This paper presents ab initio potential surface parameters and transition state theory (TST) rate constants for the reaction H2 + CH3 → H + CH4, its reverse, and all the deuterium isotopic counterparts associated with it and its reverse. The potential surface parameters are derived from accurate POLCI calculations and include vibrational frequencies, moments of inertia, and other quantities for CH3, H2, CH4, and the H-H-CH3 saddle point. TST rate constants are calculated from standard expressions and the Wigner tunneling correction. For H2 + CH3 and H2 + CD3, agreement of the rate constant with experiment is good over a broad temperature range, suggesting that the calculated 10.7 kcal/mol barrier is accurate to within about 0.5 kcal/mol. Agreement with experiment for H + CH4 using the calculated 13.5 kcal/mol reverse reaction barrier is poorer; a 12.5 kcal/mol barrier is found to provide a more reasonable estimate of the true barrier. Primary isotope effects for the deuterated analogues of H2 + CH3 are found to be correct in magnitude at high temperature, but with a weaker temperature dependence than experiment. The calculated secondary isotope effects also appear to be weaker functions of temperature than experiment, although a large uncertainty in the experimental results precludes a quantitative assessment of errors. Our analysis of isotope effects in the H + CH4 reaction is restricted to examining the branching ratios between H and D atom abstraction in the reaction of H with the mixed species CH3D, CH2D2, and CHD3. A combination of reaction path multiplicity, favorable zero point energy shifts, and a greater likelihood of tunneling causes H atom abstraction to predominate over D atom abstraction in H + CH3D and H + CH2D2, but for H + CHD3, we find that the H atom and D atom abstraction rate constants cross near 700 K, with H atom abstraction dominating at low temperatures and D atom at high.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry