Empirical Values of Resonance Energies

Empirical Values of Resonance Energies.—The tables of bond energies permit the calculation of values of the heats of formation of molecules to which a single valence-bond structure can be assigned that agree with the experimental values to within a few kcal/mole. On carrying out a similar calculation for a resonating molecule on the 

The difference between the observed heat of formation and that calculated for a single valence-bond structure for a molecule with use of the table of bond energies is an empirical value of the resonance energy of the molecule relative to the assumed valence-bond structure. 

It must be remembered that one of the conditions for resonance of molecules among several electronic structures is that the configuration of the molecule (the arrangement of the nuclei) remain constant during the electronic resonance it is the composite electronic structure that provides a single potential function determining the equilibrium configuration and modes of oscillation for the molecule. It is not possible for an amide to resonate between the structures 

The heat of formation of the benzene gas molecule from separated atoms is found from the heat of combustion (789.2 kcal/mole) and the heats of formation of the products of combustion, water and carbon dioxide, to have the value 1323 kcal/mole. The sum of the bond energies 6C—H + 3C—C + 30=C gives the value 1286 kcal/mole for the heat of formation of a hypothetical molecule with the Kekul6 struc- 

The value observed for the heat of hydrogenation of benzene is much less than this  

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