A PH-DEPENDENT SURFACE-ENHANCED RAMAN-SPECTROSCOPY INVESTIGATION OF THE ADSORPTION OF 3-ACETYLPYRIDINE

Surface-enhanced Raman spectra (SERS) of 0.1 mol l-1 3-acetylpyridine(3-ACP),0.1 mol l-1 NaCl solutions on a polycrystalline silver electrode are reported in the wavenumber range 500-1800 cm-1 as a function of pH. In order to determine the type of adsorbate (neutral or cationic species) the SER spectra are compared with the pH-dependent solution spectra and the spectrum of neat 3-ACP. In the pH region between 6.6 and 2.4, neutral 3-ACP is adsorbed end-on via the ring nitrogen atom. Furthermore, application of the image field theory to particular substituent modes, namely the carbonyl and C-CH3 vibration, leads to the conclusion of cis-configurated 3-ACP on the metal surface. At pH 1.4, the SER spectrum exhibits quite different features which are discussed in terms of an altered molecular structure in the double-layer region. An ion pair interaction as well as a planar orientation towards the electrode-the latter being feasible owing to a decrease in surface concentration-seem to be the most likely assumptions.