Electrostatics, Charge Transfer, and the Nature of the Halide-Water Hydrogen Bond

Binary halide–water complexes X^–(H₂O) are examined by means of symmetry-adapted perturbation theory, using charge-constrained promolecular reference densities to extract a meaningful charge-transfer component from the induction energy. As is known, the X^–(H₂O) potential energy surface (for X = F, Cl, Br, or I) is characterized by symmetric left and right hydrogen bonds separated by a C_2v-symmetric saddle point, with a tunneling barrier height that is < 2 kcal/mol except in the case of F^–(H₂O). Our analysis demonstrates that the charge-transfer energy is correspondingly small (< 2 kcal/mol except for X = F), considerably smaller than the electrostatic interaction energy. Nevertheless, charge transfer plays a crucial role determining the conformational preferences of X^–(H₂O) and provides a driving force for the formation of quasi-linear X^...H–O hydrogen bonds. Charge-transfer energies correlate well with measured O–H vibrational redshifts for both halide–water complexes as well as OH^–(H₂O) and NO₂^–(H₂O), providing some indication of a general mechanism.