Far-Infrared Spectrum and Normal Coordinate Analysis of α-Methanol

Abstract

The infrared spectra of α-methanol and α-methanol-d3 have been measured in the range 400–20 cm−1 at 100 and 150°K. Seven intermolecular vibrations are expected and were observed. The CH3 internal rotations were not observed. A normal coordinate analysis of the spectra has been made assuming that the potential energy has the symmetry C2h, that is, is independent of the position of the hydrogen atom in the hydrogen bond. The hydrogen-bond stretching force constant is 0.346 mdyn Å−1, which is about twice the value for the hydrogen bond in ice. This does not correlate well with the effect of hydrogen bonding on the OH stretching frequencies, nor with the dissociation energy of the dimers of methanol and water in the vapor. However, the heat of sublimation of methanol at 0°K indicates that about 8.5 kcal mole−1 are required to break the hydrogen bonds, compared with only 5.66 kcal mole−1 in ice, which is consistent with the higher force constant for methanol.