A comprehensive study on low-temperature oxidation chemistry of cyclohexane. I. Conformational analysis and theoretical study of first and second oxygen addition

To understand the low-temperature oxidation chemistry of cyclohexane, conformational analysis and theoretical study of the first and second oxygen addition are performed using quantum chemical calculations and kinetic calculations. Pressure- and temperature-dependent rate constants and branching ratios for major reaction channels are determined with RRKM/master-equation simulations over 298-2000 K and 0.01-1000 bar. The theoretical results indicate that the rapid inversion-topomerization processes facilitate fast equilibrium between axial and equatorial conformers. This can greatly counterbalance the influence of initial positions of side-chain groups in ROO, QOOH, cis-OO gamma QOOH and trans-OO gamma QOOH conformers. Conformational effects are found to be influential on the chain branching reaction sequences in second oxygen addition. The carbon ring prevents the conventional intramolecular H-transfer of cis-OO gamma QOOH conformers to yield ketohydroperoxides, as well as the inversion-topomerization from cis-OO gamma QOOH conformer to trans-OO gamma QOOH conformers. cis-OO gamma QOOH conformers mainly undergo alternative isomerization channel (cis-OO gamma QOOH ->gamma P(OOH)(2)-> alkenylhydroperoxides+OH -> oxy radical+OH+OH), while trans-OO gamma QOOH conformers have both conventional isomerization channel (trans-OO gamma QOOH -> ketohydroperoxides+OH -> oxy radical+OH+OH) and alternative isomerization channel (trans-OO gamma QOOH ->gamma P(OOH)(2)-> alkenylhydroperoxides+OH -> oxy radical+OH+OH). Kinetic calculation results also support the application of the thumb rule widely used in acyclic alkane oxidation that the rate constant of QOOH+O-2 is roughly half of that of R+O-2 in cyclic alkane oxidation, while it is indicated that estimating the rate constants of OOQOOH reactions from similar reactions of ROO may cause significant uncertainties. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.