Chemical activation in OH radical-oxidation of 1-n-alkenes

Abstract

Statistical kinetic calculations, using RRKM-master equation analysis and phase space theory, were performed to determine the importance of chemical activation in the subsequent evolution of the β-hydroxy-1-alkoxy radicals produced in the troposphere OH-oxidation of 1-n-alkenes via the exothermic reaction R–CH(OH)–CH₂O₂ + NO → R–CH(OH)–CH₂O₂NO* → R–CH(OH)–CH₂O + NO₂; the series of radicals from β-hydroxy-ethoxy to -1-hexoxy was considered. At 298 K and 1 atm the fraction of promptly reacting alkoxy radical (on a subnanosecond time-scale) decreases from 75% for the C₃-radical to 20% for the C₆-radical, the C₂-radical being a particular case (higher dissociation barrier height) with only 30% of prompt dissociation. The thermal unimolecular rate constants of these alkoxy radicals were determined by RRKM calculations of the fall-off curves. From the totality of these results it is deduced: (i) the troposphere impact of chemical activation is important for C₂-hydroxy-alkoxy radical and specially in the upper troposphere (ii) for C₃ chain length radicals, a chemical activation, by favouring the unimolecular reactions, decreases the fraction of O₂ reaction, the major effect being predicted for C₄ radical. For all radicals except C₂ radical the unimolecular reactions are predicted to be preponderant, and at low temperature the isomerisation, when possible from a –CH₂– group, is the major way of reaction.