Orientation of 1- and 2-methylimidazole on silver electrodes determined with surface-enhanced Raman scattering

Surface-enhanced Raman scattering is used to determine the potential-depend ent orientation of 1- and 2-methylimidazole on Ag electrodes. Frequency shi fts resulting from adsorption and potential changes suggest adsorption of t hese molecules through the pyridine nitrogen at positively charged surfaces . Unique nu(CH3) features appear in the SERS spectra of 1-methylimidazole a t neutral and negatively charged surfaces which suggest direct interaction of the methyl group with the surface. SERS surface selection rules are appl ied to three carefully selected groups of vibrational modes. The comparison of in-plane and out-of-plane ring modes is used to determine the orientati ons of the imidazole ring planes relative to the surface. The orientations of the methyl groups relative to the surface are elucidated by comparing th e nu(sym)(CH3) and nu(asym)(CH3) modes. Finally, comparison of the nu(N-CH3 ) to the delta(sym)(N-CH3) of 1-methylimidazole and the nu(C-CH3) to the de lta(sym)(C-CH3) of 2-methylimidazole provides information on the "side-to-s ide" tilting of these molecules. The orientation information thus obtained suggests that the methylimidazoles are attached to the electrode in a tilte d orientation at potentials positive of the potential of zero charge (PZC), become more Vertical as the potential approaches the PZC, and then are att ached to the electrode primarily through the interaction of the methyl grou p at the most negative potentials. Pronounced "butterfly" waves in the cycl ic voltammetry of these systems are interpreted in terms of adsorbate layer rearrangement. The potentials at which these processes occur are consisten t with those at which spectral changes are observed.