Molecular Orbital Theory of Electric Polarizabilities and Magnetic Susceptibilities of Hydrocarbons

Abstract

Hartree–Fock perturbation theory and the molecular orbital method are used to develop a consistent theory of the polarizabilities and susceptibilities of hydrocarbons. When each unperturbed MO is expanded in terms of a set of basis orbitals it is found that in the presence of an electric or magnetic field the corresponding perturbed orbitals have two parts, a molecular or “nonlocal” one and a “local” part which takes into account the effect of the field on the basis orbitals themselves. When certain integrals are neglected the polarizability and susceptibility can also be divided into local and nonlocal parts which is a result generally assumed without proof. For saturated hydrocarbons, the contribution due to the nonlocal terms in the polarizability is zero so that there are only local terms in the final expression, while for unsaturated molecules there are both local and nonlocal contributions. In the case of the magnetic susceptibility the molecular (nonlocal) parts of the paramagnetic term are important—and it is necessary they be treated accurately—because they cancel with terms from the diamagnetic part of the susceptibility and so give a final expression which is largely local in character.