Cyclohexene Oxidation Initiated by Ozone: Linking Ozonolysis to Low-Temperature Combustion Chemistry

Ozone-initiated oxidation of cyclohexene bridges the gap between classical alkene ozonolysis and low-temperature combustion chemistry. In this study, it was investigated in jet-stirred reactors using complementary gas-chromatographic measurements over 350-900 K, temperature-dependent synchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS) over 370-800 K for only ozone-containing mixtures, and isomer-resolved synchrotron vacuum-ultraviolet photoelectron-photoion coincidence (SVUV-PEPICO) spectroscopy at 400 K under only ozone-containing conditions. Ozone induces substantial low-temperature oxidation, including about 1,000 ppm 1,6-hexanedial, while synchrotron measurements identify cyclic ethers and ketohydroperoxides (KHPs). The temperature-dependent detection of cyclohex-2-en-1-one provides direct evidence for competition between unimolecular peroxy-radical isomerization and bimolecular radical-recombination pathways: the oxyl-radical channel accounts for 72.3% of the modeled branching at 370 K, whereas hydroperoxide formation and isomerization account for 51.5% and 29.2%, respectively, near 575 K. A kinetic model reproduces cyclohexene consumption and the principal product trends, showing that prompt fragmentation dominates the initial ozonolysis step and that subsequent cyclic-peroxy chemistry is strongly temperature dependent.