Proton transfer via hydronium and hydroxide ions in water is ubiquitous. Despite two centuries of investigation, the mechanism underlying why hydroxide diffuses slower than hydronium in water is still not well understood. We employ state-of-the-art density-functional-theory-based molecular dynamics—with corrections for non-local van der Waals interactions, and self-interaction in the electronic ground state—to model water and hydrated water ions. At this level of theory, structural diffusion of hydronium preserves the previously recognized concerted behaviour. However, by contrast, proton transfer via hydroxide is less temporally correlated, due to a stabilized hypercoordination solvation structure that discourages proton transfer. Asymmetry in the temporal correlation of proton transfer leads to hydroxide diffusing slower than hydronium.

References:

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• “Systematic study of chloride ion solvation in water using van der Waals inclusive hybrid density functional theory”. A. Bankura, B. Santra, R. A. DiStasio Jr., C. W. Swartz, M. L. Klein, and X. Wu. Mol. Phys. 113, 2842 (2015)