Dynamics and adsorption sites for guest molecules in methyl chloride hydrate

Abstract

Methyl rotational tunneling, quasielastic spectra and phonondensities of states measured with neutrons at energy transfers from35 µeV to 40 meV are used to characterize the adsorption sites and potentials ofCH3Cl guest molecules in the two kinds of cages of cubic I methyl chloride hydrate.The model of adsorption to hydrogen bonds of the ice cage explains the broadtunnel spectrum as a superposition of four bands. Up to a temperatureT∼80 K the quasielastic scattering is influenced by quantum motion. Only aboveT∼80 K does the effective linewidth follow an Arrhenius behavior with an average activationenergy. Rotational potentials contain a significant sixfold component supporting theproposed adsorption to hydrogen bonds with their mirror symmetry. Positional disorder ofprotons in the hydrogen bonds of the host ice lattice leads to potential distributions oflarge relative widths .