%0 Journal Article %A Barber, Victoria P. %A Pandit, Shubhrangshu %A Green, Amy M. %A Trongsiriwat, Nisalak %A Walsh, Patrick J. %A Klippenstein, Stephen J. %A Lester, Marsha I. %D 2018 %T Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production %U 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 %R 10.1021/jacs.8b06010.s001 %2 https://acs.figshare.com/ndownloader/files/12810812 %K Criegee intermediates control %K RR %K unimolecular decay pathways %K OO %K OH products %K unimolecular decay rate %K master equation modeling yields %K Methyl Vinyl Ketone Oxide Synthesis %K IR absorption spectra %K CH %K Four-Carbon Criegee Intermediate %K carbonyl oxide species %K MACR-OO %K hydrogen atom transfer mechanism %K MVK-OO %X 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 (CH2OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R′ = H, CH3, CHCH2) 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 O2. 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–1 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 syn-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–1) 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 syn-MVK-OO of 33 s–1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products. %I ACS Publications