The Mechanism of Methoxy Radical Oxidation by O₂in the Gas Phase. Computational Evidence for Direct H Atom Transfer Assisted by an Intermolecular Noncovalent O···O Bonding Interaction

Abstract

The mechanism of the CH 3 O • + O 2 reaction in the gas phase leading to CH 2 O + HO 2 • was studied by using high-level quantum mechanical electronic structure calculations. The CASSCF method with the 6-311G(d,p) basis set was employed for geometry optimization of 15 stationary points on the ground-state potential energy reaction surface and computing their harmonic vibrational frequencies. These stationary points were confirmed by subsequent geometry optimizations and vibrational frequencies calculations by using the CISD and QCISD methods with the 6-31G(d) and 6-311G(d,p) basis sets. Relative energies were calculated at the CCSD(T) level of theory with extended basis sets up to cc-pVTZ at the CASSCF/6-311G(d,p)-optimized geometries. In contrast to a recent theoretical study predicting an addition/elimination mechanism forming the trioxy radical CH 3 OOO • as intermediate, the oxidation of CH 3 O • by O 2 is found to occur by a direct H atom transfer mechanism through a ringlike transition structure of C s symmetry. This transition structure shows an intermolecular noncovalent O···O bonding interaction, which lowers its potential energy with respect to that of a noncyclic transition structure by about 8 kcal/mol. The 1,4 H atom transfer in CH 3 OOO • is not accompanied by HO 2 • elimination but leads to the trioxomethyl radical • CH 2 OOOH via a puckered ringlike transition structure, lying 50.6 kcal/mol above the energy of the reactants. The direct H atom transfer pathway is predicted to occur with an Arrhenius activation energy of 2.8 kcal/mol and a preexponential factor of 3.5733 × 10 - 14 molecule cm 3 s - 1 at 298 K. Inclusion of quantum mechanical tunneling correction to the rate constant computed with these parameters leads to a rate constant of 2.7 × 10 - 15 molecule - 1 cm 3 s - 1 at 298 K, in good agreement with the experimental value of 1.9 × 10 - 15 molecule - 1 cm 3 s - 1 .