Chemical activation in unimolecular reactions of some unsubstituted alkoxy radicals

In a previous work, (F. Caralp, W. Forst and M. T. Rayez, Phys Chem. Chem. Phys., 2003, 5, 476.), we presented a statistical RRKM-master equation (RRKM-ME) procedure for the calculation of the fraction of chemically activated beta-hydroxy-alkoxy radicals, produced by the reaction RO2 + NO --> RO + NO2, which undergo "prompt" unimolecular reactions (on a sub-nanosecond time-scale). In experimental relative-rate studies of thermal unimolecular reactions of several unsubstituted alkoxy radicals produced by this same reaction (H. G. Libuda, O. Shestakov, J. Theloke and F. Zabel, Phys. Chem. Chem. Phys, 2002, 4, 2579, and Geiger et al., J. Atmos. Chem. 2002, 42, 323), a systematic increase of the effective rate constant ratio (k(diss)/k(O2))(eff) with a partial pressure of O-2 was observed. This O-2 dependence was attributed to the formation and decomposition of chemically activated alkoxy radicals. In this work, it is shown that the same statistical RRKM-ME procedure quantitatively predicts the chemical activation effect observed experimentally. The good agreement between experiment and calculation confirms the interpretation of the experimental results.