Theoretical Study of OH−O2−Isoprene Peroxy Radicals

Ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the peroxy radicals arising from the OH-initiated oxidation of isoprene. Geometry optimizations of the OH−O2−isoprene peroxy radicals were performed using density functional theory at the B3LYP/6-31G** level, and individual energies were computed using second-order Møller−Plesset perturbation theory (MP2) and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G* level of theory the zero-point-corrected OH−O2−isoprene adduct radical energies are 47−53 kcal mol-1 more stable than the separated OH, O2, and isoprene reactants. In addition, we find no evidence for an energetic barrier to O2 addition and have calculated rate constants for the O2 addition step using canonical variational transition state theory (CVTST) based on Morse potentials to describe the reaction coordinate. These results provide the isomeric branching between the six isoprene−OH−O2 adduct radicals.