Strengthening the foundations of chemical reactivity theory

Abstract

Chemical reactivity theory, in its evolution from its earliest empirical beginnings to its current level of sophistication has had an immense impact on chemistry. In work of Parr and collaborators in the 1980s, the theory was grounded in a rigorous formulation of many-body theory and density functional theory, and its richness and promise were made manifest in contemporaneous work by Nalewajski and collaborators. Nevertheless, many fundamental issues remained unresolved, and the need to strengthen the foundations of reactivity theory and broaden its applicability remained. In the present chapter, the early work of Parr, Nalewajski and their coworkers is reviewed and the unresolved issues defined and enumerated. Recent work by the author and his collaborators addressing the resolution of these issues is reviewed, including both published and previously unpublished results. The principal advances discussed include explicit Kohn-Sham expressions for reactivities, the distinction between localized and extended chemical systems and the associated limitation of the ensemble formulation of density-functional theory, the distinction between isoelectronic and electron-transfer reactivities, the need for both static and dynamic reactivities, the need for nonlocal reactivity kernels, the placement of nuclear and electronic reactivities on the same footing, the identification of chemical stimuli in addition to chemical responses, and the establishment of a relationship between the total energy and the reactivities and, ultimately, the reaction pathway. While many elements of the theory have been completed, the proper definition of electron-transfer reactivities for localized systems and the relation between total energy and reactivities for overlapping reactants have not yet been found. How this might be done is briefly discussed, and it is suggested that successfully accomplishing it would complete the strengthening of the foundations of reactivity theory attempted by the author and collaborators.