Theory of resonance Raman scattering in benzene derivatives

A simple model is formulated which allows for study of the Raman excitation profiles and depolarization dispersion curves of fundamentals and overtones of Franck-Condon and pseudo-Jahn-Teller active modes in substituted benzenes in the resonance with the analogs of the 1 A_1g→1 B_2u and 1 A_1g→1 B_1u transitions of benzene. Three excited electronic states and three vibrational modes are taken into account. Of the former, one represents the B state whereas the other two are the components of the degenerate 1 E_1u state of benzene. Of the latter, one is the analog of the ring breathing mode ν₁ whereas the two others are analogs of the two components of the e_2g mode responsible for the vibronically induced moment of the optical transition to the B state (ν₈ mode for the 1 B_1u state). The substituent effect is introduced by an electronic matrix element which mixes the B state under consideration with one of the orbital components of the E state. Resonance Raman spectra resulting from the model are discussed in the light of available experimental data. The effect of the relative position of identical substituents on these spectra is also investigated. Ab initio calculations of the normal modes of chloro derivatives of benzene and of their resonance Raman activity at the S₀→S₂ transition are also presented and discussed in terms of the model.