Structure and dynamics of hydrated NH  4+ : 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 NH  4+ 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 NH  4+ 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 NH  4+ in water. This phenomenon has resulted from multiple coordination, which drives the NH  4+ to translate and rotate quite freely within its surrounding water molecules. In addition, a “structure‐breaking” behavior of the NH  4+ is well reflected by the detailed analysis on the water exchange process and the mean residence times of water molecules surrounding the ion. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1329–1338, 2005

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