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

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Abstract
The reaction CH(X 2Π)+H2⇄CH2(X 3B1)+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 k1. The data are well fit by the Arrhenius expression k1=(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, k1=(5.5±1.5)×10−16 T1.79±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 T1.54±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 ΔHfo {CH2}=92.6±0.5 kcal/mol, assuming that the CH2+H radical recombination reaction has no activation energy.