ABSTRACT
Kekulé valence structures have been used in organic chemistry ^ since 1865, when Kekulé proposed a hexagonal structure (see Figure 1) for ben zene. The continued interest in Kekulé structures" is related to their use in simple classical pictures of delocalized chemical bonding"^ " pictures that carry over to modem quantum-chemical theories of benzenoid hydrocar bons." ’^ ^
Kekulé valence stmctures are the basis of several variants of valence bond (VB) resonance-theoretical models^^"^ such as the Pauling-W heland m o d e l , t h e Simpson-Hemdon model,^®"^ ^ and the conjugated-circuit modeF® (see the next chapter). These models have been shown to be useful for predicting the properties of conjugated molecules""^’^ *^^ ’^^ ^" in spite of some rather strong criticisms.
