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An ab Initio Study of the Reaction of Propargyl Cation with Ammonia

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journal contribution
posted on 10.04.2002, 00:00 by Ramón López, Emma del Río, M. Isabel Menéndez, Tomás L. Sordo
The reaction of propargyl cation with NH3 was investigated at the CCSD(T)/6-311+G(2df,2p)//MP2-FULL/6-31+G(d,p) theory level by performing a statistical thermodynamics analysis of the theoretical results. Three different reactive channels starting at an association complex were found. The first one is the C2H4 + HCNH+ channel, which proceeds under the energy level corresponding to reactants and is the most exothermic one, −34.1 kcal/mol in Gibbs energy, but which implies complex molecular rearrangements giving rise to several adducts much more stable than reactants. The other two channels are the c-C3H2 + NH4+ channel, which is exothermic by only 3.3 kcal/mol, and the H2CCC + NH4+ channel, which presents an energy barrier of 8.8 kcal/mol owing to its endothermicity. The c-C3H2 + NH4+ channel proceeds through a TS for the cyclization of HCCCH2+ efficiently catalyzed by NH3 and presents the largest Gibbs energy barrier, 24.2 kcal/mol. For the cyclization of H2CCC, we have found two different pathways with energy barriers of 79.4 and 38.0 kcal/mol. Therefore, according to our theoretical results, obtaining a cyclic product, c-C3H2, is energetically disfavored. The product distribution experimentally reported can be rationalized assuming that at low pressure the reaction between propargyl cation and ammonia proceeds along the two reaction channels H2CCC + NH4+, with a low energy barrier, and C2H4 + HCNH+, dynamically disfavored. At higher pressure, the dynamically disfavored pathway would give rise to two channels:  C2H4 + HCNH+ and C3H3·NH3+.