The new pictorial structural covariance method for qualitative quantum chemistry. III: Fused polycyclics and their ions

Abstract

The number of non-bonding (NBMOs), bonding, and anti-bonding MOs, the HOMO-LUMO types, reactivity to electro- or nucleophiles, qualitative relative stabilities of the neutral species, their anions or cations, are readily deduced for pi-polycyclics “on the blackboard” directly from the structural formulae utilizing the recently presented pictorial quantum rules. Among others, the bicyclo [p.q.0] pi-hydrocarbons are treated extensively and are found to fall into ten distinct homolog classes. In addition to the customary pi-aromatic or pi-anti-aromatic types, classes of compounds with other behaviour (e.g. one here called “anti-Kekule”) are found. When an anti-aromatic ring is fused with another anti-aromatic or with a pi-aromatic ring, the resulting bicyclo-molecule is not anti-aromatic. In another series, the heptalene and other (4k + 3), (4k′ + 3) bicyclics have two electrons in a single NBMO, making them “anti-Kekule”. The method is able to distinguish between the properties of pairs of isomers such as (s-indacene; as-indacene) and (1.2,5.6-dibenzpentalene; 1.2,4.5-dibenzpentalene).