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Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-pinene and β-pinene by Hydroxyl Radicals

Xu, Lu and Møller, Kristian H. and Crounse, John D. and Otkjær, Rasmus V. and Kjaergaard, Henrik Grum and Wennberg, Paul O. (2019) Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-pinene and β-pinene by Hydroxyl Radicals. Journal of Physical Chemistry A, 123 (8). pp. 1661-1674. ISSN 1089-5639. doi:10.1021/acs.jpca.8b11726.

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Atmospheric oxidation of monoterpenes (emitted primarily by evergreen trees) is known to contribute to the formation and growth of aerosol particles. While recent research has tied the formation of organic aerosol to unimolecular chemistry of the organic peroxy radicals (RO_2) formed in the oxidation of monoterpenes, the fundamental physical chemistry of these RO_2 remains obscure. Here we use isomer-specific measurements and ab initio calculations to determine the unimolecular reaction rates and products of RO_2 derived from the hydroxyl radical (OH) oxidation of α-pinene and β-pinene. Among all of the structural isomers of the first-generation RO_2 from both monoterpenes, we find that the first-generation RO_2 produced following opening of the four-membered ring undergo fast unimolecular reactions (4 ± 2 and 16 ± 5 s^(–1) for α-pinene and β-pinene, respectively) at 296 K, in agreement with high-level ab initio calculations. The presence of the hydroxy group and carbon–carbon double bond in the ring-opened RO_2 enhances the rates of these unimolecular reactions, including endo-cyclization and H-shift via transition states involving six- and seven-membered rings. These reaction rate coefficients are sufficiently large that unimolecular chemistry is the dominant fate of these monoterpene-derived RO_2 in the atmosphere. In addition, the overall yields of first-generation α-pinene and β-pinene hydroxy nitrates, C_(10)H_(17)NO_4, at 296 K and 745 Torr are measured to be 3.3 ± 1.5% and 6.4 ± 2.1%, respectively, for conditions where all RO_2 are expected to react with NO ([NO] > 1000 ppbv). These yields are lower than anticipated.

Item Type:Article
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URLURL TypeDescription Information ItemOutput files of all calculations, including the xyz geometries
Xu, Lu0000-0002-0021-9876
Møller, Kristian H.0000-0001-8070-8516
Crounse, John D.0000-0001-5443-729X
Otkjær, Rasmus V.0000-0002-6094-1828
Kjaergaard, Henrik Grum0000-0002-7275-8297
Wennberg, Paul O.0000-0002-6126-3854
Additional Information:© 2019 American Chemical Society. Received: December 5, 2018; Revised: January 30, 2019; Published: January 31, 2019. We thank Eric Praske, Krystal T. Vasquez, and Yuanlong Huang for technical support. L.X., J.D.C., and P.O.W. acknowledge NASA (NNX14AP46G) and NSF (CHE-1508526) for support of this work. K.H.M., R.V.O., and H.G.K. acknowledge funding from the University of Copenhagen. K.H.M. acknowledges the financial support from the Danish Ministry for Higher Education and Science’s Elite Research Travel Grant. The authors declare no competing financial interest.
Funding AgencyGrant Number
University of CopenhagenUNSPECIFIED
Ministry for Higher Education and Science (Denmark)UNSPECIFIED
Issue or Number:8
Record Number:CaltechAUTHORS:20190131-104830327
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Official Citation:Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-Pinene and β-Pinene by Hydroxyl Radicals. Lu Xu, Kristian H. Møller, John D. Crounse, Rasmus V. Otkjær, Henrik G. Kjaergaard, and Paul O. Wennberg. The Journal of Physical Chemistry A. 2019 123 (8), 1661-1674 DOI: 10.1021/acs.jpca.8b11726
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:92540
Deposited By: George Porter
Deposited On:31 Jan 2019 23:24
Last Modified:16 Nov 2021 03:51

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