Surface-Enhanced Raman Spectroscopy as a Probe or Adsorbate-Surface Bonding: Benzene and Monosubstituted Benzenes Adsorbed at Gold Electrodes

摘要

Surface-enhanced Raman spectra of benzene and eight monosubstituted benzenes adsorbed at gold electrodes have been examined in order to probe the nature of adsorbate-surface bonding for simple aromatic molecules. Benzene appears to adsorb flat via pi adsorbate-surface interactions as evidenced by the significant (20-30/cm) downshifts in the symmetric rig breathing mode (v sub 1) and the absence of other ring modes in the SER spectra. Similar results were obtained using gold surfaces prepared by electroplating or anodic-cathodic potential sweeps in chloride, although much more stable SERS was obtained by using the latter procedure. The ring substituents -CHD3, -CH(CH3)2, -C(CH3)3, -Cl, -Br, -CN, -NO2, -COO(-) and -CHO were chosen so to provide steric perturbations or alternative binding modes to the metal surface. With the alkyl, benzoate, and aldehyde substituents, small yet significant (ca. 5-15/cm) decreases in the v sub 12 and v sub 18a characteristic ring modes along with band broadening were observed upon adsorption, consistent with attachment via the benzene ring; For the halogen substituents, smaller such downshifts were obtained along with the appearance of low-frequency modes associated with halogen-surface vibrations, implicating the role of substituent bonding in the adsorption. For the nitrile and nitro substituents, the ring modes were unaltered in frequency or bandshape upon adsorption while intense internal substituent modes were observed.