Ni2+ coordination in concentrated aqueous solutions

“Thought experiments” as dry-runs for “tough experiments”: novel approaches to the hydration behavior of oxyanions

We explore the deconvolution of correlations for the interpretation of the microstructural behavior of aqueous electrolytes according to the neutron diffraction with isotopic substitution (NDIS) approach toward the experimental determination of ion coordination numbers of systems involving oxyanions, in particular, sulfate anions. We discuss the alluded interplay in the title of this presentation, emphasized the expectations, and highlight the significance of tackling the challenging NDIS experiments. Specifically, we focus on the potential occurrence of $N{i^{2 + }} \cdots SO_4^{2 - }$ pair formation, identify its signature, suggest novel ways either for the direct probe of the contact ion pair (CIP) strength and the subsequent correction of its effects on the measured coordination numbers, or for the determination of anion coordination numbers free of CIP contributions through the implementation of null-cation environments. For that purpose we perform simulations of NiSO4 aqueous solutions at ambient conditions to generate the distribution functions required in the analysis (a) to identify the individual partial contributions to the total neutron-weighted distribution function, (b) to isolate and assess the contribution of $N{i^{2 + }} \cdots SO_4^{2 - }$ pair formation, (c) to test the accuracy of the neutron diffraction with isotope substitution based coordination calculations and X-ray diffraction based assumptions, and (d) to describe the water coordination around both the sulfur and oxygen sites of the sulfate anion. We finally discuss the strength of this interplay on the basis of the inherent molecular simulation ability to provide all pair correlation functions that fully characterize the system microstructure and allows us to “reconstruct” the eventual NDIS output, i.e., to take an atomistic “peek” (e.g., see Figure 1) at the local environment around the isotopically-labeled species before any experiment is ever attempted, and ultimately, to test the accuracy of the “measured” NDIS-based coordination numbers against the actual values by the “direct” counting.

Direct contact versus solvent-shared ion pairs in saturated NiCl2 aqueous solution: a DFT, CPMD, and EXAFS investigation

In this work, a systematic investigation of the competition coordination of H2O and Cl(-) with Ni(2+) in saturated NiCl2 aqueous solution at room temperature was conducted using density functional theory (DFT), Car-Parrinello molecular dynamics (CPMD) simulations, and extended X-ray absorption fine structure (EXAFS) spectra. The calculated results reveal that the six-coordinated structure is favorable for [NiCl(x)(H2O)(n)](2-x) (x = 0-2; n = 1-12) clusters in the aqueous phase. The hydration energy calculation shows that the six-coordinated solvent-shared ion pair (SSIP) ([Ni(H2O)6(H2O)(n-6)Cl](+)) is more stable than its contact ion pair (CIP) ([NiCl(H2O)5(H2O)(n-5)](+)) isomer as n ≥ 9 in the aqueous phase, and the six-coordinated solvent-shared ion pair with a dissociated double Cl(-) (SSIP/d) ([Ni(H2O)6(H2O)(n-6)Cl2](0)) is more preferable than its CIP ([NiCl2(H2O)4(H2O)(n-4)](0)) and solvent-shared ion pair with single dissociated Cl(-) (SSIP/s) ([NiCl(H2O)5(H2O)(n-5)Cl](0)) isomers as n ≥ 11. The six-coordinated SSIP/d ([Ni(H2O)6(H2O)(n-6)Cl2](0)) conformers are the dominant structures in a saturated NiCl2(aq) solution (NiCl2 concentration: ~5.05 mol·kg(-1), corresponding to n ≈ 11). The CPMD simulations exhibited that there are six water molecules with Ni-O distance at ~205.0 pm on average around each Ni(2+) in the first hydration sphere, even in the saturated NiCl2 aqueous solution (~5.05 mol·kg(-1)) at room temperature, and no obvious Ni-Cl interaction was found. The EXAFS spectra revealed that the first solvation shell of Ni(2+) is an octahedral structure with six water molecules tightly bound in the NiCl2(aq) solution with a concentration ranging from 1.00 to 5.05 mol·kg(-1), and there is no obvious evidence of Ni-Cl contact ion pairs. A comprehensive conclusion from the DFT, CPMD, and EXAFS studies is that there is no obvious direct contact between Ni(2+) and Cl(-), even in saturated NiCl2 aqueous solution at room temperature.

Coordination numbers of hydrated divalent transition metal ions investigated with IRPD spectroscopy

Hydration of the divalent transition metal ions, Mn, Fe, Co, Ni, Cu, and Zn, with 5-8 water molecules attached was investigated using infrared photodissociation spectroscopy and photodissociation kinetics. At 215 K, spectral intensities in both the bonded-OH and free-OH stretch regions indicate that the average coordination number (CN) of Mn(2+), Fe(2+), Co(2+), and Ni(2+) is ~6, and these CN values are greater than those of Cu(2+) and Zn(2+). Ni has the highest CN, with no evidence for any population of structures with a water molecule in a second solvation shell for the hexa-hydrate at temperatures up to 331 K. Mn(2+), Fe(2+), and Co(2+) have similar CN at low temperature, but spectra of Mn(2+)(H(2)O)(6) indicate a second population of structures with a water molecule in a second solvent shell, i.e., a CN < 6, that increases in abundance at higher temperature (305 K). The propensity for these ions to undergo charge separation reactions at small cluster size roughly correlates with the ordering of the hydrolysis constants of these ions in aqueous solution and is consistent with the ordering of average CN values established from the infrared spectra of these ions.

EXAFS studies of structural changes in fragile glasses of zinc nitrate and nickel nitrate hydrates

The ionic structures of aqueous solutions of two sets of transition metal nitrates have been studied in the liquid and glass states by EXAFS spectroscopy. Experiments were carried out on Zn(NO3)2.xH2O, with x=2, 6, 12 and NiNO.9HO over the temperature range 30<T(K)<250. The glass transition regime was monitored by means of an in-situ DSC probe. The EXAFS data were analysed by recently developed Monte Carlo procedures, enabling a discussion of the glass structure in terms of pairwise and higher order correlations. Results for the zinc nitrate hydrates show complex behaviour depending on the concentration. This behaviour is explained in terms of first hydration shell stability and NO3- penetration. This result contrasts with that for the equivalent correlation in nickel nitrate, and is taken as evidence for a more extensive free energy landscape of zinc nitrate hydrates. The results are also consistent with the known hydration properties of Zn2+ and Ni2+, and help explain why Zn2+ is biologically active in solution.

Hydrogen and higher shell contributions in Zn2+, Ni2+, and Co2+ aqueous solutions: an X-ray absorption fine structure and molecular dynamics study

A detailed investigation of the hydration structure of Zn2+, Ni2+, and Co2+ in water solutions has been carried out combining X-ray absorption fine structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. The first quantitative analysis of EXAFS from hydrogen atoms in 3d transition metal ions in aqueous solutions has been carried out and the ion-hydrogen interactions have been found to provide a detectable contribution to the EXAFS spectra. An accurate determination of the structural parameters associated with the first hydration shell has been performed and compared with previous experimental results. No evidence of significant contributions from the second hydration shell to the EXAFS signal has been found for these solutions, while the inclusion of the hydrogen signal has been found to be important in performing a quantitative analysis of the experimental data. The high-frequency contribution present in the EXAFS spectra has been found to be due to multiple scattering (MS) effects inside the ion-oxygen first coordination shell. MD has been used to generate three-body distribution functions from which a reliable analysis of the MS contributions to the EXAFS spectra of these systems has been carried out.