Valence bond theory associated with resonance

Both cis and trans conformers for ONNO are known , with cis more stable than tram. The additional stability of the cis eonformer is associated with cis 0-0 overlap in valence-bond theory, this occurs primarily via the oxygen 2p7t -orbitals (Fig. 1-5 cf. N2O4 in Section 7-3). The stabilization energy for covalent-ionic resonance of the type (57) (58) is cis 0-0 overlap dependent (Section... 

The contributions of Erich Hiickel to the development of molecular orbital theory have already been mentioned in the subsection on Germany (Section 5.4.1) the development of semi-empirical quantum mechanical treatments in organic chemistry by M. J. S. Dewar has been discussed in Section 5.5. In the early development of the application of quantum mechanics to chemistry, Linus Pauling (1901-1994)359 was pre-eminent. He was associated with CalTech for most of his career. His work before World War II generated two influential books the Introduction to Quantum Mechanics (with E. Bright Wilson, 1935)360 and The Nature of the Chemical Bond (1939).361 He favoured the valence-bond treatment and the theory of resonance. 

The concept of a sea of electrons not belonging to any particular atom is reminiscent of the resonance structures covered earlier. The valence electrons in a metal are delocalized just as they are in resonance molecules. The mobile electrons in a bar of sodium are not associated with any particular ion core, just as the electrons in the double bonds of benzene are not associated with any particular atom. To explain this phenomenon in metals, one must apply molecular orbital theory. 

This section deals with analytical properties of the Hiickel molecular orbital theory and the associated isolated molecule method of predicting the active positions in a conjugated molecule. We shall deal with polarizability coefficients defined as certain partial derivatives with respect to the coulomb and resonance integrals described in Section III. The important derivatives are those relating to the total tt electron energy and to the charges q, fi ee valences and bond orders... 

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