Cation effects on the vibrational frequencies of adsorbed thiocyanate on platinum

Infrared spectra of thiocyanate adsorbed on a platinum electrode surface were obtained in the presence of perchlorate electrolytes of various alkali metal cations. It was discovered that the vibrational frequency of the C-N stretching mode is dependent upon the nature of the supporting electrolyte cation. Two bands were observed in the 2050 to 2150 cm⁻¹ range; one band was attributed to nitrogen-bound thiocyanate, and the other to species adsorbed via the sulfur atom. Each of these bands demonstrated independent frequency dependencies on cation nature and on the applied electric field within the interfacial region. Differences were also observed in the intensity dependence of the bands on the applied potential. The results were explained in terms of changes in the distance between the outer Helmholtz plane (OHP) and the surface of the electrode, and also in terms of the possible influence of coadsorbed alkali metal cations on the vibrational frequency of thiocyanate species adsorbed through the nitrogen atom. The effects that variations in the OHP-electrode distance impart on the magnitude of the potential drop across the interface, and the influence of small changes in this potential field on the C-N stretching frequency of N- and S-adsorbed thiocyanate species, are discussed.