Other aromatic hydrocarbons and heterocyclic compounds

Isocyclic compounds. For all aromatic molecules there is resonance between different electron configurations, the number of which becomes rapidly greater for larger molecules. In particular the number of possible so-called excited configurations increases, in which, analogous to the Dewar structures in benzene, one, two or more formal bonds are present. 

For naphthalene there are three unexcited configurations of practically equal energy, one symmetrical (Erlenmeyer) and two asymmetrical (Erdmann), which correspond to the Kekule configurations. 

In addition there are also sixteen singly excited configurations and also nineteen doubly and four trebly excited configurations to a total of 42 Lewis-Langmuir configurations. 

The three first-mentioned configurations contribute (see p. 268) all (respectively 39.4 %, 30.3 % and 30.3 % ) to the stationary state, so that the ground has disappeared for a discussion on which formula is preferable. 

For anthracene and phenanthrene four and five unexcited configurations, respectively, can be written down. 

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