Unraveling Conformer-Specific Sources of Hydroxyl Radical Production from an Isoprene-Derived Criegee Intermediate by Deuteration

Abstract

Ozonolysis of isoprene, the most abundant volatile organic compounds emitted into the Earth's troposphere after methane, yields three distinct Criegee intermediates. Among these, methyl vinyl ketone oxide (MVK-oxide) is predicted to be the major source of atmospheric hydroxyl radicals (OH) from isoprene ozonolysis. Previously, Barber et al. [J. Am. Chem. Soc., 2018, 140, pp 10866−10880] demonstrated that syn-MVK-oxide conformers undergo unimolecular decay via 1,4-hydrogen (H) transfer from the methyl group to the adjacent terminal oxygen atom, followed by the prompt release of OH radical products. Here, we selectively deuterate the methyl group of MVK-oxide (d 3 -MVK-oxide) and record its IR action spectrum in the vinyl CH stretch overtone (2ν CH ) region. The resultant time-dependent appearance of OD radical products, detected by laser-induced fluorescence, demonstrates that a unimolecular decay of d 3 -MVK-oxide proceeds by an analogous 1,4-deuterium (D) atom transfer mechanism anticipated for syn conformers. The experimental spectral and temporal results are compared with the calculated IR absorption spectrum and unimolecular decay rates predicted by the Rice−Ramsperger− Kassel−Marcus (RRKM) theory for syn-d 3 -MVK-oxide, as well as the prior study on syn-MVK-oxide. The d 3 -MVK-oxide IR action spectrum is similar to that for MVK-oxide, yet exhibits notable changes as the overtone and combination transitions involving CD stretch shift to a lower frequency. The unimolecular decay rate for d 3 -MVK-oxide is predicted to be a factor of 40 times slower than that for MVK-oxide in the 2ν CH region. Experimentally, the temporal profile of the OD products reflects the slower unimolecular decay of d 3 -MVK-oxide compared to that for MVK-oxide to OH products as well as experimental factors. Both experiment and theory demonstrate that quantum mechanical tunneling plays a very important role in the 1,4-H/D-transfer processes at energies in the vicinity of the transition-state barrier. The similarities of the IR action spectra and changes in the unimolecular decay dynamics upon deuteration indicate that syn conformers make the main contribution to the IR action spectra of MVK-oxide and d 3 -MVKoxide.

Temperature-dependent kinetics of the atmospheric reaction between CH2OO and acetone

“…Criegee intermediates 1 (carbonyl oxides) are formed in the ozonolysis of unsaturated compounds in the atmosphere, [2][3][4][5][6][7][8][9][10][11] and they are very important in the atmosphere because they contribute to the formation of OH radicals [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and secondary organic aerosols. [28][29][30][31][32][33][34][35][36][37] Criegee intermediates act as oxidants, and their kinetics provide key parameters [38][39][40][41][42] for estimating the oxidative capacity of the atmosphere.…”

Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation

“…Criegee intermediates (CIs) are carbonyl oxides that play vital roles in the Earth’s troposphere, where they are formed via a [3 + 2]-cycloaddition reaction involving volatile alkenes and ozone. − CIs are implicated in enhancing the oxidizing capacity of the troposphere and in the formation of less volatile species that may condense to form secondary organic aerosols (SOAs). − …”

The photoisomerization mechanism of methacrolein oxide (MACR-OO): the cyclic dioxole formation pathway revealed

“…Because of the different relative orientations of the CQO and CQC bonds (anti/syn), MVK-OO and MACR-OO have four different configurations and the syn (CO) configuration has a much larger proportion. [21][22][23] In previous work, Lester has proposed the thermal unimolecular decay paths for the MVK-OO and MACR-OO with a combination of experiments and theoretical calculations. It was found that, for MVK-OO, the anti (CO) configurations prefer to have an H-transfer from methyl to the terminal oxygen, while the syn (CO) configuration is more likely to have a three-membered ring-closure; there exists another different rule for MACR-OO, where the syn (CO) -syn (CC) configuration (SS configuration) is predicted to form the 5-membered ring of dioxole.…”