Comparative review of structural parameters of the nearest surrounding of monoatomic cations in water and methanol media

Literature data on structure of coordination sphere of selected monoatomic cations in aqueous and methanol solutions have been reviewed and compared. The following structural parameters have been considered: coordination number, interparticle distances, the second coordination sphere features, and ionic association type. It has been revealed that for doubly charged ions the parameters of the first coordination sphere are similar in the both solvents. The second coordination sphere of the same cation consists of fewer solvating molecules located farther from the ion in the case of methanol as compared to water.     

Structural parameters of hydration of Be<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript> ions in aqueous solutions of their salts

Published data on structural characteristics of hydration of beryllium and magnesium ions in aqueous solutions of their salts under standard conditions, obtained by various methods, as well as authors’ X-ray data are reviewed. Structural parameters of the immediate environment of Be²⁺ and Mg²⁺, specifically coordination numbers, interparticle distances, and types of ionic association, are discussed. It is noted that Be²⁺ coordinates four water molecules at an average distance of 0.167 nm and Mg²⁺ coordinates six water molecules at an average distance of 0.210 nm. In aqueous solutions of their salts, both Be²⁺ and Mg²⁺ form the second coordination spheres.     

Determination of ion association constants by means of IR absorption band intensities of the molecules forming the first coordination spheres of the cations

The IR spectra of the binary systems CH₃CNLiI, CH₃CNLiClO₄ and CH₃CNLiNCS in the regions of the stretching vibrations of CC and CN bonds of acetonitrile have been recorded. The association constants have been determined on the basis of the analysis of the IR absorption band intensities of acetonitrile molecules. The values of the absorption coefficients of the molecules forming the first coordination spheres of Li⁺ cations have been estimated.     

Adsorption of Co(II), Ni(II), Cu(II), and Zn(II) on hexagonal templated zirconia obtained thorough a sol-gel process: the effects of nanostructure on adsorption features

Using zirconium tetrabutoxide, diaminedecane, and diamineoctane as precursors, a templated hexagonal zirconia matrix is synthesized and characterized by X-ray diffractometry and scanning electron microscopy. The adsorption capacity of such a matrix toward Co(II), Ni(II), Cu(II), and Zn(II) from aqueous solutions is studied. The adsorption affinity of the synthesized hexagonal templated zirconia toward the cations is Cu(II)>Zn(II) >Ni(II)>Co(II). It is also verified that the adsorption of the cations follows a Langmuir and not a Freundlich isotherm. All obtained isotherms are of type I, according to the IUPAC classification. The observed adsorption affinity sequence can be explained by taking into account the velocity constant for the substitution of water molecules into the cation coordination spheres, as well as the Irving-Williams series.     

The non-equilibrium charge screening effects in diffusion-driven systems with pattern formation

The effects of non-equilibrium charge screening in mixtures of oppositely charged interacting molecules on surfaces are analyzed in a closed system. The dynamics of charge screening and the strong deviation from the standard Debye-Hückel theory are demonstrated via a new formalism based on computing radial distribution functions suited for analyzing both short-range and long-range spacial ordering effects. At long distances the inhomogeneous molecular distribution is limited by diffusion, whereas at short distances (of the order of several coordination spheres) by a balance of short-range (Lennard-Jones) and long-range (Coulomb) interactions. The non-equilibrium charge screening effects in transient pattern formation are further quantified. It is demonstrated that the use of screened potentials, in the spirit of the Debye-Hückel theory, leads to qualitatively incorrect results.     

Structural studies of ammonia and metallic lithium-ammonia solutions

The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions.     

Flow Properties of Moderately Concentrated Solutions of Cellulose Acetate Polymer

Calculations [1] of rheological properties of polymeric solutions have usually been restricted to very dilute o r to very concentrated solutions. In either case, one starts with a model for the micro- structure. Between these two conceptual extremes lies the broad class of moderately concentrated solutions. To cover this region theoretically, two approaches have been used; 1) network theories focusing attention on entanglement junctions, and 2) single-molecule theories focusing attention on single molecules but including inter- molecular interactions. Williams, Wang, and Zimm introduced a double model; one for a particular macromolecule and another for solutions containing like molecules in which the chosen macro- molecule is suspended. This approach has turned out to be quite successful because spheres immersed in a suspension of like spheres behave as i f suspended in a Newtonian fluid, and the resulting ex- pression for viscosity is quite simple in form and qualitatively correct for moderate concentrations.     

Solvation of multivalent cations in methanol – Apparent molar volumes,expansibilities, and isentropic compressibilities

Densities of the solutions of gallium, yttrium, lanthanum, gadolinium, and lutetium trifluoromethanesulfonates in methanol have been determined at temperatures ranging from (283.15 to 313.15) K. The sound velocities in these systems have been measured at T = 298.15 K. From obtained data the standard partial molar volumes and their temperature derivatives as well as standard partial molar adiabatic compressibilities for ions in solutions have been estimated. The results have been discussed on the basis of the classical model of ion in solution and compared with our earlier findings for divalent cations. The importance of the numbers of solvent molecules in first solvation spheres of ions has been revealed.     

Metal-cation-mediated nanocrystal arrays of sandwich-type (phthalocyaninato) [tetrakis(4-pyridyl)porphyrinato] cerium complex formed at the water-chloroform interface

Regular square, wirelike, quadrate, and rodlike nanocrystal arrays of Cd2+, Hg2+, or Ag+ metal-cation-mediated sandwich-type mixed (phthalocyaninato) [5,10,15,20-tetrakis(4-pyridyl)poprhyrinato] cerium(III) double-decker complex Ce(Pc)(TPyP) have been successfully prepared at the water-chloroform interface. The nanocrystal growth processes were monitored by transmission electron microscopy (TEM), which reveals that different morphologies of nanocrystals have been fabricated from double-decker molecules connected by different kinds of metal cations, forming coordination polymers. These nanoscaled coordination polymers were characterized by FT-IR spectra and energy-dispersive X-ray spectra (EDS). EDS results clearly revealed the elements of the nanocrystals and the FT-IR spectra give evidence for the coordination interaction between the double-decker molecules and metal cations. The UV-vis absorption spectrum indicates the formation of J-aggregates of the double-decker molecules in the nanocrystals formed.     

Molecular simulation study of temperature effect on ionic hydration in carbon nanotubes

Molecular dynamics simulations have been performed to investigate the hydration of Li(+), Na(+), K(+), F(-), and Cl(-) inside the carbon nanotubes at temperatures ranging from 298 to 683 K. The structural characteristics of the coordination shells of ions are studied, including the ion-oxygen radial distribution functions, the coordination numbers, and the orientation distributions of the water molecules. Simulation results show that the first coordination shells of the five ions still exist in the nanoscale confinement. Nevertheless, the first coordination shell structures of cations change more significantly than those of anions because of the preferential orientation of the water molecules induced by the carbon nanotube. The first coordination shells of cations are considerably less ordered in the nanotube than in the bulk solution, whereas the change of the first coordination shell structures of the anions is minor. Furthermore, the confinement induces the anomalous behavior of the coordination shells of the ions with temperature. The first coordination shell of K(+) are found to be more ordered as the temperature increases only in the carbon nanotube with the effective diameter of 1.0 nm, implying the enhancement of the ionic hydration with temperature. This is contrary to that in the bulk solution. The coordination shells of the other four ions do not have such behavior in the carbon nanotube with the effective diameter ranging from 0.73 to 1.00 nm. The easier distortion of the coordination shell of K(+) and the match of the shell size and the nanotube size may play roles in this phenomenon. The exchange of water molecules in the first coordination shells of the ions with the solution and the ion diffusion along the axial direction of the nanotube are also investigated. The mobility of the ions and the stability of the coordination shells are greatly affected by the temperature in the nanotube as in the bulk solutions. These results help to understand the biological and chemical processes at the high temperature.     

Silica surfaces lubrication by hydrated cations adsorption from electrolyte solutions

Adsorption of hydrated cations on hydrophilic surfaces has been related to a variety of phenomena associated with the short-range interaction forces and mechanisms of the adhesive contact between the surfaces. Here we have investigated the effect of the adsorption of cations on the lateral interaction. Using lateral force microscopy (LFM), we have measured the friction force between a silica particle and silica wafer in pure water and in electrolyte solutions of LiCl, NaCl, and CsCl salts. A significant lubrication effect was demonstrated for solutions of high electrolyte concentrations. It was found that the adsorbed layers of smaller and more hydrated cations have a higher lubrication capacity than the layers of larger and less hydrated cations. Additionally, we have demonstrated a characteristic dependence of the friction force on the sliding velocity of surfaces. A mechanism for the observed phenomena based on the microstructures of the adsorbed layers is proposed.     

Long distance electron-transfer mechanism in peptidylglycine alpha-hydroxylating monooxygenase: a perfect fitting for a water bridge

The active sites of copper enzymes have been the subject of many theoretical and experimental investigations from a number of years. Such studies have embraced topics devoted to the modeling of the first coordination sphere at the metallic cations up to the development of biomimetic, or bioinspired, catalytic systems. At least from the theoretical viewpoint, fewer efforts have been dedicated to elucidate how the two copper cations act concertedly in noncoupled dicopper enzymes such as peptidylglycine alpha-hydroxylating monooxygenase (PHM) and dopamine beta-monooxygenase (DbetaM). In these metalloenzymes, an electronic transfer is assumed between the two distant copper cations (11 A). Recent experimental results suggest that this transfer occurs through water molecules, a phenomenon which has been theoretically evidenced to be of high efficiency in the case of cytochrome b5 (Science, 2005, 310, 1311). In the present contribution dedicated to PHM, we overpass the common theoretical approaches dedicated to the electronic and geometrical structures of sites CuM or CuH restricted to their first coordination spheres and aim at directly comparing theoretical results to the experimentally measured activity of the PHM enzyme. To achieve this goal, molecular dynamics simulations were performed on wild-type and various mutants of PHM. More precisely, we provide an estimate of the electron-transfer efficiency between the CuM and CuH sites by means of such molecular dynamics simulations coupled to Marcus theory joined to the Beratan model to approximate the required coupling matrix elements. The theoretical results are compared to the kinetics measurements performed on wild and mutated PHM. The present work, the dynamic aspects of which are essential, accounts for the experimental results issued from mutagenesis. It supports the conclusion that an electronic transfer can occur between two copper(I) sites along a bridge involving a set of hydrogen and chemical bonds. Residue Gln170 is evidenced to be the keystone of this water-mediated pathway.     

Diffusion of methanol in zeolite NaY: a molecular dynamics study

Molecular dynamics simulations were performed in order to obtain a detailed understanding of the self-diffusion mechanisms of methanol in the zeolite NaY system. We derived a new force-field term to describe the interactions between the methanol molecules and the extraframework cations. From the simulations, we show that diffusive behavior in the high-temperature range consists of a combination of both short- and long-range motions at low and intermediate loadings. This type of motion is characterized by an activation energy that decreases as the loading increases. At low loadings, we also observe short-range diffusive behavior based on a surface-mediated mechanism. The short-range behavior corresponds to motion only on the length scale of an FAU supercage, whereas the long-range behavior involves intercage diffusion. For the saturation loading corresponding to 96 methanol molecules per unit cell, only short-range motions within the same supercage predominate. Finally, the preferential arrangement of the adsorbate molecules around the extraframework cations are examined and compared with those previously deduced from experimental data.     

Identification of molecules in inorganic compounds from X-ray or neutron diffraction data and correction of their structures on the basis of vibrational spectroscopy data

A method of identification of molecules in inorganic crystals with asymmetric M-X...M bridges has been suggested on the basis of analysis of interatomic distances in the structure unit M (−X...M) _n , which includes the atoms of the first and second coordination spheres. This analysis makes it possible to discern molecules (complex anions or cations) as a groups of atoms linked with each other by short M-X bonds, whereas the atoms of neighboring groups are linked by long M...X bonds. The symmetry of such a group is often lower than it follows from X-ray or neutron diffraction data. Studying vibrational spectra affords information on the true symmetry of a molecule. The use of the method is exemplified by the rhombohedral BaTiO₃ phase.     

Sizes and properties of clusters of magnetic ferrite structures

The structures of the coordination spheres or orbits of hexagonal and cubic crystals and their sizes, coordination numbers, and coordinates of atoms and cavities have been studied. The orbits of atoms of all sublattices of octahedral and tetrahedral cavities have been calculated. The close-packed structures (FCC and HCP) of oxygen ions have been considered as basic structures. In both structures, the metal cations are distributed over octahedral and tetrahedral cavities. The developed method is used for calculating the orbits of clusters with the hexagonal crystal structure of magnetoplumbite, ilmenite, and corundum, as well as with the cubic structure of spinel and perovskite.     

Identical extraction behavior and coordination of trivalent or hexavalent f-element cations using ionic liquid and molecular solvents

The extraction of both UO2(2+) and trivalent lanthanide and actinide ions (Am3+, Nd3+, Eu3+) by dialkylphosphoric or dialkylphosphinic acids from aqueous solutions into the ionic liquid, 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide has been studied and compared to extractions into dodecane. Radiotracer partitioning measurements show comparable patterns of distribution ratios for both the ionic liquid/aqueous and dodecane/aqueous systems, and the limiting slopes at low acidity indicate the partitioning of neutral complexes in both solvent systems. The metal ion coordination environment, elucidated from EXAFS and UV-visible spectroscopy measurements, is equivalent in the ionic liquid and dodecane solutions with coordination of the uranyl cation by two hydrogen-bonded extractant dimers, and of the trivalent cations by three extractant dimers. This is the first definitive report of a system where both the biphasic extraction equilibria and metal coordination environment are the same in an ionic liquid and a molecular organic solvent.     

Unravelling the Mystery of Solid Solutions: A Case Study of 89Y Solid-State NMR Spectroscopy

Incorporating heteroatoms in functional materials is an invaluable approach to modulate their properties, assuming a solid solution is formed. However, thorough understanding of key structural information on the resulting solid solution, such as the local environment of cations and vacancies, remains a challenge. Solid-state NMR (SSNMR) spectroscopy is a powerful structural characterization tool, very sensitive to the local environment. Due to the difficulty in signal acquisition and spectral interpretation, SSNMR spectroscopy is relatively less known to chemists and materials scientists. Herein, we present an introductory review to demonstrate how to use ⁸⁹Y SS NMR spectroscopy to unravel the mystery of solid solutions. In general, ⁸⁹Y chemical shift varies with different cation/vacancy arrangements in Y coordination spheres, providing ultrafine structural information in the atomic scale. As a case study and an extreme condition, the approach demonstrated in this review can be extended to other systems.     

Theoretical evaluation of some interactions in the system of acetylene-alkali metal hydroxide-DMSO

Within the ab initio approach and with the use of density functional theory the formation of solvation shells of nondissociated alkali metals hydroxides of the corresponding cations and the hydroxide ion in dimethyl sulfoxide (DMSO) is studied. Complexes in which the alkali metal environment contains the coordinated acetylene molecule along with solvent molecules are considered. The coordination number of the hydroxide ion in DMSO is shown to be 4. It is demonstrated that solvated cations of alkali metals cannot form π-complexes with the acetylene molecule, whereas the introduction of molecular acetylene into the solvation sphere of nondissociated NaOH and KOH is possible.     

Structural parameters of close surroundings of Sr2+ and Ba2+ ions in aqueous solutions of their salts

Published data on various techniques of studying structural characteristics of close surroundings of strontium(II) and barium(II) ions in aqueous solutions of their salts under standard conditions (coordination numbers, interparticle distances, parameters of the second coordination sphere, and ionic association types) have been generalized. It has been concluded that the Sr²⁺ ion coordinates eight water molecules in the first coordination sphere, and the Ba²⁺ ion, nine water molecules, at average distances of 0.262 and 0.283 nm, respectively. Both ions form the second coordination sphere at average distances of 0.493 and 0.500 nm, respectively. There is a well-pronounced trend to the formation of ion pairs.     

Spatial structure of water over the whole region of short-range ordering

A new approach to spatial analysis of molecular arrangement over the whole region of short-range ordering is suggested. The structural properties of water are calculated by the Monte-Carlo method using the SPC/E intermolecular interaction potential. Structural correlations are considered at distances of up to 10 Å. The functions g _OO, g _OH, and g _HH, and the partial functions corresponding to different local densities of surroundings and different coordination numbers of water molecules have been obtained. An analytical procedure has been developed to find the spatial distribution of particles, and the total and partial functions have been determined for particles of the same (having identical c.n.₁ and c.n.₂) and different types. The functions are considered for layers R _min < R < R _max corresponding to different coordination spheres of water molecules (up to 10 Å). A structural model of water is suggested, in which the deviation of the 3D net of H bonds from tetrahedricity is associated with the formation of configurations complementary to icelike configurations of water. The model is supported by the results of computer simulation.