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 detectio n has been used to determine the variable-temperature dependence of the rat e constant and product branching fractions for the reaction between F(P-2) and C2H4 at P = 1 Torr nominal pressure (He). The reaction was studied at T = 202 and 236 K by monitoring the decay of C2H4 in the presence of a large excess of F(P-2). The overall rate coefficients were determined to be k(1) (202 K) = (1.7 +/- 0.4) x 10(-10) cm(3) molecule(-1) s(-1) and k(1)(236 K) = (2. 1 +/- 0.5) x 10(-10) cm(3) molecule(-1) s(-1) with the quoted uncerta inty representing total errors. Further, the branching fractions for the tw o observed reaction channels F + C2H4 --> C2H3 + HF (1a) and F + C2H4 --> C 2H3F + H (1b) were determined by quantitatively measuring the yield of C2H3 F under conditions of excess C2H4. The stabilized adduct, C2H4F, was not de tected at T = 202 K. The derived branching fractions were Gamma(1a)(202 K) = 0.25 +/- 0.09, Gamma(1b) (202 K) = 0.75 +/- 0.16, and Gamma(1a)(236 K) = 0.27 +/- 0.13, and Gamma(1b) (236 K) = 0.73 +/- 0.20, where the quoted unce rtainty represents total errors. By inclusion of k(1)(298 K) = (3.0 +/- 0.8 ) x 10(-10) cm(3) molecule(-1) s(-1), a revised value that used data from o ur previous study and Gamma(1a)(298 K) = 0.35 +/- 0.04 and Gamma(1b) (298 K ) s = 0.65 +/- 0.04 from a laser photolysis/photoionization mass spectromet ry study, we obtain the Arrhenius expressions k(1a)(T) = (7.5 +/- 4.0) x 10 (-10) exp[(-1.2 +/- 0.3)/(RT)] and k(1b)(T) = (5.2 +/- 1.0) x 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 t otal errors at 1 sigma precision errors plus 15% systematic errors. RRKM ca lculations have shown that the critical energy for H addition to C2H3F is l ess than 6 kcal mol(-1) larger than that for the addition of F to C2H4 and that the competitive decomposition of chemically activated C2H4F radicals f avor C-H bond rupture by a factor greater than 1000 over that for C-F bond rupture.