Molecular orbital studies of the energies and structure of polyatomic cations

Abstract

Recent theoretical work on the structures of small organic cations is reviewed. For the small species AH n + where A is a non-hydrogen atom, most structures have been examined to a high level, including effects of correlation in some cases. Theory indicates that C 2v or C 3v ions of this type have larger valence angles than isoelectronic neutral species. An example is the hydronium ion, H 3O +, which is predicted to be pyramidal with a bond angle close to 111°. CH 5 + is predicted to be a C 5 structure with a two-electron, three-center bond connecting a CH 3 + ion and a hydrogen molecule. The vinyl cation C 2H 3 + is found to be classical with Hartree-Fock theory, but recent studies including correlation now favor a bridged structure for this ion. The ethyl cation C 2H 5 + is also predicted to be bridged if correlation is taken into account. Structures of some of the possible isomers of the C 3 systems C 3H +, C 3H 3 +, C 3H 5 + and C 3H 7 + are also described. Finally, the relative stability of carbonium ions is discussed and it is shown that the theory gives a satisfactory account of differences between the hydride abstraction energies of various hydrocarbons.