| A statistical mechanical approach is taken to calculate the equilibrium position and orientation of a benzene molecule relative to a smooth surface, in a Lennard-Jones fluid designed to mimic the structure of water between two metal walls. These results are compared to surface adsorbate geometries experimentally derived from surface enhanced Raman spectra taken from colloidal solutions of benzene and two azabenzenes (pyrazine and pyrazine). Surface selection rules were used in accordance with the classical electrodynamic mechanism.; The surface enhanced Raman signals for benzene, pyridine, and pyrazine differ greatly from their bulk and concentrated solution phase spectra. In the case of benzene and pyrazine, bulk phase Raman forbidden bands are observed. For each molecule studied there are large changes in the relative intensities and polarization ratios for the Raman active vibrations. The appearance of Raman forbidden bands and the alteration of the intensity profiles result from the reduced symmetry surface complexes formed.; The results of the calculations show that the benzene molecule is most stable lying flat on the metal surface. The experimental results are in agreement with this finding for the non-polar molecules studied (benzene and pyrazine). The surface enhanced Raman spectra for benzene and pyrazine show that these molecules orient themselves parallel to the metal surface. The pyridine molecule, however, forms a surface complex perpendicular to the surface. |
