Adsorption sites and rotational tunneling of methyl groups in cubic I methyl fluoridewater clathrate

Abstract

Neutron spectroscopy in the μeV and meV regime and quasielastic scattering is applied to characterize the dynamics of methyl groups of methyl fluoride guest molecules in cubic I CH₃F–water clathrate. Only above T ∼ 60 K quasielastic spectra are unaffected by quantum effects. They are well described by two Lorentzians representing the CH₃F species in the small and large cages of the structure. The intensities show that both cages are completely filled. The linear broadenings with temperature follow the model of rotational diffusion. Two clearly separated tunneling bands were observed at T = 4.2 K and are also assigned to the two types of water cages. Disorder of the environment (H-bonds) is reflected in the shape of the bands. For the less hindered species housing the large cages the tunneling band can be quantitatively converted into a potential distribution function within the model of single particle rotation. Transitions to excited rotational states show the dominance of a sixfold potential term ₆ = 13 meV modified by a weak threefold term distributed around a characteristic value ₃ = 0.9 meV. The potential distribution of V₃ influences the barrier for classical reorientation only weakly in agreement with the results from quasielastic data. Adsorption sites with the guest molecules oriented towards a hydrogen bond along one of twelve local twofold axes of the cage are proposed. Such sites are consistent with the sixfold rotational potential and earlier results from methyl iodide clathrate. Rotation–translation coupling as an alternative dynamical process is excluded.