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

Abstract

Rate coefficients for the reactions C₂H with C₂H₄, C₂H₆, and H₂ are measured over the temperature range 150?359 K using transient infrared laser absorption spectroscopy. The ethynyl radical is formed by photolysis of C₂H₂ with a pulsed excimer laser at 193 nm, and its transient absorption is monitored with a color center laser on the Q₁₁(9) line of the A²Π?X²Σ transition at 3593.68 cm^-1. Over the experimental temperature range 150?359 K the rate constants of C₂H with C₂H₄, C₂H₆, and H₂ can be fit to the Arrhenius expressions k_C2H4 = (7.8 ± 0.6) × 10^-11 exp[(134 ± 44)/T], k_C2H6 = (3.5 ± 0.3) × 10^-11 exp[(2.9 ± 16)/T], and k_H2 = (1.2 ± 0.3) × 10^-11 exp[(?998 ± 57)]/T cm³ molecule^-1 s^-1, respectively. The data for C₂H with C₂H₄ and C₂H₆ indicate a negligible activation energy to product formation shown by the mild negative temperature dependence of both reactions. When the H₂ data are plotted together with the most recent high-temperature results from 295 to 854 K, a slight curvature is observed. The H₂ data can be fit to the non-Arrhenius form k_H2 = 9.2 × 10^-18T^2.17±0.50 exp[(?478 ± 165)/T] cm³ molecule^-1 s^-1. The curvature in the Arrhenius plot is discussed in terms of both quantum mechanical tunneling of the H atom from H₂ to the C₂H radical and bending mode contributions to the partition function.