Raman and far-infrared spectra of gaseous 1,1,1,3,3,3-hexafluoro-2-propanol, conformational stability and barriers to internal rotation

Abstract

The Raman (4000-50 cm −1) and far-infrared (370-50 cm −1) spectra of gaseous 1,1,1,3,3,3-hexafluoro-2-propanol, (CF 3) 2CHOH, and of the OD compound have been recorded. Particular attention is given to the OH stretching and low frequency spectral regions, where evidence is found for the existence of both the trans and gauche conformers. From simulations of observed gas phase IR band profiles, it is possible to assign the OH stretch at 3666 cm −1 to the gauche conformer and the one at 3626 cm −1 to the trans conformer (the OH bond trans to the CH bond). From relative intensities of the corres- ponding Raman lines as a function of temperature, the enthalpy difference in the gas phase is found to be 336 ± 132 cm −1 (961 ± 377 cal mol −1) with the trans conformer being more stable. The fundamental OH torsion is observed at 331.7 cm −1 for the trans conformer and at 288.9 cm −1 for the gauche conformer with both bands being identified from their shift with deuteration. From these data an asymmetric potential function is calculated which gives a trans/gauche barrier of 937 cm −1 (2.68 kcal mol −1) and a gauche/gauche barrier of 360 cm −1 (1.00 kcal mol −1). One of the CF 3 torsional modes has been observed at 90 cm −1 from which the barrier to internal rotation is estimated to be in the range and 5–6 kcal mol −1. All these data are compared to the corresponding quantities for some similar compounds.