Aromaticity of pyrene and its cyclopentafused congeners - Resonance and NICS criteria. An ab initio valence bond analysis in terms of Kekule resonance structures

The effect of cyclopentafusion on the aromatic properties of pyrene and its cyclopentafused congeners has been studied by calculating resonance energi es, using the valence bond (VB) method, and nucleus independent chemical sh ifts using DIGLO. The VB resonance energy is only slightly affected by cycl opentafusion. The resonance interactions between Kekule resonance structure s that lead to six pi electron (benzene-like) conjugated circuits have the largest contributions to the resonance energy, in favor of Clar's model. Fo r all compounds these contributions are of similar magnitude. Hence, accord ing to the resonance criterion, all compounds have the same aromatic charac ter. In contrast, the total NICS values show a decrease of aromatic charact er of the compounds in the series upon the addition of externally fused fiv e-membered rings. However, in line with the resonance criterion, the diamag netic part of the shielding tenser perpendicular to the molecular framework is nearly constant for all compounds, provided that comparable gauge origi ns are chosen. Thus, care should be taken by comparing the aromatic charact er of rings of different molecules by considering only their total NICS val ues.