Probing the Unimolecular Decay of Atmospherically Important Criegee Intermediates

Ozonolysis of alkenes is an important source of hydroxyl (OH) radicals, key oxidants in the Earth’s troposphere. Alkene ozonolysis proceeds via carbonyl oxide species known as Criegee intermediates. Infrared (IR) action spectroscopy is used to study OH production from jet-cooled, stabilized Criegee intermediates. IR activation drives the rate-limiting 1,4 H-atom transfer from a syn-alkyl substituent to the terminal oxygen of the carbonyl oxide group, followed by rapid unimolecular decay to OH, which is detected by ultraviolet (UV) laser-induced fluorescence (LIF). IR action spectra provide spectral fingerprints of the Criegee intermediates. OH appearance rates are measured following IR activation by varying the IR-UV time delay. This technique is applied to two prototypical Criegee intermediates, syn-CH3CHOO and (CH3)2COO, in three different energy regimes, including at energies significantly below the calculated transition state (TS) barrier, indicating the importance of quantum mechanical tunneling in the H-atom transfer reaction. The role of tunneling is further confirmed by a significant observed kinetic isotope effect for the D-atom transfer reaction of syn-CD3CHOO. The IR action spectroscopy technique is also extended to more complex Criegee intermediates. Methyl vinyl ketone oxide (MVK-oxide) is an unsaturated four-carbon Criegee intermediate formed from the ozonolysis of isoprene, the most abundant non-methane volatile organic compound in the atmosphere. MVK-oxide was generated via a novel synthetic method and identified by its IR action spectrum. OH appearance rate measurements validate the calculated TS barrier for the H-atom shift reaction, and provide insight into an alternative unimolecular decay mechanism. The methyl-ethyl substituted Criegee intermediate (MECI) is a saturated four-carbon Criegee intermediate and is unique among Criegee intermediates studied by IR action spectroscopy because multiple conformational forms can undergo H-atom transfer to OH. Comparisons among the Criegee intermediates studied provides insight into substituent effects on unimolecular decay. The experimental OH appearance rates across many systems are in good agreement with statistical RRKM rate calculations incorporating tunneling, validating the unimolecular decay mechanism. Finally, UV LIF is used to detect vinoxy radicals, a coproduct in the H-atom transfer reaction. LIF detection of vinoxy radicals may be a probe for alternative unimolecular chemistry of vinyl-substituted Criegee intermediates from isoprene ozonolysis