RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, AND AB-INITIO CALCULATIONS OF TRIFLUOROACETYL ISOCYANATE

The Raman (2500 to 1O 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 bee n recorded and qualitative depolarization values obtained. The observe d bands are assigned on the basis of a more stable cis conformer (isoc yanate group cis to the carbonyl bond) and the less stable trans confo rmer in the fluid phases and only the cis rotamer is present in the so lid. 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 Delta H value of 163 /- 12 cm(-1) (466 +/- 34 calmol(-1)) which should represent closely th e value of Delta H for the gas. The potential function governing confo rmational interchange has been determined from the asymmetric torsiona l frequencies and Delta H value. The values of the potential constants are: V-1 = 356 +/- 14, V-2 = 1865 +/- 60, and V-3 = -203 +/- 6cm(-1). Additionally, variable-temperature studies of the Raman spectrum of t he liquid give a Delta H of 236 +/- 30 cm(-1) (675 +/- 86 calmol(-1)). A complete vibrational assignment is proposed for the cis conformer b ased on infrared band contours, Raman depolarization data, group frequ encies, relative intensities, and normal coordinate calculations. Also , several of the fundamentals of the trans conformer have been assigne d. The experimental conformational stability, barriers to internal rot ation, 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 t o the corresponding quantities obtained for some similar molecules.