High frequency dynamics of liquid methanol and toluene. Contribution of molecular rotational diffusion to inelastic neutron scattering

Abstract

Both molecular rotational and translational diffusion may contribute to the spectra of molecular liquids measured by incoherent inelastic neutron scattering, (i.n.s.), especially in the “quasielastic” scattering region. Some experiments on two, quite different, molecular liquids, methanol (CH₃OH) and toluene (CH₃ . C₆H₅) are presented here to assess the extent to which rotational diffusion and whole molecule rotation interfere with quasi-elastic measurements of centre of mass diffusion, in the light of Larsson's recent theories. Isotopic substitution, previously introduced to help assign contributions to i.n.s. from intramolecular motions, is again found to be a valuable technique for studying the external rotations. Toluene and methanol have rather different moments of intertia and barriers to internal rotation. While the inertial effects of substitution are relatively small (except for the CH₃ group motions) the effects produced by the various deuterium permutations on the cross-sections for rotational diffusion scattering vary greatly in the two systems.

Deuterium substitution of the CH₃ group in methanol allows a reliable measurement of the centre of mass diffusion of the molecule on the neutron scattering time scales (10^–11 s-10^–12s) with present time of flight spectrometer resolutions (∼ 10%ΔE/E at E₀= 23 cm^–1). No evidence for intermolecular proton transfer in pure methanol was found.

For toluene the moment of inertia distribution is such that rotational contributions cannot be completely eliminated by deuterium substitution. Dynamical models have been made to investigate the inelastic rotational scattering in the methanol i.n.s. spectrum and some more refined computing methods for fitting quasi-elastic scattering spectra, in cases susceptible to simple diffusion analysis, are given.