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

Physical Chemistry Chemical Physics Pub Date: 2022-09-08 DOI: 10.1039/D2CP01828D

Abstract

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 ? and the next nearest neighbor hydration with hydration distance ranging from 3.9 to 4.3 ?—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)62+] 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.

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