Rate Coefficients of C2H with C2H4, C2H6, and H2 from 150 to 359 K

Rate coefficients for the reactions C 2 H with C 2 H 4, C 2 H 6, and H 2 are measured over the temperature range 150−359 K using transient infrared laser absorption spectroscopy. The ethynyl radical is formed by photolysis of C 2 H 2 with a pulsed excimer laser at 193 nm, and its transient absorption is monitored with a color center laser on the Q 11 (9) line of the A 2 Π−X 2 Σ transition at 3593.68 cm -1 . Over the experimental temperature range 150−359 K the rate constants of C 2 H with C 2 H 4, C 2 H 6, and H 2 can be fit to the Arrhenius expressions k C 2 H 4 = (7.8 ± 0.6) × 10 -11 exp[(134 ± 44)/ T ], k C 2 H 6 = (3.5 ± 0.3) × 10 -11 exp[(2.9 ± 16)/ T ], and k H 2 = (1.2 ± 0.3) × 10 -11 exp[(−998 ± 57)]/ T cm 3 molecule -1 s -1, respectively. The data for C 2 H with C 2 H 4 and C 2 H 6 indicate a negligible activation energy to product formation shown by the mild negative temperature dependence of both reactions. When the H 2 data are plotted together with the most recent high-temperature results from 295 to 854 K, a slight curvature is observed. The H 2 data can be fit to the non-Arrhenius form k H 2 = 9.2 × 10 -18 T 2.17±0.50 exp[(−478 ± 165)/ T ] cm 3 molecule -1 s -1 . The curvature in the Arrhenius plot is discussed in terms of both quantum mechanical tunneling of the H atom from H 2 to the C 2 H radical and bending mode contributions to the partition function.