Kinetics and mechanisms of the reactions of CH and CD with H₂ and D₂

The reactions CH + H₂→^k1products and CD + D ₂→^k2products were studied using a laser photolysis/laser-induced fluorescence (LIF) technique. The room-temperature rate constant for reaction (1) was found to be pressure dependent between 25 and 600 Torr. Reactions (1) and (2) were studied as a function of temperature in the range 159-658 K at a total pressure of 100 Torr. Two pathways dominate these reactions. Below 400 K, addition followed by collisional stabilization of the adduct predominates. The rate constants display negative temperature dependences and are described by k₁ = (2.37 ± 0.43) × 10 ^-12 exp[(1041 ± 85)/RT] and k₂ = (3.51 ± 0.58) × 10^-12 exp[(1030 ± 90)/RT] cm³ molecule^-1 s^-1 with E_a in units of cal mol ^-1. Above 500 K, production of CH₂ + H or CD₂ + D with an activation energy of 4 ± 2 kcal mol^-1 dominates. In the reactions of CH + D₂ and CD + H₂, anomalously fast removal of the parent radical is observed because of formation and dissociation of chemically activated addition complexes which lead to isotopically substituted parent radicals. LIF spectra of these product radicals were obtained. A transition-state-theory model was applied to a mechanism in which collision stabilization and product formation proceed through a single complex.