Some observations. on the kinetics and thermochemistry of the reactions of HO2 radicals with aldehydes and ketones

The rapid, gas phase equilibrium addition of HO2 radicals to CH2O to form the peroxy radical HOCH2OO. is in agreement with the known thermochemistry of these species. The recent study of the similar addition of HO2. to ketones shows no significant reaction, which is again in agreement with known thermochemistry. All these reactions are notable for significant dipole attraction between the reactants ranging from 3 to 7 kcal/mol. The thermochemistry shows that the hydroperoxyl alkoxy species, the primary possible adduct, is not favored by the free energy change for direct addition. This and the observed kinetics favor a concerted addition, H-atom transfer, as the transition state for the reactions. Kinetic estimates for forward and reverse reactions are in good agreement with observations. A thermochemical examination of the step-wise addition of HO2. to the carbonyl shows that the reaction proceeds through a concerted, cyclic transition state involving simultaneous H-transfer, 3 + 2 cycloaddition of HO2. to the carbonyl group. (C) 2001 John Wiley & Sons.