``Ab initio'' Molecular Dynamics Simulation of the Solvation and Transport of H(3)O+ and OH- Ions in Water

Copyright © (1995) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

Charge defects in water created by excess or missing protons appear in the form of solvated hydronium \tho \ and hydroxyl \oh \ ions. Using the method of {\it 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. For the \tho \ ion we find a dynamic solvation complex, which continuously fluctuates between an \phdo \ and an \nhqo \ structure as a result of proton transfer. The \oh \ has a predominantly planar fourfold coordination forming a \sho \ complex. Occasionally this complex is transformed in a more open tetrahedral \iho \ structure. Proton transfer is observed only for the more water-like \iho \ 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 \tho , 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 \iho \ structure, which is the solvation state showing proton transfer activity.

By: Mark Tuckerman (Univ. of PA), K. Laasonen (Univ. of PA), M. Sprik and M. Parrinello (Max-Planck Inst., Germany)

Published in: Journal of Chemical Physics, volume 103, (no 1), pages 150-61 in 1995

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