Raman and infrared spectra, conformational stability and ab initio calculations for chlorocarbonyl isocyanate

The Raman (2500—10 cm−1) and infrared (2600—50 cm−1) spectra of chlorocarbonyl isocyanate, CIC(O)NCO, were recorded for the gas and solid. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. The observed bands were assigned in terms of the more stable trans conformer (isocyanate grouptrans to the carbonyl bond) and less stable cis conformer. A conformer pair observed at 788 and 636 cm−1 in the Raman spectrum of the gas (775 and 634 cm−1 for the liquid) and assigned to the cis and trans COCI rock, respectively, was used to determine experimentally a ΔH value of 254 ± 48 cm−1 (726 ± 137 cal mol−1) and 230 ± 28 cm−1 (658 ± 80 cal mol−1) for the gas and liquid, respectively. In the far-infrared spectrum of the gas, a series of three Q-branches beginning at 83 cm−1 was observed and assigned to the fundamental torsional transition and two excited states transitions for the trans conformer. Using these three transitions and the experimentally determined ΔH value, a potential function was obtained for the interconversion of the two conformers with the following coefficients (in cm−1): V1 = −330 ± 50, V2 = 1025 ± 70, and V3 = 67 ± 40 and a trans to cis barrier of 1230 ± 150 cm−1 (3.52 ± 0.43 kcal mol−1) and a torsional frequency for the cis conformer of 69 ± 2 cm−1. The barrier to internal rotation and fundamental vibrational frequencies which were determined experimentally were compared with those obtained from ab initio Hartree—Fock gradient calculations utilizing the 6-31G* basis set. These results are compared with some previous results on this molecule.