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.

Spectroscopic monitoring of spent nuclear fuel reprocessing streams: an evaluation of spent fuel solutionsviaRaman, visible, and near-infrared spectroscopy

“…Raman spectra of the ATM-109 fuel solutions with variable HNO 3 associated for the υ 1 symmetric stretch (∼ 1050 cm −1 ), υ 3 asymmetric stretch (∼ 1350 cm −1 ), and υ 4 in-plane deformation (∼ 700 cm −1 ) modes were observed; the υ 2 band for nitrate is Raman forbidden and was not observed [40][41][42]. The 718 cm −1 band is assigned to the υ 4 in-plane deformation mode of the nitrate moiety, and has E symmetry in D 3h ; this band splits into A 1 and B 2 pairs upon descent in symmetry to C 2v when undergoing interaction with cations (such as H + , or UO 2 2+ ); the two Raman bands observed at 644 and 697 cm −1 were assigned to these modes in the 5.1 M HNO 3 spectrum.…”

Ab initio investigation on the ion‐associated species and process in Mg(NO3)2 solution

“…The very recent work reported by Xu et al has also confirmed the CIPs exist barely in the Mg(NO 3 ) 2 solution by using the molecular dynamics (MD) simulation. 49 At last, the ion-associated process is not as simple as that suggested by Eigen and Tamm, which contains the dehydration between only two aqueous ions. From Figure 8, it can be known that the TI and MI clusters possess important status in the associated process.…”

The structural independence of Raman scattering cross sections of ν1(NO3−) and ν(H2O)

“…It is believed that Na + ions tends to form CIPs while Mg 2+ usually maintains its hydration structure. Xu et al 16 suggested that the formation of CIPs in Mg(NO 3 ) 2 solutions is thermodynamically unfavorable. According to the free‐energy profile of MgNO 3 + , the free‐energy minimum for CIP is significantly higher (>9 kcal mol −1 ) than those for solvent‐shared and solvent‐separated ion pairing.…”

“…Fleissner et al 15 estimated that the molar Raman intensities of the bound and free forms of nitrate are equal. Based on this assumption, Xu et al 16 revealed a linear correlation between the sum intensity of the nitrate in‐plane deformation (IPD) bands and the concentration, indicating that the IPD intensity is not significantly sensitive to the relative amount of free or ion paired species. According to a hypothesis that RSCS is unchanged from dilute solutions to supersaturated aerosols, Zhao et al 17 investigated the state of water in supersaturated Mg(NO 3 ) 2 and NaNO 3 aerosols through the application of the Raman difference spectroscopy.…”