Theoretical study of H-abstraction and addition reactions of cyclohexene with H, OH and HO2

Abstract

Reactions with small radicals are of essential significance for alkene oxidation. In this work, the potential energy surfaces and rate constants of cyclohexene plus H, OH, HO2 radicals were thoroughly investigated. The molecular geometries and vibrational frequencies were calculated for all stationary points at BHandHLYP/6-311++G(d,p) level, and then the single point energies were refined at DLPNO-CCSD(T)/CBS level. It contained secondary alkyl, allylic, vinyl H-abstraction and addition channels. The axial and equatorial carbon sites were calculated separately. The reactant and product complexes for allylic H-abstraction and addition channels by OH and HO2 were located with lower energies than the path entrance or exit. In contrast, those complexes for pathways with H-atom had higher energies than reactants or products. The barrierless formation process of reactant complex for cyclohexene plus OH/HO2 was consider to evaluate the influences on rate constants. Rate constants were determined using canonical variational transition state theory in consideration of quantum transmission effects. Notable tunneling effects are observed for H-abstraction reactions with H and HO2 radicals, of which low-temperature rate constants are enhanced, whereas variational and recrossing coefficients of channels with OH deviate far from unity, rendering lower rate constants than those obtained at conventional transition state theory. Comparison of rate constants with similar reactions published in literatures showed that fairly agreements were achieved for H-abstraction and addition channels with H and HO2, but great gaps are noted for those with OH radical. Branching ratio analysis exhibited that addition reactions dominated at T < 1000 K and allylic H-abstraction reactions turned more significant as temperature increased.