Torsional vibrations and barriers to internal rotation for ethanol and 2,2,2-trifluoroethanol

Abstract

The far-IR (370-50 cm −1) spectra of gaseous ethanol, CH 3CH 2OH, and the O- d compound have been recorded at a resolution of 0.10 cm −1. Similar far-IR spectra have been collected for 2,2,2-trifluoroethanol, CF 3CH 2OH, and the O- d, C- d 2 and - d 3 isotopic species. In addition, the Raman (4000-50 cm −1) and the mid-IR (5000-400 cm −1) spectra have been collected for CF 3CH 2OH and the isotopic compounds in the vapor phase. A detailed examination of the torsional modes has been carried out and potential functions for the hindered internal rotation of the OH and OD groups and the CH 3 and CF 3 rotors are calculated. For ethanol, the fundamental OH torsion for the trans conformer is observed at 202.6 cm −1 whereas for the gauche conformation, the 1 −1 ← 0 + transition is found at 243.1 cm −1 and the 1 + ← 0 − transition is 195.8 cm −1. An asymmetric potential function utilizing these transitions gives values of 403 cm −1 (1.15 kcal mol −1) for the trans/gauche barrier and 399 cm −1 (1.14 kcal mol −1) for the gauche/gauche barrier with the trans conformer more stable by 42 cm −1 (120 cal mol −1) than the gauche conformer. Potential functions for the hydroxy torsions are also calculated for the ethanol O- d compound and the four 2,2,2-trifluoroethanol isotopic molecules. The CH 3 torsional transitions of trans ethanol give a V 3 barrier of 1185 ± 16 cm −1 (3.39±0.05 kcal mol −1) and the corresponding barrier for the gauche conformer is 1251±2 cm −1 (3.58±0.01 kcal mol −1). Similarly, a series of CF 3 torsional transitions was observed for the trans and gauche conformations of the CF 3CH 2OH molecule as well as for some of the isotopic species. In addition to the examination of the far-IR spectra, a series of sum and difference bands associated with the OH and OD stretching fundamentals is found in the mid-IR spectra for the 2,2,2-trifluoroethanol compounds and a possible explanation for these modes is given.