On rotational dynamics of an NH+4 ion in water

The structural, thermodynamic, and dynamical properties of a single ammonium ion, NH4+, in liquid water were investigated using molecular dynamic simulation techniques. Many-body polarizable model potentials were employed to describe the molecular interactions. The water molecules were found to form well-defined hydration shells with a preferred orientation around the NH4+ ion as expected. We also found that the average water dipole moment is enhanced in the first hydration shell. The translational and rotational motion of NH4+ was examined via the velocity, angular-momentum, and reorientation-autocorrelation functions. The computed rotational diffusion coefficients of the NH4+ ion in water, which were determined from the angular momentum autocorrelation function and the angular mean-square displacement, are 0.093 and 0.067×1012 rad2/s, respectively. These results are in good agreement with the experimental nuclear magnetic resonance value of 0.075×1012 rad2/s.

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