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-dependent orientation of 1- and 2-methylimidazole on Ag electrodes. Frequency shifts resulting from adsorption and potential changes suggest adsorption of these molecules through the pyridine nitrogen at positively charged surfaces. Unique nu(CH3) features appear in the SERS spectra of 1-methylimidazole at neutral and negatively charged surfaces which suggest direct interaction of the methyl group with the surface. SERS surface selection rules are applied to three carefully selected groups of vibrational modes. The comparison of in-plane and out-of-plane ring modes is used to determine the orientations of the imidazole ring planes relative to the surface. The orientations of the methyl groups relative to the surface are elucidated by comparing the 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 delta(sym)(C-CH3) of 2-methylimidazole provides information on the "side-to-side" tilting of these molecules. The orientation information thus obtained suggests that the methylimidazoles are attached to the electrode in a tilted orientation at potentials positive of the potential of zero charge (PZC), become more Vertical as the potential approaches the PZC, and then are attached to the electrode primarily through the interaction of the methyl group at the most negative potentials. Pronounced "butterfly" waves in the cyclic voltammetry of these systems are interpreted in terms of adsorbate layer rearrangement. The potentials at which these processes occur are consistent with those at which spectral changes are observed.