Kekule configurations

The correct amount of the resonance energy (R.E.) can hardly be deduced from experimental data, since the energy content of one separate Kekule configuration is not known (p. 198). However, we can derive fairly well the energy of cyclohexatriene, calculated on the basis of the bond energies (Table 18), that is to say, for a cyclic molecule with alternate bonds, which correspond to those in ethane and ethylene, thus... 

According to condition 2 (p. 197) the arrangement of the atoms in the KekuLE configuration must be the same as that in the stationary state in benzene, thus a plane structure with six equal C—C distances of 1.39 A. The most complete proof has now been furnished both by electron diffraction and by the interpretation of Raman and infra-red spectra that this plane regular hexagon structure is correct. 

If, however, with Pauling we neglect the difference in energy between these two different spatial structures then we can calculate for the total bond energy of one Kekule configuration (Table 18) ... 

If we attempt, in accordance with condition 3 (p. 200), to construct all possible Lewis-Langmuir (non-polar) configurations for benzene with electron pair bonds, it appears that there are five of them. Besides the two Kekule configurations there are also three Dewar configurations. 

For naphthalene there are three unexcited configurations of practically equal energy, one symmetrical (Erlenmeyer) and two asymmetrical (Erdmann), which correspond to the Kekule configurations. 

If the differences in the contributions are taken into account (p. 209), then the bond character for naphthalene through superposition of the Kekule configurations becomes ab 0.70 (0.63), be 0.30 (0.29) ai 0.30 (0.27), ij 0.39 (0.41) the bond character given between brackets is obtained if all 42 configurations with their respective weights are taken into account. (For other calculations see p. 271 and Table 24.)... 

B Valence bond method with only Kekule configurations (p. 245, Table 22a). 

In triphenylmethane with only the diverse Kekule configurations there is no resonance other than that in benzene itself. 

In triphenylmethyl chloride, besides the Kekule configurations, there are also nine configurations with the positive charge (sextet) at the ortho and para positions of the three phenyl groups. 

In benzene it is found by superposition of the five canonical configurations mentioned that each bond has a double bond character of 0.389 + 0.073 = 0.462. In view of the single o-bond always present one also speaks of total bond character equal to 1 -j- the double bond character. Though this bond character ( indice de liaison double , Daudel and Pullman) from the calculations based on the Valence Bond method is different from the bond order (Penney, Coulson), e.g. in the Molecular Orbital Method, both terms are frequently used without discrimination. In Pauling s earlier very simple concept (p. 210) the bond character of benzene is just 0.5 since there only the Kekule configurations are taken into account. 

Each carbon atom has residual angular momentum which can be quenched in a Kekule configuration, provided that all atoms are in the xy-plane, as shown in figure 9. 

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