Relation between Electronic Structure and Chemical Reactivity of Organic Molecules

This paper gives a brief history of the establishment of relations between electronic structure and chemical reactivity of molecules obtained by using quantum mechanical techniques. After a short introduction devoted to the qualitative period a more important part of the paper is concerned with the discussion of the methods which permit some estimations of the constant rates. Both static and dynamic indices are described. The effect of the solvent is taken into account. Various applications are given showing how it has become possible to predict theoretically new kinds of reactivity or even of reactions which have been really discovered later. The role of electronic computers is stressed. It is stated that quantum chemistry will become more and more important in the near future in the field of biochemistry, chemical industries and in particular pharmacology. THE QUALITATIVE PERIOD 'Ti-w study of the structure of molecules was originally carried on by chemists using methods of investigation which were essentially chemical in nature, relating to the chemical composition of substances', 'the nature of the chemical reactions in which a substance takes part1'. Following the discovery of the electron many attempts were made to build an electronic theory of the chemical bond and the famous paper of Lewis2 remains the basis of modern theories relating structure and chemical reactivity. As is well known, another important concept, that of resonance, also had an important role in the development of the theory. Two different groups of organic chemists were responsible for the first important step in that direction. On the one hand, Arndt3 and collaborators developed the theory of intermediate stages. On the other hand, Robinson4 and Ingold5 built the theory of mesomerism. 'The significant feature which these theories had in common was that they considered it possible for the true state of a molecule not to be identical with that represented by any single classical valence bond structure, but to be intermediate between those represented by two or more different valence bond structures6'.