Vibronic coupling activity in the resonance Raman spectra of alkyl benzenes

The Raman spectra of the alkyl benzene derivatives toluene, para-xylene, and mesitylene (1,3,5,-trimethylbenzene) have been obtained with 212.8 nm laser radiation. This radiation is resonant with the ¹L_a excited state of these benzenoid aromatics. The dominant dipole forbidden character of this electronic transition is reflected in the resonance Raman spectra. The additional substituent-induced dipole allowed character is clearly evident in the scattering spectra of the D_2h and C_2v derivatives as well. These results illustrate the validity of the vibronic theory of Raman scattering.