Absolute Rate Constant and Product Branching Fractions for the Reaction between F and C₂H₄ at T = 202−298 K

The discharge-flow kinetic technique coupled to mass-spectrometric detection has been used to determine the variable-temperature dependence of the rate constant and product branching fractions for the reaction between F( 2 P) and C 2 H 4 at P = 1 Torr nominal pressure (He). The reaction was studied at T = 202 and 236 K by monitoring the decay of C 2 H 4 in the presence of a large excess of F( 2 P). The overall rate coefficients were determined to be k 1 (202 K) = (1.7 ± 0.4) × 10 -10 cm 3 molecule -1 s -1 and k 1 (236 K) = (2.1 ± 0.5) × 10 -10 cm 3 molecule -1 s -1 with the quoted uncertainty representing total errors. Further, the branching fractions for the two observed reaction channels F + C 2 H 4 → C 2 H 3 + HF (1a) and F + C 2 H 4 → C 2 H 3 F + H (1b) were determined by quantitatively measuring the yield of C 2 H 3 F under conditions of excess C 2 H 4 . The stabilized adduct, C 2 H 4 F, was not detected at T = 202 K. The derived branching fractions were Γ 1a (202 K) = 0.25 ± 0.09, Γ 1b (202 K) = 0.75 ± 0.16, and Γ 1a (236 K) = 0.27 ± 0.13, and Γ 1b (236 K) = 0.73 ± 0.20, where the quoted uncertainty represents total errors. By inclusion of k 1 (298 K) = (3.0 ± 0.8) × 10 -10 cm 3 molecule -1 s -1, a revised value that used data from our previous study and Γ 1a (298 K) = 0.35 ± 0.04 and Γ 1b (298 K) = 0.65 ± 0.04 from a laser photolysis/photoionization mass spectrometry study, we obtain the Arrhenius expressions k 1a ( T ) = (7.5 ± 4.0) × 10 -10 exp[(−1.2 ± 0.3)/( RT )] and k 1b ( T ) = (5.2 ± 1.0) × 10 -10 exp[(−0.6 ± 0.1)/( RT )] in units of cm 3 molecule -1 s -1 for k and in units of kcal mol -1 for activation energy. The quoted uncertainty represents total errors at 1σ precision errors plus 15% systematic errors. RRKM calculations have shown that the critical energy for H addition to C 2 H 3 F is less than 6 kcal mol -1 larger than that for the addition of F to C 2 H 4 and that the competitive decomposition of chemically activated C 2 H 4 F radicals favor C−H bond rupture by a factor greater than 1000 over that for C−F bond rupture.