Selection Rules of the Charge Transfer Mechanism of Surface-Enhanced Raman Scattering:  The Effect of the Adsorption on the Relative Intensities of Pyrimidine Bonded to Silver Nanoclusters

Surface-enhanced Raman spectra (SERS) of pyrimidine recorded on a silver electrode have been analyzed on the basis of a resonant Raman (RR) process involving photoexcited charge transfer (CT) states of the metal−adsorbate surface complex. The main feature of the SERS of benzene and azine derivatives is the enhancement of the totally symmetric ring stretching mode 8a due to Franck−Condon contributions related to the CT transition. Although this behavior is observed in the SERS of pyrimidine, its spectrum is also characterized by the strong enhancement of the nontotally symmetric mode 8b. This peculiar feature can be explained only by the redistribution of the Franck−Condon factors between the 8ab pair of vibrations originated by the descent in symmetry occurring when pyrimidine is bonded to silver nanoclusters. This conclusion is a new evidence of the main role of the RR−CT enhancement mechanism in the SERS of aromatic molecules and shows once again the usefulness of the methodology developed by our group in order to analyze these complex spectra.