Jump Rotational Mechanism of Ammonium Ion in Aqueous Solutions

The dynamic behavior of the ammonium ion is closely related to the biological and chemical processes of life. A fast rotation of NH4+ in aqueous solution has been observed in previous experiments, which is unexpected from hydrodynamic theories because of the multiple strong hydrogen bonds (HBs) between ammonium ion and water. The mechanism behind this rotation is still not well understood. The simulations in this work show that a sudden and large-magnitude angular jump rotation occurs during the hydrogen bond exchange processes of the ammonium ion like water. The rotation of the ammonium ion can be approximately described with the extended jump model, and can be decomposed into two independent contributions: the jump rotation and the diffusive rotation of the HB frame. The rotational mobility of the ammonium ion is determined by fast jump rotation compared with the slow diffusive rotation. In addition, the contribution of the jump rotation increases with increasing NH4+ concentration. Compared with water, NH4+ prefers to exchange its HB between two water molecules without forming a HB each other.