Charge-transfer processes in surface-enhanced Raman scattering. Franck-Condon active vibrations of pyrazine

SERS spectra of pyrazine on silver electrode have been recorded and analyzed, assuming a charge transfer effect and using selection rules analogous to those of resonance Raman. With the aim of predicting the effect of this mechanism on the selective enhancement of fundamentals, a method has been proposed based on an analysis of the results of geometry optimizations carried out by ab initio calculations. The strongest SERS bands coincide with those assigned to the normal modes connecting the equilibrium geometries of the neutral molecule and the radical anion. These results support the presence of a charge transfer process from the metal to the adsorbate in the SERS spectra of pyrazine where a significant enhancement of vibrations 8a, 9a, 1, and 6a can be observed. The prediction capability of the analysis proposed here has been checked with published resonance Raman spectra of pyrazine and pyrazine-d(4). In all of the cases the strongest bands in the spectra are directly related to the largest Delta Q values obtained through the transformation Delta Q = L(-1)Delta R.