Self- and cross-reactions of β-hydroxyperoxy radicals of relevance to tropospheric monoterpene oxidation: structure–activity relationships for rate coefficients

Abstract

The laser flash photolysis technique has been used to measure rate coefficients for the self- and cross-reactions of four peroxy radicals, which have been selected as surrogate of those peroxy radicals resulting from the OH addition to monoterpenes: the 2-hydroxycyclohexylperoxy c-C6H10(OH)O2 (I), 2-hydroxy-1,2-dimethylcyclohexylperoxy c-C6H8(CH3)2(OH)O2 (II), 2-hydroxy-2-methylcyclohexylperoxy c-C6H9(OH)CH3O2 (III) and 2-hydroxy-1-methylcyclohexylperoxy c-C6H9CH3(OH)O2 (IV) radicals. Those radicals were obtained by addition of OH and O2 to cyclohexene and 1,2-dimethylcyclohexene for I and II, respectively, and to 1-methylcyclohexene for (III) and (IV). Rate constants for self-reactions of radicals I and II are: k8=(1.60±0.10)×10−12 and k12=(2.00±0.05)×10−14 (units of cm3 molecule−1 s−1, errors 2σ) at 298°K and in 1 atm air. Assuming that both the secondary radicals I and III and both the tertiary radicals II and IV have similar self-reaction rate constants, the rate constant for cross-reaction (13) between III and IV has been determined: k13=(6.15±0.33)×10−13 cm3 molecule−1 s−1. The self-reaction rate constant of the peroxy radical c-C6H8(OH)O2, formed by OH and O2 addition to 1,4-cyclohexadiene, was also estimated and found to be of same order of magnitude as k8, thus showing that the non-conjugated double bond in the radical has no significant effect on the rate constant. Structure–activity relationships are discussed, particularly for self- and cross-reactions involving secondary and tertiary peroxy radicals, in view of their relevance to tropospheric oxidation modelling of alkenes and monoterpenes. In particular, emphasis is given to the large increase in reactivity resulting from the β-OH substitution, compared to non-substituted species. As part of this work, a rate constant of (1.5±0.5)×10−14 cm3 molecule−1 s−1 was measured for the self-reaction of the β-OH substituted t-butylperoxy radical (CH3)2C(O2)CH2OH.