Direct measurement of Criegee intermediate (CH2OO) reactions with acetone, acetaldehyde, and hexafluoroacetone

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.