10.1021/jacs.8b06010.s001
Victoria
P. Barber
Victoria
P.
Barber
Shubhrangshu Pandit
Shubhrangshu
Pandit
Amy M. Green
Amy M.
Green
Nisalak Trongsiriwat
Nisalak
Trongsiriwat
Patrick J. Walsh
Patrick J.
Walsh
Stephen J. Klippenstein
Stephen J.
Klippenstein
Marsha I. Lester
Marsha I.
Lester
Four-Carbon
Criegee Intermediate from Isoprene Ozonolysis:
Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production
American Chemical Society
2018
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
2018-08-03 00:00:00
Journal contribution
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.