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Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production
journal contribution
posted on 2018-08-03, 00:00 authored by Victoria
P. Barber, Shubhrangshu Pandit, Amy M. Green, Nisalak Trongsiriwat, Patrick J. Walsh, Stephen J. Klippenstein, Marsha I. LesterThe
reaction of ozone with isoprene, one of the most abundant volatile
organic compounds in the atmosphere, produces three distinct carbonyl
oxide species (RR′COO) known as Criegee intermediates: formaldehyde
oxide (CH2OO), methyl vinyl ketone oxide (MVK-OO), and
methacrolein oxide (MACR-OO). The nature of the substituents (R,R′
= H, CH3, CHCH2) and conformations of
the Criegee intermediates control their subsequent chemistry in the
atmosphere. In particular, unimolecular decay of MVK-OO is predicted
to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis.
This study reports the initial laboratory synthesis and direct detection
of MVK-OO through reaction of a photolytically generated, resonance-stabilized
monoiodoalkene radical with O2. MVK-OO is characterized
utilizing infrared (IR) action spectroscopy, in which IR activation
of MVK-OO with two quanta of CH stretch at ca. 6000 cm–1 is coupled with ultraviolet detection of the resultant OH products.
MVK-OO is identified by comparison of the experimentally observed
IR spectral features with theoretically predicted IR absorption spectra.
For syn-MVK-OO, the rate of appearance of OH products
agrees with the unimolecular decay rate predicted using statistical
theory with tunneling. This validates the hydrogen atom transfer mechanism
and computed transition-state barrier (18.0 kcal mol–1) leading to OH products. Theoretical calculations reveal an additional
roaming pathway between the separating radical fragments, which results
in other products. Master equation modeling yields a thermal unimolecular
decay rate for syn-MVK-OO of 33 s–1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration
of several unimolecular decay pathways predicts that isomerization
to dioxole is the most likely initial step to products.