Comparative study of ethane and propane cation radicals by B3LYP density functional and high-level ab initio methods

A comparative computational study of the cation radicals of ethane and propane is made by using B3LYP density functional and high-level ab initio methods [up to QCISD(T)/6-311++G(2df,p) and CBS-APNO model chemistry]. The properties investigated include the structures and energetics of the cation radical isomers, their isotropic hyperfine coupling constants, and their carbon-carbon bond dissociation energies, as well as the vertical and adiabatic ionization potentials of the parent molecules. The computational results are compared with experimental data where possible. In general, the B3LYP method exhibits good agreement with experiment. All of the methods show that the potential energy surfaces of the two cation radicals are very flat. The electronic origin of this phenomenon and the relationship between the cation radical isomers of a given species (so-called electromers) are discussed in terms of a valence-bond model.