Aqueous divalent metal–nitrate interactions: hydration versus ion pairing

Abstract

Nitrate aqueous solutions, Mg(NO₃)₂, Ca(NO₃)₂, Sr(NO₃)₂, and Pb(NO₃)₂, are investigated using Raman spectroscopy and free energy profiles from molecular dynamics (MD) simulations. Analysis of the in-plane deformation, symmetric stretch, and asymmetric stretch vibrational modes of the nitrate ions reveal perturbation caused by the metal cations and hydrating water molecules. Results show that Pb²⁺ has a strong tendency to form contact ion pairs with nitrate relative to Sr²⁺, Ca²⁺, and Mg²⁺, and contact ion pair formation decreases with decreasing cation size and increasing cation charge density: Pb²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺. In the case of Mg²⁺, the Mg²⁺–OH₂ intermolecular modes indicate strong hydration by water molecules and no contact ion pairing with nitrate. Free energy profiles provide evidence for the experimentally observed trend and clarification between solvent-separated, solvent-shared, and contact ion pairs, particularly for Mg²⁺ relative to other cations.