Unstable intermediates. Part CXIX. Electron spin resonance studies of radiation effects upon cyanate and thiocyanate ions in alkali-metal halide crystals and in aqueous glasses

Abstract

Irradiated crystalline sodium or potassium cyanate at 77 K had e.s.r. spectra dominated by a quintet assigned to the radical N₂CO^–, and a triplet assigned to NCO^2–. The latter had a ¹³C hyperfine tensor characteristic of a bent radical as expected, since it is isoelectronic with NO₂. In contrast, aqueous glasses containing these salts gave, on irradiation, spectra which have been interpreted in terms of trapped nitrogen atoms and HCNO^– radicals. The latter radicals are characterised by an unusually large proton hyperfine coupling, which is interpreted in terms of protonation on carbon.

Potassium thiocyanate had an e.s.r. spectrum, after irradiation, which has been interpreted in terms of the dimer anion (NCS)₂^–, well known in pulse-radiolysis studies, and the anion NCS^2–. On being annealed, sharp features, thought to be characteristic of NO₃ molecules, developed. The radical (NCS)₂^– is assigned a σ*-structure on the basis of its ³³S hyperfine tensor components. The radical CNS^2–, like CNO^2–, is shown to be strongly bent by analysis of its ¹³C tensor components.

Aqueous glasses gave mainly HCNS^– at 77 K, which is magnetically very similar to HCNO^–. Nitrogen atoms were not detected. The radicals CNO and CNS were not positively identified in these studies, although their formation could not be ruled out.