Novel OH roaming pathway in the unimolecular decay of alkyl- substituted criegee intermediates T. Liu 1 , M. Zou 1 , M. F. Vansco 2 , S. N. Elliot 2 , C. R. Markus 3,4 , R. Almeida 5 , K. Au 5 , L. Sheps 5 , D. L. Osborn 5,6 , C. J. Percival 3 , C. A. Taatjes 5 , S. J. Klippenstein 2 , R. L. Caravan 2 , and Marsha I. Lester 1 1 Department of Chemistry, University of Pennsylvania, USA, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, USA, 3 NASA Jet Propulsion Laboratory, California Institute of Technology, USA, 4 Division of Chemistry and Chemical Engineering, California Institute of Technology, USA, 5 Combustion Research Facility, Sandia National Laboratories, USA, 6 Department of Chemical Engineering, University of California, USA Anthropogenic and biogenic alkenes, abundant volatile organic compounds emitted into the atmosphere, react with ozone to generate short-lived, highly reactive carbonyl oxide intermediates (R 1 R 2 C=O + O − ), known as Criegee intermediates. Unimolecular decay of alkyl-substituted Criegee intermediates generally proceeds via intramolecular 1,4 H-atom transfer from an alkyl group (R 1 , R 2 ) to the terminal O-atom, transiently forming a vinyl hydroperoxide, followed by O-O bond fission that releases hydroxyl (OH) radicals.While unimolecular decay of Criegee intermediates to OH products can occur promptly or following thermalization, recent experimental and theoretical studies suggest that the separating OH radical can also reorient, roam, and add to the vinyl group, resulting in roaming-induced isomerization to hydroxycarbonyl products. 1-3 The present work identifies stable hydroxybutanone products arising from OH roaming in the unimolecular decay of the methyl-ethyl substituted Criegee intermediate (CH 3 )(CH 3 CH 2 )C=O + O − , along with products derived from C-C fragmentation. Kinetic time profiles, exact masses, and photoionization spectra enable identification of roaming products generated under thermal conditions. The experiments utilize multiplexed photoionization mass spectrometry with tunable vacuum ultraviolet radiation at the Advanced Light Source (Lawrence Berkeley National Laboratory). Complementary theoretical calculations validate the OH roaming pathway leading to hydroxybutanone and other products. References 1. Taatjes, C. A.; Liu, F.; Rotavera, B.; Kumar, M.; Caravan, R.; Osborn, D. L.; Thompson, W. H.; Lester, M. I., Hydroxyacetone Production From C3 Criegee Intermediates. J. Phys. Chem. A 2017, 121 , 16-23. 2. Kuwata, K. T.; Luu, L.; Weberg, A. B.; Huang, K.; Parsons, A. J.; Peebles, L. A.; Rackstraw, N. B.; Kim, M. J., Quantum Chemical and Statistical Rate Theory Studies of the Vinyl Hydroperoxides Formed in trans-2-Butene and 2,3-Dimethyl-2- butene Ozonolysis. J. Phys. Chem. A 2018, 122 , 2485-2502. 3. Barber, V. P.; Pandit, S.; Green, A. M.; Trongsiriwat, N.; Walsh, P. J.; Klippenstein, S. J.; Lester, M. I., Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production. J. Am. Chem. Soc. 2018, 140 , 10866-10880.
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