[Paper Review] Why does hydronium diffuse faster than hydroxide in liquid water?

Mohan Chen, Lixin Zheng+6 more|arXiv (Cornell University)|Jan 24, 2018

Spectroscopy and Quantum Chemical StudiesPhysics and Astronomy58 references86 citations

TL;DR

The study uses advanced density functional theory-based molecular dynamics to show that hydronium diffuses faster than hydroxide due to concerted proton transfer in hydronium versus stepwise transfer in hydroxide, which is stabilized by a non-planar hyper-coordinated solvation structure.

ABSTRACT

Proton transfer via hydronium and hydroxide ions in water is ubiquitous. It underlies acid-base chemistry, certain enzyme reactions, and even infection by the flu. Despite two-centuries of investigation, the mechanism underlying why hydronium diffuses faster than hydroxide in water is still not well understood. Herein, we employ state of the art Density Functional Theory based molecular dynamics, with corrections for nonlocal van der Waals interactions, and self-interaction in the electronic ground state, to model water and the hydrated water ions. At this level of theory, structural diffusion of hydronium preserves the previously recognized concerted behavior. However, by contrast, proton transfer via hydroxide is dominated by stepwise events, arising from a stabilized hyper-coordination solvation structure that discourages proton transfer. Specifically, the latter exhibits non-planar geometry, which agrees with neutron scattering results. Asymmetry in the temporal correlation of proton transfer enables hydronium to diffuse faster than hydroxide.

Motivation & Objective

• Motivate understanding of differing diffusion rates of hydronium and hydroxide in water.
• Investigate proton transfer mechanisms for both ions with accurate electronic structure methods.
• Elucidate how solvation structure influences proton transfer dynamics.
• Connect findings to neutron scattering data and existing structural diffusion concepts.

Proposed method

• Perform state-of-the-art density functional theory based molecular dynamics.
• Include nonlocal van der Waals corrections and self-interaction corrections.
• Model water and hydrated hydronium/hydroxide ions to capture electronic structure effects.
• Analyze concerted versus stepwise proton transfer mechanisms.
• Relate transfer dynamics to observed solvation geometries and asymmetries.

Experimental results

Research questions

• RQ1 Why does hydronium diffuse faster than hydroxide in liquid water?
• RQ2 What proton-transfer mechanisms dominate hydronium and hydroxide transport under accurate electronic structure modeling?
• RQ3 How do solvation structures influence the rate and pathway of proton transfer for each ion?

Key findings

• Hydronium undergoes concerted proton transfer contributing to faster diffusion.
• Hydroxide transfer is dominated by stepwise events due to a stabilized hyper-coordinated solvation structure.
• Hydroxide’s hyper-coordination exhibits non-planar geometry, aligning with neutron scattering results.
• Asymmetry in temporal correlation of proton transfer favors hydronium diffusion.
• Nonlocal van der Waals and self-interaction corrections are essential for accurate mechanistic description.