Rate Coefficients and Kinetic Isotope Effect for the C2H Reactions with NH3 and ND3 in the 104−294 K Temperature Range

Abstract

Reactions of C2H with NH3 and ND3 are studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193 nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Δ) chemiluminescence from the C2H + O2 reaction. The rate constants for the C2H + NH3 and C2H + ND3 reactions are measured at three temperatures, 104 ± 5 K, 165 ± 15 K, and 296 ± 2 K. Measured rate constants are fit to power law expressions, k(T) = A(T/298)n, for ease of comparison with the results for the related CN + NH3 reaction and to emphasize the importance of the attractive part of intermolecular interaction potential in the reaction mechanism. The rate constants are (2.9 ± 0.7) × 10-11 × (T/298 K)(−0.90 ± 0.15) cm3 molecule-1 s-1 and (1.1 ± 0.2) × 10-11 × (T/298 K)(−0.82 ± 0.026) cm3 molecule-1 s-1 for NH3 and ND3, respectively. A large kinetic isotope effect is observed, k(C2H + NH3)/k(C2H + ND3) = 2.0 ± 0.2, which within experimental uncertainty does not depend on the temperature in the 104−296 K range. Previous theoretical work shows that a hydrogen abstraction channel, C2H + NH3 → C2H2 + NH2, is a possible mechanism for the C2H + NH3 reaction since the minimum energy path for this channel does not have an activation barrier. This theoretical prediction is consistent with the strong negative temperature dependence of the rate coefficients for the C2H + NH3 reaction observed in this work, which clearly shows that the C2H + NH3 reaction does not have a barrier.