Theoretical Study of the Phenoxy Radical Recombination with the O(³P) Atom, Phenyl plus Molecular Oxygen Revisited

Quantum chemical calculations of the C₆H₅O₂ potential energy surface (PES) were carried out to study the mechanism of the phenoxy + O(³P) and phenyl + O₂ reactions. CASPT2(15e,13o)/CBS//CASSCF(15e,13o)/DZP multireference calculations were utilized to map out the minimum energy path for the entrance channels of the phenoxy + O(³P) reaction. Stationary points on the C₆H₅O₂ PES were explored at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311++G** level for the species with a single-reference character of the wave function and at the CASPT2(15e,13o)/CBS//B3LYP/6-311++G** level of theory for the species with a multireference character of the wave function. Conventional, variational, and variable reaction coordinate transition-state theories were employed in Rice-Ramsperger-Kassel-Marcus master equation calculations to assess temperature- and pressure-dependent phenomenological rate constants and product branching ratios. The main bimolecular product channels of the phenoxy + O(³P) reaction are concluded to be para/ortho-benzoquinone + H, 2,4-cyclopentadienone + HCO and, at high temperatures, also phenyl + O₂. The main bimolecular product channels of the phenyl + O₂ reaction include 2,4-cyclopentadienone + HCO at lower temperatures and phenoxy + O(³P) at higher temperatures. For both the phenoxy + O(³P) and phenyl + O₂ reactions, the collisional stabilization of peroxybenzene at low temperatures and high pressures competes with the bimolecular product channels.