Rationalisation of Relative »Ring-Current« Sizes in Polycyclic, Conjugated Hydrocarbons

A theoretical basis is established for relating the incidence of relatively \u27high\u27 and \u27low\u27 »ring-current« intensities (as encountered, for example, in the central rings of peropyrene (I) and perylene (II), respectively) to intuitive Valence-Bond Resonance- Theory (VB-RT) ideas about \u27bond fixation\u27. In the present treatment, this aim is not achieved by devising a »ring-current« formalism that is itself actually based on a VB-RT wave-function: the philosophy adopted here is (a) to identify, and classify, those occasions on which simple Molecular-Orbital (MO) theory predicts specific rings in conjugated hydrocarbons to have exceptionally high and low »ring-currents«, and then (b) to use topological arguments that rely simply on the carbon-atom connectivity of the conjugated system in question to show that the situations identified in (a), above, are just those in which the absence, or presence, of VB-RT \u27bond-fixation\u27 is to be expected