Jean Hoarau

π‐electron calculations of polarizabilities in conjugated systems

Finite‐field technique has been applied to the calculation of π molecular polarizabilities within the Pariser–Parr–Pople Hamiltonian. This formalism allows to analyze large oligomers containing up to 400 atoms, and asymptotic behaviors may be attained in some cases. We have investigated the role of the polymer size, the backbone geometries, the presence of neutral and charged defects (solitons, polarons, bipolarons), the chemical nature of the monomeric unit on the polarizabilities. Generally, the polarizabilities are not deeply modified by a change of the geometrical characteristics, and may lead to huge values for sufficiently large chains. Besides, the charge of the system is the leading factor which determines the values of this property. The evolution of the polarizability with the number N of π atomic centers, αu depends largely on the charge, and on the defect. For the neutral systems, the polarizability per monomeric unit αu increases smoothly, and then exhibits an asymptotic behavior with N. For ...

Extended Huckel calculations of carbon chemical shifts with the aid of Gauge-Invariant Atomic Orbitals

Abstract 13 C shielding constants have been calculated by means of the Extended Huckel Theory using Gauge-Invariant Atomic Orbitals (GIAO) and including a large number of terms arising from the Hamiltonian and orbital expansions. The results indicate that the method can account for the general trends of 13 C chemical shifts. It is shown that an approximation in which the paramagnetic contribution is calculated, by retaining a determined type of two-centre terms, gives a correct description of the observed effects.

Extended London theory of diamagnetism. I. General theory and saturated hydrocarbons

In this paper a method is proposed to calculate diamagnetic susceptibilities of organic compounds. This method uses the extended Huckel formalism generalized to include the perturbation due to a magnetic field H. Parameters are varied to obtain agreement with experimental results. The diamagnetic properties of the alkanes are examined and all the terms contributing to the susceptibility are analyzed. The importance of a coupling term between matrix elements of first order in H and London terms is shown. Some results are discussed, particular to methane and the cycloalkanes.

Etude variationnelle d'une double perturbation. Application au calcul des constantes de couplage entre spins nucleaires

The influence of the perturbed variational function upon the calculation of a double perturbation is shown and the convergence of the results is analysed when this function is developed upon a basis set. In coupling constant calculations, the singularity of the Fermi contact term leads to a divergence if a correct variational calculation is performed. The different works concerning this subject are analysed and a method is proposed.

Etude variationnelle d'une double perturbation. Application au calcul des constantes de couplage entre spins nuclaires@@@Double perturbation variational study. Application to nuclear spin-spin coupling constants: II. Mthode de la fonction d'onde perturbe@@@II. Perturbed wavefunction method

Fermi contact contribution to nuclear spin coupling constants of the H-D molecule is calculated with two kinds of variational functions which include a singularity at the nuclei. With the first function the total perturbed second order energy is minimized, while with the second one we have used the minimum condition of the self-coupling energy. These calculations necessitate a perturbation operator having a weaker singularity than the classical Fermi operator. We have used the Gregson, Hall, and Rees operator.

π‐electron polarizabilities of infinite organic polymers

The contribution of the π electrons to the polarizabilities of finite and infinite systems have been calculated for infinite one‐dimensional chains modeling organic polymers, such as polyacetylene and various polyheterocycles. In the first part, results obtained previously using a finite‐field technique within the Huckel and Pariser–Parr–Pople (PPP) models applied to polymers containing up to 400 atoms have been compared to results obtained from the sum‐over‐states (SOS) perturbation theory. The calculated polarizabilities depend on the type of monomer following polyacetylene ≥ polythiophene ≥ polypyrrole. This result is qualitatively independent on the type of model or formalism. In the second part, we present results of the application of the Genkin–Mednis formalism within the PPP model to the calculation of the π polarizability of infinite one‐dimensional chains. Numerical difficulties arise from the phase of the linear‐combination‐of‐atomic‐orbital (LCAO) coefficients corresponding to different k poin...

Extended Hückel calculation of proton chemical shifts with the aid of gauge invariant atomic orbitals. Application to hydrocarbons

A theory of proton magnetic shielding constants is developed within the Extended Huckel formalism using gauge invariant atomic orbitals. The major part of the terms arising from the hamiltonian and orbitals expansions is evaluated. The method is applicable to relatively large molecules and the calculated chemical shifts agree to generally better than 1 p.p.m. with the experimental values. The results for alkanes, ethylenic compounds and benzene are analysed.

Extended London theory of diamagnetism Unsaturated hydrocarbons

The extended London theory of diamagnetism is applied to the calculation of the susceptibilities and the anisotropies of unsaturated hydrocarbons. The agreement with experimental results is good.

Comparative study on the convergence of the nuclear spin--spin coupling constant in the H--D molecule

A numerical estimate is given of the improvement in the determination of nuclear spin–spin coupling constant of HD molecule, using the method of Schwartz? The variation perturbation technique employing the Fermi‐contact operator is discussed.(AIP)

Application des orbitales atomiques perturbées au calcul de la constante de couplage entre les spins nucléaires de la molécule HD

Nuclear spin-spin coupling constant JHD of the deuterated hydrogen molecule is determined by minimizing the self-coupling energy evaluated with the Gregson, Hall and Rees operator. The family of variational functions used is suggested by the form of the hydrogen-like atomic orbitals perturbed by this operator.The comparison of the results with those previously obtained with a different family of functions shows that it is necessary for those calculations to use a perturbated function which is as perfectly adapted to the perturbation in the immediate neighbourhood of the nucleus, as in the remainder of the molecular domain.