Theoretical study of the reaction of the ethynyl radical with ammonia (C2H + NH3): hydrogen abstraction versus condensation

Abstract

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–NH₃ (−7.6) → TS (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.