Clar's Aromatic Sextet Theory Revisited via Molecular Electrostatic Potential Topography

Abstract

Full characterization of the molecular electrostatic potential (MESP) topography of the π-regions of 12 polycyclic benzenoid hydrocarbons (PBHs) is carried out. Benzene is endowed with the most perfect circular distribution of π-delocalization, and the hexagonal rings of other systems possess varying degrees of lesser π-delocalization. The topographical features describe Clar's aromatic sextet theory very well and simplify the aromatic characterization of each ring of a PBH system. The concepts such as “aromatic dilution” observed for polyacene series and the “empty ring” in triphenylene, perylene, and coronene are clearly brought out from this study. The positions of (3, +3) critical points (CPs) are always observed very close to shorter bonds, providing valuable hints at how the π-electrons are shared among the carbons. Further, average values of the MESP at CPs calculated for each ring ( ) and for the whole molecule ( ) bear linear correlation with the local aromaticity values estimated by Li and Jiang and the hardness values reported by Zhou and Parr for the global aromaticity, respectively. Thus the mapping of the MESP topography provides an elaborate characterization of the π-regions of PBH systems and a description of the intimately connected aromaticity.