Electronic Spectroscopy, Photochemistry, And Reactivity Of Isoprene-Derived Criegee Intermediates
Ozonolysis is an important sink of alkenes in Earth’s troposphere, leading to the formation of highly reactive carbonyl oxide (R1R2C=O+O-) species known as Criegee intermediates. Ozonolysis of isoprene, the most abundant non-methane volatile organic compound emitted into the atmosphere, can generate three distinct Criegee intermediates: formaldehyde oxide (CH2OO), methyl vinyl ketone oxide (MVK-oxide), and methacrolein oxide (MACR-oxide). Due to the abundance of isoprene in the atmosphere, the unimolecular and bimolecular reactions of Criegee intermediates may significantly impact the composition of the troposphere. The laboratory synthesis and direct detection of MVK-oxide and MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MVK-oxide and MACR-oxide are characterized on their first *← transition using a ground-state depletion method under jet-cooled conditions. These Criegee intermediates exhibit broad ultraviolet-visible (UV-vis) spectra with strong absorption (ca. 10-17 cm2 molec-1). Electronic excitation of Criegee intermediates results in nonadiabatic coupling to repulsive potentials and prompt release of O 1D products. Velocity map imaging is used to determine the angular and velocity distributions of the O-atom products following UV-vis excitation of the isoprene-derived Criegee intermediates. UV-vis transient absorption spectroscopy permits study of the bimolecular reactions of MVK-oxide with SO2, formic acid, and water vapor under thermal conditions. Complimentary experiments using multiplexed photoionization mass spectrometry (MPIMS) identify products resulting from reaction of MVK-oxide with SO2 and formic acid. The reaction of MVK-oxide with deuterated formic acid reveals multiple reaction channels including adduct formation and formic acid catalyzed isomerization yielding a vinyl hydroperoxide. Through a combination of experiment, theory, and global modeling, syn conformers of MVK-oxide are shown to survive high-humidity tropospheric environments and play a role in sulfuric acid formation and formic acid removal. In contrast, anti-MVK-oxide and syn-MACR-oxide conformers are predicted to be removed rapidly from the atmosphere via electrocyclic ring closure to form a cyclic dioxole, which subsequently decays to oxygenated hydrocarbon radical products. These radicals react rapidly with oxygen and their stable carbonyl products are detected using MPIMS.
Jessica M. Anna