ChemInform Abstract: Torsional Vibrations and Barriers to Internal Rotation for Ethanol and 2,2,2-Trifluoroethanol.

Abstract

Abstract The far-IR (370-50 cm −1 ) spectra of gaseous ethanol, CH 3 CH 2 OH, 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 3 CH 2 OH, 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 3 CH 2 OH 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 3 CH 2 OH 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.