Solvent Structure and the Extended Range Hydration of Cr3+ in Aqueous Solution

Unraveling the Hydration Shell Structure and Dynamics of Group 10 Aqua Ions

We present ab initio simulations based on subsystem DFT of group 10 aqua ions accurately compared against experimental data on hydration structure. Our simulations provide insights into the molecular structures and dynamics of hydration shells, offering recalibrated interpretations of experimental results. We observe a soft, but distinct second hydration shell in Palladium (Pd) due to a balance between thermal fluctuations, metal-water interactions, and hydrogen bonding. Nickel (Ni) and platinum (Pt) exhibit more rigid hydration shells. Notably, our simulations align with experimental findings for Pd, showing axial hydration marked by a broad peak at about 3 Å in the Pd-O radial distribution function, revising the previously sharp "mesoshell" prediction. We introduce the "hydrogen bond dome" concept to describe a resilient network of hydrogen-bonded water molecules around the metal, which plays a critical role in the axial hydration dynamics.

Unveiling the structure of aqueous magnesium nitrate solutions by combining X-ray diffraction and theoretical calculations

The structure of aqueous magnesium nitrate solution is gaining significant interest among researchers, especially whether contact ion pairs exist in concentrated solutions. Here, combining X-ray diffraction experiments, quantum chemical calculations and ab initio molecular dynamics simulations, we report that the [Mg(NO₃)₂] molecular structure in solution from the coexistence of a free [Mg(H₂O)₆]²⁺ octahedral supramolecular structure with a free [NO₃(H₂O)_n]^- (n = 11-13) supramolecular structure to an [Mg²⁺(H₂O)_n(NO₃^-)_m] (n = 3, 4, 5; m = 3, 2, 1) associated structure with increasing concentration. Interestingly, two hydration modes of NO₃^--the nearest neighbor hydration with a hydration distance less than 3.9 Å and the next nearest neighbor hydration with hydration distance ranging from 3.9 to 4.3 Å-were distinguished. With an increase in the solution concentration, the hydrated NO₃^- ions lost outer layer water molecules, and the hexagonal octahedral hydration structure of [Mg(H₂O)₆²⁺] was destroyed, resulting in direct contact between Mg²⁺ and NO₃^- ions in a monodentate way. As the concentration of the solution further increased, NO₃^- ions replaced water molecules in the hydration layer of Mg²⁺ to form three-ion clusters and even more complex chains or linear ion clusters.

Ion hydration and association in aqueous potassium tetrahydroxyborate solutions

Potassium tetrahydroxyborate solution is a significant material in the borate solution family, but there is limited knowledge about hydration structures and interactions of K⁺, [B(OH)₄^-], and water. In this study, the X-ray diffraction measurements of potassium tetrahydroxyborate solutions have been made. The experimental structure factors are subjected to Empirical Potential Structure Refinement (EPSR) modeling to reveal the details of ion hydration and association in the aqueous solutions. This study shows that the O(W)-O(W) distance of water in the studied solutions ranges from 2.82 to 2.76 Å with a coordination number that ranges from 4.7 ± 1.4 to 3.1 ± 1.3 when the value of the water-salt molar ratio (WSR) is decreased from 30 to 6. The addition of ions slightly affects the tetrahedral structure of water even when the concentration of ions is high. The first hydration distance of K⁺ remained at ∼2.67 Å, whereas the value of the coordination number (CN) decreased from 5.4 ± 1.3 to 3.9 ± 1.5 when the concentration of the borate solution was increased. The hydration ability of K⁺ was weak and almost did not have a fixed local hydration structure. The pair distribution function (PDF) of g_B-O(W)(r) shows that [B(OH)₄^-] has a broad hydration distance from 2.9 to 5.4 Å because of the complex interactive relationship between K⁺, [B(OH)₄^-] and water. There is a competitive hydration between K⁺ and [B(OH)₄^-]. Both the X-ray diffraction and DFT-based calculations confirm that the main species is monodentate contact ion pairs when WSR = 30, bidentate contact ion pairs when WSR = 14, and triple contact ion pairs when WSR = 6. These results will provide a new understanding about potassium tetrahydroxyborate solution.

The structure of water in solutions containing di- and trivalent cations by empirical potential structure refinement

Empirical potential structure refinement (EPSR) has been used to build experimentally consistent models of a range of electrolyte solutions containing di- and trivalent cations: Cu(ClO4)2 at concentrations of 0.5 and 2.0 m, and Cr(NO3)3, YCl3 and LaCl3 at a concentration of 1.0 m. The resulting models are used to investigate the perturbation of these electrolytes on the pair distribution and triplet angle correlations between solvent water molecules, compared with those found in the pure solvent. The results elucidate the differences that derive from the reflected range of highly structured local cation environments and provide a complementary viewpoint on the hydration shell geometries.

The X-ray absorption spectroscopic model of the copper(II) imidazole complex ion in liquid aqueous solution: a strongly solvated square pyramid

Cu K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near-edge structure (MXAN) analyses were combined to evaluate the structure of the copper(II) imidazole complex ion in liquid aqueous solution. Both methods converged to the same square-pyramidal inner coordination sphere [Cu(Im)(4)L(ax)](2+) (L(ax) indeterminate) with four equatorial nitrogen atoms at EXAFS, 2.02 ± 0.01 Å, and MXAN, 1.99 ± 0.03 Å. A short-axial N/O scatterer (L(ax)) was found at 2.12 ± 0.02 Å (EXAFS) or 2.14 ± 0.06 Å (MXAN). A second but very weak axial Cu-N/O interaction was found at 2.9 ± 0.1 Å (EXAFS) or 3.0 ± 0.1 Å (MXAN). In the MXAN fits, only a square-pyramidal structural model successfully reproduced the doubled maximum of the rising K-edge X-ray absorption spectrum, specifically excluding an octahedral model. Both EXAFS and MXAN also found eight outlying oxygen scatterers at 4.2 ± 0.3 Å that contributed significant intensity over the entire spectral energy range. Two prominent rising K-edge shoulders at 8987.1 and 8990.5 eV were found to reflect multiple scattering from the 3.0 Å axial scatterer and the imidazole rings, respectively. In the MXAN fits, the imidazole rings took in-plane rotationally staggered positions about copper. The combined (EXAFS and MXAN) model for the unconstrained cupric imidazole complex ion in liquid aqueous solution is an axially elongated square-pyramidal core, with a weak nonbonded interaction at the second axial coordination position and a solvation shell of eight nearest-neighbor water molecules. This core square-pyramidal motif has persisted through [Cu(H(2)O)(5)](2+), [Cu(NH(3))(4)(NH(3),H(2)O)](2+), (1, 2) and now [Cu(Im)(4)L(ax))](2+) and appears to be the geometry preferred by unconstrained aqueous-phase copper(II) complex ions.

Boroxol rings from diffraction data on vitreous boron trioxide

There has been a considerable debate about the nature of the short range atomic order in vitreous B(2)O(3). Some authorities state that it is not possible to build a model of glassy boron oxide of the correct density containing a large number of six-membered rings which also fits experimental diffraction data, but recent computer simulations appear to overrule that view. To discover which view is correct I use empirical potential structure refinement (EPSR) on existing neutron and x-ray diffraction data to build two models of vitreous B(2)O(3). One of these consists only of single boron and oxygen atoms arranged in a network to reproduce the diffraction data as closely as possible. This model has less than 10% of boron atoms in boroxol rings. The second model is made up of an equimolar mixture of B(3)O(3) hexagonal ring 'molecules' and BO(3) triangular molecules, with no free boron or oxygen atoms. This second model therefore has 75% of the boron atoms in boroxol rings. It is found that both models give closely similar diffraction patterns, suggesting that the diffraction data in this case are not sensitive to the number of boroxol rings present in the structure. This reinforces recent Raman, ab initio, and NMR claims that the percentage of boroxol rings in this material may be as high as 75%. The findings of this study probably explain why some interpretations based on different simulation techniques only find a small fraction of boroxol rings. The results also highlight the power of EPSR for the extraction of accurate atomistic representations of amorphous structures, provided adequate additional, non-scattering data (such as Raman and NMR in this case) are available.

Solvation structure and ion complexation of La3+ in a 1 molal aqueous solution of lanthanum chloride

H/D isotopic substitution neutron scattering and X-ray scattering have been used to investigate the short and intermediate range solution structure in a 1 m aqueous solution of lanthanum chloride. To improve the reliability of the local structural information on the cation environment, information has been incorporated from Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy data into the applied analytical framework. The markedly different structural sensitivities of the experimental probes allow the construction of a detailed three-dimensional atomistic model using the Empirical Potential Structure Refinement (EPSR) technique. The results show that at the investigated concentration La(3+) is hydrated by eight water molecules and one chloride ion, forming an inner-sphere ion complex in which the water molecules maintain angular configurations consistent with a tricapped trigonal prism configuration. This local geometry considerably disrupts the bulk solvent structure.