Vibrational spectra and conformations of isopropyl thiocyanate and its d6 and d7 deuterated analogues

The Raman and IR spectra of isopropyl thiocyanate, isopropyl-1,1,1,3,3,3,-d6 thiocyanate and isopropyl-d7 thiocyanate were recorded as vapours, liquids and amorphous and crystalline solids. IR matrix isolation spectra were recorded in argon and in nitrogen matrices at 14 K, using the hot nozzle technique. Two conformers with Cs, symmetry (anti) and C1, (gauche) were present in the vapour and liquid states whereas only one conformer (anti) existed in the crystal. The enthalpy difference was measured to be ΔH° (gauche-anti) = 2.4 ± 0.2 kJ mol−1 in the liquid from variable-temperature Raman spectra. In argon matrices a value of 2.3 ± 0.3 kJ mol−1 was obtained in the temperature range 300–600 K, similar to the value in nitrogen matrices, both assumed to be valid for the vapour phase. Annealing experiments indicated the barrier of gauche–anti conversion to be around 9 kJ mol−1. Quantum chemical calculations with the basis sets STO–3G, 3–21G* and 6–31G* were carried out The energy difference between the conformers was calculated and scaled ab initio force constants were employed in normal coordinate calculations. Complete assignments of Raman and IR bands belonging to the anti conformer of the parent molecule and the isotopomers were carried out. Only a limited number of bands characteristic of the gauch conformer were observed since most of its Raman and IR bands overlapped those of the anti conformer.