A series of C2-symmetrjc homoderivatives of the cyclo C9H_9~+ 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.
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