Interfacial study of benzenesulfinate chemisorbed on silver.

The oxidative stability of self-assembled monolayer (SAM) of thiols on silver was examined with matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). A benzenesulfinate (BS) monolayer on silver was also prepared and investigated with MALDI-TOF-MS and surface-enhanced Raman scattering (SERS). The presence of sulfinate, sulfonate, and thiosulfonate fragment ions reveals that thiolate monolayers oxidize in the air. The relative abundance of C6H5SO2- and C6H5SO3- species in the mass spectra of the monolayer with different air-exposure times provides an estimation of the oxidation progress. In the vibrational spectrum, the large red shift of v(SOO-), combined with the lack of a shift of v(C-S) upon adsorption, indicates its bidentate O-coordination. The orientation of the confined molecules, based on the Raman surface selection rules, was derived from the preferential enhancement of the different functional groups. The benzene ring of BS was found almost normal to the metal surface as a result of the intermolecular forces; that geometry excludes the possibility of the pi-system of the benzene ring from participating as another binding site. The SER spectra of BS obtained via a selective etching process confirms these observations.