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The atmospherically important reaction of hydroxyl radicals with methyl nitrate: a theoretical study involving the calculation of reaction mechanisms, enthalpies, activation energies, and rate coefficients

The atmospherically important reaction of hydroxyl radicals with methyl nitrate: a theoretical study involving the calculation of reaction mechanisms, enthalpies, activation energies, and rate coefficients
The atmospherically important reaction of hydroxyl radicals with methyl nitrate: a theoretical study involving the calculation of reaction mechanisms, enthalpies, activation energies, and rate coefficients
A theoretical study, involving the calculation of reaction enthalpies and activation energies, mechanisms and rate coefficients, has been made of the reaction of hydroxyl radicals with methyl nitrate, an important process for methyl nitrate removal in the earth's atmosphere. Four reaction channels were considered:- formation of H2O + CH2ONO2, CH3OOH + NO2, CH3OH + NO3, and CH3O + HNO3 . For all channels, geometry optimization and frequency calculations were carried out at the M06-2X/6-31+G** level, while relative energies were improved at the UCCSD(T*)-F12/CBS level. The major channel is found to be the H abstraction channel, to give the products H2O + CH2ONO2. The reaction enthalpy (ΔH298KRX) of this channel is computed as -17.90 kcal.mol-1. Although the other reaction channels are also exothermic, their reaction barriers are high (> 24 kcal.mol-1) and therefore these reactions do not contribute to the overall rate coefficient in the temperature range considered (200-400 K). Pathways via three transition states have been identified for the H abstraction channel. Rate coefficients were calculated for these pathways at various levels of variational transition state theory (VTST) including tunneling. The results obtained are used to distinguish between two sets of experimental rate coefficients, measured in the temperature range 200-400K, one of which is approximately an order of magnitude greater than the other. This comparison, as well as the temperature dependence of the computed rate coefficients, shows that the lower experimental values are favoured. The implications of the results to atmospheric chemistry are discussed.
1089-5639
6554–6567
Ng, Maggie
bc203fa1-3eb7-4b63-b1e9-78faca2e5c30
Mok, Daniel K.W.
49a4e516-0e71-4f59-a3ec-bd607b47ef33
Lee, Edmond P.F.
28f39876-705a-4fa5-b340-db5658ce1503
Dyke, John
46393b45-6694-46f3-af20-d7369d26199f
Ng, Maggie
bc203fa1-3eb7-4b63-b1e9-78faca2e5c30
Mok, Daniel K.W.
49a4e516-0e71-4f59-a3ec-bd607b47ef33
Lee, Edmond P.F.
28f39876-705a-4fa5-b340-db5658ce1503
Dyke, John
46393b45-6694-46f3-af20-d7369d26199f

Ng, Maggie, Mok, Daniel K.W., Lee, Edmond P.F. and Dyke, John (2017) The atmospherically important reaction of hydroxyl radicals with methyl nitrate: a theoretical study involving the calculation of reaction mechanisms, enthalpies, activation energies, and rate coefficients. Journal of Physical Chemistry A, 121 (35), 6554–6567. (doi:10.1021/acs.jpca.7b05035).

Record type: Article

Abstract

A theoretical study, involving the calculation of reaction enthalpies and activation energies, mechanisms and rate coefficients, has been made of the reaction of hydroxyl radicals with methyl nitrate, an important process for methyl nitrate removal in the earth's atmosphere. Four reaction channels were considered:- formation of H2O + CH2ONO2, CH3OOH + NO2, CH3OH + NO3, and CH3O + HNO3 . For all channels, geometry optimization and frequency calculations were carried out at the M06-2X/6-31+G** level, while relative energies were improved at the UCCSD(T*)-F12/CBS level. The major channel is found to be the H abstraction channel, to give the products H2O + CH2ONO2. The reaction enthalpy (ΔH298KRX) of this channel is computed as -17.90 kcal.mol-1. Although the other reaction channels are also exothermic, their reaction barriers are high (> 24 kcal.mol-1) and therefore these reactions do not contribute to the overall rate coefficient in the temperature range considered (200-400 K). Pathways via three transition states have been identified for the H abstraction channel. Rate coefficients were calculated for these pathways at various levels of variational transition state theory (VTST) including tunneling. The results obtained are used to distinguish between two sets of experimental rate coefficients, measured in the temperature range 200-400K, one of which is approximately an order of magnitude greater than the other. This comparison, as well as the temperature dependence of the computed rate coefficients, shows that the lower experimental values are favoured. The implications of the results to atmospheric chemistry are discussed.

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Accepted/In Press date: 9 August 2017
e-pub ahead of print date: 9 August 2017
Additional Information: Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”.

Identifiers

Local EPrints ID: 413528
URI: http://eprints.soton.ac.uk/id/eprint/413528
ISSN: 1089-5639
PURE UUID: b5325174-3f00-4d64-9a89-ad45018d90b8
ORCID for John Dyke: ORCID iD orcid.org/0000-0002-9808-303X

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Date deposited: 25 Aug 2017 16:31
Last modified: 07 Oct 2020 05:52

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Contributors

Author: Maggie Ng
Author: Daniel K.W. Mok
Author: Edmond P.F. Lee
Author: John Dyke ORCID iD

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