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Relationships between third-order reactivity indicators in the closed system [N, v(r)], open system [μ, v(r)], and density [ρ(r)] pictures are derived. Our method of derivation unifies and extends known results. Among the relationships is a link between the third-order response of the energy to changes in the density and the quadratic response of the density to changes in external potential. This provides a link between hyperpolarizability and the system’s sensitivity to changes in electron density. The dual descriptor is a unifying feature of many of the formulas we derive. [ABSTRACT FROM AUTHOR]

Abstract: Guided by experimentally observed solvolysis rates, 66 different reactivity indicators for predicting the quality of organic leaving groups are tested. None of these indicators is completely satisfactory. Most of the proposed indicators work well when the leaving group is a halogen, but poorly in the other cases (carboxylates, sulphates, phosphinates). This suggests that it is very difficult to devise simple reactivity indicators for the intrinsic quality of a leaving group, and supports recent work on molecule-dependent models for nucleofugality. [Copyright &y& Elsevier]