Ab Initio Chemical Kinetics for the CH3+ O(3P) Reaction and Related Isomerization–Decomposition of CH3O and CH2OH Radicals.

Thekinetics and mechanism of the CH3+ O reaction andrelated isomerization–decomposition of CH3O andCH2OH radicals have been studied by ab initio molecularorbital 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 reactionshows that the CHO + H2products can be directly generatedfrom CH3O by the TS3 → LM1 → TS7 →LM2 → TS4 path, in which both LM1 and LM2 are very loose andTS7 is roaming-like. The result for the CH2O + H reactionshows that there are three low-energy barrier processes includingCH2O + H → CHO + H2via H-abstractionand CH2O + H → CH2OH and CH2O + H → CH3O by addition reactions. The predictedenthalpies of formation of the CH2OH and CH3O radicals at 0 K are in good agreement with available experimentaldata. Furthermore, the rate constants for the forward and some keyreverse reactions have been predicted at 200–3000 K under variouspressures. Based on the new reaction pathway for CH3+O, the rate constants for the CH2O + H and CHO + H2reactions were predicted with the microcanonical variationaltransition-state/Rice–Ramsperger–Kassel–Marcus(VTST/RRKM) theory. The predicted total and individual product branchingratios (i.e., CO versus CH2O) are in good agreement withexperimental data. The rate constant for the hydrogen abstractionreaction of CH2O + H has been calculated by the canonicalvariational transition-state theory with quantum tunneling and small-curvaturecorrections to be k(CH2O + H →CHO + H2) = 2.28 × 10–19T2.65exp(−766.5/T) cm3molecule–1s–1for the 200–3000 K temperaturerange. The rate constants for the addition giving CH3Oand CH2OH and the decomposition of the two radicals havebeen calculated by the microcanonical RRKM theory with the time-dependentmaster equation solution of the multiple quantum well system in the200–3000 K temperature range at 1 Torr to 100 atm. The predictedrate constants are in good agreement with most of the available data. [ABSTRACT FROM AUTHOR]