S. Obst et H. Bradaczek, MOLECULAR-DYNAMICS STUDY OF THE STRUCTURE AND DYNAMICS OF THE HYDRATION SHELL OF ALKALINE AND ALKALINE-EARTH METAL-CATIONS, Journal of physical chemistry, 100(39), 1996, pp. 15677-15687
Molecular dynamics simulations of hydrated alkaline and alkaline-earth
metal cations at room temperature (T = 300 K) were carried out using
the CHARMM22 force field, Dynamic and static properties of systems con
taining one ion and 123 or 525 water molecules were investigated by an
alysis of trajectories of 1 ns duration and compared to experimental a
nd theoretical results. In addition, the size and the direction of the
elementary motions of both the ions and the water molecules were inve
stigated on the scale of the integration time step of 1 fs. Comparison
between systems of different size revealed that for the larger system
the diffusion coefficient and the number of hydrogen bonds were incre
ased. Radial pair distribution functions and coordination numbers are
in good agreement with X-ray and neutron scattering data. The diffusio
n coefficient D of bulk TIP3P water in a system with 528 water molecul
es was by one-fourth higher than the experimental value. Minor differe
nces of-approximately 10% between experimental and simulated diffusion
coefficients were found for Li+, Na+, K+, and Mg2+. On the other hand
, D was underestimated by the simulation for Ca2+ and Sr2+ by as much
as 30%. On the average, 2.9 hydrogen bonds per bulk water molecule wer
e found. The observed order of residence times for the monovalent ions
, tau(Li+) > tau(Na+) > tau(K+), is in good agreement with the literat
ure. Although tau was expected to increase with decreasing mass of the
ion, the exchange of water molecules from the solvation shell of Mg2 occurred much faster than for Ca2+.