The CH ₃ + HO ₂ reaction: First-principles prediction of its rate constant and product branching probabilities

The reaction of the CH ₃ radical with HO ₂ has been investigated by means of ab initio molecular orbital theory and variational RRKM theory calculations. The reaction can take place by several product channels producing (a) CH ₄ + O ₂ ( ³ Σ _g ^- ) and (b) CH ₄ + O ₂ ( ¹ Δ) by direct H abstraction and (c) CH ₃ O + OH and (d) CH ₂ O + H ₂ O by an association/decomposition mechanism via CH ₃ OOH. The bimolecular reaction rate constants for the formation of these products have been calculated for the temperature range 300-3000 K and found to be pressure independent up to 50 atm. The Arrhenius equations for the two major channels a and c were found to be strongly curved; they can be represented by k _a = 4.23 × 10 ^-16 T ^1.25 exp(828/T) for 300-800 K, k _a = 3.02 × 10 ^-21 T ^2.83 exp(1877/T) for 800-3000 K, and k _c = 2.97 × 10 ^-10 T ^-0.24 exp(182/T) for 300-1000 K and 1.02 × 10 ^-13 T ^0.76 exp(1195/T) for 1000-3000 K, in units of cm ³ molecule ^-1 s ^-1 . In the abstraction channel a, the effect of multiple reflections above its van der Waals complex (CH ₃ ⋯HO ₂ ), which lies 1.9 kcal/mol below the reactants with a 1.2 kcal/mol barrier leading to the formation of the CH ₄ + O ₂ ( ³ Σ _g ^- ) products, was found to be quite significant at low temperatures (T < 300 K). In addition, the predicted rate constant for the unimolecular decomposition of CH ₃ OOH agrees closely with the available experimental data using the heats of formation of CH ₃ O (5.4 ± 0.5 kcal/mol) and CH ₃ OOH (-29.0 ± 1.0 kcal/mol) calculated with the isodesmic method at 0 K.