Aqueous divalent metal-nitrate interactions: hydration versus ion pairing

M Xu and JP Larentzos and M Roshdy and LJ Criscenti and HC Allen, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 10, 4793-4801 (2008).

DOI: 10.1039/b807090n

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.

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