Hydration of NH$_4^+$ in Water: Bifurcated Hydrogen Bonding Structures and Fast Rotational Dynamics

TL;DR

This study uses advanced simulations to reveal that ammonium ions in water have bifurcated hydrogen bonds that facilitate rapid rotation, providing insights into ion hydration and dynamics at the molecular level.

Contribution

It introduces a detailed molecular-level understanding of NH$_4^+$ hydration and rotational mechanisms using ab initio simulations and quantum calculations, highlighting the role of bifurcated hydrogen bonds.

Findings

NH$_4^+$ hydration involves bifurcated hydrogen bonds.

Rotational switching occurs via simultaneous bifurcated bond exchange.

Results align with experimental measurements of fast ion rotation.

Abstract

Understanding the hydration and diffusion of ions in water at the molecular level is a topic of widespread importance. The ammonium ion (NH$_4^+$) is an exemplar system that has received attention for decades because of its complex hydration structure and relevance in industry. Here we report a study of the hydration and the rotational diffusion of NH$_4^+$ in water using ab initio molecular dynamics simulations and quantum Monte Carlo calculations. We find that the hydration structure of NH$_4^+$ features bifurcated hydrogen bonds, which leads to a rotational mechanism involving the simultaneous switching of a pair of bifurcated hydrogen bonds. The proposed hydration structure and rotational mechanism are supported by existing experimental measurements, and they also help to rationalize the measured fast rotation of NH$_4^+$ in water. This study highlights how subtle changes in the electronic structure of hydrogen bonds impacts the hydration structure, which consequently affects the dynamics of ions and molecules in hydrogen bonded systems.