Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method

Quantum statistics of identical nuclei in molecules gives rise to nuclear spin isomers. Except for methanol (CH3OH) that has internal rotation (i.e., torsion), all the studied molecules so far have overall rotational symmetry. Methanol does not rotate around the overall rotational symmetry axis but...

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Autores principales:	Li-Sha Ma, Zhen-Dong Sun
Formato:	article
Lenguaje:	EN
Publicado:	Elsevier 2021
Materias:	Conversion of nuclear spin isomers Methanol Internal axis method Torsion tunneling Quantum relaxation Physics QC1-999
Acceso en línea:	https://doaj.org/article/eaa2fdbc983e4b6bb59f416a0f3b33f8
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spelling	oai:doaj.org-article:eaa2fdbc983e4b6bb59f416a0f3b33f82021-11-06T04:27:20ZConversion of ortho and para nuclear spin isomers of methanol studied by internal axis method2211-379710.1016/j.rinp.2021.104959https://doaj.org/article/eaa2fdbc983e4b6bb59f416a0f3b33f82021-12-01T00:00:00Zhttp://www.sciencedirect.com/science/article/pii/S2211379721009761https://doaj.org/toc/2211-3797Quantum statistics of identical nuclei in molecules gives rise to nuclear spin isomers. Except for methanol (CH3OH) that has internal rotation (i.e., torsion), all the studied molecules so far have overall rotational symmetry. Methanol does not rotate around the overall rotational symmetry axis but the methyl group axis. The identical nuclei in the methyl group produce a specific type of nuclear spin isomers of the ortho-CH3OH and para-CH3OH in methanol. Conversions between these two spin isomers have been observed in the experiment and a theory based on the rho-axis method (RAM) for partially explaining their conversions has been developed by us recently. However, besides RAM the internal axis method (IAM) is a classical method commonly used by many researchers and the coupling of the angular momenta between torsion and overall rotation in methanol can be completely separated from each other in the IAM. In this paper, as a series work we present a general theoretical model about conversion of ortho–para methanol using the IAM based on quantum relaxation theory. The expressions of the ortho–para state-mixing strengths induced by the nuclear spin–spin and nuclear spin–rotation interactions have been analytically deduced and numerically calculated. We found that the ortho–para states-mixings and ortho–para conversion can be induced by all these two intramolecular interactions whereas the nuclear spin–rotation interaction plays a major role. The conversion rate at temperature 300 K and pressure 1 Torr summed over eight ortho–para level pairs with gaps less than 2.7 GHz is calculated to be 1.4 × 10−2 s−1, which is close to the measured value of 2.1 (3) × 10−2 s−1, indicating the quantum relaxation is the leading process in the spin isomers conversions of methanol and suggesting our theory to be valid for other gaseous methanol-like molecules having torsion tunneling of symmetrical group. The most probable gateways in methanol for collision-induced population transfers are found to be levels-pairs in the ground-state A–E species of (υt J, K, p) = (0, 24, 4, 1)−(0, 25, 2, 1) by nuclear spin–spin coupling, (0, 22, 5, p)−(0, 21, 6, −1) and (3, 34, 1, 0)−(3, 35, 0, 1) by nuclear spin–rotation coupling.Li-Sha MaZhen-Dong SunElsevierarticleConversion of nuclear spin isomersMethanolInternal axis methodTorsion tunnelingQuantum relaxationPhysicsQC1-999ENResults in Physics, Vol 31, Iss , Pp 104959- (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Conversion of nuclear spin isomers Methanol Internal axis method Torsion tunneling Quantum relaxation Physics QC1-999
spellingShingle	Conversion of nuclear spin isomers Methanol Internal axis method Torsion tunneling Quantum relaxation Physics QC1-999 Li-Sha Ma Zhen-Dong Sun Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
description	Quantum statistics of identical nuclei in molecules gives rise to nuclear spin isomers. Except for methanol (CH3OH) that has internal rotation (i.e., torsion), all the studied molecules so far have overall rotational symmetry. Methanol does not rotate around the overall rotational symmetry axis but the methyl group axis. The identical nuclei in the methyl group produce a specific type of nuclear spin isomers of the ortho-CH3OH and para-CH3OH in methanol. Conversions between these two spin isomers have been observed in the experiment and a theory based on the rho-axis method (RAM) for partially explaining their conversions has been developed by us recently. However, besides RAM the internal axis method (IAM) is a classical method commonly used by many researchers and the coupling of the angular momenta between torsion and overall rotation in methanol can be completely separated from each other in the IAM. In this paper, as a series work we present a general theoretical model about conversion of ortho–para methanol using the IAM based on quantum relaxation theory. The expressions of the ortho–para state-mixing strengths induced by the nuclear spin–spin and nuclear spin–rotation interactions have been analytically deduced and numerically calculated. We found that the ortho–para states-mixings and ortho–para conversion can be induced by all these two intramolecular interactions whereas the nuclear spin–rotation interaction plays a major role. The conversion rate at temperature 300 K and pressure 1 Torr summed over eight ortho–para level pairs with gaps less than 2.7 GHz is calculated to be 1.4 × 10−2 s−1, which is close to the measured value of 2.1 (3) × 10−2 s−1, indicating the quantum relaxation is the leading process in the spin isomers conversions of methanol and suggesting our theory to be valid for other gaseous methanol-like molecules having torsion tunneling of symmetrical group. The most probable gateways in methanol for collision-induced population transfers are found to be levels-pairs in the ground-state A–E species of (υt J, K, p) = (0, 24, 4, 1)−(0, 25, 2, 1) by nuclear spin–spin coupling, (0, 22, 5, p)−(0, 21, 6, −1) and (3, 34, 1, 0)−(3, 35, 0, 1) by nuclear spin–rotation coupling.
format	article
author	Li-Sha Ma Zhen-Dong Sun
author_facet	Li-Sha Ma Zhen-Dong Sun
author_sort	Li-Sha Ma
title	Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
title_short	Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
title_full	Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
title_fullStr	Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
title_full_unstemmed	Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
title_sort	conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method
publisher	Elsevier
publishDate	2021
url	https://doaj.org/article/eaa2fdbc983e4b6bb59f416a0f3b33f8
work_keys_str_mv	AT lishama conversionoforthoandparanuclearspinisomersofmethanolstudiedbyinternalaxismethod AT zhendongsun conversionoforthoandparanuclearspinisomersofmethanolstudiedbyinternalaxismethod
_version_	1718443951672786944

Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method

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