Laboratory Studies of the Hydrogen Kinetic Isotope Effects (KIES) of the Reaction of Non-Methane Hydrocarbons with the OH Radical in the Gas Phase

AThe hydrogen kinetic isotope effects (KIEs) of the reactions of 15 non-methane hydrocarbons (NMHCs) with the OH radical were measured at 298 (plus-minus) 2 K. The measurements were made using NMHCs without artificial isotopic labeling or enrichment. The following average hydrogen KIE values, in per mil (%0), were obtained: 29.8 (plus-minus) 2.1 (toluene), 51.6 (plus-minus) 2.1 (nbutane), 97.3(plus-minus) 12.5 (i-butane), 63.2 (plus-minus) 5.9 (cyclopentane), 10.5 (p-xylene), 26.8 (plus-minus) 3.5 (ethylbenzene), 65.9 (plus-minus) 7.0 (n-pentane), 79.5 (plus-minus) 9.6 (cyclohexane), 52.8 (plus-minus) 5.0 (n-hexane), 38.9 (plus-minus) 7.8 (n-heptane), 33.4 (plus-minus) 3.1 (n-octane), 29.6 (plus-minus) 1.6 (n-nonane), and 29.0 (plus-minus) 5.3 (ndecane). The KIEs for reactions of two alkenes (cyclohexene and 1-heptene) could not be determined accurately due to interference from reaction with ozone, but nevertheless the results clearly show that the KIEs for reaction of alkenes with OH are significantly lower than those for saturated hydrocarbons. The KIEs for reaction of alkanes are smaller than isotope effects reported in literature for the reactions of NMHCs artificially labeled with Adeuterium. The main reason for this difference is the reduced probability for reaction at a Alabeled site for compounds with close to natural deuterium abundance, although some impact Aof secondary isotope effects cannot be ruled out. Still, the KIEs for NMHCs with natural or Aclose to natural abundance of deuterium are of sufficient magnitude to allow determination Aof the extent of chemical processing of hydrocarbons in the atmosphere using methods Aanalogous to stable carbon KIE studies. Furthermore, it is shown that combining stable Ahydrogen and stable carbon isotope ratio data has the Apotential to also provide valuable Ainformation regarding the stable isotope ratios of emissions, and specifically to test one Aof the key assumptions of the stable isotope hydrocarbon clock, the absence of significant Avariations of the stable isotope ratio for the emitted NMHCs.