Graph-Theoretical Models of Complex Reaction Mechanisms and their Elementary Steps

Abstract

Chemical reactivity, which can be viewed as the capability of chemical species of any kind to undergo chemical transformations, has always been a key problem in theoretical chemistry. Any progress in understanding reactivity not only enriches chemical knowledge but also has important practical implications. Numerous methods have been developed to assess reactivity quantitatively, and it is not the aim of this chapter to review all of them. Yet, some basic approaches are to be mentioned. These include first of all various quantum chemical concepts and results, such as the classical work of Dewar and Simonetta,1,2 the frontier orbital theory of Fukui et al.,3–5 orbital symmetry principles,6 the isolobal concept of Hoffmann,7 and molecular hardness (softness) concept of Parr and Pearson.8–10 Correlation analysis has also contributed greatly to this area.11–14 The experimental reactivity measure most commonly used is the rate constant of the reaction in which the compound of interest is involved. Advances in contemporary experimental techniques made possible the precise direct measurements of the rate constants of chemical reactions, including elementary reactions of electron15,16 and proton17,18 transfer, and steps involving radicals,19 ions,20 and metal complexes.21–24