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 ε values, where ε = (KIE – 1) × 1000‰, and KIE = k12/k13. The following average stable-carbon KIEs were obtained: (8.40 ± 0.11) ‰ (isoprene), (7.67 ± 0.28) ‰ (MACR), and (7.87 ± 0.08) ‰ (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 ε values for the reactions of isoprene and 1-heptene with ozone agree with measurements in a previous study, but the values presented here have a substantially improved accuracy. The ε 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) observed in previous studies. MACR and MVK had ε values that were somewhat below the expected range of values predicted by the 1/NC dependence found for alkenes.