Hyperpolarizabilities of substituted conjugated molecules. II. Substituent effects and respective σ–π contributions

Introduced in a one-electron self-consistent model (D–C field perturbated INDO Hamiltonian), symmetry considerations exhibit σ and π electron contributions to second order polarizabilities of planar molecules. A family of benzene monosubstitutes is explored, their mesomeric moments and π second order hyperpolarizabilities are seen to account for the donor or acceptor nature of the substituent radical in accordance with a previous model and experiments. Influence of the INDO parametrization is studied for nitrobenzene, where an artificial variation of the oxygen affinity yields a satisfactory value for the benzene third order hyperpolarizability.