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

Reactions of C 2 H with NH 3 and ND 3 are studied at low temperature using a pulsed Laval nozzle apparatus. The C 2 H radical is prepared by 193 nm photolysis of acetylene, and the C 2 H concentration is monitored using CH(A 2 Δ) chemiluminescence from the C 2 H + O 2 reaction. The rate constants for the C 2 H + NH 3 and C 2 H + ND 3 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 + NH 3 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) cm 3 molecule -1 s -1 and (1.1 ± 0.2) × 10 -11 × ( T /298 K) (−0.82 ± 0.026) cm 3 molecule -1 s -1 for NH 3 and ND 3, respectively. A large kinetic isotope effect is observed, k (C 2 H + NH 3 )/ k (C 2 H + ND 3 ) = 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, C 2 H + NH 3 → C 2 H 2 + NH 2, is a possible mechanism for the C 2 H + NH 3 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 C 2 H + NH 3 reaction observed in this work, which clearly shows that the C 2 H + NH 3 reaction does not have a barrier.