Physical Chemistry Chemical Physics vol:6 issue:16 pages:4111-4117
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.