EXAFS Investigation of Inner- and Outer-Sphere Chloroaquo Complexes of Cr3+ in Aqueous Solutions

Chiral Templating in a Hybrid Chromium Chloride Hydrate

The synthesis and characterization of three new hybrid metal halide hydrates in which mer-[CrIIICl3(H2O)3]0 cocrystallizes alongside α-methylbenzylammonium chloride are described. The enantiomorphic crystals, ((R)-(+)-α-methylbenzylammonium)2(mer-[CrCl3(H2O)3])Cl2 ((R)-1) and ((S)-(-)-α-methylbenzylammonium)2(mer-[CrCl3(H2O)3])Cl2 ((S)-1), have C2221 space group symmetry and show mirrored circular dichroism signals. The racemate, (rac-α-methylbenzylammonium)2(mer-[CrCl3(H2O)3])Cl2 ((rac)-1), adopts a polar structure with Cm space group symmetry in which enantiomers are related by mirror planes within organic bilayers. Alongside detailed crystallography and magnetism of each compound, the optical properties of the mer-[CrIIICl3(H2O)3]0 unit are revisited. Understanding the intermolecular forces that stabilize each of these crystal structures lends insights into crystal engineering methodologies for stabilizing noncentrosymmetric hybrid metal halides.

Analysis of the First Ion Coordination Sphere: A Toolkit to Analyze the Coordination Sphere of Ions

Rapid and accurate approaches to characterizing the coordination structure of an ion are important for designing ligands and quantifying structure-property trends. Here, we introduce AFICS (Analysis of the First Ion Coordination Sphere), a tool written in Python 3 for analyzing the structural and geometric features of the first coordination sphere of an ion over the course of molecular dynamics simulations. The principal feature of AFICS is its ability to quantify the distortion a coordination geometry undergoes compared to uniform polyhedra. This work applies the toolkit to analyze molecular dynamics simulations of the well-defined coordination structure of aqueous Cr³⁺ along with the more ambiguous structure of aqueous Eu³⁺ chelated to ethylenediaminetetraacetic acid. The tool is targeted for analyzing ions with fluxional or irregular coordination structures (e.g., solution structures of f-block elements) but is generalized such that it may be applied to other systems.

Speciation of chromium aqua and chloro complexes in hydrochloric acid solutions at 298 K

The distribution of metal aqua and chloro complexes is fundamental information for analysis of a chemical reaction involving these complexes. The present study investigated the speciation and structures of chromium aqua and chloro complexes using the thermodynamic model fitting analysis of UV-vis/X-ray absorption spectra. The existence of a negatively charged species was examined by adsorbability of chromium species on a strong base anion exchanger. In the results, a planar square [CrIII(H2O)4]3+, a planar square or distorted tetrahedral [CrIIICl(H2O)3]2+, a trigonal bipyramidal [CrIIICl3(H2O)2]0, and a distorted octahedral [CrIIICl4(H2O)2]- were confirmed and the thermodynamic parameters of complexation reactions were quantitatively determined. Investigation of the evolution of speciation of chromium aqua and chloro complexes in a pH 1 solution suggested the existence of [CrIIICl2(H2O) m ]+ (m = 2 or 3) during the hydration process, which diminished in the equilibrium state. The kinetic analysis deserves further investigation to understand the speciation process quantitatively.

Enhancing the analysis of disorder in X-ray absorption spectra: application of deep neural networks to T-jump-X-ray probe experiments

Many chemical and biological reactions, including ligand exchange processes, require thermal energy for the reactants to overcome a transition barrier and reach the product state. Temperature-jump (T-jump) spectroscopy uses a near-infrared (NIR) pulse to rapidly heat a sample, offering an approach for triggering these processes and directly accessing thermally-activated pathways. However, thermal activation inherently increases the disorder of the system under study and, as a consequence, can make quantitative interpretations of structural changes challenging. In this Article, we optimise a deep neural network (DNN) for the instantaneous prediction of Co K-edge X-ray absorption near-edge structure (XANES) spectra. We apply our DNN to analyse T-jump pump/X-ray probe data pertaining to the ligand exchange processes and solvation dynamics of Co2+ in chlorinated aqueous solution. Our analysis is greatly facilitated by machine learning, as our DNN is able to predict quickly and cost-effectively the XANES spectra of thousands of geometric configurations sampled from ab initio molecular dynamics (MD) using nothing more than the local geometric environment around the X-ray absorption site. We identify directly the structural changes following the T-jump, which are dominated by sample heating and a commensurate increase in the Debye-Waller factor.

Reactivity of substituted benzotrichlorides toward granular iron, Cr(II), and an iron(II) porphyrin: A correlation analysis

Cross-correlations of rate constants between a system of interest and a better-defined one have become popular as a tool in studying transformations of organic pollutants. A slope of unity (if the correlation is conducted on a log-log basis) in such plots has been invoked as evidence of a common mechanism. To explore this notion, benzotrichloride and several of its substituted analogues were reacted with Cr(H2O)6(2+), an iron(II) porphyrin (iron meso-tetra(4-carboxyphenyl)porphine chloride, Fe(II)TCP), and granular iron. The first two reductants react with organohalides by dissociative inner sphere single-electron transfer, while mechanism(s) for organohalide reduction by granular iron are still debated. Apartfrom sterically hindered compounds, good correlations were obtained in comparing any two systems, although slopes (on a log-log basis) deviated from unity. We argue that a slope of unity is neither necessary nor sufficient evidence of a common mechanism. Overall rate constants may be composite entities, consisting in part of rate or equilibrium constants for adsorption onto surfaces or for precursor formation in solution; these components may differ between systems in their susceptibility to substituent effects. Cross-correlations may prove useful in predicting reactivity in the absence of steric effects, but additional evidence is required in deducing reaction mechanisms.

Kinetic Studies of Chromium (VI) Binding to Carboxylic Acid- and Methyl Ester-Functionalized Silica/Water Interfaces Important in Geochemistry

Real-time kinetic measurements of hexavalent chromium binding to fused silica surfaces functionalized with carboxylic acid and methyl ester terminal groups are performed in situ using resonantly enhanced surface second harmonic generation (SHG) at pH 7 and 300 K. These functional groups were chosen because of their high abundance in humic acids and related biopolymers. Kinetic measurements are conducted in the submonolayer regime using chromate solution concentrations ranging from 1 x 10(-6) to 2 x 10(-5) M. The adsorption rates were analyzed using the standard Langmuir model and the Frumkin-Fowler-Guggenheim model. The desorption kinetics are consistent with a first-order process. These results indicate that hexavalent chromium mobility in carboxylic acid- and ester-rich soil environments increases with decreasing chromate concentrations. Based on the measured half-lives of the adsorbed Cr(VI) species, remobilization of bound hexavalent chromium due to natural or anthropogenic events that lower the chromate concentration in the aqueous phase can occur within minutes.

Time-dependent uptake, distribution and biotransformation of chromium(VI) in individual and bulk human lung cells: application of synchrotron radiation techniques

Chromium(VI) is a human carcinogen, primarily affecting the respiratory tract probably via active transport into cells, followed by the reduction to Cr(III) with the formation of DNA-damaging intermediates. Distribution of Cr and endogenous elements within A549 human lung adenocarcinoma epithelial cells, following treatment with Cr(VI) (100 microM, 20 min or 4 h) were studied by synchrotron-radiation-induced X-ray emission (SRIXE) of single freeze-dried cells. After the 20-min treatment, Cr was confined to a small area of the cytoplasm and strongly co-localized with S, Cl, K, and Ca. After the 4-h treatment, Cr was distributed throughout the cell, with higher concentrations in the nucleus and the cytoplasmic membrane. This time-dependence corresponded to approximately 100% or 0% clonogenic survival of the cells following the 20-min or 4-h treatments, respectively, and could potentially be explained by a new cellular protective mechanism. Such processes may also be important in reducing the potential hazards of Cr(III) dietary supplements, for which there is emerging evidence that they exert their anti-diabetic effects via biological oxidation to Cr(VI). The predominance of Cr(III) was confirmed by micro-XANES spectroscopy of intracellular Cr hotspots. X-ray absorption spectroscopy (XANES and EXAFS, using freeze-dried cells after the 0-4-h treatments) was used to gain insight into the chemical structures of Cr(III) complexes formed during the intracellular reduction of Cr(VI). The polynuclear nature of such complexes (probably with a combination of carboxylato and hydroxo bridging groups and O-donor atoms of small peptides or proteins) was established by XAFS data analyses.

Geometric hydration shells for anionic platinum group metal chloro complexes

Solvation shells surrounding complex inorganic anions have not been extensively studied and are often mentioned with an amorphous picture in mind. We use a computational model previously validated against experimental results and ab initio quantum calculations (Lienke, A.; Klatt, G.; Robinson, D.; Koch, K. R.; Naidoo, K. J. Inorg.Chem. 2001, 40, 2352-2357) to investigate the nature of the hydration shells about simple platinum group metal chloro complexes ([PtCl(6)](2-), [RhCl(6)](3-), [PtCl(4)](2-), and [PdCl(4)](2-)). Our simulations show that the hydration shells surrounding these complexes are symmetric and take on familiar geometric forms. We find that only the [RhCl(6)](3-) complex has a clearly defined second hydration shell while the [PtCl(6)](2-), [PtCl(4)](2-), and [PdCl(4)](2-) second hydration shells are more diffuse.

Investigation of Aquo and Chloro Complexes of UO(2)(2+), NpO(2)(+), Np(4+), and Pu(3+) by X-ray Absorption Fine Structure Spectroscopy

U, Np, and Pu L(II,III)-edge X-ray absorption fine structure (XAFS) spectra were collected for the UO(2)(2+), NpO(2)(+), Np(4+), and Pu(3+) ions as a function of chloride concentration in aqueous solution. At low chloride concentration, the hydration numbers and corresponding bond lengths for the different ions are as follows: UO(2)(2+), N= 5.3, R = 2.41 Å; NpO(2)(+), N = 5.0, R = 2.50 Å; Np(4+), N = 11.2, R = 2.40 Å; Pu(3+), N = 10.2, R = 2.51 Å. As the Cl(-) concentration increases, inner-sphere Cl(-) complexation occurs, resulting in a decrease in the hydration numbers and an expansion of the actinide-oxygen (water) bond lengths. The Pu(3+) ion shows only a decrease in hydration number (40%) and no inner-sphere Cl(-) complexation for [Cl(-)] < 14 M. For concentrations up to 10-14 M Cl(-), the average Cl(-) coordination numbers and bond lengths are as follows: UO(2)(2+), N = 2.6, R = 2.73 Å; NpO(2)(+), N = 1.0, R = 2.84 Å; Np(4+), N = 2.0, R = 2.61 Å. Structural changes are observed in the near-edge spectral region as shown by significant changes in the white line intensities upon Cl(-) complexation. For ions with similar structures, i.e. Pu(3+) and Np(4+) or the actinyl ions NpO(2)(+) and UO(2)(2+), positive energy shifts are observed with increasing oxidation state. The ability to use XAFS speciation results to calculate equilibrium constants and the relationship of these results to previous studies are discussed.