Structural and Dynamic Properties of Solvated Hydroxide and Hydronium Ions in Water from Ab Initio Modeling

TL;DR

This study uses ab initio molecular dynamics with the SCAN functional to accurately model the structure and dynamics of solvated hydroxide and hydronium ions in water, revealing their distinct solvation and proton transfer behaviors.

Contribution

It demonstrates that the SCAN functional reliably describes water ions' properties, offering a computationally efficient alternative to more expensive methods.

Findings

Hydroxide stabilizes a pot-like solvation structure with four or five hydrogen bonds.

Hydronium donates three hydrogen bonds and prefers concerted proton transfer.

Hydroxide diffuses slower than hydronium, aligning with experimental observations.

Abstract

Predicting the asymmetric structure and dynamics of solvated hydroxide and hydronium in water has been a challenging task from ab initio molecular dynamics (AIMD). The difficulty mainly comes from a lack of accurate and efficient exchange-correlation functional in elucidating the amphiphilic nature and the ubiquitous proton transfer behaviors of the two ions. By adopting the strongly-constrained and appropriately normed (SCAN) meta-GGA functional in AIMD simulations, we systematically examine the amphiphilic properties, the solvation structures, the electronic structures, and the dynamic properties of the two water ions. In particular, we compare these results to those predicted by the PBE0-TS functional, which is an accurate yet computationally more expensive exchange-correlation functional. We demonstrate that the general-purpose SCAN functional provides a reliable choice in describing the two water ions. Specifically, in the SCAN picture of water ions, the appearance of the fourth and fifth hydrogen bonds near hydroxide stabilizes the pot-like shape solvation structure and suppresses the structural diffusion, while the hydronium stably donates three hydrogen bonds to its neighbors. As a result, hydroxide prefers the stepwise proton transfer (PT) while hydronium prefers the concerted PT behavior. Consequently, hydroxide diffuses slower than hydronium in water, which is consistent with the experiments.