Ab initio study of the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2) and related C(4)H(10)(2+) isomers.

Ab initio calculations at the MP4(SDTQ)/6-311G**//MP2/6-31G** level were performed to study the structures and stabilities of the dimer of ethyl cation, (C 2 H 5 + ) 2 , and related C 4 H 1 0 2 + isomers. Two doubly hydrogen bridged diborane type trans 1 and cis 2 isomers were located as minima. The trans isomer was found to be more favorable than cis isomer by only 0.6 kcal/mol. Several other minima for C 4 H 1 0 2 + were also located. However, the global energy minimum corresponds to C-H (C 4 position) protonated 2-butyl cation 10. Structure 10 was computed to be substantially more stable than 1 by 31.7 kcal/mol. The structure 10 was found to be lower in energy than 2-butyl cation 13 by 34.4 kcal/mol.