title: Aqueous divalent metal-nitrate interactions: hydration versus ion pairing.
abstract: 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.
authors.affiliation: Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, OH 43210, USA.
authors.forename: Man
authors.initials: M
authors.lastname: Xu
authors.forename: James P
authors.initials: JP
authors.lastname: Larentzos
authors.forename: Mazen
authors.initials: M
authors.lastname: Roshdy
authors.forename: Louise J
authors.initials: LJ
authors.lastname: Criscenti
authors.forename: Heather C
authors.initials: HC
authors.lastname: Allen