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−O 2 −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−O 2 −isoprene adduct radical energies are 47−53 kcal mol -1 more stable than the separated OH, O 2, and isoprene reactants. In addition, we find no evidence for an energetic barrier to O 2 addition and have calculated rate constants for the O 2 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−O 2 adduct radicals.