VIBRATIONAL SPECTROSCOPIC INVESTIGATION OF BENZOIC-ACID ADSORBED ON SILVER

The surface-enhanced Raman scattering (SERS) of benzoic acid has been investigated in aqueous silver sol and at a silver electrode. The adso rption behavior of benzoic acid seemed very dependent on the nature of substrate. On the electrode surface, the molecule was found to adsorb preferentially, after deprotonation, via the oxygen lone pair electro ns, assuming a perpendicular orientation with respect to the surface. In contrast, the benzoate species appeared more likely to take a flat or tilted orientation on the sol surface. The same observation could b e made for 4-methylbenzoic acid. According to the ab initio quantum me chanical calculation which modeled the silver surface as an Ag-5(+) cl uster, the perpendicular orientation appeared to be energetically more favorable than the flat orientation. Although the origin of such a di fferent adsorption mechanism could not be clarified, it was tentativel y ascribed to the different surface morphology along with the intrinsi c difference in the nature of SERS active sites on the two surfaces.