Abstract
The Raman (3100β10 cm^β1^) and infrared (3100β30 cm^β1^) spectra of acetyl isocyanate, CH~3~C(O)NCO, and the d~3~βisotopomer were recorded for the gases and solids. Additionally, the Raman spectra of the liquids were recorded and qualitative depolarization values were obtained. The observed bands are assigned on the basis of the more stable cis (syn) conformer (isocyanate group cis to the carbonyl bond) and less stable trans (anti) conformer in the vapor state. ΞH of the gas is estimated to be 433 cm^β1^ [1.24 kcal mol^β1^ (1 kcal = 4.184 kJ)] from the relative intensity of the conformer pair at 800 and 763 cm^β1^. From the temperature dependence of the Raman spectrum of the liquid, two conformer pairs were used to determine experimentally a ΞH value of 370 Β± 60 cm^β1^ (1.06 Β± 0.17 kcal mol^β1^) but now with the trans conformer the more stable form. In the annealed crystalline solid, only the trans conformer remains. The fundamental asymmetric torsion of the cis conformer was observed at 79 cm^β1^ with two excited states falling to lower frequency. Utilizing these data and ΞH for the gas the potential function governing the conformer interchange was estimated. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies and normal coordinate calculations. The experimental confirmational stability, barriers to internal rotation and fundamental vibrational frequencies are compared with those obtained from ab initio HartreeβFock gradient calculations employing the RHF/6β31G* basis set and to the corresponding quantities obtained for some similar molecules.