Surface-enhanced Raman scattering studies of 2- and 4-vinylpyridines adsorbed on silver colloids

Surface enhanced Raman scattering (SERS) has been the subject of intense research in recent years and has become a potentially valuable technique to gain information on the nature of adsorbate-surface interactions. There are several aspects of SERS which provide insight into the fundamental enhancement mechanisms, the nature of the bonding to the surface, and the orientation of the adsorbed species on the surface. We report here SERS studies of 2- and 4-vinylpyridines adsorbed on colloidal silver particles. We have studied the frequency shifts, relative intensities of the observed transitions, enhancements factors, and the effect on SERS of the position of the vinyl substituent on the pyridine ring.;Strong SERS was obtained for both 2- and 4-vinylpyridines and significant differences were observed in the spectra of the 4- as compared with the 2-substituted compound. Furthermore, for the case of 4-vinylpyridine, the observation of two different sets of spectra strongly suggest the presence of two different modes of interaction with the surface.;In order to interpret the SERS spectra and use the surface selection rules effectively it is essential to have available complete vibrational assignments and knowledge of the eigenvectors for each of the molecules. Normal mode calculations offer a complementary tool in the interpretation of vibrational spectra. Therefore we have carried out normal coordinate analyses on the 2- and 4-vinylpyridines. Through a comprehensive and comparative study of various available force fields, a valence force field was developed and refined for the vinylpyridines. The calculated frequencies are in excellent agreement with the experimental results.;A systematic comparison of neat, solution and SERS spectra along with the use of surface selection rules, the complete vibrational assignment and the knowledge of the normal modes have enabled us to obtain information pertaining to the vibrational motions of the species on the surface, the conformational behaviour of the molecules in the adsorbed state, and the nature of the bonding between the adsorbates and the substrate.