Analysis of the Potential Dependent Surface-Enhanced Raman Scattering of p‑Aminothiophenol on the Basis of MS-CASPT2 Calculations

The effect of adsorbate concentration on the potential dependent SERS spectra of p-aminothiophenol (pATP) recorded on a silver electrode has been studied using NaClO4 as electrolyte. Multiconfigurational second-order perturbation calculations (MS-CASPT2) have been performed in order to help the analysis of the experimental results by computing resonance Raman spectra of selected structural models of the metal–adsorbate surface complex. In order to avoid as far as possible any photochemical reaction, a long wavelength exciting line of 785 nm is used in the experiments. It is found that the spectra are dependent on adsorbate concentration and dominated by a resonant charge transfer (CT) mechanism, where the charge is always transferred from the adsorbate to the metal. The relative SERS enhancements are due to Franck–Condon factors related to the CT process, and there are not intensified bands through Herzberg–Teller contributions. Furthermore, the Raman signals of the SERS recorded at low concentration arise from at least three different molecular species: (i) pATP bonded to silver electrode through sulfur atom (Agn-S–-Ph-NH2), (ii) pATP bonded to silver electrode through both sulfur and nitrogen atoms (Agn-S–-PhNH2‑Agm), and (iii) the azo derivative p,p′-dimercaptoazobenzene (or its nitrene precursor).