The kinetics of the reactions of 1-and 2-butoxy radicals have been studied using a slow-flow photochemical reactor with GC-FID detection of reactants and products. Branching ratios between decomposition, CH3CH(O*)CH2CH3 --> CH3CHO + C2H5, reaction (7), and reaction with oxygen, CH3CH(O*)CH2CH3+ O2 --> CH3C(O)C2H5+ HO2, reaction (6), for the 2-butoxy radical and between isomerization, CH3CH2CH2CH2O* --> CH2CH2CH2CH2OH, reaction (9), and reaction with oxygen, CH3CH2CH2CH2O* + O2 --> C3H7CHO + HO2, reaction (8), for the 1-butoxy radical were measured as a function of oxygen concentration at atmospheric pressure over the temperature range 250-318 K. Evidence for the formation of a small fraction of chemically activated alkoxy radicals generated from the photolysis of alkyl nitrite precursors and from the exothermic reaction of 2-butyl peroxy radicals with NO was observed. The temperature dependence of the rate constant ratios for a thermalized system is given by k7/k6= 5.4 x 10(26) exp[(-47.4 +/- 2.8 kJ mol(-1))/RT] molecule cm(-3) and k9/k8= 1.98 x 10(23) exp[(-22.6 +/- 3.9 kJ mol(-1))/RT] molecule cm(-3). The results agree well with the available experimental literature data at ambient temperature but the temperature dependence of the rate constant ratios is weaker than in current recommendations.

Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals with NO have been determined over the temperature range (261-305 K) and at 1 bar pressure from the photo-oxidation of the iodoalkane precursors in air-NO mixtures. Yields were observed to increase with decreasing temperature and, contrary to previous observations, along the series primary < secondary congruent with tertiary. Our results suggests a significant temperature dependence for the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an extension in the temperature range over which this reaction has been studied experimentally in the past.

Relative-rate experiments have been carried out, at several different temperatures between 240 and 340 K, to determine the Arrhenius rate parameters for the isomerization of the 1-pentoxyl, 2-pentoxyl, and 5-methyl-2-hexoxyl radicals. Isomerization rates were measured relative to the bimolecular reaction of each alkoxyl radical (RO•) with O2. The order of reactivity, in terms of the rate of reaction for isomerization, was 5-methyl-2-hexoxyl > 1-pentoxyl > 2-pentoxyl, reflecting the relative-strengths of tertiary, secondary, and primary C−H bonds, i.e., the nature of the bond cleaved in the isomerization process. In addition to this, the measured Arrhenius activation barriers, and reported barriers for bimolecular reactions of the methoxyl radical with several simple hydrocarbons, are shown to correlate linearly with the bond strength of the cleaving C−H bond. The A-factors obtained for the three isomerization processes also scale linearly with the number of available abstractable H atoms, with a value of 3.0 × 1010 s-1 per H atom. This value combined with the activation barrier correlation constitutes a structure activity relationship for the estimation of the kinetics of the isomerization of simple alkoxyl radicals, at temperatures pertaining to the Earth's troposphere.

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