Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH 3 CHOO

The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H 2 CO 2 , was recently detected and monitored in the laboratory. Su et al. (p. 174 ; see the Perspective by Vereecken ) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177 ; see the Perspective by Vereecken ) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms. The reaction kinetics of an intermediate implicated in atmospheric ozone chemistry has been measured in the laboratory. [Also see Perspective by Vereecken ] Although carbonyl oxides, “Criegee intermediates,” have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH 2 OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH 3 CHOO. Moreover, we independently probed the two distinct CH 3 CHOO conformers, syn- and anti - , both of which react readily with SO 2 and with NO 2 . We demonstrate that anti -CH 3 CHOO is substantially more reactive toward water and SO 2 than is syn -CH 3 CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn -CH 3 CHOO with water, and the rate constant for reaction of anti -CH 3 CHOO with water is measured as 1.0 × 10 −14 ± 0.4 × 10 −14 centimeter 3 second −1 .