Electron delocalization ring current maps

As tools for measuring electron delocalization, we opt for the visual inspection of the occupied orbitals and especially the so-called domain-averaged Fermi-hole (DAFH) analysis and MCI. Both of these require availability of atomic overlap matrices, where the overlap between two molecular orbitals is obtained in atom condensed form. This requires the definition of an atom in the molecule and in this study we chose the Ftirshfeld-I method. Finally, we introduce ring current maps in the ipso-centric approach. 

It has been shown that there is no direct relation between electron delocalization and the presence of a ring current, other than that a delocalized system is a necessary but not sufficient condition to lead to a ring current. In the case of Al " derived compounds, there is clearly both o and n electron delocalization although only the a system gives rise to a ring current. The analysis of CMO-NICS data is shown to lead to a different conclusion although this method also contains occupied-occupied terms and is not so easily interpretable as ipsocentric ring current maps. 

Annulene is fairly planar and shows little alternation of the single and double bonds. The extent of the delocalization of its ir electrons is pictured in the electrostatic potential map in the margin. Like benzene, it can be described by a set of two equivalent resonance forms. In accord with its aromatic character, the molecule is relatively stable and undergoes electrophilic aromatic substimtion. It also exhibits a benzene-Uke ring-current effect in its NMR spectrum (see Problems 64 and 65). 

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