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

Rate coefficients for the reaction C2H + CH4 → C2H2 + CH3 and C2H + CD4 → C2HD + CD3 are measured over the temperature range 154−359 K using transient infrared laser absorption spectroscopy. Ethynyl radicals are produced by pulsed laser photolysis of C2H2 in a variable temperature flow cell, and a tunable color center laser probes the transient removal of C2H (X2Σ+ (0,0,0)) in absorption. The rate coefficients for the reactions of C2H with CH4 and CD4 both show a positive temperature dependence over the range 154−359 K, which can be expressed as kCH4 = (1.2 ± 0.1) × 10-11 exp[(−491 ± 12)/T] and kCD4 = (8.7 ± 1.8) × 10-12 exp[(−650 ± 61)/T] cm3 molecule-1 s-1, respectively. The reaction of C2H + CH4 exhibits a significant kinetic isotope effect at 300 K of kCH4/kCD4 = 2.5 ± 0.2. Temperature dependent rate constants for C2H + C2H2 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 kC2H2 = 8.6 × 10-1...