VIBRATIONAL AND ROTATIONAL SPECTRA, CONFORMATIONS AND AB-INITIO CALCULATIONS OF THIOCYANATOETHENE

Infrared spectra of thiocyanatoethene in the 4000-400 cm(-1) region in the vapour and liquid states and of the amorphous and crystalline solids at low temperatures were obtained. Additional IR spectra of the compound were recorded when isolated in argon and nitrogen matrices at 5 K using the hot nozzle technique. Raman spectra in the liquid phase at various temperatures were recorded and the amorphous and crystalline solids were investigated at liquid nitrogen temperature. Microwave spectra of thiocyanatoethene were recorded in the 10-39 GHz region by means of Stark modulation. Areas of special interest were also investigated by use of the RFMWDR double resonance technique. Ab initio quantum chemical calculations were carried out with the 6-31G* basis set. The spectral data were interpreted in terms of an equilibrium in the vapour and liquid states between a syn and a gauche conformer, while syn was the only conformer present in the crystal. An approximate enthalpy difference of 3-6 kJ mol(-1) in the vapour and 5.7 +/- 1.0 kJ mol(-1) in the liquid was determined, with the syn conformer being the more stable. The microwave spectrum of the syn conformer was assigned, giving rotational constants in good agreement with the results of the ab initio calculations, whereas we were pot able to assign unambiguously the spectrum of the gauche conformer. The internal energy difference between the syn and gauche conformers was determined to be 4.4 +/- 1.0 kJ mol(-1).