SERS and DFT study of 4,4′-biphenyl dicarboxylic acid on silver surfaces: The orientation and vibrational assignment

The spectrum of 4,4′-biphenyl dicarboxylic acid (BPDC) adsorbed on silver surfaces has been measured by surface-enhanced Raman scattering (SERS). Ordinary Raman spectra of neat sample in solid state and in basic condition have also been taken for comparison. The observed Raman bands were assigned by examining normal mode vectors and wavenumbers from density functional theory calculations. The adsorption of BPDC on Ag through both carboxylate groups has been identified from investigation of the carboxylate vibrational bands in the spectra. In addition, absence of C–H stretching bands in the SERS spectrum reveals parallel orientation of biphenyl ring against the Ag surface and broadened, red-shifted ring stretching bands suggest that adsorption should take place through interaction of π-systems of the phenyl rings with Ag.