Roland Lissillour

Electronic structure of Chevrel Phases: Emphasis on the part played by the cluster twisting angle through a LCAO-EHT tight binding study

Abstract The electronic structure of PbMo6S8 has been investigated using the so-called EHT-tight-binding method. An idealized cubic and an experimental distorted geometry are used, with standard atomic orbital parameters and diagonal H-matrix elements. A full band structure is presented in the idealized case. In both cases, the density of state curves computed using an integration over the Brillouin zone are also presented. The results show the importance of the twisting angle on the electronic structure near the Fermi level.

Méthode de Hückel étendue aux électrons de valence s, p, d

A new E.H.T. program, with an s, p and d orbital basis, is used for the study of ferrocene molecule and its mono and dichloro derivatives. For ferrocene, this method gives results with a precision comparable with more sophisticated calculations. The study of the variation of total energy, including an interatomic Madelung correction, as a function of rotational angle of the cyclopentadienyl rings, shows the existence of several stable conformations. Dipole moments calculated according to an expression previously calibrated are in good agreement with the experimental results.

Electronic structure of ZrNi4P2 and related compounds with ZrFe4Si2-type structure

Extended Huckel molecular orbital and tight-binding band structure calculations have been performed on ZrNi 4 P 2 and then applied through a rigid band model to related compounds with the same ZrFe 4 Si 2 -type structure. The electronic structure has been built up step by step, from fragments, in order to obtain a better understanding of the main interactions which govern the evolution of the lengths of the metal-metal bonds and the lattice cohesion. The role of the 3d metal electron count and that of the ligand matrix are displayed and discussed

Linear dependence ofXα eigenvalues on occupation number application to the determination of the ligand field integrals

A linear relation between theXα eigenvalues and the occupation number of the ligand field states in CrCl4 is established and used to calculate mono- and bimolecular integrals of ligand field interest. A formula for calculating 10 Dq and Koide-Pryces parameter (ε) from theXα eigenvalues is given. The method is generalized for anydN system.

Electronic structure of the pseudo-one-dimensional superconductor Tl2Mo6Se6 according to SCF scattered-wave Xα computations

Abstract The electronic structure of Tl 2 Mo 6 Se 6 has been investigated using the Xα cluster approach. The ground-state eigenvalues show that the Fermi level lies in a single degenerated band which supports the experimental results. Contour maps show the anisotropic character of the wawe functions of this Fermi level. The metal-metal and metal-selenides interactions have been discussed.

A PM3 versus AM1 and RHF versus UHF study of the radical reactivity of vinyl monomers

Abstract A quantum mechanical study of the methyl radical addition to ethylene, butadiene and propene was carried out using AM1 and PM3 in the RHF and UHF schemes. The results show that only PM3/UHF is able to reproduce correctly the relative activation energies of the reactions under consideration. The work was then extended using PM3/UHF to acrylonitrile, methyl acrylate, vinyl chloride and benzene.

Theoretical analysis of the metal-metal bond alternation phenomenon leading to M4 tetrahedral metal clusters in GaMo4S8-type compounds

Abstract The Mo4 tetrahedral metal clusters observed in the crystal structure of GaMo4S8-type compounds are described as arising from the deformation of a highly symmetrical structure with MoS6 octahedral local stoichiometry. Then Mo4 clusters occur at the crossing of MoS4 distorted linear chains made of MoS6 octahedra sharing their opposite edges. The main factors (orbital interactions and electron counts) governing the pairing distortions, which lead to a metal—metal alternation phenomenon and then to cluster formation, are analysed through extended Huckel molecular orbital calculations. It is shown that the tetrahedral clustering corresponds to a multidirectional Peierls distortion in an extended network.

Theoretical study of the methylic reactivity of substituted alkenes

Abstract A PM3/UHF study of the methylic reactivity of a series of about fifty alkenes was carried out. Several MP2/6-31G∗∗ ab initio calculations were made for comparison. The semi-empirical method of calculation is proven to give good estimates of the reactivity under consideration. The computations brought to light the reactivity enhancement of alkenes when substituted on the same carbon of the double bond, whereas a lessening is observed when both carbons are substituted. An interpretation in terms of deformation (DBF) and interaction (INT) energies is made. Contrarily to phenyl, the vinyl group improves notably the reactivity of methylated and chlorinated mono or disubstituted alkenes. The prediction is made for the reactivity of more than fifteen compounds which were not studied experimentally.

Extended Hckel theory: A new population analysis and its application to forecast chemical bond lengths

Although widely used, the population analysis proposed by Mulliken has been contested by several authors. A new analysis, very easily computed on the orthogonal basis, is here proposed and applied to the EHT wave function.Under its usual presentation, the EHT method is unable to directly evaluate bond lengths through an energy minimum condition. However, it is possible to settle an empirical quadratic relation between the bond lengthRrs and a quantity calledprs, similar to a bond population. Such relations are given for bonds of the CC, CN, CO, CS, CF, CCl, CBr, CH, NO and OH types.The examination of the variation of the bond population under a variation of the bond length has enabled us to prove that this semi-empirical relation was usable in an iterative process: starting from bond lengths taken from any systematic table, it is possible, for a given molecule, to evaluate the bond length consistent with experimental values within an accuracy of 0.03 Å. Some examples, concerning cyclic or acyclic molecules and various kinds of bonds, are given.