Theoretical Study of the Ion−Molecule Reaction of the Vinyl Cation with Ethane

An ab initio study was performed of the C 2 H 3 + + C 2 H 6 ion−molecule reaction, which has received considerable experimental attention. The corresponding C 4 H 9 + potential energy surface (PES) was studied at the MP2/6-31G(d,p) level of theory, and single-point calculations on the MP2 geometries were carried out at the CCSD(T)/6-31G(d,p) and MP2/6-311+G(3df,2p) levels. According to our results, the initial interaction of ethane with the vinyl cation proceeds through a barrierless hydride transfer from C 2 H 6 to give a C 2 H 4 ···C 2 H 5 + π-complex. This complex is located in a fluxional region of the PES corresponding to different π-C 4 H 9 + complexes around 60 kcal/mol under reactants. In consonance with previous experimental and theoretical work, it is observed that this region of the C 4 H 9 + PES is separated from the most stable C 4 H 9 + tert -butyl cation by an energy barrier of about 19 kcal/mol. Different product channels for the title reaction are accessible from the C 4 H 9 + π-complexes via H 2 or CH 4 elimination and fragmentation processes, the lowest energy profiles corresponding to the C 2 H 5 + and C 3 H 5 + exothermic channels. Gibbs energy reported in this work suggests that entropic contributions may play an important role to determine the ratio of the secondary ions corresponding to different product channels, in agreement with experimental results.