Relative Rate Coefficients for the Reactions of HO₂ Radicals with Hydroxy-Substituted Alkyl Peroxy Radicals, and for Reactions of OH Radicals with Hydroxy Hydroperoxides

Rate coefficients for the reaction of hydroperoxy radicals (HO 2 ) with alkyl peroxy radicals derived from a number of 1-alkenes and isoprene have been measured relative to the rate coefficients for their reaction with NO by measurement of the reaction products. The derived ratios for k (RO 2 + HO 2 )/ k (RO 2 + NO), relative to the peroxy radicals derived from OH + 1-butene, are ethene (0.69 ± 0.07); propene (0.92 ± 0.07); pentene (1.19 ± 0.07); and isoprene (0.98). Assuming that the rate coefficients for the reactions of the peroxy radicals with NO are all 9 × 10 –12 cm 3 molecule –1 s –1, and the rate coefficient for HO 2 with hydroxybutyl peroxy is (1.5 ± 0.15) × 10 –11 cm 3 molecule s –1, the derived rate coefficients are (in units of 10 –11 cm 3 molecule –1 s –1 ): hydroxyethyl peroxy (1.03 ± 0.21); hydroxypropyl peroxy (1.37 ± 0.24); hydroxybutyl peroxy (1.50 ± 0.15); hydroxypentyl peroxy (1.78 ± 0.29); and hydroxyisoprenyl peroxy (1.47 ± 0.29). Quantum chemical calculations confirmed the large reaction rate coefficients, which are influenced by tunneling in the transition state. Further experiments measured the rate coefficients for hydroxy radicals with organic hydroxy hydroperoxides.