Role of Herzberg–Teller Vibronic Coupling in Surface-Enhanced Resonance Raman Spectra of 4,4′-Diaminotolane with Nearly Close Molecular and Charge-Transfer Transitions

We have investigated the surface-enhanced resonance Raman spectroscopy (SERRS) for 4,4′-diaminotolane absorbed on silver experimentally and theoretically. Experimental observation shows greatly enhanced b_g symmetric modes ν₂₆ and ν₂₇, which are silent in normal Raman spectroscopy and SERS on gold. The dependence of the surface-enhanced Raman spectroscopy (SERS) spectra on five excitation lines has also been registered and classified for single bands into three profiles referring to the different contributions of resonant transitions. Theoretical calculations based on the time-dependent path integral formalism by including the Herzberg–Teller correction reproduce the experimental spectra with good agreement. The importance of the Herzberg–Teller term to improve the pattern of the spectra and the enhancement of totally and non-totally symmetric modes are recognized and elucidated. The strong charge-transfer transition with nearly close molecular excitations in this compound creates an opportunity to obtain insight into the combination of molecular and charge-transfer transitions and their effect on the chemical mechanism of SERS.