Carbon Kinetic Isotope Effects in the Gas Phase Reactions of Light Alkanes and Ethene with the OH Radical at 296 ± 4 K

Abstract

The carbon kinetic isotope effects (KIEs) of the room-temperature reactions of several light alkanes and ethene with OH radicals were measured in a 30 L PTFE reaction chamber at ambient pressure using gas chromatography coupled with online combustion and isotope ratio mass spectrometry (GCC−IRMS). For simplicity, KIEs are reported in per mil according to ε (‰) = (k12/k13 − 1) × 1000. The following average KIEs were obtained, (all in ‰): ethane, 8.57 ± 1.95; propane, 5.46 ± 0.35; n-butane, 5.16 ± 0.67; methylpropane, 8.45 ± 1.49; n-pentane, 2.85 ± 0.79; methylbutane, 2.91 ± 0.43; n-hexane, 2.20 ± 0.07; n-heptane, 1.96 ± 0.26; n-octane, 2.13 ± 0.39; cyclopentane, 1.84 ± 0.13; cyclohexane, 4.46 ± 0.51; methylcyclopentane, 1.77 ± 0.53; ethene, 18.6 ± 2.9. As well, the room-temperature rate constant for the reaction of methylcyclopentane + OH, not previously reported in the literature, was determined using relative rates: (8.6 ± 2.2) × 10-12 cm3 molecule-1 s-1, including the estimated 25% uncertainty in the rate constant for cyclopentane + OH. KIE values for propane, n-butane and n-hexane have been reported previously [J. Geophys. Res. [Atmos.] 2000, 105, 29329]. Our KIE for n-hexane is in agreement with the previous measurement, but our values for propane and n-butane are both higher. The dependence between the KIE and chemical structure is discussed, and a method for estimating unknown carbon KIEs for the reactions of light alkanes with OH radicals is presented. With only one exception, predictions using this method agree within a factor of 2 of the experimental KIE results.