Impact of Conformational Structures on Low-Temperature Oxidation Chemistry for Cyclohexyl Radical: A Theoretical and Kinetic Modeling Study on First Oxygen Addition

Abstract

This work aims to examine the crucial role of inherent conformational configurations for cyclohexyl radical in low-temperature oxidation chemistry by theoretical calculation and kinetic modeling, which was missing in previous studies. Potential energy surface for cyclohexyl + O2 was precisely explored by high-level composite quantum methods, temperature- and pressure-dependent rate coefficients were predicted by RRKM/master-equation analysis over 200~2000 K and 0.001~100 atm. Detailed kinetic model for cyclohexane oxidation was constructed by incorporating Boltzmann-weighted rate coefficients based on conformer equilibrium. Results show that the addition of O2 molecule onto cyclohexyl in chair and twist-boat forms yields chair-axial, twist-boat-axial and twist-boat-isoclinal adducts accordingly. Axial and isoclinal preferences in three adducts facilitate 1,5-H transfer, while only twist-boat-isoclinal conformation proceed with 1,6-H transfer. Dissociations for cyclohexylperoxy and hydroperoxycyclohexyl species exhibit distinctive conformational-dependent features, and ring-opening reactions prefer to occur in equatorial conformations with lower steric hindrance. Kinetic predictions reveal the importance for isomerization in cyclohexylperoxy following 1,5- > 1,6- > 1,4-H transfer, and that for OH eliminations following 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 contribution at high temperatures. Most rapid inversion-topomerization allows for equilibrium between various conformers in cyclohexylperoxy and hydroperoxycyclohexyl, and thus facilitates to take their partition function contributions into kinetics. New model fairly well reproduces cyclohexane oxidation measurements in jet-stirred reactor and laminar flame speeds for cyclohexane/air mixtures.