Ab initio simulation of rotational dynamics of solvated ammonium ion in water

We have performed an ab initio molecular dynamics simulation of the rotatio nal dynamics of NH4+ ion in water. This work was motivated by the experimen tal evidence that the solvated NH4+ rotates rather fast, despite the expect ed formation of strong hydrogen bonds with water. We find that NH4+ is on a verage coordinated with five water molecules. Four water molecules form a l ong-lived tetrahedral cage around the ion, each molecule being hydrogen-bon ded with one proton of NH4+. The fifth water molecule is much more mobile a nd occasionally exchanges with one of the four molecules in the tetrahedral cage. The hydrogen bonding of NH4+ with water is strong enough to prevent the free rotation of the ion, which instead tumbles in a sequence of discon tinuous rotational jumps associated with the exchange of two water molecule s in the tetrahedral cage. The simulated rotational dynamics is consistent with nuclear magnetic resonance data and encourages the use of ab initio si mulations to describe the solvation of ions in water.