Nuclear spin conversion in gaseous methanol

Abstract

Molecules with identical nuclei exist in nature in the form of nuclear spin isomers. Most known are the spin isomers in molecules having overall rotational symmetry. There are molecules that do not have overall rotational symmetry but instead contain symmetrical groups of atoms that perform torsion tunneling (i.e., internal rotation) around their local symmetry axes. Quantum statistics gives rise in such molecules to a specific type of spin isomers based on the torsion tunneling of symmetrical groups of atoms. Recently, enrichment and conversion of nuclear spin isomers in methanol (${\mathrm{CH}}_{3}\mathrm{OH}$) that has torsion tunneling of the methyl group was experimentally investigated by Z.-D. Sun et al. [Nat. Commun. 6, 6877 (2015)]. In the present paper a theoretical model for the methanol spin isomers and their conversion is developed. The model is based on the quantum relaxation theory confirmed to be valid for molecules with overall rotational symmetry. The methanol spin isomer conversion induced by hyperfine spin-spin and spin-rotation interactions is considered. The most important level pair for the methanol isomer conversion is found to be the $({J}^{\ensuremath{'}}=25,{K}^{\ensuremath{'}}=2,{\ensuremath{\sigma}}^{\ensuremath{'}}=1)--(J=24,K=4,p=1)$ pair in the ground torsion state ${v}_{t}=0$. The spin-rotation interaction in methanol is estimated to be equal to 1.6 kHz.