Theoretical study of the ion-molecule reaction of the vinyl cation with ethane

An ab initio study was performed of the C2H3+ + C2H6 ion-molecule reaction, which has received considerable experimental attention. The corresponding C4H9+ potential energy surface (PES) was studied at the MP2/6-31G(d,p) leve l of theory, and single-point calculations on the MP2 geometries were carri ed out at the CCSD(T)/6-31G(d,p) and MP2/6-311SG(3df,2p) levels. According to our results, the initial interaction of ethane with the vinyl cation pro ceeds through a barrierless hydride transfer from C2H6 to give a C2H4... C2 H5+ pi-complex. This complex is located in a fluxional region of the PES co rresponding to different pi-C4H9+ complexes around 60 kcal/mol under reacta nts. In consonance with previous experimental and theoretical work, it is o bserved that this region of the C4H9+ PES is separated from the most stable C4H9+ tert-butyl cation by an energy barrier of about 19 kcal/mol. Differe nt product channels for the title reaction are accessible from the C4H9+ pi -complexes via H-2 or CH4 elimination and fragmentation processes, the lowe st energy profiles corresponding to the C2H5+ and C3H5+ exothermic channels . Gibbs energy reported in this work suggests that entropic contributions m ay play an important role to determine the ratio of the secondary ions corr esponding to different product channels, in agreement with experimental res ults.