Nucleus-independent chemical shifts and local aromaticities in large polycyclic aromatic hydrocarbons

Central rings of coronene and kekulene are known to be paratropic with small positive nucleus-indepedent chemical shift (NICS) values at the centers although they are (4n + 2)-membered rings. However, careful inspection of the bond resonance energy (BRE) pattern in each π-system revealed that all constituent rings are aromatic in nature. Individual rings were found to sustain diatropic currents even if some bond currents are vanishingly small. Thus, NICS values at the ring centers of large polycyclic π-systems, including fullerenes, cannot always be used as indicators of local aromaticity. BRE is better suited for estimating the degree of local aromaticity in polycyclic π-systems.