The mechanisms responsible for spin isomer con version in CH4 and some of the ramifications of such a conversion are investigated. The approach to equilibrium of an unstable mixture of spin isomers suggests possible transitions responsible for the conversion. Transitions between the lowest states belonging to spin isomers as well as transitions between excited, nearly degenerate, states occur. The temperature dependence of the mean square nuclear spin determines the hindered rotational energy levels whose energies are below 18 K. Previously published data agree with the specific heat and entropy calculated from the rearrangement of the molecules among these hindered rotational energy levels.

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The various contributions to the second moment in methane, CH4, are investigated. A general expression for the intermolecular second moment is obtained. Using this, the intramolecular second moment was measured using a series of observations during spin isomer conversion. The value found, near 1.5 G2, suggests restricted motion of a preferred axis of reorientation. A search for the onset of the intramolecular second moment shows that it occurs in the same region as the onset of birefringence.