Llano Reference Structure

Figure 10. Resonance energies per Jt electron in units of electronvolts for the conjugated monocyclic hydrocarbons using the Dewar—de Llano reference structure with two bond types and the Pariser—Parr—Pople computational method.

Dewar and de Llano then used the additivity of eq 10 to define a polyene-like reference structure for computing the resonance energy of any conjugated hydrocarbon. For example, in the case of benzene, the reference is cyclohexatriene whose heat of atomization is given by eq 10 so that... 

In fact Dewar and de Llano concluded their paper that introduced the successful reference structure (column 3 of Figure 6) with there no longer seems any point at all in carrying out calculations by less refined procedures, in particular the HMO method or variants of it . They thus appear to attribute their success to the change in computational method from HMO to Pariser—Parr—Pople rather than to their important switch in reference structure. 

However, use of the HMO computational method together with a Dewar-like reference structure (column 4 of Figure 6) gives an even better result and shows that Dewar and de Llano s success was caused by their new reference structure, not by their use of the PPP instead of the HMO computational method. 

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