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

Abstract

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[η¹-HCO₃](H₂O)₄⁺ and Mg[η¹-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⁺.