A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution

The hydration of the alkali metal ions in aqueous solution has been studied by large angle X-ray scattering (LAXS) and double difference infrared spectroscopy (DDIR). The structures of the dimethyl sulfoxide solvated alkali metal ions in solution have been determined to support the studies in aqueous solution. The results of the LAXS and DDIR measurements show that the sodium, potassium, rubidium and cesium ions all are weakly hydrated with only a single shell of water molecules. The smaller lithium ion is more strongly hydrated, most probably with a second hydration shell present. The influence of the rubidium and cesium ions on the water structure was found to be very weak, and it was not possible to quantify this effect in a reliable way due to insufficient separation of the O–D stretching bands of partially deuterated water bound to these metal ions and the O–D stretching bands of the bulk water. Aqueous solutions of sodium, potassium and cesium iodide and cesium and lithium hydroxide have been studied by LAXS and M–O bond distances have been determined fairly accurately except for lithium. However, the number of water molecules binding to the alkali metal ions is very difficult to determine from the LAXS measurements as the number of distances and the temperature factor are strongly correlated. A thorough analysis of M–O bond distances in solid alkali metal compounds with ligands binding through oxygen has been made from available structure databases. There is relatively strong correlation between M–O bond distances and coordination numbers also for the alkali metal ions even though the M–O interactions are weak and the number of complexes of potassium, rubidium and cesium with well-defined coordination geometry is very small. The mean M–O bond distance in the hydrated sodium, potassium, rubidium and cesium ions in aqueous solution have been determined to be 2.43(2), 2.81(1), 2.98(1) and 3.07(1) Å, which corresponds to six-, seven-, eight- and eight-coordination. These coordination numbers are supported by the linear relationship of the hydration enthalpies and the M–O bond distances. This correlation indicates that the hydrated lithium ion is four-coordinate in aqueous solution. New ionic radii are proposed for four- and six-coordinate lithium­(I), 0.60 and 0.79 Å, respectively, as well as for five- and six-coordinate sodium­(I), 1.02 and 1.07 Å, respectively. The ionic radii for six- and seven-coordinate K⁺, 1.38 and 1.46 Å, respectively, and eight-coordinate Rb⁺ and Cs⁺, 1.64 and 1.73 Å, respectively, are confirmed from previous studies. The M–O bond distances in dimethyl sulfoxide solvated sodium, potassium, rubidium and cesium ions in solution are very similar to those observed in aqueous solution

A Coordination Chemistry Study of Hydrated and Solvated Cationic Vanadium Ions in Oxidation States +III, +IV, and +V in Solution and Solid State

The coordination chemistry of hydrated and solvated vanadium­(III), oxovanadium­(IV), and dioxovanadium­(V) ions in the oxygen-donor solvents water, dimethyl sulfoxide (DMSO), and <i>N</i>,<i>N</i>′-dimethylpropyleneurea (DMPU) has been studied in solution by extended X-ray absorption fine structure (EXAFS) and large-angle X-ray scattering (LAXS) and in the solid state by single-crystal X-ray diffraction and EXAFS. The hydrated vanadium­(III) ion has a regular octahedral configuration with a mean V–O bond distance of 1.99 Å. In the hydrated and DMSO-solvated oxovanadium­(IV) ions, vanadium binds strongly to an oxo group at ca. 1.6 Å. The solvent molecule trans to the oxo group is very weakly bound, at ca. 2.2 Å, while the remaining four solvent molecules, with a mean V–O bond distance of 2.0 Å, form a plane slightly below the vanadium atom; the mean OVO_perp bond angle is ca. 98°. In the DMPU-solvated oxovanadium­(IV) ion, the space-demanding properties of the DMPU molecule leave no solvent molecule in the trans position to the oxo group, which reduces the coordination number to 5. The OVO bond angle is consequently much larger, 107°, and the mean VO and V–O bond distances decrease to 1.58 and 1.97 Å, respectively. The hydrated and DMSO-solvated dioxovanadium­(V) ions display a very distorted octahedral configuration with the oxo groups in the cis position with a mean VO bond distance of 1.6 Å and a OVO bond angle of ca. 105°. The solvent molecules trans to the oxo groups are weakly bound, at ca. 2.2 Å, while the remaining two have bond distances of 2.02 Å. The experimental studies of the coordination chemistry of hydrated and solvated vanadium­(III,IV,V) ions are complemented by summarizing previously reported crystal structures to yield a comprehensive description of the coordination chemistry of vanadium with oxygen-donor ligands

On the Structure and Volumetric Properties of Solvated Lanthanoid(III) Ions in Amide Solutions

The coordination chemistry and the volumetric properties of three representative lanthanoid­(III) ionslanthanum­(III), gadolinium­(III), and lutetium­(III)have been studied in three amide solvents with gradually increasing spatial demand upon coordination: <i>N</i>,<i>N</i>-dimethylformamide (dmf) < <i>N</i>,<i>N</i>-dimethylacet­amide (dma) < <i>N</i>,<i>N</i>-dimethylpropionamide (dmp). Large angle X-ray scattering (LAXS) and EXAFS have been used to determine the structure of the solvated lanthanoid­(III) ions in solution, further supplemented with a crystallographic study on octakis­(<i>N</i>,<i>N</i>-dimethylacetamide)­lanthanum­(III) triflate, [La­(dma)₈]­(CF₃SO₃)₃. The selection of ions and solvents allows an estimate of the steric congestion effects on the resulting coordination number, CN, ranging from nine for lanthanum­(III) ions in dmf to seven for the smaller lutetium­(III) ion in space-demanding dma. The standard partial molar volumes of the solvated lanthanoid­(III) ions in water and dmf are reflected in the CNs, as these solvent molecules are small enough to not interfere with each other upon coordination. However, the larger and more space-demanding dma displays a different pattern with an almost constant standard partial molar volume and a decreasing CN, counterbalancing the difference in ionic radius of the lanthanoid­(III) ion

On the Structure and Volumetric Properties of Solvated Lanthanoid(III) Ions in Amide Solutions

The coordination chemistry and the volumetric properties of three representative lanthanoid­(III) ionslanthanum­(III), gadolinium­(III), and lutetium­(III)have been studied in three amide solvents with gradually increasing spatial demand upon coordination: <i>N</i>,<i>N</i>-dimethylformamide (dmf) < <i>N</i>,<i>N</i>-dimethylacet­amide (dma) < <i>N</i>,<i>N</i>-dimethylpropionamide (dmp). Large angle X-ray scattering (LAXS) and EXAFS have been used to determine the structure of the solvated lanthanoid­(III) ions in solution, further supplemented with a crystallographic study on octakis­(<i>N</i>,<i>N</i>-dimethylacetamide)­lanthanum­(III) triflate, [La­(dma)₈]­(CF₃SO₃)₃. The selection of ions and solvents allows an estimate of the steric congestion effects on the resulting coordination number, CN, ranging from nine for lanthanum­(III) ions in dmf to seven for the smaller lutetium­(III) ion in space-demanding dma. The standard partial molar volumes of the solvated lanthanoid­(III) ions in water and dmf are reflected in the CNs, as these solvent molecules are small enough to not interfere with each other upon coordination. However, the larger and more space-demanding dma displays a different pattern with an almost constant standard partial molar volume and a decreasing CN, counterbalancing the difference in ionic radius of the lanthanoid­(III) ion

Chromium(III) Complexation to Natural Organic Matter: Mechanisms and Modeling

Chromium is a common soil contaminant, and it often exists as chromium­(III). However, limited information exists on the coordination chemistry and stability of chromium­(III) complexes with natural organic matter (NOM). Here, the complexation of chromium­(III) to mor layer material and to Suwannee River Fulvic Acid (SRFA) was investigated using EXAFS spectroscopy and batch experiments. The EXAFS results showed a predominance of monomeric chromium­(III)-NOM complexes at low pH (<5), in which only Cr···C and Cr–O–C interactions were observed in the second coordination shell. At pH > 5 there were polynuclear chromium­(III)-NOM complexes with Cr···Cr interactions at 2.98 Å and for SRFA also at 3.57 Å, indicating the presence of dimers (soil) and tetramers (SRFA). The complexation of chromium­(III) to NOM was intermediate between that of iron­(III) and aluminum­(III). Chromium­(III) complexation was slow at pH < 4: three months or longer were required to reach equilibrium. The results were used to constrain chromium-NOM complexation in the Stockholm Humic Model (SHM): a monomeric complex dominated at pH < 5, whereas a dimeric complex dominated at higher pH. The optimized constant for the monomeric chromium­(III) complex was in between those of the iron­(III) and aluminum­(III) NOM complexes. Our study suggests that chromium­(III)-NOM complexes are important for chromium speciation in many environments

Amorphous Calcium Carbonate Constructed from Nanoparticle Aggregates with Unprecedented Surface Area and Mesoporosity

Amorphous calcium carbonate (ACC), with the highest reported specific surface area of all current forms of calcium carbonate (over 350 m² g^–1), was synthesized using a surfactant-free, one-pot method. Electron microscopy, helium pycnometry, and nitrogen sorption analysis revealed that this highly mesoporous ACC, with a pore volume of ∼0.86 cm³ g^–1 and a pore-size distribution centered at 8–9 nm, is constructed from aggregated ACC nanoparticles with an estimated average diameter of 7.3 nm. The porous ACC remained amorphous and retained its high porosity for over 3 weeks under semi-air-tight storage conditions. Powder X-ray diffraction, large-angle X-ray scattering, infrared spectroscopy, and electron diffraction exposed that the porous ACC did not resemble any of the known CaCO₃ structures. The atomic order of porous ACC diminished at interatomic distances over 8 Å. Porous ACC was evaluated as a potential drug carrier of poorly soluble substances in vitro. Itraconazole and celecoxib remained stable in their amorphous forms within the pores of the material. Drug release rates were significantly enhanced for both drugs (up to 65 times the dissolution rates for the crystalline forms), and supersaturation release of celecoxib was also demonstrated. Citric acid was used to enhance the stability of the ACC nanoparticles within the aggregates, which increased the surface area of the material to over 600 m² g^–1. This porous ACC has potential for use in various applications where surface area is important, including adsorption, catalysis, medication, and bone regeneration