PH DEPENDENT RAMAN-SPECTRA OF NICOTINIC-ACID - SPECTRA OF AQUEOUS-SOLUTIONS AND SURFACE ENHANCED RAMAN-SPECTRA ON SILVER ELECTRODES

Raman spectra of nicotinic acid have been measured in aqueous solutions in the pH-range between 1.0 and 12.5. Raman band assignments between 500 cm-1 and 1750 cm-1 are carried out by referring to the compound's solid state Raman spectrum and to spectra of related molecules. Changes of the Raman spectra with solution pH are correlated to the structure change of the nicotinic acid molecule. The surface enhanced Raman spectra (SERS) of nicotinic acid (NA) on a polycrystalline silver electrode are reported in the range 500-1750 cm-1 as a function of pH and applied potential. The results are presented in order to determine the type of adsorbed species (anion, cation, zwitterion). Therefore the SER spectra are compared with the pH dependent solution spectra of NA. These investigations show - in the presence of a non specifically adsorbing electrolyte (0.1 mol l-1 Na2SO4) - the nicotinate anion as the only surface active species detectable within the pH range of 1.4 to 12.5. A detailed analysis of the SER spectra reveals, that the anion exhibits two different molecular orientations towards the metal electrode, a presumably planar alignment dominating at low and medium pH values and a perpendicular one at low proton concentrations. In the intermediate range both orientations exist side by side. The ratio of planar to perpendicular oriented molecules depends on applied potential and increases with growing cathodic polarisation.