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(NO 3 ) 2 ] molecular structure in solution from the coexistence of a free [Mg(H 2 O) 6 ] 2+ octahedral supramolecular structure with a free [NO 3 (H 2 O) n ] - ( n = 11-13) supramolecular structure to an [Mg 2+ (H 2 O) n (NO 3 - ) m ] ( n = 3, 4, 5; m = 3, 2, 1) associated structure with increasing concentration. Interestingly, two hydration modes of NO 3 - -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 3 - ions lost outer layer water molecules, and the hexagonal octahedral hydration structure of [Mg(H 2 O) 6 2+ ] was destroyed, resulting in direct contact between Mg 2+ and NO 3 - ions in a monodentate way. As the concentration of the solution further increased, NO 3 - ions replaced water molecules in the hydration layer of Mg 2+ to form three-ion clusters and even more complex chains or linear ion clusters.