A first principles molecular dynamics simulation of the hydrated magnesium ion

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Abstract

First principles molecular dynamics has been used to investigate the solvation of Mg2+ in water. In agreement with experiment, we find that the first solvation shell around Mg2+ contains six water molecules in an octahedral arrangement. The electronic structure of first solvation shell water molecules has been examined with a localized orbital analysis. We find that water molecules tend to asymmetrically coordinate Mg2+ through one of the oxygen lone pair orbitals and that the first solvation shell dipole moments increase by 0.2 Debye relative to pure liquid water.

Introduction

Nature commonly uses metal ions to activate chemical bonds and makes them more amenable to undergo various chemical reactions. For example, in many enzymatic reactions a metal ion is used to activate water molecules that are involved in hydrolysis reactions. Frequently in these types of reactions, the hydrolyzing ability of the ion is somewhat proportional to the charge to ionic radius ratio (Zm/rn) [1]. Mg2+ has a small ionic radius and is considered a `hard' divalent cation. Thus, Mg2+ has a reasonable hydrolyzing effect and is able to lower the pKa of water to 11.4 [2]. Although there have been many experimental [3], [4], [5] and computational [6], [7], [8] studies on magnesium, most investigations concerning the solvation of Mg2+ at a microscopic level have focused on gas phase clusters. Thus, little is known as to how and to what extent Mg2+ alters the properties of its surrounding water molecules in the bulk phase.

The aqueous solvation of Mg2+ has been studied experimentally with several methods, including X-ray diffraction [9], NMR [10], and Raman spectroscopy [11]. From these measurements, it is known that Mg2+ prefers to coordinate six water molecules in an octahedral arrangement [9], and that the rate of water exchange from the first coordination shell is 5.3×105s−1 at 25° [12]. However, the detailed orientation of water molecules around Mg2+ is not known because the scattering lengths of different magnesium isotopes are too similar to effectively use neutron diffraction-based isotopic substitution [13], [14]. In the literature, indirect measurements based on isomorphic substitution with Ni2+ have been used to study the solvation properties of Mg2+[14]. Another approach for examining the microscopic details of ion solvation is the use of computer simulation to complement the experimental measurements. However, the utility of simulation methods depends critically on the quality of the simulation model that is employed.

Here we report on a study of Mg2+ solvation as carried out with a first principles molecular dynamics simulation. We compare the properties of the water molecules directly surrounding Mg2+ to the available experimental data. In addition, the changes that Mg2+ causes in the structural and electronic properties of the directly coordinated water molecules are investigated and compared to the simulation water molecules not directly coordinated to Mg2+, which we refer to here as `bulk' waters.

Section snippets

Computational methods

First principles Car-Parrinello molecular dynamics [15] with the PBE generalized gradient approximation [16] was used since this approach has been shown to accurately reproduce the properties of liquid water [17] and the solvation of other ions such as sodium [18]. In our simulation, we considered 53 water molecules and one Mg2+ in a cubic box of 11.74 Å in length, to which periodic boundary conditions were applied. The starting configuration was generated from a previous simulation of pure

Results and discussion

Information on the structural properties of the environment surrounding an ion in solution can be determined from pair radial distribution functions, gαβ(r), which represent the probability, relative to a random distribution, of finding an atom of type β at a distance of r from an atom of type α. The radial distribution function gMgO(r) calculated from our first principles molecular dynamics simulation is shown in Fig. 1. Also shown in Fig. 1 is the running integration number, n(r), which

Conclusions

In summary, first principles molecular dynamics has been used to investigate the solvation of Mg2+ in liquid water. By directly using quantum mechanically derived potentials that allow for polarization effects in a self-consistent manner, the effect of Mg2+ on water has been studied in detail. The structural properties of the first solvation shell around Mg2+, which contains six water molecules in an octahedral arrangement, are in very good agreement with experimental measurements. However, in

Acknowledgments

This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

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