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■ On the Aromatic Stability of Azulenofurans, Azulenopyrroles and Azulenothiophenes

Sonja Nikolic, Albin Juric, and Nenad Trinajstic

*Department of Chemistry, University of Missouri-Kansas City, Kansas City, Missouri 64110, U.S.A.

Abstract

The aromatic stabilities of isomeric azulenofurans, azulenopyrroles and azulenothiophenes have been studied by three theoretical models of aromaticity: the graphical model, the conjugated circuits model and the topological resonance energy model. All three models predict that molecules with the delocalized underlying carbon structure are more stable than those with the quinoid structure. In each class of positional isomers, azulenopyrroles are always predicted to be more stable than the correspending isomeric azulenothiophenes, and these more stable than the corresponding azulenofurans. However, all azulenofurans, azulenopyrroles and azulenothiophenes are predicted to be less aromatic than the parent hydrocarbon 2,3-benzazulene. It is suggested that the differences in stability between azulenofurans, azulenopyrroles and azulenothiophenes is related to the differences in stability between the heterocyclic parts of the molecules, i.e. furan, pyrrole and thiophene, since the carbon parts are identical.