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The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol.

ABSTRACT: Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces - ?45% of reaction products are obtained via intersystem crossing of a pre-product complex - which demands experimental determinations of product branching. Here we report direct measurements of methanol from this reaction. A branching fraction below 15% is established, consequently highlighting a large gap in the understanding of global methanol sources. These results support the recent high-level theoretical work and substantially reduce its uncertainties.

SUBMITTER: Caravan RL

PROVIDER: S-EPMC6195545 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol.

Caravan Rebecca L RL Khan M Anwar H MAH Zádor Judit J Sheps Leonid L Antonov Ivan O IO Rotavera Brandon B Ramasesha Krupa K Au Kendrew K Chen Ming-Wei MW Rösch Daniel D Osborn David L DL Fittschen Christa C Schoemaecker Coralie C Duncianu Marius M Grira Asma A Dusanter Sebastien S Tomas Alexandre A Percival Carl J CJ Shallcross Dudley E DE Taatjes Craig A CA

Nature communications 20181019 1

Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces - ∼45% ...[more]

PMID: 30341291

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Project description:Understanding the mechanism of hydrazine oxidation reaction by OH radical along with the rate constants of all possible pathways leads to explain the fate of hydrazine in the atmosphere. In this article, the comprehensive mechanisms and kinetics of the hydrazine plus hydroxyl radical reaction have been investigated theoretically at different temperatures and pressures. To achieve the main goals, a series of high levels of quantum chemical calculations have been widely implemented in reliable channels of the H-abstraction, SN2, and addition/elimination reactions. The energy profile of all pathways accompanied by the molecular properties of the involved stationary points has been characterized at the MP2, M06-2X, and CCSD(T)/CBS levels. To estimate accurate barrier energies of the H-abstraction channels, large numbers of the CCSD (T) calculations in conjunction with various augmented basis sets have been implemented. The direct dynamic calculations have been carried out using the validated M06-2X/maug-cc-pVTZ level, and also by the CCSD(T) (energies) + MP2 (partition functions) level. The pressure-dependent rate constants of the barrierless pathways have been investigated by the strong collision approach. Therefore, the main behaviors of the N2H4 + OH reaction have been explored according to the influences of temperature and pressure on the computed rate coefficients within the well-behaved theoretical frameworks of the TST, VTST, and RRKM theories. It has been found that the H-abstraction mechanism (to form N2H3) is dominant relative to the SN2 reaction and OH-addition to the N center of N2H4 moiety (to form H2NOH + NH2). The computed high pressure limit rate constant of the main reaction pathway, k(298.15) = 7.31 × 10-11 cm3 molecule-1 s-1, has an excellent agreement with the experimental value (k (298.15) = (6.50 ± 1.3) × 10-11 cm3 molecule-1 s-1) recommended by Vaghjiani. Also, the atmospheric lifetime of hydrazine degradation by OH radicals has been demonstrated to be 32.80 to 1161.11 h at the altitudes of 0-50 km. Finally, the disagreement in the calculated rate constants between the previous theoretical study and experimental results has been rectified.

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The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol.

Publications

The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol.

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