Rate coefficients for the reactions of OH radicals with C₃–C₁₁ alkanes determined by the relative-rate technique

Abstract

Rate coefficients for the reactions of OH radicals with C3–C11 alkanes were determined using the multivariate relative-rate technique. A total of 25 relative-rate coefficients at room temperature and 24 Arrhenius expressions in the temperature range of 273–323 K were obtained. Notably, a new room temperature relative-rate coefficient for 3-methylheptane that had not been previously reported was determined, and the obtained kOH value (in units of 10−12 cm3 molec.−1 s−1) was 7.71 ± 0.35. Interestingly, whilst results for n-alkanes agreed well with available structure–activity relationship (SAR) calculations of Kwok and Atkinson (1995), Neeb (2000), Wilson et al. (2006), Jenkin et al. (2018), and McGillen et al. (2020), the three cycloalkanes (cyclopentane, methylcyclopentane, cyclohexane) and one branched alkane (2,2,4-trimethylpentane) were found to be less reactive than predicted by the SAR approach. Conversely, the SAR estimates for 2,3-dimethylbutane were approximately 25 % lower than the experimental values, with the exception of those estimated by the Wilson group, highlighting that there may be additional factors that govern the reactivity of highly branched alkanes that are not captured by current SAR techniques. Arrhenius expressions (in units of cm3 molec.−1 s−1) for the reactions of various branched alkanes with OH radicals were determined for the first time: 2-methylheptane, 1.37±0.48×10-11exp⁡-209±100/T, and 3-methylheptane, 3.54±0.45×10-11exp⁡-374±49/T. The reactivity relation of saturated alkanes with OH radicals and chlorine atoms was obtained: log⁡10k(Cl+alkanes)=0.569×log⁡10k(OH+alkanes)-3.111 (R2 = 0.86). In addition, the rate coefficients for the 24 previously studied OH + alkanes reactions were consistent with existing literature values, demonstrating the reliability and efficiency of this method for the simultaneous investigation of gas-phase reaction kinetics.