Theory of Nuclear Spin Conversion in the β Phase of Solid Methane

The dominant mechanism for spin conversion in solid CH₄ in the β phase is found to be a hybrid process in which intramolecular dipolar interactions mix the nuclear spin states, and the intermolecular octupole interaction, modulated by phonon emission, causes transitions between orientational states. The conversion rate calculated for free rotor molecules is 31 to 54% per hour. The mechanism is shown to be inefficient for ordered molecules which are estimated to convert at least 1000 times slower.