Portions of the potential energy surface (PES) related to the reaction between the ethynyl radical and ammonia (C2H + NH3) have been investigated in detail using both MO and DFT methods up to geometry optimizations using the coupled-cluster theory with large basis sets. Several (C2H4N) intermediates and transition structures for unimolecular rearrangements between them have been characterized. Calculations at the CCSD(T)/6-311++G(3df,2p) + ZPE level show that the C2H + NH3 reaction has two main entrance channels: H-abstraction and condensation. The relative energies (kcal mol-1) along the H-abstraction pathway are as follows: 1 C2H + NH3 (0) → pre-reaction complex CO2 (-2.9) → TS (-1.8) → post-reaction complex CO3 (-28.4) → HCCH + NH2 (-26.6). This channel thus starts by formation of a weak complex HCC...H3N, which after H-atom transfer gives rise to another weak complex between the products, HCCH⋯NH2. The energies (kcal mol-1) along the condensation pathway are: 1 C2H + NH3 (0) → pre-association complex CO1 (-6.1) → TS (-3.0) → adduct HCC-NH 3 (-7.6) → (4.3) → H2N-CCH + H (-14.2). Although both complex CO1 and primary adduct HCC-NH3 are slightly more stable than CO2 and CO3, the transition structure for conversion of the adduct has a substantially higher energy than the reactants and is fairly rigid, whereas the transition state for H-abstraction lies below the reactant limit and is rather loose. Therefore, H-abstraction is calculated to be clearly favored over condensation at all temperatures. The predicted barrier-free main channel is consistent with recent experimental results showing the title reaction to be a fast process exhibiting a negative temperature dependence. In view of the small energy barrier related to the novel condensation pathway, it might contribute at high temperatures in a significant way to the products formation.
