Mean-field resonating-valence-bond theory for unpaired π-electrons in benzenoid carbon species

A qualitative resonance-theoretic view is presented for the description of a variety of conjugated π-network species identified with “subgraphs” (either finite or infinite) of the graphite network. Within the framework of this resonance theory, simple rules are described to provide qualitative information: On ground-state spin multiplicities; on patterns of ground-state spin density; and on exchange splittings to low-lying “spin-flipped” excited states. Beyond ordinary benzenoid molecules, illustrative applications are noted to a diversity of extended species, including: Differently structured edges on semi-infinite graphite; corner structures where edges along different directions meet; conjugated polymer-strip ends; and local defect vacancy structures in extended graphite. The variety of simple resonance-theoretic predictions are compared against a semiempirical unrestricted Hartree–Fock view of some quantitative tight-binding molecular-orbital-theoretic computations. Agreement in predictions from the resonance- and band-theoretic viewpoints is taken to engender reliability of the so coincident predictions. A traditional organic chemical resonance-theoretic view is thence conveniently reformulated and brought to bear on several extended nano-structured systems to reveal systematic patterns of π-electronic behavior.