Sigma bonding patterns

Two structures are in equilibrium if the structures are related by differing atomic connectivity (i.e., different sigma bonding pattern) and differing electron density distribution, as in the case of tautomers. 

Simple indices, which encode information on sigma bonded electrons that can be observed directly from bonding patterns in structural formulae of PAHs. 

It is easy to determine the hybridization of an atom given a Lewis structure. First, you count the number of pairs of free electrons and the number of sigma bonds (single bonds). Do not count double bonds, since they do not affect the hybridization of the atom. Once the total of these two is determined, the hybridization pattern is as follows ... 

As depicted in 5a and 5b, the sigma skeletal NBOs (three Gch) one acc, and one aco) are all formally doubly occupied, whereas the n NBOs of the C-C-0 moiety exhibit a characteristic keto-enol or allylic-like pattern. Table 7.4 shows that in-plane a and P spin NBOs are nearly identical (hence, perfectly paired), and comparison of Table 7.1 and Table 7.2 shows the high transferability of skeletal sigma bonds from parent closed-shell species. 

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