Low-temperature autooxidation of cyclopentanone and 3-pentanone: the critical role of competing radical chain branching and chain inhibiting pathways

The oxidation of two archetypical ketones, cyclopentanone and 3-pentanone, is explored experimentally at 4500 torr (6 bar) and 400-650 K, and computationally, using theory-based master equation calculations. We use H abstraction by photolytic Cl atoms in a high-pressure flow reactor to form a mixture of radicals that are resonantly stabilized by the carbonyl group and other, non-stabilized radicals. Subsequent coupled O₂ addition, isomerization, and decomposition pathways are elucidated by time-resolved synchrotron VUV photoionization mass spectrometry. Reaction intermediates - peroxy radicals (RO₂ and O₂QOOH) and hydroperoxides - are characterized experimentally and by calculated photoionization pathways. These, and other closed-shell products, are quantified using a global carbon balance approach. Cyclopentanone oxidation produces almost exclusively cyclopent-2-enone + HO₂<i>via</i> RO₂ decomposition. For 3-pentanone, stabilized diketohydroperoxides dominate the products at 500-600 K. Between 600-650 K unsaturated hydroperoxides, pent-1-en-3-one, and acrylic acid also emerge as major products. Automated high-level kinetics calculations of the 1st and 2nd O₂ addition reactions to pentan-3-on-1-yl and pentan-3-on-2-yl radicals are performed using the KinBot kinetics workflow software. We reveal the dominant reaction networks, mediated by three out of the six possible QOOH intermediates. Our results show that unsaturated hydroperoxide production and unconventional, Korcek-like hydroperoxide decomposition pathways should be included in the 3-pentanone oxidation mechanism.