Structure and dynamics of the hydrated magnesium ion and of the solvated magnesium carbonates: insights from first principles simulations

We report first principles molecular dynamics simulations based on the density functional theory and the Car-Parrinello method to study the structures and dynamics of the hydrated Mg2+ ion and of the solvated MgHCO3+ and MgCO3 complexes in aqueous solution. According to these simulations, the first hydration shell of the hydrated magnesium ion consists of six water molecules, whereas in the solvated magnesium bicarbonate and magnesium carbonate complexes the Mg2+ is mostly five-coordinated, which indicates that when coordinated to magnesium the HCO3- and CO32- anions reduce its the coordination sphere. Our simulations show that the structures of the most stable monomers of magnesium bi-carbonate and magnesium carbonate in solution are Mg[eta(1)-HCO3](H2O)(4)(+) and Mg[eta(1)-CO3](H2O)(4), i.e. the preferred hydration number is four, while the (bi-) carbonate is coordinated to the magnesium in a monodentate mode. The analysis of the exchange processes of the water molecules in the first and second hydration shell of Mg2+ shows that the HCO3- or CO32- ligands affect the dynamics of the magnesium coordination spheres by making its hydration shell more labile''. Furthermore, molecular dynamics simulations of the non-associated Mg2+/Cl- pair in water suggest that, despite negligible differences in the coordination spheres of Mg2+, the chloride anion has a significant influence on the water exchange rates in the second hydration shell of Mg2+.