A note on the enumeration of Kekulé structures in a class of coronoids (original) (raw)

An extension of the Gordon-Davison algorithm to the enumeration of Kekul6 valence structures in a class of simple coronoids is outlined. Introdllction The number of KekulC valence structures K for conjugated hydrocarbons is an important structural parameter [l-3]. In the early days of organic quantum chemistry, it was already recognized that K represents a measure of molecular stability (see, for example, Ref. 4). For example, anthracene and phenanthrene (with K = 4 and 5, respectively) illustrate qualitatively that molecules with greater K have greater aromatic stability. It has also been suggested that 1ogK is proportional to the molecular resonance energy (RE) [5]. The parameter K is of interest in many theoretical studies of conjugated systems, e.g. for calculation of Pauling bond orders [6,7] or for the conjugated circuit approach to molecular RE [8,9]. There are a number of methods available for counting KekulC structures [ 1,2, lo]. However, there is also continuing interest in simple schemes for finding K [l l-191. We are particularly interested in algorithms that have simplicity and