Probing the Unimolecular Reactions of Atmospherically Important Criegee Intermediates

Ozonolysis of alkenes, which originate from both biogenic and anthropogenic sources, is an important atmospheric source of hydroxyl (OH) radicals, key oxidants in the Earth's troposphere. Alkene ozonolysis proceeds through a class of highly reactive carbonyl oxide species known as Criegee intermediates. Upon formation, Criegee intermediates undergo a series of complex reactions, which result in many products, including free radicals and secondary organic aerosols. Unimolecular decay of Criegee intermediates often leads to OH products, and thus impacts the atmospheric HO_x radical budget and tropospheric oxidation capacity. In this work, jet-cooled, stabilized Criegee intermediates are interrogated using infrared (IR) action spectroscopy in order to characterize their unimolecular reaction dynamics. IR activation drives a hydrogen transfer reaction that ultimately results in unimolecular decay to OH products, which are detected by UV laser-induced fluorescence. Energy-dependent OH appearance rates are measured following IR activation of a series of prototypical substituted Criegee intermediates. Recent work is focused on methyl vinyl ketone oxide, the predominant unsaturated four-carbon Criegee intermediate formed from isoprene ozonolysis, as well as a saturated four-carbon Criegee intermediate with methyl and ethyl substituents. These two systems are shown to have significantly different unimolecular reactivity. Importantly, quantum mechanical tunneling is shown to play a significant role in these reactions, enhancing the reaction rates by several orders of magnitude at atmospherically relevant temperatures under thermal conditions. Comparison across the series of differently substituted Criegee intermediates provides insight into the conformational and substituent effects associated with the unimolecular decay of Criegee intermediates to OH products.