25th International Congress of Pure and Applied Chemistry. by Y. Marcus

By Y. Marcus

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More recently the orbital correlation diagrams of Woodward and Hoffmann13 have had an immense impact. These show the connection of the molecular orbitals of the reactant with those of the product for concerted reactions of symmetric molecules. Fifty years of quantum chemistry 43 Fig. 1. Correlation diagram for homonuclear diatonic molecules. Orbital energies are schematically plotted against internuclear separation, from the "united atom" to separated atoms. ' ' 90° ,80° Angle HAH Fig. 2. Walsh diagram for HAH molecules.

I am now speaking of properties depending on the electronic motion. There is an important point to make here. A number, whether coming from theory or experiment, is really useless if we have no idea of its accuracy. We have at present no practical, reliable method for calculating error bounds for quantum mechanical quantities (except upper bounds for total energy). The only validation for a given approximate method is the agreement with experi­ ment. This places greater emphasis on the one hand upon the availability of reliable and accurate experimental data and on the other hand on the application of the method to a large and varied group of substances.

It stressed the so-called valence bond approach and the concepts of resonance, partial ionic character, overlap of atomic orbitals, hybridization and the directional properties of orbitals and covalent bonds, electronegativity of atoms, constancy of empirical bond lengths and bond energies, the hydrogen bond, and a remarkable set of principles for the structure of complex ionic crystals. The main theme was the concept of resonance. It is an interesting paradox that in the subject index of The Nature of the Chemical Bond under orbital one does not find listed the term molecular orbital, although the concept is mentioned on one page.

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