Measurement of absolute unimolecular and bimolecular rate constants for CH3CHOO generated by the trans-2-butene reaction with ozone in the gas phase

Ozone-alkene reactions form vibrationally excited Criegee intermediates (of the form R1R2COO), some of which, once thermalized, are thought to react w ith SO2, H2O, NOx, aldehydes, and alcohols. Several studies using relative rate techniques or ab initio calculations have resulted in estimates for th e rate coefficients of reactions of the thermalized biradicals. The ranges of measured and estimated rate coefficients span 2-6 orders of magnitude, d epending on the reaction partner. Using an atmospheric pressure flow reacto r, we have made the first absolute rate coefficient determinations for reac tions of a thermalized Criegee intermediate, measuring rates for unimolecul ar decomposition and reaction with acetaldehyde. For the thermalized CH3-CH OO formed in trans-2-butene ozonolysis, values for k(dec) = (7)over dot 6 s (-1) and k(ald) = 1.0 x 10(-12) cm(3) molecule(-1) s(-1), accurate to withi n a factor of 3 and 6, respectively, were obtained.