Proton Motions in Methanol by Cold Neutron Scattering

Abstract

Cold neutron scattering data are reported for methyl alcohol in the liquid phase at room temperature. The quasielastic scattering was interpreted using the Larsson and Bergstedt model, that takes into account intramolecular motions and molecular diffusion. On the basis of this model, one finds for the relaxation time of the hindered rotation of the CH3 group within the molecule a value of 2.4 × 10−12 sec. The analysis of the quasielastic scattering according to the L-B model explains in a consistent way our experimental results in a range of momentum transfers of about 0.80–1.55 Å−1. In the inelastic region some structure is observed at energy transfers of 22, 17, and 5 meV. The 17-meV energy transfer is associated with the 1 → 0 transition of the torsional oscillations of the methyl group. The activation energy for the above motion was calculated to be E=1.3 kcal/mol, in good agreement with the value of the barrier height for internal rotation of the CH3 in methanol, obtained by microwave methods.