Surface-enhanced Raman spectra of pyrazine, pyrimidine, and pyridazine adsorbed on silver sols

Raman spectra of pyrazine, pyrimidine, and pyridazine adsorbed on silver sols have been obtained and compared with existing data from corresponding experiments on Ag electrode. These two techniques give similar results except for pyrazine for which strong bands, observed only in the SER spectrum on the electrode, may be due to reduction products coadsorbed on the Ag surface. Chemisorption plays a role in the absorption of diazines as evidence of Ag-N bond formation was found in the SERS of all three molecules. Both N atoms of pyridazine are bound to the substrate and this explains an enhancement, for the latter molecule, of 2 orders of magnitude larger than for the other two compounds. The presence of low-frequency Ag-N modes and predictions of surface selection rules support “edge on”, rather than flat, orientation of diazines on the colloidal particles.