The effects of solvents on the stretching vibrations of the thiocyanate and isothiocyanate groups

Abstract

The frequency and intensity of the pseudo-antisymmetric -NCS stretching vibration and the -C≡N stretching vibration in -SCN have been measured for the methyl and phenyl compounds of both isomers in a range of solvents. It is shown that the intensity change in the thiocyanates follows a function of the Buckingham type, but that the change in the intensity of the isothiocyanates is complicated by a further effect which could be due to specific solvent/solute interaction. This interaction is reflected in the large frequency shift of the isothiocyanates whereas for the thiocyanates the shift is only very small. However, mixed-solvent studies indicate that the interaction is not strong enough to be classed as a hydrogen bond. Parallel solvent studies on the analogous vibration of the -NCO group indicate that this group, rather surprisingly, is barely solvent-sensitive at all.