Quantum Chemistry, Reaction Kinetics, and Tunneling Effects in the Reaction of Methoxy Radicals with O₂

Abstract

The reaction of the methoxy radical with O₂ is the prototype for the reaction of a range of larger alkoxy radicals with O₂ in the lower atmosphere. This reaction presents major challenges to quantum chemistry, with CCSD­(T) overpredicting the barrier height by about 7 kcal/mol in the complete basis set limit. CCSD­(T) calculations also indicate that the CH₃OOO^• analog of the HOOO^• radical is energetically unstable with respect to CH₃O^• + O₂, a finding that seems unlikely. The previous successful prediction of the barrier height using CCSD­(T)/cc-pVTZ energies at CASSCF/6-311G­(d,p) geometries is shown to rely on the use of a metastable Hartree–Fock reference wave function. The performance of several density functionals is explored and B3LYP is selected to examine the role of tunneling, including the competition between small curvature tunneling (SCT) and large curvature tunneling (LCT). SCT is found to be sufficient to describe tunneling, in contrast to the typical findings for bimolecular hydrogen-abstraction reactions. The previously proposed mechanism of a cyclic transition state yields rate constants for CH₃O^• + O₂ that faithfully reproduces the experimentally derived Arrhenius pre-exponential term. Predictions of the branching ratios for the competing reactions CH₂DO^• + O₂ → CHDO + HO₂ (1a) and CH₂DO^• + O₂→ CH₂O + DO₂ (1b) are also in good agreement with experiment