Low-Temperature Rate Coefficients of C₂H with CH₄ and CD₄ from 154 to 359 K

Abstract

Rate coefficients for the reaction C 2 H + CH 4 → C 2 H 2 + CH 3 and C 2 H + CD 4 → 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 .