A hardness and softness theory of bond energies and chemical reactivity

Abstract

Publisher Summary To determine the path and the products of a given reaction the potential energy surface that is associated with the interacting species should be calculated to obtain the reaction coordinate that allows establishing the path followed by the reacting molecules to reach the transition state and the final products. Chemists have developed intuitive concepts and simple theories that have allowed them to understand the behavior of molecules under different circumstances, their reactive sites, and possible reaction mechanisms. The objective of the present chapter is to make use of density functional theory to develop a simple unified approach to bond energies, activation energies, and chemical reactivity, to show that the hardness and softness concepts provide fundamental information about the changes in energy associated with the interaction of chemical species. Basic aspects of density functional theory has been reviewed and it has been shown that within the framework provided by this theory, the fundamental equations for chemical events may be expressed in terms of several reactivity parameters. It has been shown that the total electronic energy may be expressed in terms of effective valence energy contribution and core contribution. The former depends mainly on the chemical potential and the hardness of the system, and accounts for the main energy changes that occur in a chemical interaction, while the latter depends mainly on the core densities of the interacting molecules, and it approximately cancels the nuclear–nuclear interaction energy. The energy expression has then been used to examine the principle of maximum hardness, and the principle of hard and soft acids and bases, and has been used to analyze bond, activation, and reaction energies in terms of hardness differences. Finally the implications of the present approach in the cases of catalyzed reactions and reactions in solution have been discussed in the chapter.