Calculation of the ortho-para conversion rate of gaseous methanol by the internal axis
methodZ.-D. Sun and L.-S. Ma
School of Physics, Shandong University, China
School of Physics and Electrical Engineering, Kashi University, China
In astrophysics and astrochemistry, the ortho-para conversion rate is an important parameter
which is related to the ortho-to-para population ratio (OPR) and the nuclear-spin temperatureof nuclear-spin isomers of molecules in the interstellar medium (ISM) [1]. The methanol(CH3OH) molecule is a major source of interstellar organic species due to its abundance in theISM. Although conversion rate of nuclear-spin isomers in gaseous methanol has been
experimentally measured recently [2], the quantum calculation of the ortho-para conversion
rate of methanol molecules in the gas phase, the conversion mechanism in the nuclear-spin
isomers conversion of methanol, and the most probable gateways of spin conversion inmethanol have not yet theoretically studied by the internal axis method (IAM) that is aclassical method commonly used by many researchers.
In molecular physics, methanol is one of the simplest asymmetric-top molecules withhindered internal rotation (i.e. torsion) [3]. Methanol does not rotate around the overallrotational symmetry axis but the methyl (CH3) group axis. In the IAM, the CH3-group and theOH-group in methanol relatively rotate in the opposite directions with each other, and thecoupling of the angular momenta between torsion and overall rotation in the Hamiltonian ofmethanol can therefore be completely separated from each other. In accordance with Pauli’sprinciple in quantum mechanics, the identical nuclei in the methyl group produce a specifictype of nuclear spin isomers of the ortho-CH3OH and para-CH3OH in methanol. In this study,
we have developed a theoretical model by the IAM for the description of isomer conversions
by molecular collisions in gaseous methanol. The model is based on the quantum-relaxation
theory confirmed to be valid for the standard nuclear-spin isomers based on the molecularrotational states [4]. The expressions of the ortho-para state-mixing strengths induced byintramolecular nuclear-spin−nuclear-spin and nuclear-spin−rotation interactions in methanol
have been analytically deduced and numerically calculated. We found that the ortho-parastates-mixings and ortho-para conversion can be induced by both of these two intramolecular
hyperfine interactions whereas the nuclear-spin−rotation interaction plays a major role. The
conversion rates at room temperature low pressures over eight ortho-para level pairs havebeen calculated to be closely to the measured values and the most probable gateways of spin
conversion in methanol by collision-induced population transfers are found.
Acknowledgements

We express sincere thanks to P. L. Chapovsky for valuable guidance and discussions with us when
he visited our laboratory. This work was jointly supported by the National Natural Science Foundation ofChina (Grants No. 91536105, No. 11174186, and No. 11074147) and the Tianshan Scholar Program andthe FY2021 JSPS Invitational Fellowship for Research in Japan in a short-term program (S21002).

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