Atmospheric autoxidation is increasingly important in urban and suburban North America
Gas-phase autoxidation—regenerative peroxy radical formation following intramolecular hydrogen shifts—is known to be important in the combustion of organic materials. The relevance of this chemistry in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures. Here, we combine computational and experimental approaches to investigate the rate of autoxidation for organic peroxy radicals (RO_2) produced in the oxidation of a prototypical atmospheric pollutant, n-hexane. We find that the reaction rate depends critically on the molecular configuration of the RO_2 radical undergoing hydrogen transfer (H-shift). RO_2 H-shift rate coefficients via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, respectively) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1 s^(−1) at 296 K, while the 1,4 H-shift is calculated to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coefficients of these reactions increase rapidly with temperature and exhibit a large, primary, kinetic isotope effect. The observed H-shift rate coefficients are sufficiently fast that, as a result of ongoing NO_x emission reductions, autoxidation is now competing with bimolecular chemistry even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temperatures are elevated.
© 2017 National Academy of Sciences. Published under the PNAS license. Edited by Marsha I. Lester, University of Pennsylvania, Philadelphia, PA, and approved November 8, 2017 (received for review September 7, 2017). Published online before print December 18, 2017. We thank Kristian H. Møller for helpful discussions related to the implementation of MC-TST. J.C.H. thanks the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry for support. We acknowledge funding from National Science Foundation Grant CHE-1508526 as well as from the University of Copenhagen. Author contributions: J.D.C., H.G.K., and P.O.W. designed research; E.P., R.V.O., J.D.C., and H.G.K. performed research; J.C.H., B.M.S., and P.O.W. contributed new reagents/analytic tools; E.P., R.V.O., and J.D.C. analyzed data; and E.P., R.V.O., J.D.C., H.G.K., and P.O.W. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1715540115/-/DCSupplemental.
Published - PNAS-2018-Praske-64-9.pdf
Supplemental Material - pnas.1715540115.sapp.pdf