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(NO(3))(2), Ca(NO(3))(2), Sr(NO(3))(2), and Pb(NO(3))(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 Pb(2+) has a strong tendency to form contact ion pairs with nitrate relative to Sr(2+), Ca(2+), and Mg(2+), and contact ion pair formation decreases with decreasing cation size and increasing cation charge density: Pb(2+) > Sr(2+) > Ca(2+) > Mg(2+). In the case of Mg(2+), the Mg(2+) -OH(2) 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 Mg(2+) relative to other cations.

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