Conformational dependence of vibrational and molecular nonlinear optical properties in substituted benzenes:: the role of π-electron conjugation and back-donation

In this article we focus on the conformational dependence of molecular hyperpolarizabilities and on the related infrared and Raman intensities for molecules where pi electron delocalization and/or back-donation by oxygen lone pairs can occur. Ab initio calculations at the Hartree-Fock level have been performed on anisole and 4-nitro-anisole with 3-21G and 6-31G** basis sets. Vibrational properties and second and third order molecular polarizabilities have been studied as a function of the torsional angle describing the rotation of the O-(CH3) group about the phi-O bond. In these systems two competing effects can take place, namely back-donation and conjugation. From the study of infrared intensities it is shown that no back-donation of electrons from the oxygen lone pairs takes place, independently of the conformation of the methoxy-group. These results are used in the interpretation of the infrared spectrum of many compounds which show a characteristic strong and low frequency infrared CH stretching vibration, previously assigned to a back-donation effect. However, conjugation dependent optical properties, such as the vibrational pattern, the intensity of the collective modes, the first (beta) and second order (gamma) hyperpolarizabilities are almost constant with respect to changes of the torsional angle. (C) 1999 Elsevier Science B.V. All rights reserved.