Ammonium ions in alkali metal halide crystals: tunnelling and spin relaxation

We have used low energy inelastic neutron scattering spectroscopy to examine the tunnelling spectroscopy of the ammonium ion in the (NH4)0.02Rb(x)K(0.98-x)I system. The concentration of different species were varied as x increased, this was followed systematically and the first consistent assignment scheme for these features is given. Differences were also found for the relaxation rate of the spin temperature inversions that could be generated in these species. At a critical concentration--about x = 0.04 mole fraction--the relaxation rates of the species changed dramatically.     

A view and a review of melting of alkali metal halide crystals

Theoretical models for the melting of solids are inadequate because relatively little is known about the structures of liquids formed and the factors that control this phase transformation. In the present analysis of fusion phenomenon, usually considered to be a physical change, it is pointed out that, for many solids (e.g., metals and some simple ionic salts) melting involves the redistribution of primary valence bonds. Accordingly, this review includes examination some more chemical aspects of the controls of melting. The available data show that enthalpy and density changes during liquefaction and solidification of the metallic elements and of the alkali halides are small. From quantitative consideration of these values, it is concluded that ordered packing arrangements of atoms, ions, or molecules, comparable with those of crystals, must be extensively retained into the melt. The energy and molar volume changes on melting are too small to allow significant departure, in the liquid, from the regular, efficient space-filling arrays that characterize crystalline solids. The set/liq model for melting (dynamic equilibria between alternative ordered structures) is proposed to account for the properties of the liquid. A detailed and critical comparison of melting with solid state decompositions considers the kinetics and the mechanisms of the changes that occur during the supply/removal of energy to/from the melt/crystal contact interface. Other relevant aspects of melting are discussed including the factors that determine the magnitudes of the melting points of individual solids.     

The vibrational spectra of sulfate ions in alkali halide crystals

Single crystals of KCl and KBr grown from melts doped with magnesium, calcium, barium and lead sulfates at about 0.1 mole per cent have been examined in the fundamental vibrational region of the sulfate ion with an infrared grating spectrometer at 35 and −190°C. Some samples employed sulfate enriched in ³⁴S to about 45 per cent. Except in the case of magnesium, the infrared absorption spectra can be interpreted on the basis of C_2v symmetry arising from the perturbation of the sulfate T_d symmetry by an adjacent divalent cation, i.e. the barium, calcium and lead ions form ion pairs with the sulfate within the alkali halide lattice.The triply degenerate SO stretching and bending modes are split into three unequal components; the observed frequencies in the stretching region are in reasonable quantitative agreement with values calculated on the basis of changes in the bond stretching force constants induced by the adjacent excess positive charge of the metal ions; such changes are calculated to be of the order of Δf = 1 mdynÅ compared with observed values ranging from 0.6 to 0.9 mdynÅ.     

A view and a review of the melting of alkali metal halide crystals

Summary A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described in Part 1 [1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order [1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively reviewed in [3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides [3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded that the set/liq model for melting, proposed in [1, 2], accounts for some important properties of the phase diagrams presented in [3].     

A view and a review of the melting of alkali metal halide crystals

Summary Although melting is a most familiar physical phenomenon, the nature of the structural changes that occur when crystals melt are not known in detail. The present article considers the structural implications of the changes in physical properties that occur at the melting points, T_m, of the alkali halides. This group of solids was selected for comparative examination because the simple crystal lattices are similar and reliable data are available for this physical change. For most of these salts, the theoretical lattice energies for alternative, regular ionic packing in 4:4, 6:6 and 8:8 coordination arrangements are comparable. Density differences between each solid and liquid at T_mare small. To explain the pattern of quantitative results, it is suggested that the melt is composed of numerous small domains, within each of which the ions form regular (crystal-type) structures (regliq). The liquid is portrayed as an assemblage of such domains representing more than a single coordination structure and between which dynamic equilibria maintain continual and rapid transfers of ions. T_mis identified as the temperature at which more than a single (regular) structure can coexist. The interdomain (imperfect and constantly rearranging) material (irregliq) cannot withstand shear, giving the melt its fluid, flow properties. From the physical evidence, it is demonstrated that the structural changes on melting are small: these can accommodate only minor modifications of the dispositions of all, or most, ions or larger changes for only a small fraction. This proposed representation, the set/liq melt model, may have wider applicability.     

Ab initio calculations for absorption and emission energies of alkali halide crystals doped with thallium

We calculate transition energies associated with optical properties of thallium doping in alkali halide crystals via an atomic cluster of minimal size where an sp‐valence‐shell impurity enters as a substitutional defect in the model crystal. Hartree–Fock (HF), density functional theory (DFT), and configuration interaction (CI) [CIS (CI with single excitation) and QCISD (single plus double and quadruple excitation)] calculations are performed to theoretically obtain the absorption and emission energies as vertical transitions evaluated at the ground and first excited‐state optimized geometries, respectively, where the optimization is carried out separately with the HF and DFT methods. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 785–790, 2000     

Absolute complex formation cross sections for alkali—alkali halide collisions: a Monte Carlo trajectory study

A classical Monte Carlo trajectory study has been performed, where the potential energy function used is an ideal dipole potential with its center displaced from the center of mass. The potential shape depends on two parameters. For one choice of parameters, the theoretical Roach and Child surface is well approximated at large and intermediate distances. Several other choices of parameter values have also been used. Complex formation cross sections and angular distributions at the centrifugal barrier have been determined, and a comparison with the theoretical quantities for a spherically symmetric potential is made. The previously proposed mechanisms, assumed to explain the experimentally found low values of σ_c and the quantity of Γ, have not been found to be significant. A dynamical picture is used to interpret the present results. It also provides an interpretation of the previous experimental results. In this picture rotational energy transfer at the barrier is of great importance.     

Formation of molecular π-complexes upon the mechanochemical reaction of the freshly formed surface of alkali halide crystals with some organic compounds

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, p. 2646, November, 1988.     

Current and future perspectives in research on lignocellulose biodegradation

The present article is an attempt to briefly summarize the findings of the meeting held at the Glasshouse Crops Research Institute on the 15th and 16th September, 1983. Over 120 people from a range of scientific disciplines attended and both formal presentations were given and poster sessions took place. Currently the lignocellulose biodegradation field is at an exciting stage where breakthroughs in one or several areas of the fundamental work will have immediate effects on the range of technologies that are currently possible for the utilization of plant wastes.     

Some flow effects in continuum theory for smectic liquid crystals

This paper presents a constrained theory for smectic C liquid crystals that may be useful for the analysis of some effects in these materials. The theory is based on two simplifying assumptions, namely that the layers although deformed remain of constant thickness, and also that the tilt with respect to the layer normal remains fixed. The equilibrium version of the theory proves to be a non-linear generalization of the earlier Orsay theory, and promises to model a number of static effects satisfactorily. Here our aim is to examine preliminary predictions based on the corresponding dynamic theory, where some progress proves possible for shear flow, and also for a shear wave reflection-refraction experiment useful for the measurement of some viscous coefficients.     

Vibronic and resonance Raman spectra of NO2impurity ions in the body-centered alkali halide crystals

The low-temperature resonance secondary radiation spectrum as well as the absorption and luminescence excitation spectra of NO₂⁻ impurity ions in cesium halides have been studied. The energy relaxation processes and NO₂⁻ equilibrium orientation and reorientation problems have also been discussed. It was shown that the systems under study were characterized by average Stokes' losses and strong lattice distortions, exemplified by the Generation of a number of low-frequency local and pseudolocal vibrations. The inhomogeneous broadening in CsCl-NO₂⁻ and CsI-NO₂⁻ spectra was extremely large for the simple molecular impurity systems, leading to the interesting peculiarities of energy relaxation processes. Unlike some alkali halide crystals with NaCl structure the impurity NO₂⁻ does not rotate in the lattices with CsCl structure. The NO₂⁻ equilibrium orientation in cesium halides was fixed: both the molecular axis and the axis perpendicular to the molecular plane were directed in (100) directions of the crystal.     

Some past achievements and future perspectives in main group chemistry

A survey of some recent developments and past achievements in low-valent main group chemistry is presented. Some emerging implications of this area of chemistry in materials science, catalysis and new reagent development are also discussed.     

Some future works of research in electrochemistry

Journal of Solid State Electrochemistry - From the brief critical overview of such energy conversion processes as fuel cell (FC), electrolysis of water/hydrochloric acid and discharge/charge of...     

From conception to realization: an historial account of graphene and some perspectives for its future

There has been an intense surge in interest in graphene during recent years. However, graphene-like materials derived from graphite oxide were reported in 1962, and related chemical modifications of graphite were described as early as 1840. In this detailed account of the fascinating development of the synthesis and characterization of graphene, we hope to demonstrate that the rich history of graphene chemistry laid the foundation for the exciting research that continues to this day. Important challenges remain, however; many with great technological relevance.     

A set of molecular models for alkali and halide ions in aqueous solution

This work presents new molecular models for alkali and halide ions in aqueous solution. The force fields were parameterized with respect to the reduced liquid solution density at 293.15 K and 1 bar, considering all possible ion combinations simultaneously. The experimental target data are reproduced with a high accuracy over a wide range of salinity. The ion models predict structural properties of electrolyte solutions well, such as pair correlation functions and hydration numbers. The force fields provide good predictions of the properties studied here in combination with different models for water.     

Structural and dynamic properties of concentrated alkali halide solutions: a molecular dynamics simulation study

The physicochemical properties of alkali halide solutions have long been attributed to the collective interactions between ions and water molecules in the solution, yet the structure of water in these systems and its effect on the equilibrium and dynamic properties of these systems are not clearly understood. Here, we present a systematic view of water structure in concentrated alkali halide solutions using molecular dynamics simulations. The results of the simulations show that the size of univalent ions in the solution has a significant effect on the dynamics of ions and other transport properties such as the viscosity that are correlated with the structural properties of water in aqueous ionic solution. Small cations (e.g., Li+) form electrostatically stabilized hydrophilic hydration shells that are different from the hydration shells of large ions (e.g., Cs+) which behave more like neutral hydrophobic particles, encapsulated by hydrogen-bonded hydration cages. The properties of solutions with different types of ion solvation change in different ways as the ion concentration increases. Examples of this are the diffusion coefficients of the ions and the viscosities of solutions. In this paper we use molecular dynamics (MD) simulations to study the changes in the equilibrium and transport properties of LiCl, RbCl, and CsI solutions at concentrations from 0.22 to 3.97 M.     

Lower limit for disappearance rate of F-centers in irradiated alkali halide compounds

Conclusions A study was made of the destruction rate of the F-centers In NaCl, KC1, KBr, and Csl monocrystals at room and higher temperatures; the rate constants and activation energies were measured. The process for the destruction of the F-centers is independent of the temperature below a certain temperature.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1164–1166, May, 1976.     

Determination of alkali and halide monovalent ion parameters for use in explicitly solvated biomolecular simulations

Alkali (Li(+), Na(+), K(+), Rb(+), and Cs(+)) and halide (F(-), Cl(-), Br(-), and I(-)) ions play an important role in many biological phenomena, roles that range from stabilization of biomolecular structure, to influence on biomolecular dynamics, to key physiological influence on homeostasis and signaling. To properly model ionic interaction and stability in atomistic simulations of biomolecular structure, dynamics, folding, catalysis, and function, an accurate model or representation of the monovalent ions is critically necessary. A good model needs to simultaneously reproduce many properties of ions, including their structure, dynamics, solvation, and moreover both the interactions of these ions with each other in the crystal and in solution and the interactions of ions with other molecules. At present, the best force fields for biomolecules employ a simple additive, nonpolarizable, and pairwise potential for atomic interaction. In this work, we describe our efforts to build better models of the monovalent ions within the pairwise Coulombic and 6-12 Lennard-Jones framework, where the models are tuned to balance crystal and solution properties in Ewald simulations with specific choices of well-known water models. Although it has been clearly demonstrated that truly accurate treatments of ions will require inclusion of nonadditivity and polarizability (particularly with the anions) and ultimately even a quantum mechanical treatment, our goal was to simply push the limits of the additive treatments to see if a balanced model could be created. The applied methodology is general and can be extended to other ions and to polarizable force-field models. Our starting point centered on observations from long simulations of biomolecules in salt solution with the AMBER force fields where salt crystals formed well below their solubility limit. The likely cause of the artifact in the AMBER parameters relates to the naive mixing of the Smith and Dang chloride parameters with AMBER-adapted Aqvist cation parameters. To provide a more appropriate balance, we reoptimized the parameters of the Lennard-Jones potential for the ions and specific choices of water models. To validate and optimize the parameters, we calculated hydration free energies of the solvated ions and also lattice energies (LE) and lattice constants (LC) of alkali halide salt crystals. This is the first effort that systematically scans across the Lennard-Jones space (well depth and radius) while balancing ion properties like LE and LC across all pair combinations of the alkali ions and halide ions. The optimization across the entire monovalent series avoids systematic deviations. The ion parameters developed, optimized, and characterized were targeted for use with some of the most commonly used rigid and nonpolarizable water models, specifically TIP3P, TIP4P EW, and SPC/E. In addition to well reproducing the solution and crystal properties, the new ion parameters well reproduce binding energies of the ions to water and the radii of the first hydration shells.     

Nanovaccines for cancer immunotherapy: Current knowledge and future perspectives

Cancer immunotherapy harnesses the immune system to attack tumors and has received extensive attention in recent years. Cancer vaccines as an important branch of immunotherapy are designed for delivering tumor antigens to antigen-presenting cells (APCs) to stimulate a strong immune response to against tumors, representing a potentially therapeutic and prophylactic effect with the long-term anti-cancer benefits. Nevertheless, the disappointing outcomes of their clinical use might be attributed to dilemma in antigen selection, immunogenicity, lymph nodes (LNs) targeting ability, lysosomal escape ability, immune evasion, etc. Nanotechnology, aiming to overcome these barriers, has been utilized in cancer vaccine development for decades. Numerous preclinical and clinical studies demonstrate positive results in nanomaterials-based cancer vaccines with considerable improvement in the vaccine efficacy. In this review, we systematically introduced the characteristics of nanovaccines and highlighted the different types of nanomaterials used for cancer vaccine design. In addition, the opportunities and challenges of the emerging nanotechnology-based cancer vaccines were discussed.