ABSTRACT
I. THE CONCEPT OF CONJUGATED CIRCUITS A graph-theoretical analysis of Kekulé valence structures produced the
concept of conjugated circuits.^ This concept may be in tro d u c e d ,fo r ex ample, by using the notion of cyclic path.^^ Imagine a Kekulé structure of some polycyclic conjugated hydrocarbon. Start at any carbon atom on the Kekulé structure and traverse any path through bonds, returning to the starting position. If the cyclic path consists of alternating single and double bonds, this path is defined to be a conjugated circuit; otherwise it is not. In other words, the conjugated circuits are those circuits within the individual Kekulé structure in which there is a regular alternation of formal CC single and double bonds. Thence, the conjugated circuits are necessarily of even length.
