| Abstract
| - The mechanism of the CH3O• + O2 reaction in the gas phase leading to CH2O + HO2• was studiedby 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 potentialenergy reaction surface and computing their harmonic vibrational frequencies. These stationary points wereconfirmed by subsequent geometry optimizations and vibrational frequencies calculations by using the CISDand QCISD methods with the 6-31G(d) and 6-311G(d,p) basis sets. Relative energies were calculated at theCCSD(T) level of theory with extended basis sets up to cc-pVTZ at the CASSCF/6-311G(d,p)-optimizedgeometries. In contrast to a recent theoretical study predicting an addition/elimination mechanism forming thetrioxy radical CH3OOO• as intermediate, the oxidation of CH3O• by O2 is found to occur by a direct H atomtransfer mechanism through a ringlike transition structure of Cs symmetry. This transition structure shows anintermolecular noncovalent O···O bonding interaction, which lowers its potential energy with respect to thatof a noncyclic transition structure by about 8 kcal/mol. The 1,4 H atom transfer in CH3OOO• is not accompaniedby HO2• elimination but leads to the trioxomethyl radical •CH2OOOH 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 tooccur with an Arrhenius activation energy of 2.8 kcal/mol and a preexponential factor of 3.5733 × 10-14molecule cm3 s-1 at 298 K. Inclusion of quantum mechanical tunneling correction to the rate constant computedwith these parameters leads to a rate constant of 2.7 × 10-15 molecule-1 cm3 s-1 at 298 K, in good agreementwith the experimental value of 1.9 × 10-15 molecule-1 cm3 s-1.
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