A theoretical study of deuterium isotope effects in the reactions H₂ + CH₃ and H + CH₄

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.