  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.
