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

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 . [ , 2018, 140, pp 10866-10880] demonstrated that MVK-oxide conformers undergo unimolecular decay 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 (-MVK-oxide) and record its IR action spectrum in the vinyl CH stretch overtone (2ν) region. The resultant time-dependent appearance of OD radical products, detected by laser-induced fluorescence, demonstrates that a unimolecular decay of -MVK-oxide proceeds by an analogous 1,4-deuterium (D) atom transfer mechanism anticipated for 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 --MVK-oxide, as well as the prior study on -MVK-oxide. The -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 -MVK-oxide is predicted to be a factor of 40 times slower than that for MVK-oxide in the 2ν region. Experimentally, the temporal profile of the OD products reflects the slower unimolecular decay of -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 conformers make the main contribution to the IR action spectra of MVK-oxide and -MVK-oxide.