Pauling-Wheland resonance energies

The number of Kekule stmctures (Kekule structure count, KSC) of a conjugated hydrocarbon gives information on the thermodynamic stability and chemical reactivity and is used for the computation of various structural indices for ben-zenoid hydrocarbons " the Pauling bond order, PBO the stability index SI, SI = KSC the r-electron energy, Kekul6 structures are considered in valence-bond resonance-theoretical models such as the Pauling-Wheland model, the... 

Pauling, L. Wheland, G.W. The Nature of the Chemical Bond. V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals J. Chem. Phys. 1933, 1, 362-374 Errata, ibid, 1934, 2, 482. 

In the hands of Pauling this treatment has been successful in correlating many facts in terms of a number of possible structural i ormulae, and it has been widely developed by Wheland and many others. Sometimes the valence bond structures betv een which resonance is considered to take place are natural and plausible, such as the two Kekule forms of benzene on other occasions it is tiecessary to postulate less plausible structures of liigh energy such as the ionic forms of carbon dioxide see Long i ). The... 

Pauling, L. and Wheland, G.W. (1933) The nature of the chemical bond. V. The quantum-mechanical calculation of the resonance energy of benzene and naphthalene and hydrocarbon free radicals. 

Pauling and Wheland did not hesitate to make bold simplifications and approximations as long as this enabled them to extend the scope of the theory. Simultaneously, this quantum mechanical treatment was complemented and supported by an extensive amount of empirical information on resonance energies. The interplay between theoretical and empirical considerations had become crucial for the development and extension of a program that, in order to be of any help to chemists, had to show its usefulness in dealing with a great variety of molecules, from the simplest to the more complex molecules of organic chemistry. 

Pauling and Wheland used perturbation theory to calculate the wave function representing the normal state of the benzene molecule and to find an expression for its resonance energy. They assumed that the wave function could be written as a linear combination of five wave functions that represented the five independent canonical structures contributing to the normal state of the molecule. The principal contributions were made by the two Kekul structures and the rest by the three different forms assumed by the Dewar structure. But it was clear that "in a sense all structures based... 

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