Aromaticity in Heterocyclic and Inorganic Benzene Analogues

Abstract

Recently, we presented a molecular orbital (MO) model of aromaticity that explains, in terms of simple orbital-overlap arguments, why benzene (C6H6) has a regular structure with delocalized double bonds. Here, we show that the same model and the same type of orbital-overlap arguments also account for heterocyclic and inorganic benzene analogues, such as s-triazine (C3N3H3), hexazine (N6), borazine (B3N3H6), boroxine (B3O3H3), hexasilabenzene (Si6H6), and hexaphosphabenzene (P6). Our MO model is based on accurate Kohn–Sham density-functional theory (DFT) analyses of the bonding in the seven model systems, and how the bonding mechanism is affected if these molecules undergo geometrical deformations between regular, delocalized ring structures and distorted ones with localized double bonds. It turns out that also in the heterocyclic and inorganic benzene analogues, the propensity of the π electrons is always to localize the double bonds, against the delocalizing force of the σ electrons. The latter in general prevails, yielding the regular, delocalized ring structures. Interestingly, we find one exception to this rule: N6.