Investigation of the vibrational properties of OCN- on a silver electrode by in situ synchrotron far infrared spectroscopy and visible-infrared sum frequency generation spectroscopy

The vibrational properties of cyanate ions, OCN-, adsorbed on a polycrystalline silver electrode in neutral aqueous solution have been studied by in situ VIS-IR sum frequency generation (SFC) and by synchrotron far-IR spectroscopy. The SFG spectra show two bands, one in the region 2100-2130 cm(-1) in the potential range from -0.6 to 0 V vs. SCE, and another in the region 2200-2240 cm(-1) in the potential range from 0 to 4 V. These bands are assigned to the antisymmetric stretching mode of OCN- ions bound to the Ag surface. The lower wavenumber band is assigned to cyanate N-bound to a bridging surface site, whereas the higher wavenumber band is assigned to cyanate in a terminal N-bonding mode. These assignments are consistent with the results of earlier IR absorption studies. The far-IR spectrum shows a broad band at about 360 cm(-1) at electrode potentials above 0 V. This is assigned to the v(AgN) mode of terminally bound cyanate, in accordance with the assignment for the VIS-IR SFG band observed at these potentials. The far-IR spectrum also shows a sharper band at 90 cm(-1) that might arise from a bending (or restricted rotation) of cyanate on the silver surface.