Infrared and Raman spectroscopic study of terephthalic acid adsorbed on silver surfaces

Surface-enhanced Raman scattering (SERS) of terephthalic acid (TPA) has been investigated in aqueous silver sol and at a silver electrode. From the SERS spectral pattern, TPA has been found to chemisorb on the sliver surface via Ag-COO-bonds after deprotonation even in a very acidic medium, taking a flat orientation with respect to the surface. The same conclusion can be made from the infrared spectrum of TPA self-assembled on a vacuum evaporated silver film. Negligible difference in the frequency of nu(s)(COO-) mode between the ordinary Raman spectrum of TPA(2-) and its SERS spectrum is attributed to the disruption of conjugation caused by the surface adsorption. The symmetry based electromagnetic SERS selection rule is not applicable to the present system. Also, comparing the relative enhancements of the in-plane and out-of-plane modes upon surface adsorption has not been found helpful in determining the orientation of benzene derivatives adsorbed on metal surfaces. (C) 1997 Elsevier Science B.V.