Tunneling states in phase III of solid partially deuterated methanes. Determination by inelastic neutron scattering and correlation with thermodynamic data

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Abstract
Energies of tunneling states in solid CH3D, CH2D2, and CHD3 in the range 0<ε<200 μeV were observed directly at low temperatures (T→1.5 K) by high resolution neutron scattering. An isotope effect was found which is broadly consistent with theoretical predictions. This is in contrast to the ’’view’’ obtained previously from the results of heat capacity measurements on the same solids at low temperatures: such an isotope effect was not apparent. By application of a new pocket state formalism that takes account of differences in energy for different orientations of the molecules under anisotropic fields, it is shown that the two kinds of experimental data can be correlated. The important general result is the achievement of a much deeper understanding than available heretofore of orientational and tunneling states in solid methane. Because the structure, and hence the site symmetries, of the lowest temperature phase (phase III) have not been established unambiguously, a relatively simple model is used to analyze the experimental results. It assumes equal proportions of sites of C2 and Cs symmetry and it gives a remarkably good description of the essential features of both the neutron scattering and the thermodynamic data. The heat capacities of CH2D2 and CH3D are not, however, reproduced within their probable accuracies at all temperatures. Moreover, some features of the neutron scattering spectra are broader than those predicted by the model. However, these are viewed as relatively minor imperfections in a model for what must be a structure that has a far more complex sublattice arrangement than our model assumes.

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