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 values, where = (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 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 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 values that were somewhat below the expected range of values predicted by the 1/NC dependence found for alkenes.