Decomposition of 3-methylcyclohexene radicals: Beginning of its mechanism development

Abstract Cyclohexenes are important oxidation intermediates for cyclohexanes and precursors of aromatic hydrocarbons. 3-methylcyclohexene is one of important intermediate products of methylcyclohexane oxidation. In order to improve the understanding of oxidation for 3-methylcyclohexene, the decomposition pathways of 3-methylcyclohexene radical isomers were investigated using ab initio theoretical method. The molecular geometric structures of stationary points on the potential energy surfaces were optimized at B3LYP/6–311++G(d,p) level and then the single point energies were extrapolated using complete basis set (CBS) based on the results obtained using MP2 and QCISD(T) methods with basis sets of cc-pVDZ and cc-pVTZ. The reactivity of reaction channels are discussed with the energy barriers on potential energy surfaces. It is observed the decompositions of allylic radicals have clearly high barrier heights. The products of decomposition of 3-methylcyclohexene radicals are alkyl and alkenyl chain radicals via ring opening reactions, cyclic diolfins plus H atom via C–H bond fission, and cyclohexadienes plus methyl via side chain dissociation. Then it is shown that energies delivered with various quantum chemistry methods are of great similarity. Rate constants of major channels for all radicals were computed with information of stable and saddle points collected on the PESs. The high pressure limits of ring opening and dehydrogenation reactions are comparable for all alkyl radicals, while the other two allylic radicals possess much distinguished values. Finally, the pressure-dependent rate constants are computed based on RRKM/ME theory. The rate constants of ring opening reactions are compared with literature data for similar molecules.