Raman and infrared spectra, conformational stability, barriers to internal rotation and ab initio calculations of fluorocarbonyl isocyanate

The Raman (3100–10 cm−1) and infrared (3100–30 cm−1) spectra of fluorocarbonyl isocyanate, FC(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 are assigned on the basis of the more stable cis conformer (isocyanate group cis to the carbonyl bond) in the fluid states but only the trans conformer remains in the annealed solid. From temperature dependence studies of the Raman spectra of the gas and liquid, two pairs of conformer peaks were used to determine Δ H values of 258 ± 47 cm−1 (738 ± 134 cal mol−l) and 140 ± 29 cm−1 (400 ± 83 cat mol−1), respectively, with the cis conformer the more stable rotamer in both phases. Additionally, variable‐temperature studies of the infrared spectrum of the sample dissolved in liquified Kr gave ΔH = 150 ± 28 cm−1 (429 ± 80 cal mol−1), which should closely represent the ΔH value for the gas. A complete vibrational assignment is proposed for both conformers based on infrared bard contours, Raman depolarization data, group frequencies, relative intensities and normal coordinate calculations. The experimental conformational stability, barriers to internal rotation, structural parameters and fundamental vibrational wavenumbers were 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 to the corresponding quantities obtained for some similar molecules.