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