Halogen versus halide electronic structure

Halide anions X^- are known to show a decreasing proton affinity (PA), as X descends in the periodic table along series F, Cl, Br and I. But it is also well-known that, along this series, the halogen atom X becomes less electronegative (or more electropositive). This corresponds to an increasing energy of the valence np atomic orbital (AO) which, somewhat contradictorily, suggests that the electron donor capability and thus the PA of the halides should increase along the series F, Cl, Br, I. To reconcile these contradictory observations, we have carried out a detailed theoretical analysis of the electronic structure and bonding capability of the halide anions X^- as well as the halogen radicals X^•, using the molecular orbital (MO) models contained in Kohn-Sham density functional theory (DFT, at SAOP/TZ2P as well as OLYP/TZ2P levels) and ab initio theory (at the HF/TZ2P level). We also resolve an apparent intrinsic contradiction in Hartree-Fock theory between orbital-energy and PA trends. The results of our analyses are of direct relevance for understanding elementary organic reactions such as nucleophilic substitution (S_N2) and base-induced elimination (E2) reactions.