On the Relationship. Between Localized Molecular Orbitals and Kekule Structures*

Localized molecular orbitals for n-systems of conjugated polycyclic hydrocarbons have been calculated using both intrinsic and external localization criteria relying only on molecular topology. For benzenoid aromatic compounds the intrinsic localized molecular orbital arrangement can always be associated to the Kekule structure of maximal Kekule index value. However hydrocarbons containing four-membered rings show some exceptions to this correspondence, though the localized orbital array always corresponds to the most important Kekule structure as predicted by the extended Fries rule. External localization sums show a good linear correlation with the Kekule index, though different slopes are found for benzenoid and cyclobutadiene-containing systems. The results obtained for the canonical valence bond structures of naphthalene suggest that the external localization sum could be a good alternative to the Kekule index.