Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production Victoria P. Barber Shubhrangshu Pandit Amy M. Green Nisalak Trongsiriwat Patrick J. Walsh Stephen J. Klippenstein Marsha I. Lester 10.1021/jacs.8b06010.s001 https://acs.figshare.com/articles/journal_contribution/Four-Carbon_Criegee_Intermediate_from_Isoprene_Ozonolysis_Methyl_Vinyl_Ketone_Oxide_Synthesis_Infrared_Spectrum_and_OH_Production/6985247 The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR′COO) known as Criegee intermediates: formaldehyde oxide (CH<sub>2</sub>OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R′ = H, CH<sub>3</sub>, CHCH<sub>2</sub>) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized mono­iodo­alkene radical with O<sub>2</sub>. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm<sup>–1</sup> is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For <i>syn</i>-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol<sup>–1</sup>) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for <i>syn</i>-MVK-OO of 33 s<sup>–1</sup> (298 K, 1 atm). For <i>anti</i>-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products. 2018-08-03 00:00:00 Criegee intermediates control RR unimolecular decay pathways OO OH products unimolecular decay rate master equation modeling yields Methyl Vinyl Ketone Oxide Synthesis IR absorption spectra CH Four-Carbon Criegee Intermediate carbonyl oxide species MACR-OO hydrogen atom transfer mechanism MVK-OO