Calculated “London” π-electron magnetic properties of some “perturbed” annulenes, their dications and dianions

Abstract

The “periphery model” for planar polycyclic conjugated systems is tested by applying the method of Coulson et al. for calculating π-electron magnetic properties of such species to a selection of polycyclic neutral molecules, and their respective dianions and dications, all of which appear to have the attributes necessary for them to be considered “perturbed” annulenes. Because some of the species investigated have the potential to be strongly paramagnetic in their calculated π-electron magnetic properties, it was considered appropriate to base this application of the approach of Coulson et al. on a Pariser-Parr-Pople wavefunction that made resonance integrals iteratively self-consistent with respect to calculated bond orders. With one exception (which is discussed) these computations broadly support an analogy, in the context of conjugated systems that may be regarded as “perturbed annulenes”, of Sondheimer's observation that the dianions (and, by implication, the dications) of genuine (mono-cyclic) [4 n]-annulenes are diamagnetic, whereas doubly charged [4 n + 2]-annulenes are paramagnetic, in their predicted “London” (“ring current”) π-electron magnetic properties. These conclusions are not inconsistent with the available experimental data on 1H NMR chemical shifts; it is, however, pointed out that the latter are not necessarily directly comparable with the present calculations because, in the case of the non-alternant and/or multiply-charged species included in this study, other factors such as non-uniform π-electron charge densities are thought likely to make as significant a contribution to the experimentally observed 1H NMR chemical shifts in these systems as the “ring current” effects that are actually being considered here.