Semi-empirical extrapolation and estimation of rate constants for abstraction of H from methane by H, O, HO, and O2

It has been concluded that for extrapolating rate constants of atom transfer reactions to and from high temperatures, a useful form of the rate constant is k=AT2 exp(−C/T), where A and C are fitted constants. For k/cm3/(mol s) and TK, on the basis of previous experimental data, the values of log A and C for the following reactions are: H+CH4=H2+CH3, log A=7.15, C=4449; O+CH4=OH+CH3 log A=6.71, C=3240; HO+CH4 =H2O+CH3, log A=6.93, C=1485 and O2+CH4=O2H+CH3, log A=6.93, C=26153. At all temperatures, abstraction by HO is faster than by O. The form of the rate constant is equivalent to assuming that ΔCp 0‡, the heat capacity at the constant pressure of activation, is zero. When ΔCp O‡, was estimated and assumed to be constant (in principle, a more accurate assumption than ΔCp O‡ =0), the fit to the experimental data was slightly worse. It is confirmed that at 400 to 700 K, the kinetic and thermodynamic equilibrium constants for the reaction H+CH4 =H2+CH3 are significantly different. (At 1340 K, they are in agreement.)