Unveiling the structure of aqueous magnesium nitrate solutions by combining X-ray diffraction and theoretical calculations

The structure of aqueous magnesium nitrate solution is gaining significant interest among researchers, especially whether contact ion pairs exist in concentrated solutions. Here, combining X-ray diffraction experiments, quantum chemical calculations and ab initio molecular dynamics simulations, we report that the [Mg(NO3)(2)] molecular structure in solution from the coexistence of a free [Mg(H2O)(6)](2+) octahedral supramolecular structure with a free [NO3(H2O)(n)](-) (n = 11-13) supramolecular structure to an [Mg2+(H2O)(n)(NO3-)(m)] (n = 3, 4, 5; m = 3, 2, 1) associated structure with increasing concentration. Interestingly, two hydration modes of NO3--the nearest neighbor hydration with a hydration distance less than 3.9 angstrom and the next nearest neighbor hydration with hydration distance ranging from 3.9 to 4.3 angstrom-were distinguished. With an increase in the solution concentration, the hydrated NO3- ions lost outer layer water molecules, and the hexagonal octahedral hydration structure of [Mg(H2O)(6)(2+)] was destroyed, resulting in direct contact between Mg2+ and NO3- ions in a monodentate way. As the concentration of the solution further increased, NO3- ions replaced water molecules in the hydration layer of Mg2+ to form three-ion clusters and even more complex chains or linear ion clusters.