Surface-enhanced Raman scattering of thiobenzoic acid and thiobenzamide
have been recorded on three different silver colloids in order to find the
chemical species responsible for the spectra and to detect differences in the
adsorption with respect to their oxygen counterparts, benzoic acid and
benzamide, respectively. Very significant and unexpected shifts of opposite
sign between the Raman and SERS wavenumbers have been detected. By
comparing the experimental and DFT calculated wavenumbers, it can be
concluded that the acid is bonded to the metal as thiobenzoate through the sulfur
atom with unidentate coordination. SERS spectra of thiobenzamide can be
explained by assuming that it is adsorbed as azanion, like in the case of
benzamide, linking to the metal through the sulfur and nitrogen atoms of the
ionized thiocarboxamide group. In order to support these conclusions,
B3LYP/LanL2DZ force field calculations for different complexes of silver
cations with the thiobenzoate anion, the neutral thiobenzamide as well as its
azanion have been carried out. Additionally, the 8a;nring mode is the most
enhanced band in the SERS of both adsorbates pointing to the participation of
a metal-to-molecule resonant charge transfer mechanism.
