Graph theory and molecular orbitals. VII. The role of resonance structures

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Abstract
The relations between the simplest variants of MO and VB theory are discussed. It is shown that there is a unique principle causing all the cases of congruity between these two theories‐Kekulé structures being related to the permutations contained in the molecular graph [Eqs. (6) and (7)]. The class of benzenoid hydrocarbons where both theories are substantially equivalent is rigorously defined using graph theory. A number of topological regularities for these hydrocarbons are proved. Thus, the Dewar‐Longuet‐Higgins equation, the proof of the Ruedenberg's and Pauling's bond orders, the relation between the VB and MO spin and charge density, and Heilbronner's formula are obtained. The limits of validity for all these results are strictly determined.

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