Ab initio chemical kinetics for the CH₃ + O(³P) reaction and related isomerization-decomposition of CH₃O and CH₂OH radicals

The kinetics and mechanism of the CH₃ + O reaction and related isomerization-decomposition of CH₃O and CH₂OH radicals have been studied by ab initio molecular orbital theory based on the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVTZ, CCSD/aug-cc-pVDZ, and G2M//B3LYP/6-311+G(3df,2p) levels of theory. The predicted potential energy surface of the CH₃ + O reaction shows that the CHO + H₂ products can be directly generated from CH₃O by the TS3 → LM1 → TS7 → LM2 → TS4 path, in which both LM1 and LM2 are very loose and TS7 is roaming-like. The result for the CH₂O + H reaction shows that there are three low-energy barrier processes including CH₂O + H → CHO + H₂ via H-abstraction and CH₂O + H → CH₂OH and CH₂O + H → CH₃O by addition reactions. The predicted enthalpies of formation of the CH₂OH and CH₃O radicals at 0 K are in good agreement with available experimental data. Furthermore, the rate constants for the forward and some key reverse reactions have been predicted at 200-3000 K under various pressures. Based on the new reaction pathway for CH₃ + O, the rate constants for the CH₂O + H and CHO + H₂ reactions were predicted with the microcanonical variational transition-state/Rice-Ramsperger-Kassel-Marcus (VTST/RRKM) theory. The predicted total and individual product branching ratios (i.e., CO versus CH₂O) are in good agreement with experimental data. The rate constant for the hydrogen abstraction reaction of CH₂O + H has been calculated by the canonical variational transition-state theory with quantum tunneling and small-curvature corrections to be k(CH₂O + H → CHO + H₂) = 2.28 × 10^-19 T^2.65 exp(-766.5/T) cm³ molecule^-1 s^-1 for the 200-3000 K temperature range. The rate constants for the addition giving CH₃O and CH₂OH and the decomposition of the two radicals have been calculated by the microcanonical RRKM theory with the time-dependent master equation solution of the multiple quantum well system in the 200-3000 K temperature range at 1 Torr to 100 atm. The predicted rate constants are in good agreement with most of the available data.