Science Abstract

Abstract

Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H₃O ⁺ and OH^- ions. Using the method of ab initio molecular dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liquid. In our ab initio molecular dynamics approach, the interatomic forces are calculated each time step from the instantaneous electronic structure using density functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H₃O ⁺ ion, we find a dynamic solvation complex, which continuously fluctuates between a H₅O₂⁺ and a H₉O₄⁺ structure as a result of proton transfer. The OH^- has a predominantly planar fourfold coordination forming a H₉O₅^- complex. Occaisionally this complex is transformed into a more open tetrahedral H₇O₄^- structure. Proton transfer is observed only for the more waterlike H₇O₄^- complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure -- and not the constituent particles -- of the charged complex migrates through the hydrogen bond network. For H₃O ⁺, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate-limiting step for OH^- diffusion is the formation of the H₇O₄^- structure, which is the solvation state showing proton transfer activity.