Experimental Determination of Resonance Energies

Experimentally it is possible to obtain a value for the energy of a molecule (the heat of formation), but calculation of the energy of a non-resonating, reference structure introduces a number of complications. 

An alternative, and widely used method of calculating resonance energies is from bond energies. By taking a standard set of bond energies the heat of formation of the non-resonating reference structure (i.e. one of the resonance structures) can be calculated, and compared with that of the actual molecule. The resonance energy of benzene, En would be calculated from the relationship 

More important than the need to define the reference state, is an appreciation that the resonance energies 6alculated by either method include not only delocalization energies, but also energy terms 

A treatment of compressional energy for the benzene case was given by Coulson and Altman in 1952, and this is summarized diagrammatically in Fig. 3. 

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