A Conceptual Density Functional Theory-Based Maximum Nucleophilicity Principle

We introduce a variational formulation of nucleophilicity within the framework of conceptual density functional theory (CDFT), thus proposing a maximum nucleophilicity principle (MNP), in formal analogy to the established minimum electrophilicity principle (MEP). Starting from a third-order Taylor expansion of the electronic energy with respect to the number of electrons at constant external potential, we extend our recently proposed global nucleophilicity index N_G derived from the chemical potential μ, hardness η, and hyperhardness γ. We demonstrate that this index follows a well-defined variational condition for electron-loss processes. An explicit functional differential expression is derived for N_G[N, v(r)], enabling a thermodynamically consistent description of nucleophilic behavior. The principle is evaluated across a set of six benchmark reactions. The results confirm the concurrent validity of MNP and MEP as complementary conceptual tools for understanding electron transfer in chemical reactivity.