Molecular mechanics calculation on the hydroxyl stretching frequency in saturated alcohols

Abstract

The frequencies of the hydroxyl stretching vibration, ν OH, of the OH rotamers of methanol, ethanol, propan-2-ol, t-butanol and adamantan-1-ol in the gas phase have been calculated applying molecular mechanics methods. A discussion is devoted to the assignment of the observed ν OH frequencies to the rotamers in ethanol and propan-2-ol in view of the discrepancy in published data. The calculations were performed with the Allinger MM2 force field. It proved to be necessary to include electrostatic interactions between point charges located at the atomic nuclei to obtain calculated results that are in good agreement with experimental measurements. The calculation of these interactions and the introduction in the minimization procedure of the energy is described. Minor adaptation of the force field was needed to fit the observed frequencies. With the introduced modifications a systematic increase of about 0.3 kcal mol −1 is found in the calculated steric energies of the conformers. The relative rotamer populations derived from the energy difference between OH conformers in the molecules ethanol and propan-2-ol agree with experimental IR results.