The ground state as well as low-lying excited states of the zinc-dimer are studied using ab initio calculations. Spectroscopic constants and potential curves from all-electron, multi-configurational second-order perturbation calculations are compared to coupled cluster including triples corrections and averaged coupled pair functional results as well as available experimental data. Scalar relativistic effects are included through the use of the one-electron Douglas–Kroll operator. Spin–orbit coupling is accounted for perturbatively using the atomic mean-field spin–orbit operator. Both scalar relativistic and spin–orbit transition dipole moments are presented. The importance of correlation of 3d-orbitals is demonstrated, and recommended values of spectroscopic constants for the ground state are provided.

Activation of methane by oxidative addition and sigma-bond metathesis has been investigated for (N-N)M(CH3) (M = Pd+, Pt+, Rh+, Ir+, Rh, Ir; N-N = (HN=CH-CH=NH) using different density functional approaches. The pathway of oxidative addition is in general favored, the exceptions being Pd+ and Rh+. Oxidative addition is clearly more favorable for the third-row metal complexes than those of the second row. The third-row metal complexes also tend to have a lower activation barrier for sigma-bond metathesis than those of the second row. In each case, the oxidative addition is preceded by formation of a sigma complex. The bonding energies of these complexes are significantly stronger for the cationic systems. (C) 2003 Wiley Periodicals, Inc.

We have tested the performance of four-component relativistic density functional theory (DFT) by calculating spectroscopic constants (re, ωe, and ωexe) and dipole moments μ0 in the vibrational ground state for a selected set of 14 molecules: the hydrogen halides HX, the dihalogens X2, as well as the interhalogens XY (X,Y=F, Cl, Br, and I). These molecules have previously been studied by four-component relativistic wave function based methods by Visscher and co-workers [J. Chem. Phys. 108, 5177 (1998); 104, 9040 (1996); 105, 1987 (1996)]. We have used four different nonrelativistic functionals at the DZ and TZ basis set level. What is perhaps the most striking result of our study is the overall good performance of the local density approximation functional SVWN5; at the triple zeta basis set level it predicts bond lengths re, harmonic frequencies ωe, anharmonicities ωexe, and dipole moments μ0 with relative errors of 0.46%, 0.39%, −16.3%, and −0.74%, respectively. The corresponding values for the B3LYP hybrid functional are 1.27%, −2.10%, −20.4%, and 4.71%. The two generalized gradient approximation functionals PW86 and BLYP show a less convincing performance, characterized by a systematic overestimation of bond lengths and underestimation of harmonic frequencies. We show that only the constant term is modified in second-order vibrational perturbation theory upon the inclusion of a linear term, corresponding to the choice of a nonstationary reference geometry. Upon shifting the reference geometry from the optimized to the experimental geometry the calculated harmonic frequencies are significantly improved, whereas the anharmonicities are basically unchanged. Dipole moments calculated at the experimental geometry at the B3LYP/TZ level appear to be remarkably accurate with a mean relative error of −1.1% and a standard deviation of less than 4%. Our study reveals that anharmonicities are quite sensitive to the numerical integration scheme employed in the DFT calculations, and for the interhalogens we had to modify the Becke partitioning scheme by using atomic adjustments along the lines of the atom in molecules approach of Bader.

We have studied the effect of relativity, electron correlation and the lanthanide contraction on the spectroscopic constants, dissociation energies and dipole moments of the homologous series CsAu, CsAg, and CsCu. We observe a relativistic strengthening of the bond in all cases, particularly for CsAu, but all three compounds are predicted to exist on both the relativistic and nonrelativistic levels of theory with bond strengths more than 1 eV. The effect of the lanthanide contraction on the bonding in CsAu has been studied using a pseudoatom model of the Au atom and is shown to contribute to the strength and polarity of the bond, albeit to a lesser degree than relativity. We present a new estimate of the experimentally derived value of the CsAu dissociation energy using spectroscopic constants calculated at the coupled cluster CCSD(T) level. The new value (2.53 eV) is slightly lower than the previous estimate by Busse and Weil (2.58 eV) and is in excellent agreement with the corresponding CCSD(T) value. We have employed 4-component density functional theory at the B3LYP level, and the spectroscopic constants calculated with this method are in good agreement with coupled cluster results. For dipole moments the B3LYP values appear on the other hand to be too low and this requires further investigation.

Dynamic dipole polarizabilities for the ground 4 1S and the low-lying 4 1,3P and 5 1,3S excited states of Zn are calculated by the time-dependent gauge-invariant method and compared with other experimental and theoretical results. The wavefunctions are obtained from multi-reference configuration-interaction calculations using a two-electron relativistic pseudopotential. Core-valence polarization is accounted for by the use of a semi-empirical core-valence potential. Core-polarization effects are also considered when calculating the oscillator strengths using a modified dipole transition operator. Long-range coefficients for the molecular states of Zn2 dissociating into: 4 1S + 4 1S; 4 1S + 4 3P; 4 1S + 4 1P; 4 1S + 5 1S and 4 1S + 5 3S are presented.

Dynamic dipole polarizabilities for the ground 4 S-1 and the low-lying 4 P-1,P-3 and 5 S-1,S-3 excited states of Zn are calculated by the time-dependent gauge-invariant method and compared with other experimental and theoretical results. The wavefunctions are obtained from multi-reference configuration-interaction calculations using a two-electron relativistic pseudopotential. Core-valence polarization is accounted for by the use of a semi-empirical core-valence potential. Core-polarization effects are also considered when calculating the oscillator strengths using a modified dipole transition operator. Long-range coefficients for the molecular states of Zn-2 dissociating into: 4 S-1 + 4 S-1; 4 S-1 + 4 P-3; 4 S-1 + 4 P-1; 4 S-1 + 5 S-1 and 4 S-1 + 5 S-3 are presented.

IR spectroscopic measurements:have been used to chart the reactions activated by irradiating argon matrices doped with methyltrioxorhenium, CH3ReO3 (1), first with light sit wavelengths near 254 nm and subsequently with broad-band UV-visible light (200 less than or equal to lambda less than or equal to 800 nm). Assignments of the IR absorption bands are made and. their carriers identified on the basis of experiments with CD3ReO3 or (CH3ReO3)-C-13 and by comparison either with the vibrational. properties forecast by density functional theory (DFT) calculations or with those of related molecules. Photoexcitation at lambda = ca. 254 nm results in tautomerization of CH3ReO3 to the methylidene derivative H2C=Re(O)(2)OH (2), the properties of which have been determined, partly by experiment and partly by DFT calculations. The mechanism of the change is discussed, as is the relevance of the tautomeric product to the catalytic action of CH3ReO3 in olefin metathesis. Broad-band UV-visible radiation leads to the formation of a rhenium carbonyl, 3, possibly the Re(V) compound H2Re(CO)(O)OH.

The two iso-electronic molecules NpO4- and UO42- have been investigated by means of different ab initio methods: Hartree-Fock, Moller-Plesset perturbation theory at the second order, complete active space perturbation theory at the second order (CASPT2), coupled pair functional, coupled cluster single double with perturbative contributions from triple excitations, and density functional theory with hybrid functionals. Relativistic effective core potentials have been used in all calculations. NpO4- is a square planar molecule, whereas UO42- has a tetrahedral structure. The 5f orbitals, and, in particular, their lower energy in the neptunium compound that make them more available to form covalent bonds, play a crucial role in explaining the different structures of the two compounds.

The molecular structures of WCl6, WCl5, WCl4, WCl3 and the dimer W2Cl6 have been optimised by density functional theory calculations at the B3LYP level using the quasi-relativistic electron core potential in combination with basis sets of DZ+P quality. The experimental, octahedral structure and the vibrational frequencies of the hexachloride are well reproduced by the calculations. Calculations on the pentachloride under D3h symmetry indicated an orbitally degenerate (2E″) ground state with bond distances in good agreement with experiment. The Jahn–Teller (J–T) energy of distortion to C2v symmetry was calculated to 1.8 kJ mol−1, an order of magnitude smaller than the estimated spin–orbit (L–S) coupling energy. This is in agreement with an earlier suggestion that J–T distortion is this molecule is quenched by L–S coupling. The ground state of WCl4 is found to be a tetrahedral (Td) triplet, that of WCl3 a trigonal planar (D3h) quartet and that of W2Cl6 to be a metal–metal bonded, ethane-shaped singlet. The structures indicated for these molecules are very different from those previously derived from gas electron diffraction data. Investigation by mass spectrometry indicates, however, that the composition of the vapours is much more complex and quite different from what was assumed during the structure analysis. The experimental structures are therefore open to question.

The dissociative chemisorption of molecular hydrogen on the gold surface, modeled by a seven atom cluster, has been studied using explicitly correlated wave functions. The spin-orbit effect on the barrier was calculated using the microscopic spin-orbit Hamiltonian. The reaction is found to be endothermic by about 33kcal/mol with a barrier to dissociative chemisorption of about 45kcal/mol. The barrier to associative desorption is estimated to be of the order of 12kcal/mol. The spin-orbit effect on the barrier was 1.2kcal/mol and estimated to be insignificant for the infinite surface.

The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second–order Møller–Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work.

The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second-order Moller-Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work.

Fully relativistic configuration-interaction calculations, based bn the Dirac-Coulomb Hamiltonian, were performed on the ground state of HCl, the Cl 2p-ionized HCl+, and the 3p(pi, sigma)(-2) states of HCl2+. Calculated ionization energies and bond lengths were found to be in good agreement with recent experimental results and previous calculations. The potential-energy curves were used in calculating vibrational profiles of selected Auger transitions. Nonadiabatic effects in the spin-orbit-induced avoided level crossing were investigated using diabatic electronic basis sets.

Fully relativistic, self-consistent field calculations, based on the Dirac-Coulomb-Gaunt Hamiltonian, were performed on the ground state of HBr, the Br 3d-ionized HBr+, and the 4p(pi,sigma)(-2) states of HBr2+. Correlation in the ground and valence-excited states and partially in the Br 3d ionized states was described using a configuration-interaction (CI) method. Calculated ionization energies and bond lengths were found to be in good agreement with recent experimental results. The distortion in the vibrational bands of the 3d(-1)-->4p pi(-2) Auger transitions due to lifetime vibrational interference was verified through ab initio calculations. Bands due to the transitions to bound and continuum vibrational states of the same electronic state were reproduced by calculations and compared with experimentally determined profiles. The nonadiabatic effects in the spin-orbit-induced avoided level crossing were investigated using adiabatic and diabatic electronic basis sets.

The geometrical and electronic structure of (H3SiO)3Si−O−R clusters (R=H, B(CH3)2, Al(CH3)2, and ZnCH3; n=0, 1, or 2) modeling a −OR group chemisorbed on a SiO2 surface was studied theoretically with the use of ab initio quantum chemical calculations at the MP2 and B3LYP levels. Various modes of coordination of the organometallic groups at the SiO2 surface were considered. For the Al-containing surface group, two stable structures were found: an open structure and a cyclic structure with the Al atom involved in additional coordination with one of the neighboring oxygen atoms. At the best computational level, only one stable structure was located for the B- and Zn-containing surface fragments, in which the central atom of the surface group (Zn or B) only weakly interacts with the second surface oxygen atom. The calculated vibrational frequencies were compared with the experimental ones and, on this basis, the possible reaction pathways of chemical modification were discussed.

Matrix-isolated [CH3ReO3] tautomerises to [H2C=Re(O)(2)OH] under the influence of UV light; the carbene has been characterised in its normal and H-2- and C-13-enriched isotopic forms by its IR spectrum with results well replicated by quantum chemical calculations.

The structures of the UO2(aq)(2+) ion and of the uranium(VI) hydroxide complex(es) formed in strongly alkaline solution have been investigated theoretically using molecular-orbital based quantum chemical methods, and experimentally using EXAFS methodology. Relativity was included explicitly through the Douglas-Kroll transformation. The uranium atom was described at the ECP level, using the AIMP methodology. The structures of [UO2(H2O)(5)](2+), and the hydroxide complexes, viz., [UO2(OH)(4).(H2O)](2-.)[UO2(OH)(4)](2-).(H2O), [UO2(O)(OH)(2)](2-). 2(H2O), and [UO2(OH)(5)](3-), were optimized at the SCF level, using gradient techniques, while the relative stabilities were calculated at the TvIP2 level of approximation. The third structure contains three coordinated ligands, one of which is an oxide ion, in the plane perpendicular to the linear UO2-unit. Complexes of this type have not been experimentally identified for U(VI); however, they are formed for the iso-electronic Np(VII). The experimental EXAFS data indicates that the complex(es) formed is(are) mononuclear. The number of coordinated ligands in the equatorial plane is 4.5 +/- 0.4, while the bond distances are the same within the experimental errors, as in a previous study of [Co(NH3)(6)3(+)](2)[UO2(OH)(4)(2-)](3). 2H(2)O, by Clark et aI. An EXAFS model where the coordination number is fixed to four, is only marginally less precise than the model without constraints on the coordination number. This fact together with the close agreement between experimental and theoretically observed variations in bond distances between the different structure models provides a strong indication for the formation of [UO2(OH)(4)](2-) in solution. This is an unusual coordination number for uranium(VI) complexes, previously found in sterically crowded systems such as UO2Cl42-.

The molecular structure of NbCl5 was determined experimentally by gas electron diffraction and computationally by structure optimisation of D3h models. The bond distances obtained by ab initio calculations with very large basis sets, relativistic effects included through the one-electron Douglas–Kroll method and all electrons correlated at the MP2 level and by gas electron diffraction are: (calc/exp) Nb–Clax=230.7/230.6(5)pm and Nb–Cleq=227.0/227.5(4)pm.

An ab initio study of the structure, binding energies, electronic distribution, vibrational frequencies, and thermodynamic parameters of possible gas-phase complexes of silicon tetrafluoride with water (1:1, 2:1, and 1:2) in different conformations has been performed at the RHF, B3LYP, MP2, and MP4(sdq) levels with a variety of basis sets up to cc-pVTZ. On this basis, the thermodynamic stability, assignment of observed infrared bands of SiF4/H2O adducts in matrixes and in the gas phase, and activation energies of the elementary reactions leading to polyhydroxy derivatives and siloxane are discussed. The most thermodynamically stable structure is a 1:1 complex with the most accurate estimate of the binding energy of 2.7 kcal/mol. This complex is characterized by a small amount of electron transfer from the water molecule to silicon tetrafluoride and by small participation of the silicon d-orbitals in the coordination bond. A single stable conformation has been found for complex 2SiF(4). H2O whereas the complex SiF4. 2H(2)O can exist in two conformations (distorted octahedral symmetry), distinguished by positions of the water molecules. The energetically preferred conformation of SiF4. 2H(2)O is that with the water molecules in cis positions of an octahedron. The activation energy for the hydrolysis of first Si-F bond is 21 kcal/mol. The most thermodynamically preferred product at the first stages of hydrolysis is the hexafluorodisiloxane SiF3-O-SiF3 (activation energy 30 kcal/mol) rather than polyhydroxy derivatives.

The structural model of the trimethyldioxorhenium molecule, Me3ReO2 (1), has been revised on the basis of analysis of its H-1, C-13, and O-17 NMR and vibrational spectra and of its gas electron diffraction (GED) pattern. The results are consistent with the molecular symmetry C-s; in the new model both oxo ligands together with one methyl group are located in the equatorial plane of a distorted trigonal bipyramid. Structure optimization by density functional theory (DFT) calculations and least-squares refinement to the GED data yield the valence angles (calc/expt; eq = equatorial; ax = axial; av = average) angle CeqReO = 118.0/118.5(10)degrees and angle CeqReCax = 74.3/73.5(11)degrees. The pseudoaxial Re-C bond distance is found to be shorter than the equatorial one, viz., Re-C-ax = 2.130/2.122(6) Angstrom versus Re-C-eq = 2.193/ 2.199(22) Angstrom, and Re=O-av = 1.739/1.703(3) Angstrom. It is suggested that the distortion from trigonal bipyramidal to edge-bridged tetrahedral coordination geometry is driven by the need for the axial C atoms to achieve optimal overlap with both the d(z)2 and d(yz) orbitals on the Re atom. The DFT calculations indicate that the axial methyl groups are tilted in such a manner that the angle ReCH valence angles in the ReC3 plane are reduced to 100.8 degrees. It is suggested that this tilting is due in part to bent Re-C-ax bonds and in part to weak C-H ... Re agostic interactions.