STRUCTURE AND ENERGETICS OF C2H4BR+ - ETHYLENEBROMONIUM ION VS BROMOETHYL CATIONS

The cyclic and acyclic isomers of C2H4Br+ are compared by using ab initio quantum mechanical techniques, including the use of electron correlation and polarization functions. The cyclic bromonium ion is found to be more stable than the acyclic 1-bromoethyl cation by 1.5 kcal/mol, in very good agreement with experiment. A transition state for the interconversion of these two forms is reported, the energy barrier being ~25 kcal/mol. The relative energies of the cyclic and acyclic minima are remarkably insensitive to basis set and electron correlation effects, validating the results of previous low-level studies. The 2-bromoethy cation does not exist as a minimum on the potential energy surface and spontaneously collapses to the bromonium ion upon torsion. © 1990, American Chemical Society. All rights reserved.