Simple Models of Charge-Transfer Reactivity

Finite-difference expressions for the chemical potential (negative electronegativity) and hardness (inverse softness) descriptors of molecular and reactive donor-acceptor systems are summarized and chemically “biased” (informed) and “unbiased” (uninformed) estimates of charge-transfer (CT) descriptors in A(acid)B(base)¬ systems are reexamined. The former recognizes the chemical characteristics of reactants and the chemical-potential discontinuity, while in the latter no prior knowledge of such kind is used. The biased chemical potential and fragment hardness descriptors are interpreted in terms of the frontier-electron orbitals, and equivalence of predictions in both treatments is demonstrated using the electronegativity-equalization principle. Two-state description of CT involves a statistical mixture of initial state NCT = 0 = A0, B0 of the polarized (mutually closed) reactants in R+ = (A+B+), and one of admissible final states for the full electron transfer, NCT = 1, in the forward B0A0 or reverse A0B0 directions, leading to ion-pairs B0A0 = NCT = 1 = A1, B+1 and A0B0 = NCT = 1 = A+1, B1. Parabolic interpolation between energies of the integral-N states identifies the process activation and reaction energies, predicts the equilibrium amount of CT and stabilization energy it generates