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Direct measurement of Criegee intermediate (CH2OO) reactions with acetone, acetaldehyde, and hexafluoroacetone
Ist Teil von
Physical chemistry chemical physics : PCCP, 2012-08, Vol.14 (3), p.1391-14
Ort / Verlag
Cambridge: Royal Society of Chemistry
Erscheinungsjahr
2012
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Criegee biradicals,
i.e.
, carbonyl oxides, are critical intermediates in ozonolysis and have been implicated in autoignition chemistry and other hydrocarbon oxidation systems, but until recently the direct measurement of their gas-phase kinetics has not been feasible. Indirect determinations of Criegee intermediate kinetics often rely on the introduction of a scavenger molecule into an ozonolysis system and analysis of the effects of the scavenger on yields of products associated with Criegee intermediate reactions. Carbonyl species, in particular hexafluoroacetone (CF
3
COCF
3
), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH
2
OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH
2
I radicals, which react with O
2
to yield CH
2
OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH
2
OO and the formation of reaction products are monitored. The rate coefficient for CH
2
OO + hexafluoroacetone is
k
1
= (3.0 ± 0.3) × 10
11
cm
3
molecule
1
s
1
, supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde,
k
2
= (9.5 ± 0.7) × 10
13
cm
3
molecule
1
s
1
, and with acetone,
k
3
= (2.3 ± 0.3) × 10
13
cm
3
molecule
1
s
1
, are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH
2
OO with acetone and hexafluoroacetone. Their photoionization spectra are interpreted with the aid of quantum-chemical and Franck-Condon-factor calculations. No secondary ozonide was observable in the reaction of CH
2
OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).
Absolute rate coefficients are directly measured for reactions of formaldehyde oxide with several carbonyl compounds.