Molecular dynamics of methanol

Abstract

The far infra-red power absorption coefficients of methanol in the pure and dilute solution states have been measured at 293 K in the wavenumber range 20–230 cm^-1 with interferometry and laser spectroscopy. The liquid state spectrum shows a peak at 120 cm^-1 which shifts to below about 80 cm^-1 upon extreme dilution in cyclohexane. In such solutions a second major peak is observed at 210 cm^-1, which probably corresponds to the COH torsion of single molecules as observed at 200 cm^-1 by Lake and Thompson for methanol vapour. The results are interpreted in a semi-quantitative manner using molecular dynamics simulation. The methanol pair-potential is modelled roughly with a 3×3 site-site representation, made up to Lennard-Jones cores and fixed point charges. While this gives a fairly acceptable result at short-times or far infrared frequencies, it does not continue to do so at longer times. This has been attributed to an insufficient consideration for the long-range hydrogen-bonding in the model.