Nuclear-Spin Conversion and Vibration-Rotation Spectra of Methane in Solid Argon

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Abstract
The ν3 and ν4 infrared spectra of methane in an argon matrix have been studied. The assignment of vibration-rotation features enables the methane rotational energies to be determined. The spacings of these levels, which are discussed in terms of crystal-field effects, suggest that methane is a hindered rotor in its solid matrix. The vibration-rotation features exhibit time-dependent absorption changes. These changes which are evidence of triplet →quintet nuclear-spin conversion that accompanies the J = 1→J = 0 rotational relaxation, follow first-order kinetics with a half-life of 90 min. The half-life is unchanged by addition of up to 1% N2 to the matrix but decreases to 3 min with only 0.2% O2. Two mechanisms are proposed which are in order of magnitude agreement of observed relaxation rates. In the absence of paramagnetic impurities, spin-spin interaction within the molecule mixes spin states and allows J = 1→J = 0 to relax into the crystal lattice. In the presence of O2 the spin states are mixed by the magnetic-field gradient which is set up across CH4.