Vibrational Spectrum of Cyanate Ion in Various Alkali Halide Lattices

Abstract

The infrared absorption spectra of dilute solid solutions of cyanate ion in KI, KBr, KCl, and NaCl single crystals have been examined between 600 and 5000 cm—1 at temperatures ranging from 150 to 480°K. Over fifty distinct maxima have been observed and assigned (including numerous isotopic, combination, and hot bands) in the most concentrated sample, for which KBr was the solvent. A strong Fermi resonance between 2ω2 and ω1 occurs. The vibrational energy has been expressed in terms of the eleven fundamental frequency and anharmonicity constants appropriate to a linear, unsymmetric triatomic molecule. Force constants in the KI solvent lattice are fNC=15.51, fCO=11.03, f′=1.35, and (fα/l1l2)=0.506 mdyne/A, a result which shows very clearly that the NC bond is of higher order than the CO bond in the cyanate ion, and that the latter bond is notably weaker in cyanate ion than in CO2, where its force constant is 16.0. In general, the fundamental frequencies increase as the lattice constant decreases. Fairly detailed calculations show that although the induction energy term arising from the dipoles induced in the solvent by the vibrating cyanate ion is not negligible, the observed variation of frequency cannot be explained by the induction term alone and must involve the short-range, repulsive forces.