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

Abstract

The ${\nu }_3$ and ${\nu }_4$ 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 $\text{J\hspace{0.17em}=\hspace{0.17em}1→J\hspace{0.17em}=\hspace{0.17em}0}$ 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 $\text{J\hspace{0.17em}=\hspace{0.17em}1→J\hspace{0.17em}=\hspace{0.17em}0}$ 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.