Four-Carbon
Criegee Intermediate from Isoprene Ozonolysis:
Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production
Victoria
P. Barber
Shubhrangshu Pandit
Amy M. Green
Nisalak Trongsiriwat
Patrick J. Walsh
Stephen J. Klippenstein
Marsha I. Lester
10.1021/jacs.8b06010.s001
https://acs.figshare.com/articles/journal_contribution/Four-Carbon_Criegee_Intermediate_from_Isoprene_Ozonolysis_Methyl_Vinyl_Ketone_Oxide_Synthesis_Infrared_Spectrum_and_OH_Production/6985247
The
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 (CH<sub>2</sub>OO), methyl vinyl ketone oxide (MVK-OO), and
methacrolein oxide (MACR-OO). The nature of the substituents (R,R′
= H, CH<sub>3</sub>, CHCH<sub>2</sub>) 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 O<sub>2</sub>. 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<sup>–1</sup> 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 <i>syn</i>-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<sup>–1</sup>) 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 <i>syn</i>-MVK-OO of 33 s<sup>–1</sup> (298 K, 1 atm). For <i>anti</i>-MVK-OO, theoretical exploration
of several unimolecular decay pathways predicts that isomerization
to dioxole is the most likely initial step to products.
2018-08-03 00:00:00
Criegee intermediates control
RR
unimolecular decay pathways
OO
OH products
unimolecular decay rate
master equation modeling yields
Methyl Vinyl Ketone Oxide Synthesis
IR absorption spectra
CH
Four-Carbon Criegee Intermediate
carbonyl oxide species
MACR-OO
hydrogen atom transfer mechanism
MVK-OO