Measurements of the¹²C/¹³C kinetic isotope effects in the gas-phase reactions of light alkanes with chlorine atoms

The carbon kinetic isotope effects (KIEs) of the reactions of several light non-methane hydrocarbons (NMHC) with Cl atoms were determined at room temperature and ambient pressure. All measured KIEs, defined as the ratio of the Cl reaction rate constants of the light isotopologue over that of the heavy isotopologue (^Clk₁₂/ ^Clk₁₃) are greater than unity or normal KIEs. For simplicity, measured KIEs are reported in per mil according to ^Clk₁₂ = ^Clk₁₃ -1) × 1000% unless noted otherwise. The following average KIEs were obtained (all in %): 10.73 ± 0.20 (ethane), 6.44 ± 0.14 (propane), 6.18 ± 0.18 (methylpropane), 3.94 ± 0.01 (n-butane), 1.79 ± 0.42 (methylbutane), 3.22 ± 0.17 (n-pentane), 2.02 ± 0.40 (n-hexane), 2.06 ± 0.19 (n-heptane), 1.54 ± 0.15 (n-octane), 3.04 ± 0.09 (cyclopentane), 2.30 ± 0.09 (cyclohexane), and 2.56 ± 0.25 (methylcyclopentane). Measurements of the ¹²C/ ¹³C KIEs for the Cl atom reactions of the C₂-C₈ n-alkanes were also made at 348 K, and no significant temperature dependence was observed. To our knowledge, these ¹²C/¹³C KIE measurements for alkanes + Cl reactions are the first of their kind. Simultaneous to the KIE measurement, the rate constant for the reaction of each alkane with Cl atoms was measured using a relative rate method. Our measurements agree with published values within ± 20%. The measured rate constant for methylcyclopentane, for which no literature value is available, is (2.83 ± 0.11) × 10^-10 cm³ molecule^-1 s^-1, 1σ0 standard error. The ^Clε values presented here for the C₂-C₈ alkanes are an order of magnitude smaller than reported methane ^Clε values (Geophys. Res. Lett., 2000, 27, 1715), in contrast to reported ^OHε values for methane (J. Geophys. Res. (Atmos.), 2001, 106, 23, 127) and C2-C8 alkanes (J. Phys. Chem. A, 2004, 108, 11537), which are all smaller than 10%. This has important implications for atmospheric modeling of saturated NMHC stable carbon isotope ratios. ¹³C-structure reactivity relationship values ( ¹³C-SRR) for alkane-Cl reactions have been determined and are similar to previously reported values for alkane-OH reactions.