The application of SERS to the determination of relative adsorption strengths of nitrogen heterocycles on silver electrodes

Abstract

Research was undertaken to explore the application of surface enhanced Raman scattering (SERS) to the understanding of electrosorption phenomena. In particular, the relative strength of interactions between the functional groups of imidazole (N₁, N₂ and the π orbitals) on Ag electrodes was examined. This information is useful in understanding how the presence of these functional groups contributes to the relative adsorption strength of nitrogen heterocycles. A Raman spectrometer equipped with a charge-coupled device (CCD) detector was required to obtain reproducible SERS spectra in this research. It was also necessary to obtain accurate Raman shifts so that small (ca. 1-2 cm⁻¹ vibrational frequency changes between adsorbed and solution species could be detected. Therefore, significant effort was expended to develop calibration and spectral acquisition procedures which would provide acceptable accuracy and efficiency. Instrumental factors affecting Raman spectral calibration were studied. Available Raman shift calibration standards are reviewed and improved Raman shift data for these standards are reported. Several methods for the conversion of CCD position (pixel number) data to wavelength and Raman shifts are appraised using both experimental and simulated Raman data. SERS spectra for imidazole, 1-methylimidazole, and 2-methylimidazole support the conclusion that these molecules are adsorbed to the Ag electrode through the "pyridine nitrogen" (N₃). This evidence includes vibrational frequency shifts and orientations of these molecules deduced from the consideration of SERS surface selection rules. These data also suggest that the π orbital of the C=N bond interacts with the electrode at potentials near -0.25 V (versus SCE reference electrode) producing a tilt of the ring relative to the surface at these potentials. At potentials near the potential of zero charge (ca. -0.80 V), this interaction is minimized and these molecules assume a more vertical position. At the most negative potentials examined, the methylimidazoles interact with the surface predominantly through the methyl group. Preliminary work on the application of SERS to obtain surface coverage information was performed. An increase in SERS intensity with increasing solution concentration suggests adsorption isotherm-like behavior.