Theoretical Study of the Effect of Hydrogen-Bonding on the Stability and Vibrational Spectrum of Isolated 2,2,2-Trifluoroethanol and Its Molecular Complexes

Abstract

The structural and spectroscopic effects of hydrogen bonding on isolated 2,2,2-trifluoroethanol (TFE) and its molecular complexes are theoretically investigated at the MP2/aug-cc-pVDZ level. As a result, previous interpretations of the relative stability of the trans and gauche conformers of the isolated molecule are revised. We show that the prevalence of the gauche form is due to a decrease of repulsion forces, rather than to the formation of an intramolecular hydrogen bond. We find that the instability of the trans geometry is caused by repulsion forces between the oxygen electronic pair and the fluorine atom clouds, which are significantly stronger in trans -TFE. Molecular agents capable of weakening the repulsion produce stabilization. These results lead to a reinterpretation of the stabilizing factors of halogenated compounds. To analyze complexation, two small molecules (water and ammonia) have been chosen. Water can form four different molecular aggregates with TFE. The most stable corresponds to a species where H 2 O acts as hydrogen donor and TFE presents the cis−gauche conformation, forming two intramolecular hydrogen bonds. For NH 3, the cis−gauche conformation loses stability, because of steric hindrance. In this case, TFE varies the relative stability of its conformers, with trans -TFE becoming the preferred structure. Hydrogen bond formation between NH 3 and trans -TFE produces vibrational shifts of −354, −17, and +518 cm -1 for the OH stretching, the OH bending, and the OH torsion, in agreement with the experimental findings. We found complexation to produce an important variation of the position of the infrared bands corresponding to the hydroxyl group.