Alain Strich

Bulk properties or not: The electronic structure of small metal clusters

The dependence of the properties of small copper clusters on their size, and their relationships to the properties of the bulk metal have been studied through ab initio SCF calculations (with the emphasis on the clusters Cu8 and Cu13). The basis set used is of double‐zeta quality for the valence shells 3d and 4s. The analysis focuses on the following properties of the clusters: geometrical structure, binding energy, and distribution of energy levels. For the 13‐atom cluster, the icosahedron is found more stable than the cubo‐octahedron corresponding to the fcc structure of the bulk metal. The binding energy per atom increases almost linearly with the number of atoms of the cluster. From the orbital energy values, the sets of 3d and 4s levels are well separated for Cu8 and just begin to overlap for Cu13. This situation looks rather different from the one for the bulk metal where the s band totally overlaps the d band. The relationship between the orbital energies from the ab initio SCF calculations and the...

Gaussian basis sets for the transition metals of the first and second series

Gaussian basis sets consisting of, respectively, (13s, 7p, 5d) and (14s, 8p, 7d) Gaussian functions have been optimized for the transition metal atoms of the first and second series. The optimization criteria and the applicability of these atomic sets for molecular calculations are discussed.

An ab initio LCAO-MO-SCF study of reaction paths for proton transfer in ammonium aqueous solution

The possible mechanisms for proton transfer in ammonium aqueous solutions are discussed through ab initio LCAO-MO-SCF calculations for the following hydrogen-bonded complexes : [NH4 + … NH3] ; [NH4 + … OH2] ; [NH4 + … OH2 … OH2] ; [NH4 + … OH2 … NH3] and [H2O … NH4 + … OH2 … OH2]. The energy curve along the reaction coordinate is drawn for the first three systems. A double well potential curve is obtained for the two symmetrical systems with a very low barrier to proton transfer : 2·9 kcal/mole for the system [NH4 + … NH3] and 4·3 kcal/mole for the system [NH4 + … H2O … NH3]. For both systems the exchange mechanism involves three successive steps : association, transfer and dissociation. Solvation may affect the energetics of the first and third steps. For the unsymmetrical system NH4 + + H2O, the energy would increase continuously during the steps of proton transfer and dissociation. Hence the process of proton transfer between an ammonium ion and a water molecule may take place in solution only if assis...

Electronic spectroscopy of HRe(CO)5: a CASSCF/CASPT2 and TD-DFT study

Abstract The low-lying excited states of HRe(CO) 5 have been calculated at the CASSCF/CASPT2 and TD-DFT level of theory using relativistic effective core potentials (ECP) or ab initio model potentials (AIMP). The theoretical absorption spectrum is compared to the experimental one. Despite the similarity between the experimental absorption spectra of HMn(CO) 5 and HRe(CO) 5 in the UV/visible energy domain it is shown that the assignment differs significantly between the two molecules. The low-lying excited states of HRe(CO) 5 correspond to 5d→π * CO excitations whereas the spectrum of HMn(CO) 5 consists mainly of 3d→3d and 3d→ σ * Mn–H excitations. If the CASPT2 and TD-DFT results are quite comparable for the lowest excited states, the upper part assignment is more problematic with the TD-DFT method.

Quantum chemical study of the electronic structure of NiCH2+ in its ground state and low-lying electronic excited states

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.

A theoretical study of co insertion reactions: an assessment of electron correlation effects

Abstract MP2 and CAS SCF calculations for CO insertion reactions of the type RMCO → M(COR), R = H, CH 3 ; M = Mn, Pd + are reported. Non-dynamical correlation appears to be quite important, involving essentially the metal-carbon π bond in the Mn(I) systems and the metal-carbon and metal-hydrogen σ bonds in the Pd(II) systems.

A CAS SCF CCI study of the lowest excited states of HMn(CO)5 and Fe(CO)5

Abstract Contracted CI calculations, based on CAS SCF reference wavefunctions, are reported for the lowest excited states of HMn(CO) 5 and Fe(CO) 5 . The results are used to discuss the photochemical reactions of these organometallics.

The electronic spectroscopy of transition metal di-hydrides H2M(CO)4(M = Fe,Os): a theoretical study based on CASSCF/MS-CASPT2 and TD-DFT

Transition energies to the low-lying singlet electronic excited states of H2M(CO)4 (M = Fe,Os) are calculated at the CASSCF/MS-CASPT2 level of theory using relativistic effective core potentials in the ab initio model potential (AIMP) approach. The main features of the absorption spectra of both molecules differ significantly. The spectrum of H2Fe(CO)4 is dominated by low-lying excited states corresponding mainly to 3dFe → σ*u and 3dFe → σ*g excitations calculated between 39 310 cm−1 (4.9 eV) and 43 210 cm−1 (5.4 eV). These allowed transitions contribute to the shoulder observed in the experimental spectrum around 270 nm (37 040 cm−1) and are responsible for the photoreactivity of H2Fe(CO)4 at 254 nm (39 370 cm−1). In contrast the lowest part of the absorption spectrum of the osmium analog H2Os(CO)4 is characterized by a high density of metal–ligand-charge-transfer (MLCT) states between 47 220 cm−1 (5.9 eV) and 52 430 cm−1 (6.55 eV) and corresponding to 5dOs → π*CO excitations. The transitions corresponding to 5dOs → σ*u and 5dOs → σ*g excitations are displaced to the upper part of the absorption spectrum of H2Os(CO)4 (beyond 60 000 cm−1 or 166 nm) and cannot induce H2 elimination under irradiation at 254 nm like in the iron complex. A series of sigma-bond–ligand-charge-transfer (SBLCT) states corresponding to σg → π*CO and σu → π*CO excitations not present in H2Fe(CO)4 is found exclusively in the spectrum of H2Os(CO)4 beyond 55 000 cm−1. The upper part of the spectrum of H2Fe(CO)4 (below 220 nm) is assigned to intense MLCT transitions corresponding to 3dFe → π*CO excitations. The MS-CASPT2 and TD-DFT methods agree qualitatively as far as the assignment and transition energies of the low-lying states are concerned in both molecules whereas a comparison of the calculated oscillator strengths is more problematic. The transitions are generally underestimated at the TD-DFT level, an effect that is even more pronounced for the charge transfer excitations to the carbonyl ligands. The high density of singlet electronic states in these simple transition metal hydrides carbonyls illustrates the complexity of the electronic spectroscopy in this class of molecules. This explains the poor resolution of these overcrowded spectra and the difficulty in understanding the photoreactivity of these molecules.

Electric properties of the water molecule in 1A1, 1B1, and 3B1 electronic states: Refined CASSCF and CASPT2 calculations

SummaryThe dipole moments and dipole polarizabilities of the 1A1, 1B1, and 3B1 electronic states of the water molecule have been calculated by using the CASSCF approach followed by the evaluation of the dynamic electron correlation contribution by the second-order perturbation scheme CASPT2. All calculations have been carried out in a specifically extended ANO basis set which accounts for the Rydberg character of the two excited states. In order to estimate the correctness and accuracy of the present data a scan over a variety of different active spaces for the CASSCF wave function has been made. The present results are superior to earlier CASSCF calculations, although their qualitative features remain essentially the same. The dipole moments in 1B1 and 3B1 states are predicted to be about 0.49 a.u. and 0.33 a.u., respectively, and have the opposite orientation with respect to the ground state dipole moment. The dipole polarizability tensors of the excited states are characterized by high anisotropy and are dominated by the in-plane component perpendicular to the symmetry axis. All their components are found to be about an order of magnitude larger than those of the ground state polarizability tensor. The excitation energy dependence on the choice of the active orbital space in the CASSCF reference function is also considered and the analysis of the present data concludes in the concept of what is called the mutually compatible active spaces for the two states involved in excitation. All CASPT2 results are in good agreement with the results of recent calculations carried out in the framework of the open-shell coupled cluster formalism. This agreement confirms the high efficiency of the CASSCF/CASPT2 approach to the treatment of the electron correlation effects.

The metal-carbon triple bond in Cl(Co)4crCH: A CAS SCF study of neutral carbyne complexes

Abstract Calculated SCF and CAS SCF potential curves are reported for the ground state of the Cl(CO)4CrCH carbyne complex. The totality of the computed bond energy (115 kcal mole ) is recovered from near-degeneracy correlation effects mainly involving the metal-carbyne π bonds. The Cr-C π population is almost equally distributed between the two atoms, but the σ bond retains the character of a donation from the carbyne carbon to chromium.