The Quantitative Treatment of Resonance in Aromatic Molecules

If there were only two orbitals and two electrons, as in the hydrogen molecule, the interaction energy would be just the resonance energy associated with the interchange of the two electrons between the two orbitals. This is the situation in ethylene the two pz electrons here convert the single bond into a double bond. Let us designate this pt resonance energy by the symbol a.15 

The resonance energy for the two Kekuld structures in benzene can be calculated in terms of a by neglecting all interactions except those 

14 These orbitals are given in a footnote of Sec. 4-4 for the orbital divided equally among the three bonds. In benzene it is probable that the strong C—C bonds, with interatomic distances smaller than the single-bond value, use more of the 8 orbital than does the H—C bond. 

18 In this discussion, contrary to the usual custom, a has been used to represent the magnitude of the resonance energy of two p, electrons, taken with positive sign. 

However, it is found on examination of the problem that consideration must also be given, in addition to the Kekul6 structures A and B, to the three structures C, D, and E of the Dewar type  

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