Chiral Aromaticities. A Topological Exploration of Mobius Homoaromaticity

A series Of C-2-symmetric homoderivatives of the cyclo C9H9+ cation first identified by Schleyer as Mobius aromatic are shown to themselves sustain Mobius 4n-pi-electron homoaromaticity. Analogous double-twist Mobius bis-homoaromatics follow a 4n+2 electron rule. AIM (atoms-in-molecules) and ELF (electron localization function) analysis of the electron topology in the region of the homobond of these systems reveals that the presence of a AIM bond-critical point in this region is not mandatory, it being unstable to subtle variations in the local electron density induced by local or remote substituents, and which can in turn induce self-annihilation or creation of a pair of bond and ring critical points. The same substituent-induced annihilation/creation of such a BCP/RCP pair can also be observed in the nonclassical norbornyl cation. We suggest that the ELF and ELF pi thresholds for any basin found in the homoregion are better indicators of the delocalized nature of the homoaromatic interaction and the aromaticity of the system.