Surface-Enhanced Raman Scattering Study of Quinoxaline-2,3-dithiol and Disodium Bis(quinoxaline-2,3-dithiolato)nickelate(II) Adsorbed on Silver Surface

Adsorption of quinoxaline-2,3-dithiol (H2QS2) and disodium bis(quinoxaline-2,3-dithiolato)nickelate(II) (Na2-[Ni(QS2)2]) on a silver surface has been studied by SERRS as well as FT-SERS techniques. The surface Raman spectra of H2QS2 are very similar to the corresponding normal Raman spectra of aqueous solutions of disodium quinoxaline-2,3-dithiolate, suggesting that H2QS2 dissociates on a silver surface to the quinoxaline-2,3-dithiolate ion. Normal-mode analysis based on the results of ab initio calculation shows that in-plane modes are selectively enhanced, indicating a perpendicular orientation for the adsorbed dithiolate ion with respect to the surface. The FT-SERS spectra of H2QS2 are found to be very weak on colloidal silver at all stages of aggregation, whereas intense spectra are observed when adsorbed on chemically deposited silver films, indicating that the surface Raman spectrum is mainly enhanced through electromagnetic mechanism for near-infrared excitation. The vibrational frequencies of Na2[Ni(QS2)2] are found to be virtually unaffected by adsorption, indicating only very weak interaction with silver surface.