Surface-Enhanced Raman Scattering of the Acid-Base Forms of Imidazole on Ag

Surface-enhanced Raman scattering (SERS) of adsorbed imidazole (pH 9) and the imidazolium ion (pH 2) on Ag electrodes is reported and compared to the normal Raman spectra of the corresponding solution and solid species. Changes in frequency and relative intensity of the imidazole bands upon adsorption and as a function of potential are consistent with imidazole adsorption primarily through the pyridine N. Some interaction of the C=N π orbital with Ag is also suggested. Consideration of surface selection rules suggests that the imidazole is tilted with respect to the surface normal at potentials positive of the potential of zero charge and assumes a more vertical orientation as the potential is made more negative. Photolysis of adsorbed imidazole results in a number of background features in the SERS spectrum. In addition, intense SERS bands are observed when using imidazole that has not been rigorously purified. These are proposed to be due to an impurity present in the imidazole supplied by a number of manufacturers. These bands have been previously misinterpreted as unique imidazole surface species. The SERS response of the imidazolium ion to changes in the electrode potential is consistent with imidazolium coadsorption with the chloride ion.