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 molecular dynamics simulations, we report that the [Mg(NO)] molecular structure in solution from the coexistence of a free [Mg(HO)] octahedral supramolecular structure with a free [NO(HO)] ( = 11-13) supramolecular structure to an [Mg(HO)(NO)] ( = 3, 4, 5; = 3, 2, 1) associated structure with increasing concentration. Interestingly, two hydration modes of NO-the nearest neighbor hydration with a hydration distance less than 3.9 Å and the next nearest neighbor hydration with hydration distance ranging from 3.9 to 4.3 Å-were distinguished. With an increase in the solution concentration, the hydrated NO ions lost outer layer water molecules, and the hexagonal octahedral hydration structure of [Mg(HO)] was destroyed, resulting in direct contact between Mg and NO ions in a monodentate way. As the concentration of the solution further increased, NO ions replaced water molecules in the hydration layer of Mg to form three-ion clusters and even more complex chains or linear ion clusters.