Kinetic study of the reaction CH(X ²Π)+H₂⇄ CH₂(X ³B₁)+H in the temperature range 372 to 675 K

The reaction CH(X ²Π)+H₂⇄CH ₂(X³B₁)+H was studied over the temperature range 372 to 675 K. Laser-induced fluorescence detection of CH radicals allowed direct determination of absolute rate constants for the forward reaction k ₁. The data are well fit by the Arrhenius expression k ₁=(2.38±0.31)×10^-10 exp[-(1760±60)/T] cm³/s over the temperature range studied. A transition state theory (RRKM) calculation yields the following form for k₁, k ₁=(5.5±1.5)×10^-16 T ^1.79±0.04 exp[-(840±30)/T] cm³/s for the temperature range 300 to 2500 K, thus allowing cautious extrapolation to combustion temperatures. The rate constants for the reverse reaction are evaluated by utilization of the equilibrium and the forward rate constants. The data for k_-1 are well fit by k_-1=(4.7±0.6) ×10^-10 exp[-(370±60)/T] cm³/s over the temperature range 372 to 675 K. A transition state theory calculation yields the following form for k_-1, k_-1=(6.4±1.1) ×10^-15 T^1.54±0.02 exp[(430±20)/T] cm³/s for the temperature range 300 to 2500 K. The heat of formation of CH₂ at 0 K was determined to be ΔH_fo {CH ₂}=92.6±0.5 kcal/mol, assuming that the CH₂+H radical recombination reaction has no activation energy.