Impact of conformational structures on low-temperature oxidation chemistry for cyclohexyl radicals: a theoretical and kinetic modeling study on first oxygen addition

Abstract

This work aims to investigate the crucial role of the inherent conformations of cyclohexyl radical in low-temperature oxidation chemistry through theoretical calculations and kinetic modeling, which has not been explored previously. Potential energy surface for cyclohexyl + O₂ was precisely examined using high-level composite quantum methods, and temperature- and pressure-dependent rate coefficients were predicted via RRKM/master-equation analysis in the range of 200–2000 K and 0.001–100 atm, respectively. A detailed kinetic model for cyclohexane oxidation was constructed by incorporating Boltzmann-weighted rate coefficients based on the equilibrium of conformers. Results show that the addition of an O₂ molecule onto cyclohexyl in the chair and twist-boat forms yields chair-axial, twist-boat-axial and twist-boat-isoclinal adducts accordingly. Axial and isoclinal preferences in the three adducts facilitate the 1,5-H transfer, while only the twist-boat-isoclinal conformation proceeds with the 1,6-H transfer. The dissociations of cyclohexylperoxy and hydroperoxycyclohexyl species exhibit distinctive conformational-dependent features, and ring-opening reactions preferably occur in equatorial conformations with lower steric hindrance. Kinetic predictions reveal the importance of isomerization in cyclohexylperoxy in the order 1,5- > 1,6- > 1,4-H transfer, while that for OH eliminations follows the order 1,2- > 1,4- > 1,3-epoxycyclohexane cyclization at evaluated temperatures and pressures. Stabilization and HO₂ elimination in cyclohexylperoxy separately predominate the overall oxidation mechanism at correspondingly low and high temperatures, while OH elimination and hydroperoxycyclohexyl stabilization have minor contributions at high temperatures. The most rapid inversion-topomerization allows for equilibrium between various conformers in cyclohexylperoxy and hydroperoxycyclohexyl, thereby facilitating the inclusion of partition function contributions into kinetics. The new model reproduces cyclohexane oxidation measurements in jet-stirred reactors and laminar flame speeds for cyclohexane/air mixtures fairly well.