Graph theory and additivity in delocalized aromatic molecules

10 Graph theory and additivity in delocalized (aromatic) molecules 

In some early applications [96] to ETs involving compounds with quin-one-like compounds Q, QA Q -A Q=, it was necessary to examine some data on the formation constants of the semiquinone QH, Q + QH2 — 2QH (The H is typically attached to an O or an N.) My impression, after looking at available data, was that the equilibrium constant was approximately the same, provided all three species had the same charge. Looking at the structures, one could see that every atom in a molecule on the left in this reaction corresponded to one on the right that had the same nearest and next nearest neighbor. I then looked at many examples of other pairs of compounds, which I termed conformal pairs and found that the total of heat of combustion of a pair was approximately the same as its conformal pair [97]. 

Comment In chemistry it is nice to see generalizations from experiment and to find them in unexpected places. The approximate additivity of delocalized systems, described above, provides one such example. With new developments in electronic structure treatments, such as density functional methods, one might hope for a yet deeper insight. 

It is a real pleasure to dedicate this article to John Albery and to recognize his influential and joyous contributions to this interaction of experimentalists and theorists. It is a pleasure too to acknowledge the many contributions of my coworkers whose work is described in this chapter. This research was supported by the National Science Foundation and by the Office of Naval Research and I am very pleased to acknowledge that support. 

The following references are given as examples, rather than being comprehensive. 

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