Kinetics of the C₂H₃ + H₂ ⇄ H + C₂H₄ and CH₃ + H₂ ⇄ H + CH₄ Reactions

The kinetics of the reactions C2H3 + H2 → H + C2H4 (1) and CH3 + H2 → H + CH4 (2) have been studied in the temperature ranges 499−947 K (reaction 1) and 646−1104 K (reaction 2) and He densities (6−18) × 1016 atoms cm-3 by laser photolysis/photoionization mass spectrometry. Rate constants were determined in time-resolved experiments as a function of temperature. Ethylene was detected as a primary product of reaction 1. Within the above temperature ranges the experimental rate constants can be represented by Arrhenius expressions k1 = (3.42 ± 0.35) × 10-12 exp(−(4179 ± 67 K)/T) cm3 molecule-1 s-1 and k2 = (1.45 ± 0.18) × 10-11 exp(−(6810 ± 102 K)/T) cm3 molecule-1 s-1. Experimental values of k2 are in agreement with the available literature data. The potential energy surface and properties of the transition state for reactions (1, −1) were studied by ab initio methods. Experimental and ab initio results of the current study were analyzed and used to create a transition state model of the reaction. The resulting model provides the temperature dependencies of the rate constants for both direct (1) and reverse (−1) reactions in the temperature range 200−3000 K: k1 = 1.57 × 10-20T2.56 exp(−(2529 K)/T) cm3 molecule-1 s-1, k-1 = 8.42 × 10-17T1.93 exp(−(6518 K)/T) cm3 molecule-1 s-1. Data on reactions 1 and −1 available in the literature are analyzed and compared with the results of the current study.