Nuclear spin–lattice relaxation in solid methane and its deuterated modifications

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Abstract
The proton and deuteron spin–lattice relaxation time T1 has been measured in CH4, CD4, CHD3, CH3D, and CH4–CD4 mixtures and CH4–krypton mixtures as a function of temperature between 1.2 °K and 60 °K. The aim of this study was to obtain information on the two low-temperature phase transitions characteristic of methane. Discontinuities in the plot of T1 versus temperature were observed for all the systems studied at the upper phase transition, and also for CHD3 and CH3D at the lower phase transition, but not for CH4 and CD4 at the lower phase transition. Analysis of the data revealed that the strong temperature dependence of T1 is associated with molecular reorientation, but the classical theory of molecular reorientation proved to be inadequate to explain the numerical values of T1 obtained for all the systems studied. It is suggested that a more complete analysis of the data requires a much more detailed theory of solid methane. In particular, more quantitative theoretical calculations of the energy levels of methane in the crystal field are required, and the role of the lattice vibrations in causing molecular reorientations must be elucidated before the data presented in this paper can be properly interpreted.
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