Infrared Spectrum and Structure of the Thiocyanate Ion

The infrared absorption spectra of aqueous and solid KNCS have been studied (the latter in detail) from 280–10 000 cm—1. The fundamental frequencies of NCS— in aqueous solution are v1 = 2066, v2 = 470, and v3 = 743 cm—1. In the solid, the degeneracy of the bending vibration is removed and four fundamentals are observed at 2053, 484, 470, and 749 cm—1, respectively. Many combination bands were observed for solid KNCS from which the anharmonicity constants and the frequencies for infinitesimal amplitudes of vibration were calculated. The fundamental frequencies of the C13 and S34 isotopic species were observed also. From the zero-order frequencies, force constants were calculated for NCS— in KNCS as kN–C = 15.95 md/A, kC–S = 5.18 md/A, k12 = +0.9 md/A, and kα/RC–NRC–S = 0.311 and 0.300 md/A. With the use of Badger's rule the bond distances are calculated to be RN–C = 1.17 A, RC–S = 1.61 A. These distances indicate a considerable amount of double bond character in the C–S bond.