  Effects of presence of ions, at moderate to high concentrations, on dynamical
properties of water molecules are investigated through classical molecular
dynamics simulations using two well known non-polarizable water models.
Simulations reveal that the presence of magnesium chloride (MgCl$_2$) induces
perturbations in the hydrogen bond network of water leading to the formation of
bulk-like domains with \textquoteleft defect sites\textquoteright~on boundaries
of such domains: water molecules at such defect sites have less number of
hydrogen bonds than those in bulk water. Reorientational autocorrelation
functions for dipole vectors of such defect water molecules are computed at
different concentrations of ions and compared with system of pure water.
Earlier experimental and simulation studies indicate significant differences in
reorientational dynamics for water molecules in the first hydration shell of
many dissolved ions. Results of this study suggest that defect water molecules,
which are beyond the first hydration shells of ions, also experience
significant slowing down of reorientation times as a function of concentration
in the case of MgCl$_2$. However, addition of cesium chloride(CsCl) to water
does not perturb the hydrogen bond network of water significantly even at
higher concentrations. This difference in behavior between MgCl$_2$ and CsCl is
consistent with the well-known Hofmeister series.
