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 fluoro carbonyl isocyanate, FC(O)NCO, were recorded for the gas and solid. Ad ditionally, the Raman spectrum of the liquid was recorded and qualitat ive depolarization values were obtained. The observed bands are assign ed on the basis of the more stable cis conformer (isocyanate group cis to the carbonyl bond) in the fluid states but only the trans conforme r remains in the annealed solid. From temperature dependence studies o f the Raman spectra of the gas and liquid, two pairs of conformer peak s were used to determine DELTAH values of 258 +/- 47 cm-1 (738 +/- 134 cal mol-1) and 140 +/- 29 cm-1 (400 +/- 83 cal mol-1), respectively, with the cis conformer the more stable rotamer in both phases. Additio nally, variable-temperature studies of the infrared spectrum of the sa mple dissolved in liquified Kr gave DELTAH = 150 +/- 28 cm-1 (429 +/- 80 cal mol-1), which should closely represent the DELTAH value for the gas. A complete vibrational assignment is proposed for both conformer s based on infrared band contours, Raman depolarization data, group fr equencies, relative intensities and normal coordinate calculations. Th e experimental conformational stability, barriers to internal rotation , structural parameters and fundamental vibrational wavenumbers were c ompared with those obtained from ab initio gradient calculations emplo ying the RHF/3-21G, RHF/6-31G and/or MP2/6-31G* basis sets and to the corresponding quantities obtained for some similar molecules.