The stable-carbon kinetic isotope effects of the reactions of isoprene, methacrolein, and methyl vinyl ketone with ozone in the gas phase

The stable-carbon kinetic isotope effects (KIEs) for the gas-phase reactions of isoprene, methacrolein (MACR), and methyl vinyl ketone (MVK) with ozone were studied in a 25 L reaction chamber at 298 ± 2 K and ambient pressure. The time dependence of both the stable-carbon isotope ratios and the concentrations was determined using a gas chromatography combustion isotope ratio mass spectrometry (GCC–IRMS) system. The volatile organic compounds (VOCs) used in the KIE experiments had natural-abundance isotopic composition thus KIE data obtained from these experiments can be directly applied to atmospheric studies of isoprene chemistry. All 13C/12C KIEs reported herein are as per mille ɛ values, where ɛ = (KIE − 1) × 1000‰, and KIE = k12/k13. The following average stable-carbon KIEs were obtained: (8.40 ± 0.11)‰ (isoprene), (8.38 ± 0.42)‰ (MACR), and (8.01 ± 0.07)‰ (MVK). The stable-carbon KIE values of three 1-alkenes, which were used as reference compounds for relative rate experiments, were also determined: (5.48 ± 0.09)‰ (1-heptene), (4.67 ± 0.17)‰ (1-octene), and (4.59 ± 0.56)‰ (1-nonene). The ɛ values for the reactions of isoprene and 1-heptene with ozone agree with measurements in a previous study [Iannone, R., Anderson, R.S., Rudolph, J., Huang, L., Ernst, D., 2003. The carbon kinetic isotope effects of ozone–alkene reactions in the gas-phase and the impact of ozone reactions on the stable carbon isotope ratio of alkenes in the atmosphere. Geophysical Research Letters 30, 1684, doi: 10.1029/2003GL017221.], but the values presented here have a substantially improved accuracy. The ɛ values for 1-octene and 1-nonene reactions with ozone have not been measured before and closely follow the 1/NC dependence (where NC represents the number of carbon atoms in the alkene) derived in the aforementioned study. MACR and MVK had ɛ values that were somewhat below the expected range of values predicted by the 1/NC dependence found for alkenes.