Differentiated adsorption of thiobenzoic acid and thiobenzamide on silver nanoparticles determined by SERS spectroscopy

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 246; p. 119048
• Main Authors: López-Ramírez, María Rosa, Aranda, Daniel, López-Tocón, Isabel, Soto, Juan, Castro, José Luis, Otero, Juan Carlos
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05-02-2021
• Subjects: Adsorption; Benzamide; Benzoic; DFT; Raman; SERS; Silver sol; Thiobenzamide; Thiobenzoic; Benzoic; Silver sol; DFT; Adsorption; SERS; Thiobenzoic; Raman; Thiobenzamide; Benzamide
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2020.119048

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;νring 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.
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•This is the first comparative report of the SERS of thiobenzoic and thiobenzamide.•Discussion has been based on DFT calculated wavenumbers of different species and their complexes with a silver cation.•Thiobenzamide is adsorbed deprotonated like benzoic acid.•A differentiated behaviour in the adsorption of aromatic acids and amides is found when an oxygen is replaced by a sulfur atom.