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

Article abstract of DOI:10.1016/0022-2860(94)08362-2

The Raman (2500 to 10 cm^-1) and infrared (2500 to 30 cm^-1) spectra of trifluoroacetyl isocyanate, CF3C(O)NCO, have been recorded for the gas and solid.Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained.The observed bands are assigned on the basis of a more stable cis conformer (isocyanate group cis to the carbonyl bond) and the less stable trans conformer in the fluid phases and only the cis rotamer is present in the solid.From the variable-temperature studies of the infrared spectrum of the sample dissolved in liquified krypton, the conformer pair at 1796 and 1781 cm^-1 has been used to determine the ΔH value of 163 +/- 12 cm^-1 (466 +/- 34 cal mol^-1) which should represent closely the value of ΔH for the gas.The potential function governing conformational interchange has been determined from the asymmetric torsional frequencies and ΔH value.The values of the potential constants are: V₁ = 356 +/- 14, V₂ = 1865 +/- 60, and V₃ = -203 +/- 6 cm^-1.Additionally, variable-temperature studies of the Raman spectrum of the liquid give a ΔH of 236 +/- 30 cm^-1 (675 +/- 86 cal mol^-1).A complete vibrational assignment is proposed for the cis conformer based on infrared band contours, Raman depolarization data, group frequencies, relative intensities, and normal coordinate calculations.Also, several of the fundamentals of the trans conformer have been assigned.The experimental conformational stability, barriers to internal rotation, structural parameters, 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 to the corresponding quantities obtained for some similar molecules.