Criegee intermediate reaction rate constants

However, the relative importance of these reactions is highly uncertain. Table 6.12 gives one estimate of the range of possible rate constants for these reactions of the Criegee intermediate and the calculated lifetimes of the intermediate under some typical atmospheric conditions. Reaction with water vapor is expected to be the major fate of the Criegee intermediate, with per-... 

TABLE 6.12 Range of Reported Rate Constants for the Reactions of the Criegee Intermediate with Some Gases" and Associated Lifetimes of the Criegee Intermediate under Polluted Tropospheric Conditions... 

One can estimate the relative contribution of the Criegee intermediate (Cl) to SOz oxidation in the gas phase in the troposphere. The absolute value of the rate constant for the reaction of the Cl with SOz is not known, with estimates ranging from 1.7 X 10 " to 3 X 10 15 cm2 molecule-1 s-1 (Hatakeyama and Akimoto, 1994). Using the highest value and a concentration of the Cl of 1 X 105 molecules cm"3, one obtains 10"6 s"1 for the first-order rate of removal of S02 by this reaction. This can be compared to the rate of removal of SOz by reaction with 1 X 106 OH radicals cm3, which is also 10 6 s-1 using the effective bimolecular rate constant cited earlier. Using the lower estimates for the CI-S02 rate constant, which is more reasonable, would lower its contribution proportionately. 

The S02 oxidation rates in the units of % h 1 expected from the gas-phase reactions of S02 with OH and the Criegee intermediate can be computed as shown in Box 8.1 for comparison to rates of oxidation observed in field studies (see Section C.l), if the rate constants and the concentrations of OH and Criegee intermediates are known. 

Ozone plays a major role in the degradation of unsaturated VOCs in the troposphere, especially during night-time. The rate constants of the ozonolysis of a variety of alkenes have been reported [1]. However, in most instances the fate of the primary products of the ozonolysis is unknown, although the secondary reaction products are of crucial importance for the overall understanding of the alkene/ozone chemistry. The classical Criegee mechanism of the ozonolysis reaction involves the primary ozonide (POZ, 1,2,3-trioxolane), which cleaves to the Criegee intermediate (carbonyl O oxide) and a carbonyl compound [2, 3]. The secondary ozonide (SOZ, 1,2,4-trioxolane) is formed from these components in a [l,3]-dipolar cycloaddition reaction. 

The CBS-QB3, MCG3, RRKM/master equation, and transition-state theory (TST) calculations were made for a detailed analysis of cyclopropene ozonolysis, treating all possible conformers of all intermediates and transition structures (TSs). The TST rate constant indicated that approximately 90% of the reaction proceeded through the endo-TS. It was predicted that approximately one-third of activated syn Criegee intermediates (Cls) would cyclize to dioxiranes despite the fact that the barriers to dioxirane formation were higher than the barriers to the 1,4-hydrogen shift that would lead to vinyl hydroperoxides and "OH. This helped to reduce the predicted OH yield for cyclopropene ozonolysis to 44%. It was also predicted that approximately 20% of either the endo-primary ozonide (PO) or its syn Cl derivatives would isomerize to the exo-PO or anti Cls. ... 

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