Resonance Raman studies of the ¹L_a state of 1,2,3-trisubstituted benzene derivatives: Lack of an induced transition moment

Resonance Raman spectra of 1,2,3-trichlorobenzene and 1,2,3-trimethylbenzene obtained with excitation in the region of the ¹L_a electronic state exhibit the pattern of a vibronically induced transition similar to that observed for benzene and symmetric 1,3,5-trisubstituted derivatives in contrast to the allowed (B-type) pattern observed for monosubstituted and 1,4-disubstituted species. This behavior is consistent with early ideas of Sklar, Platt, Förster, Moffitt and Petruska of the perturbative vector nature of the allowed intensity component resulting from substituent effects in benzene derivatives. The present work represents the first quantitative demonstration of the applicability of these ideas to the ¹L_a state of benzene derivatives.