Resonance Raman study of solvent dynamics in electron transfer. I. Betaine-30 in CH3CN and CD3CN

Raman excitation profiles for nine vibrational modes of the solvatochromic dye betaine-30 have been measured in CH3CN and CD3CN solution at wavelengths that span the S0→S1 charge transfer transition. Though the absorption spectra of the dye are the same in the protonated and deuterated forms of the solvent, Raman cross sections for all modes were found to be lower in CD3CN solution than in CH3CN. Wave packet theory has been applied to model the absorption and Raman profiles, using a solvent dephasing model which accounts for bimodal solvent dynamics. The solvent isotope effect on the Raman intensities is interpreted in terms of different amplitudes of the fast solvent response in the protonated and deuterated solvents, and possible coupling of low-frequency solute and solvent modes. The solvent reorganization energy greatly exceeds that due to internal vibrational modes of betaine-30, and most of the internal reorganization is due to the torsional modes of the solute.