Isomer- resolved unimolecular dynamics of the hydroperoxyalkyl intermediate (•QOOH) in cyclohexane oxidation

• 成果类型: Article
• 署名作者: Qian, Yujie; Roy, Tarun Kumar; Jasper, Ahren W.; Sojdak, Christopher A.; Kozlowski, Marisa C.; Klippenstein, Stephen J.; Lester, Marsha I.
• 署名单位: University of Pennsylvania; United States Department of Energy (DOE); Argonne National Laboratory
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-8635
• DOI: 10.1073/pnas.2401148121
• 发表日期: 2024-04-16
• 关键词: low-temperature oxidation; kinetics; oh; radicals; autoignition; combustion; driven; no

摘要

The oxidation of cycloalkanes is important in the combustion of transportation fuels and in atmospheric secondary organic aerosol formation. A transient carbon- centered radical intermediate (center dot QOOH) in the oxidation of cyclohexane is identified through its infrared fingerprint and time- and energy- resolved unimolecular dissociation dynamics to hydroxyl (OH) radical and bicyclic ether products. Although the cyclohexyl ring structure leads to three nearly degenerate center dot QOOH isomers ((3- , gamma-, and delta-QOOH), their transition state (TS) barriers to OH products are predicted to differ considerably. Selective characterization of the (3-QOOH isomer is achieved at excitation energies associated with the lowest TS barrier, resulting in rapid unimolecular decay to OH products that are detected. A benchmarking approach is employed for the calculation of high- accuracy stationary point energies, in particular TS barriers, for cyclohexane oxidation (C6H11O2), building on higher - level reference calculations for the smaller ethane oxidation (C2H5O2) system. The isomer- specific characterization of (3-QOOH is validated by comparison of experimental OH product appearance rates with computed statistical microcanonical rates, including significant heavy - atom tunneling, at energies in the vicinity of the TS barrier. Master- equation modeling is utilized to extend the results to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclohexane combustion.