Reactivity criteria in charge sensitivity analysis

Abstract

The charge sensitivity analysis, based upon the hardness/softness concepts and the chemical potential (electronegativity) equalization principle established within the density functional theory, is used as a diagnostic tool for probing trends in the chemical reactivity of large molecular systems. The new criteria are reviewed with special emphasis on two-reactant reactivity concepts, which explicitly take into account the interaction between reactants in a general donor-acceptor system, and the collective charge displacement coordinate systems that give the most compact description of the charge reorganization accompanying chemical reactions. The global collective populational reference frames discussed include populational normal modes, minimum energy coordinates, and the relaxational modes; the reactant reference frames include internal modes of reactants as well as externally decoupled and inter-reactant-coupling modes. The charge-coupling information in the atoms-in-molecules resolution is modelled by the hardness matrix, which provides the canonical input data for determining a series of chemically interesting probes for diagnosing reactivity trends. A survey of these concepts includes the global treatment of molecular systems and the reactant-resolved description of general reactive systems. The molecular-fragment development emphasizes the relaxational influence of one reactant upon another, reflected by the off-diagonal charge sensitivities that measure the responses of one reactant to charge displacements in the other. Both open (exchanging electrons with the reservoir) and closed (preserving the number of electrons) reactive systems are investigated. Stability criteria for the equilibrium charge distribution in such systems are summarized and their implications discussed. The ground-state mapping relations between geometrical (nuclear position) and electronic (atomic charge) degrees of freedom are related to the Gutmann rules of structural chemistry. Illustrative examples are given of the application of these reactivity concepts to model catalytic clusters of transition metal oxide surfaces and large adsorbates (toluene).