1 research outputs found
Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH<sub>3</sub>)<sub>2</sub>COO
The
Criegee intermediate acetone oxide, (CH<sub>3</sub>)<sub>2</sub>COO,
is formed by laser photolysis of 2,2-diiodopropane in the presence
of O<sub>2</sub> and characterized by synchrotron photoionization
mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy.
The rate coefficient of the reaction of the Criegee intermediate with
SO<sub>2</sub> was measured using photoionization mass spectrometry
and pseudo-first-order methods to be (7.3 ± 0.5) × 10<sup>–11</sup> cm<sup>3</sup> s<sup>–1</sup> at 298 K and
4 Torr and (1.5 ± 0.5) × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup> at 298 K and 10 Torr (He buffer). These
values are similar to directly measured rate coefficients of <i>anti</i>-CH<sub>3</sub>CHOO with SO<sub>2</sub>, and in good
agreement with recent UV absorption measurements. The measurement
of this reaction at 293 K and slightly higher pressures (between 10
and 100 Torr) in N<sub>2</sub> from cavity ring-down decay of the
ultraviolet absorption of (CH<sub>3</sub>)<sub>2</sub>COO yielded
even larger rate coefficients, in the range (1.84 ± 0.12) ×
10<sup>–10</sup> to (2.29 ± 0.08) × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup>. Photoionization mass spectrometry
measurements with deuterated acetone oxide at 4 Torr show an inverse
deuterium kinetic isotope effect, <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = (0.53 ± 0.06), for reactions with SO<sub>2</sub>, which may be consistent with recent suggestions that the
formation of an association complex affects the rate coefficient.
The reaction of (CD<sub>3</sub>)<sub>2</sub>COO with NO<sub>2</sub> has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) ×
10<sup>–12</sup> cm<sup>3</sup> s<sup>–1</sup> (measured
with photoionization mass spectrometry), again similar to rate for
the reaction of <i>anti</i>-CH<sub>3</sub>CHOO with NO<sub>2</sub>. Cavity ring-down measurements of the acetone oxide removal
without added reagents display a combination of first- and second-order
decay kinetics, which can be deconvolved to derive values for both
the self-reaction of (CH<sub>3</sub>)<sub>2</sub>COO and its unimolecular
thermal decay. The inferred unimolecular decay rate coefficient at
293 K, (305 ± 70) s<sup>–1</sup>, is similar to determinations
from ozonolysis. The present measurements confirm the large rate coefficient
for reaction of (CH<sub>3</sub>)<sub>2</sub>COO with SO<sub>2</sub> and the small rate coefficient for its reaction with water. Product
measurements of the reactions of (CH<sub>3</sub>)<sub>2</sub>COO with
NO<sub>2</sub> and with SO<sub>2</sub> suggest that these reactions
may facilitate isomerization to 2-hydroperoxypropene, possibly by
subsequent reactions of association products