ANOMALOUS METHOXY RADICAL YIELDS IN THE FLUORINE+METHANOL REACTION .2. THEORY

We have carried out ab initio calculations using Moller-Plesset perturbation theory and several basis sets for the title hydrogen abstraction reaction in order to understand the theoretical basis for the anomalously large methoxy (product resulting from hydroxyl-side attack) vs hydroxymethyl (product resulting from methyl-side attack) yields observed experimentally. Of the three unique abstraction sites on methanol, we have located a saddle point for the syn-methyl channel, a hilltop for the anti-methyl channel, and a minimum (hydrogen-bonded complex) for the hydroxyl channel. The calculated classical barrier height was 1.89 kcal/mol for the methyl-side attack and negligible for the hydroxyl-side attack. Using conventional transition-state theory, the rate coefficients (in units of cm3 molecule-1 s-1) for methoxy production vary from 2.83 x 10(-11) at 298 K to 8.99 x 10(-11) at 960 K. The calculated methoxy branching fractions are consistent with the observations in paper 1 of this series and vary from 0.70 at 298 K to 0.59 at 960 K. Our analysis of these results provides a qualitative explanation for the anomalously large methoxy yields. The limitations of this approach imposed by the approximations used in both the electronic structure as well as rate constant calculations are discussed.