Bond orders in metalloporphyrins

A new concept of bond order density of shared electron pairs is introduced to study the electronic structure in molecules. This BODSEP analysis is applied to the distortion of benzene toward its building blocks (acetylenes), metalloporphyrins and two recent examples from the literature. The S12g/TZ2P results show the gradual disappearance of the bonds while distorting benzene and strengthening the bonds in acetylene to reach a final triple bond value. A large range of bond orders are observed for the metalloporphyrins, which are consistent with aromaticity indices from the literature [Can. J. Chem. 2009, 87, 1063]; with an ideal value of 1.5 for aromatic molecules (because of resonance), it suggests that (BODSEP) bond orders might be used for aromaticity measures. Finally, the delocalized bonds in porphyrins are localizing in the corrphycenes, and bond order strengths for different spin states of dipyrrolonaphthyridinedione have been assigned, which differ from the original description.