Solid parahydrogen is an excellent matrix for matrix-isolation spectroscopy because of its high spectral resolution. Here, we describe rovibrational structure and the nuclear spin conversion of CH₄ embedded in parahydrogen crystals studied by infrared absorption spectroscopy. The vibration-rotation absorptions of CH₄ exhibit time-dependent intensity changes at 4.8 K. These changes are interpreted to be a result of the I = 1 -> I = 2 nuclear spin conversion which accompanies the J = 1 -> J = 0 rotational relaxation. The half-lifetime of the upper J = 1 rotational state is unchanged by the addition of up to 2 % orthohydrogen molecules, but decreases with more than 10 % orthohydrogen molecules. The increase of the decay rate at higher orthohydrogen concentration indicates that the magnetic field gradient across CH₄ caused by orthohydrogen molecules mixes the nuclear spin states which accelerate the conversion.

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