s) (ax) cannot be explained assuming that the law relating A(s) to the metal-ligand distance, R , for the octahedral FeF63- unit can be transferred to the present case involving a lower symmetry . The present analysis casts doubts on the conclusions previously reached using the empirical superposition model.