Franck–Condon Dominates the Surface-Enhanced Raman Scattering of 3‑Methylpyridine: Propensity Rules of the Charge-Transfer Mechanism under Reduced Symmetry

The analysis of photoinduced charge transfer (CT) processes in a particular SERS spectrum is not a trivial task since each system must be studied separately and no universal selection rules can be proposed for this kind of enhancement mechanism. Despite this, we found out in previous studies that the SERS-CT spectra of benzene-like molecules are mainly characterized by the strong enhancement of the totally symmetric 8a ring-stretching vibration and we have used this as a propensity rule to recognize the presence of resonant CT processes in the SERS of these adsorbates. However, it is necessary to cautiously consider the relevant symmetry of the system under study before applying this simplified rule. When the molecule shows a low symmetry (or the relevant symmetry is reduced by the adsorption on the metal as it was demonstrated in the case of pyrimidine), the selective intensification of the 8a mode can be disguised by the redistribution of the CT enhancement with another close vibration in wavenumber, namely the 8b mode, usually a non-totally symmetric fundamental. 3-Methylpyridine is an illustrative example of this given that similar SERS enhancement of both 8a and 8b modes can be quantitatively explained through Franck–Condon factors without having to resort to Herzberg–Teller contributions.