Far infrared spectrum, barrier to internal rotation, and ab initio calculations for 3,3,3-trifluoropropene

The far infrared spectrum of gaseous 3,3,3-trifluoropropene, CH2CHCF3, between 350 and 30 cm(-1) has been recorded at a resolution of 0.10 cm(-1). The torsional fundamental for the CF, rotor is observed at 83.52 cm(-1). There are three and possibly four "hot bands", which accompany this fundamental falling to lower wavenumbers with decreasing intensities. Utilizing these torsional data along with structural parameters obtained from MP2/6-311+ G** ab initio calculations to calculate the kinetic constant F, the barrier to internal rotation has been determined to be 630 +/- 10 cm(-1) (7.54 +/- 0.12 kJ/mol) and the V-6 term is relatively small with a value of - 30 +/- 5 cm(-1) (-0.36 +/- 0.06 kJ/mol). These results are compared to the CH3 barrier of propene as well as to those of several CF3 rotors. The harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from both ab initio RHF/6-31G*; and MP2/6-31G* calculations and the results compared. Additionally, predicted quantities are compared to the corresponding experimental values when appropriate as well as with some corresponding results for some similar molecules. (C) 2000 Elsevier Science B.V. All rights reserved.