Frontier Configurations and a New Classification of Annulenes

Abstract

Organic “diradicals” are important for the synthetic chemist who wants to exploit them as precursors of target molecules, for the mechanistic chemist who seeks to unravel reaction pathways, for the quantitative theoretician who is anxious to test different computational schemes on such molecules because of the formalistic intricacies involved, and for the qualitative theoretician who seeks to understand how these species are bound. Specialists of the latter two types most often adhere to MO theory and they discuss the electronic properties of “diradicals” in the following way: They depart from Hückel MO theory and point out why neglect of interelectronic repulsion renders it inapplicable to problems involving “diradicals”. Then, the discussion shifts to the SCF-MO level and various formal drawbacks and resulting pitfalls are recognized. Finally, one is forced to examine the problem at the SCF-MO-CI level, something which guarantees that the potential audience of the paper is exponentially reduced and that the ensuing discussion is rendered cumbersome and lengthy. In a recent work,¹ we advanced the argument that qualitative Valence Bond theory has the formal correctness and conceptual clarity which can allow one to dispense with problems which are hard to deal with within the MO theoretical framework in the space of a paragraph or two. In particular, in treating homonuclear systems involving relatively weak pi bonds, one can use the Approximate Heitler-London (AHL) theory outlined in the original monograph.