No abstract

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.

Rate coefficients for the reaction C 2 H + CH 4 f C 2 H 2 + CH 3 and C 2 H + CD 4 f C 2 HD + CD 3 are measured over the temperature range 154-359 K using transient infrared laser absorption spectroscopy. Ethynyl radicals are produced by pulsed laser photolysis of C 2 H 2 in a variable temperature flow cell, and a tunable color center laser probes the transient removal of C 2 H (X 2 Σ + (0,0,0)) in absorption. The rate coefficients for the reactions of C 2 H with CH 4 and CD 4 both show a positive temperature dependence over the range 154-359 K, which can be expressed as k CH 4 ) (1.2 ( 0.1) × 10 -11 exp[(-491 ( 12)/T] and k CD 4 ) (8.7 ( 1.8) × 10 -12 exp[(-650 ( 61)/T] cm 3 molecule -1 s -1 , respectively. The reaction of C 2 H + CH 4 exhibits a significant kinetic isotope effect at 300 K of k CH 4 /k CD 4 ) 2.5 ( 0.2. Temperature dependent rate constants for C 2 H + C 2 H 2 were also remeasured over an increased temperature range from 143 to 359 K and found to show a slight negative temperature dependence, which can be expressed as k C 2 H 2 ) 8.6 × 10 -16 T 1.8 exp [(474 ( 90)/T] cm 3 molecule -1 s -1 .