Raman and infrared spectra, conformational stability, and ab initio calculations of acetyl isothiocyanate

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The Raman (3100 to 10 cm−1) and infrared (3100 to 30 cmt-1) spectra of acetyl isothiocyanate, CH3C(O)NCS, and the d3 isotopomer have been recorded for the fluid and solid phases with qualitative depolarization values obtained from the Raman spectra of the liquids. The observed bands are assigned on the basis of the more stable trans conformer (isothiocyanate group trans to the carbonyl bond) and the less stable cis conformer in the liquid state with the trans conformer remaining in the crystalline solid. The conformational stability could not be determined for the vapor state owing to the lack of vapor phase data. Variable temperature studies of the Raman spectrum of the liquid gives a ΔH value of 145 ± 21 cm−1 (415 ± 60 cal mol−1). A complete vibrational assignment is proposed for the trans conformer based on infrared band contours, Raman depolarization data, group frequencies, relative intensities and normal-coordinate calculations. Also, several of the fundamentals of the cis conformer have been assigned. The experimental conformational stability, and fundamental vibrational frequencies are compared with those obtained from ab initio gradient calculations employing the RHF/3-21G*, RHF/6-31G* and/or MP2/6-31G* basis sets and with the corresponding quantities obtained for some similar molecules.