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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.

Thermochemical and kinetic parameters for 1,5-H isomerisation reactions of alkoxy radicals up to C₈ have been determined theoretically using density functional theory. Pressure dependence (through RRKM statistical calculations) as well as tunneling corrections have been taken into account. The results of calculations are validated by available experimental relative rate constants. These results show that the set of alkoxy radicals studied can be divided into three categories according to the H-abstraction site involved in the isomerisation reaction (primary, secondary and tertiary). Values for kinetic parameters: pre-exponential factors, activation energies and rate constants are proposed for each category. In particular, the following rate constant values are predicted: k_isom = 6.2 × 105 s−1, 9.3 × 106 s−1 and 4.5 × 108 s−1 for 1,5-H transfer from a primary group (–CH₃), secondary group (–CH₂–) and tertiary group (>CH–), respectively, at 298 K and 1 atm pressure. An uncertainty factor of about 5 is estimated for calculated rate constants. These results corroborate Atkinson's recommendations except for the third group for which our value is two orders of magnitude larger. Another result of this study is that the pressure dependence of the rate constant for the isomerisation reaction is weak except for abstraction of a tertiary H-atom where k_isom (298 K, 1 atm) is 40% of the infinite pressure rate constant. It can be also stressed that, where the isomerisation is possible, it will always be the dominant pathway with respect to the reaction with O₂, but it may be in competition with the decomposition reaction. We show that this is also the case in upper tropospheric conditions (0.2 atm and 220 K).

The reactivity of the CH₃C(O)OCH(O&z.rad;)CH₃ secondary alkoxy radical, main intermediate derived from the oxydation by OH of ethyl acetate has been studied using the quantum chemistry density functional theory. Our calculations show that the major reaction pathway is a one-step mechanism where the H-atom α to the oxygenated function is abstracted by the carbonyl oxygen while the C–O bond of the ester function is breaking. This is the first computational evidence of the occurrence of the α-ester rearrangement proposed by Tuazon et al. (E. C. Tuazon, W. P. L. Carter, S. M. Aschmann, R. Atkinson, Int. J. Chem. Kinet., 1991, 24, 1003) leading to the formation of acetic acid observed experimentally. Compared to the other possible reactions, our calculations predict that, among all the unimolecular processes that the alkoxy radical can undergo, this rearrangement is, by far, the most favoured one. Moreover, in the conditions of the lower troposphere (1 atm and 298 K), this chemical process is expected to occur, rather than reaction with O₂.

Simple expressions for predicting independently the kinetic and thermodynamic parameters for the decomposition reactions of alkoxyl radicals are proposed. These relationships have been parametrized, using quantum chemistry ab initio BAC-MP4 and density functional theory calculated activation energies and reaction enthalpies on a set of linear and branched C₁ to C₅ alkoxyl radical decomposition processes. Once parametrized, and validated against experimental data, the proposed relationships can be extrapolated to larger alkoxyl radical decomposition reactions relevant to atmospheric modelling. They only involve the ionisation potential of the departing alkyl group of the alkoxyl radicals and the parameter n_H, defining whether the radical is primary, secondary or tertiary. One of the main interests of these findings is to predict with a reasonable accuracy the activation energies without knowing a priori the enthalpies of the reactions. In addition, RRKM calculations have shown that rate constant corrections for fall-off behavior are less than a factor of 2, at room temperature and atmospheric pressure, which is within experimental uncertainties. Nevertheless, Arrhenius pre-exponential factors can be very low at that pressure, compared with their high pressure limit value. The relationships proposed in this work provide kinetic parameters that are in reasonable agreement with available experimental data.

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 β-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 kdisskO2effwith a partial pressure of O2was observed. This O2dependence 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.