Role of Charge Transfer in the Structure and Dynamics of the Hydrated Proton

Although it has long been recognized that multiple water molecules strongly associate with an extra proton in bulk water, some models and conceptual frameworks continue to utilize the classical hydronium ion (H3O+) as a fundamental building block. In this work, the nature of the hydronium ion in aqueous systems is examined using an ab initio energy decomposition analysis (EDA) that evaluates both the magnitude of and energetic stabilization due to charge transfer among H3O+ and the surrounding water molecules. The EDA is performed on structures extracted from dynamical bulk-phase simulations and used to determine how frequently the pure hydronium ion, where the excess charge is primarily localized on H3O+, occurs under dynamic conditions. The answer is essentially never. The energetic stabilization of H3O+ due to charge delocalization to neighboring water molecules is found to be much larger (16-49 kcal/mol) than for other ions (even Li+) and to constitute a substantial portion (20-52%) of the complex's binding energy. The charge defect is also shown to have intrinsic dynamical asymmetry and to display dynamical signatures that can be related to features appearing in IR spectra.