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In this chemically activated reaction, the time scales for product formation versus collisional deactivation of the vibrationally excited adduct are explicitly considered. Furthermore, the subsequent addition of O2 is also incorporated within a single master equation, so as to investigate doubly activated peroxyl radical formation. The major reaction product of OH addition to MACR is the HOCH2C•(CH3)CHO radical formed via addition to the outer (β) carbon. This radical is predominantly in the Z isomer although around a third of the population is quenched as the higher-energy E isomer. Calculated rate constants agree well with experiment when using M06−2X/aug-ccpVTZ barrier heights, but are somewhat overpredicted using G3SX energies. The overall rate constant is controlled by competition between dissociation of the MACR···OH van der Waals complex back to reactants and isomerization on to MACR−OH adducts, which takes place on a time scale of several nanoseconds, but collisional deactivation of the MACR−OH adducts occurs on a time scale that is around an order of magnitude longer. When O2 addition is included in the master equation, we observe that the MACR−OH adducts are removed by reaction with O2 on a similar time scale to collisional deactivation. Around 50% of the subsequent peroxyl radical population is formed with some identifiable excess vibrational energy above singly activated [MACR−OH−O2]*, with around 20% provided with an additional 20 kcal mol−1 (>40 kcal mol−1 relative to quenched MACR−OH−O2) that can go into further unimolecular reaction. This double activation process is expected to lead to some prompt unimolecular decomposition of excited [MACR−OH−O2]** peroxyl radicals to yield products including hydroxyacetone and methylglyoxal, regenerating the initiating OH radical in the process. |
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