Surface-Enhanced Raman Scattering of Benzoate Anion Adsorbed on Silver Nanoclusters: Evidence of the Transient Formation of the Radical Dianion

Abstract

Surface-enhanced Raman scattering (SERS) of benzoate recorded on silver electrode have been analyzed on the basis of a charge transfer (CT) enhancement mechanism similar to a resonance Raman (RR) process where a photoinduced electron transfer between a metal and an adsorbate takes place. Given that benzoic acid (BzH) is adsorbed on silver as benzoate anion (Bz−), this resonant CT process implies the transient formation of the corresponding radical dianion (Bz•2−). The dependence of the relative SERS intensities on the electrode potential has been analyzed on the basis of ab initio calculations of the electronic properties of the isolated adsorbate in its ground and excited CT states (Bz − and Bz•2−, respectively). Moreover, configuration interaction singles (CIS) calculations have also been carried out for the Ag2−benzoate (Ag2−Bz−) supermolecule, which shows two excited singlets, namely CT0;1B1 and CT1;1A2, where the properties of the adsorbate are quite similar to those of the radical dianion of the isolated species. The theoretical calculations predict the selective enhancement of modes 8a and νs(CO2−) under CT resonant conditions in close agreement with the SERS results. This confirms once again the relevance of resonant CT processes in the SERS of aromatic molecules and the usefulness of this technique in the study of very unstable species such as radical anions or dianions of aromatic adsorbates.