Bond resonance energy , definition

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

More recently, Mo and Wu [51] used this type of description for benzene and cyclobutadiene as well. They also optimised the geometries of these molecules and of the ones with localised bonds. In their calculation of resonance- and stabilisation energies, they took another set of structures for the description of the cyclohexatriene, which leads again to a different definition of the resonance energy. 

Our working definition of aromatic compounds has included cyclic compounds containing conjugated double bonds with unusually large resonance energies. At this point we can be more specific about the properties that are required for a compound (or an ion) to be aromatic. 

The classical definition of resonance energy obtained from a reference structure containing carbon-carbon isolated double bonds with ir-electron energy of ethylene [Streitweiser, 1961] ... 

A definition of resonance energy which relates jr-energies to the Pauling - bond order 7t by the following expression ... 

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