Structure and dynamics of hydrated NH4+:: An ab initio QM/MM molecular dynamics simulation

A combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation has been performed to investigate solvation structure and dynamics of NH4+ in water. The most interesting region, the sphere includes an ammonium ion and its first hydration shell, was treated at the Hartree-Fock level using DZV basis set, while the rest of the system was described by classical pair potentials. On the basis of detailed QM/MM simulation results, the solvation structure of NH4+ is rather flexible, in which many water molecules are cooperatively involved in the solvation shell of the ion. Of particular interest, the QM/MM results show fast translation and rotation of NH4+ in water. This phenomenon has resulted from multiple coordination, which drives the NH4+ to translate and rotate quite freely within its surrounding water molecules. In addition, a "structure-breaking" behavior of the NH4+ is well reflected by the detailed analysis on the water exchange process and the mean residence times of water molecules surrounding the ion. (c) 2005 Wiley Periodicals, Inc.