Homoaromaticity versus Moebius Aromaticity

The structures of cyclooctatrienyl cation 1a, 1,3,5-cycloheptatriene 2a, norcaradiene 2b, cyclohexadienyl anion 3a, homocyclopentadienyl anion 3b, cyclopentadiene dianion 4a, and homocyclobutadiene dianion 4c were determined by ab initio MO theory at the RHF level with use of the split-valence 3-21 G basis set. All molecules, but 3a, prefer nonplanar structures. Homoaromaticity was found not to be important in anionic and neutral systems and has not been observed at all in cation 1. It can be concluded that Mobius aromaticity is preferred over homoaromaticity in stabilizing the investigated systems. The topological analysis of the electronic charge distribution supports this conclusion.