Showing papers on "Ionic bonding published in 1979"

The stability of ionic crystal surfaces

Abstract: When there is a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal, lattice sums in the electrostatic energy diverge and the calculated surface energy is infinite. The cause of this divergence is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types. Type 1 is neutral with equal numbers of anions and cations on each plane and type 2 is charged but there is no dipole moment perpendicular to the surface because of the symmetrical stacking sequence. Both these surfaces should have modest surface energies and may be stable with only limited relaxations of the ions in the surface region. The type 3 surface is charged and has a dipole moment in the repeat unit perpendicular to the surface. This surface can only be stabilised by substantial reconstruction. These conclusions are important for the analysis of the surface structure of ionic crystals.

Staphylococcal nuclease: proposed mechanism of action based on structure of enzyme-thymidine 3',5'-bisphosphate-calcium ion complex at 1.5-A resolution.

Abstract: The structure of the staphylococcal nuclease (EC 3.1.4.7)—thymidine 3′,5′-bisphosphate—Ca2+ (enzyme—inhibitor) complex has been extended to 1.5-A resolution by using much additional data and a phase refinement scheme based on an electron-density map modification procedure. By correlating this structure with the known properties of the enzyme, a mechanism of action is proposed that involves nucleophilic attack on phosphorus by a water molecule, which is bound to Glu-43, in line with the 5′-CH2O(H) leaving group. The carboxylate of Glu-43 promotes this attack by acting as a general base for the abstraction of a proton from the attacking water molecule. Nucleophilic attack is further facilitated by polarization of the phosphodiester by an ionic interaction between a Ca2+ ion and a phosphate oxygen atom and by four hydrogen bonds to phosphate oxygen atoms from guanidinium ions of Arg-35 and Arg-87. These interactions may also catalyze the reaction by lowering the energy of a trigonal bipyramidal transition state. The hydrolysis of nucleic acid substrate proceeds by cleavage of the 5′—P—O bond to yield a free 5′-hydroxyl group and a terminal, 3′-phosphate monoester group. In the inhibitor complex the only general acid group found in a position to donate a proton to the leaving 5′-oxygen is the guanidinium ion of Arg-87. Alternative proton donors, presently lacking direct structural support, could be the phenolic hydroxyl group of Tyr-113 or a water molecule. The precision and rigidity of the location of the reactants at the active site and the probable dual binding and catalytic roles of the guanidinium ions of Arg-35 and Arg-87 are especially noteworthy.

Experimental studies of molecular interactions between nitrogen bases of nucleic acids.

Abstract: New experimental results concerning molecular interactions between the nitrogen bases of nucleic acids in the crystalline phase and in vacuo are reported. The temperature dependence of the evaporation rate is measured for solid species. The sensitivity of conventional methods of sublimation heat measurements was improved essentially using a quartz resonator serving as a precise sensor of evaporation rate. Sublimation heats were found for both canonical bases and a number of their derivatives. The in vacuo formation of base associates interacting through hydrogen bonds was observed with a field mass spectrometer. The dimer formation enthalpies, which are indicative of a stronger attraction in complementary pairs compared with noncomplementary ones, were derived from the temperature dependence of ionic currents. Hydrogen-bound complexes of more intricate associates (base trimers and aqueous molecules associates) were studied. The energy gain in the formation of trimers of identical molecules was shown to be larger (per base molecule) than that for dimers.

Vibrational spectroscopy of cation‐site interactions in phosphate glasses

Abstract: Thirty different binary metal oxide glasses having the metaphosphate composition have been prepared containing cations from the following groups: alkali metals, alkaline earths, transition metals, and lanthanide and actinide metals. Far infrared absorption assigned to the cation vibration in its oxygen cage has been measured for these glasses. Empirical ionic models are proposed correlating the absorption maximum with the cation mass, charge, and ionic radius. Discrepancies between the observed and predicted vibrational frequency indicate a more covalent interaction between the metal cation and the glass network. Raman intensities and vibrational frequencies of the network metaphosphate vibrations have been obtained and provide additional evidence about the cation–site interaction.

The surface energies, surface tensions and surface structure of the alkali halide crystals

Abstract: Calculations have been made on the (100) and (110) surfaces of the alkali halides using recently derived empirical potentials. The ionic positions were relaxed and the surface energies and surface tensions obtained are compared with previous calculations and the available experimental data. Two different short-range potentials were used with each surface studied, enabling a comparison to be made. The sensitivity of the results to the potential is examined. The problems associated with the charged (111) surfaces are discussed and observed trends in surface energy and tension are described in terms of the surface structure. The calculated surface structure gives the characteristic opposite displacements of cations and anions and is considered in detail for both the (100) and (110) faces of the lithium halides.

An absolute method for the determination of the persistence length of native DNA from electron micrographs

Abstract: Information on spatial correlation in the tangent direction along electron microscope images of filamentous molecule is shown to be obtainable by the analysis of statistical fluctuations in curvature, yielding an absolute measure of the persistence parameter amicro. The relationship of amicro, a local, microscopic parameter, to the persistence length introduced by Kratky and Porod is discussed. The hypotheses underlying the assumed theoretical model concern (1) the shape of the angle distribution, assumed to be Gaussian; (2) the passage from a three- to a two-dimensional situation, which is supposed to occur by deformation of the flexible chain in a manner that preserves the memory of the spatial correlation in orientation (except for the blocking of one degree of freedom); and (3) the adsorption conditions, which should meet the equilibrium requirement as closely as possible. The analytical method has been checked on computer simulated “Gaussian” molecules: the study of the simulated sample was essential in solving the problems connected with minimum statistics requirements and the effect of the reading error. Experimental images obtained for T2 DNA fragments at different ionic strengths by Kleinschmidt's adsorption technique have been analyzed by means of an automatic flying spot digitizer, the “Precision Encoder and Pattern Recognition.” The results show that adsorbed molecules do in fact “remember” the rigidity they possessed in solution and that the Gaussian hypothesis is well verified. Consequently, the slopes of log cosθ(l) or θ2(l) may be used indifferently in the estimate of amicro. The dependence of this parameter on ionic strength in the range explored shows the expected behavior.

Fast Ionic Transport in Solids

Abstract: The discovery of inorganic solids with ionic conductivities comparable to those of aqueous electrolytes has revolutionized solid-state electrochemistry. Sodium beta alumina, a Na+ conductor, and LixTiS2, an intercalation compound with simultaneous Li+ and electronic conductivity, are two of the best and most versatile fast ionic conductors. A wide variety of cations can replace Na+ in beta alumina and Li+ in LixTiS2 and change the properties of the materials. Sodium beta alumina and LixTiS2 are currently used in the development of high-energy density batteries for electric vehicles and electrical utility load leveling. Current research in solid ionic conductors is exploring new intercalation compounds, solid polymer electrolytes, and alkali ion and proton transport in crystalline solids.

Analysis of cooperativity and ion effects in the interaction of quinacrine with DNA.

Abstract: The interaction of quinacrine with calf thymus DNA was monitored at several different ionic strengths using spectrophotometric and equilibrium dialysis techniques. The binding results can be explained, assuming each base pair is a potential binding site, using a model containing two negative cooperative effects: (1) ligand exclusion at binding sites adjacent to a filled binding site and (2) ligand–ligand negative cooperativity at adjacent filled binding sites. The logarithm of the observed equilibrium constant (Kobs) determined by this model varies linearily with log[Na+], as predicted by the ion condensation theory for polyelectrolytes. When the log Kobs plot is correlated for sodium release by DNA in the intercalation conformational change, the predicted number of ion pairs between the ligand and DNA is approximately two, as expected for the quinacrine dication. Even though Kobs depends strongly on ionic strength, the ligand negative cooperativity parameter ω was found to be indpendent of ionic strength within experimental error. This finding is also in agreement with the ion condensation theory, which predicts a relatively constant amount of condensed counterion on the DNA double helix over this ionic strength range. Drugs would, therefore, experience a relatively constant ionic environment when complexed to DNA even though the ionic conditions of the solvent could change considerably.

Defect properties of ionic solids. II. Point defect energies based on modified electron-gas potentials

Abstract: For pt.I see ibid., vol.10, no.9, p.1431 (May 1977). A modified form of the electron-gas approximation is proposed for the calculation of interionic potentials in the solid state. On the basis of these potentials perfect lattice and defect properties of a number of solids are calculated and compared both with experiment and with the results derived from other theoretical methods. The cohesive energies, lattice constants and compressibilities of a wide range of oxides are considered, while the defect energies for NaCl, MgO, MnO, CaF2 and MgF2 are examined in detail. From the results presented it is concluded that for solids such as the alkali and alkaline-earth halides and the alkaline-earth oxides, which are largely ionic, the modified electron-gas approximation is a reliable non-empirical method for the calculation of interionic potentials.

Corresponding states for ionic fluids

Abstract: Corresponding states ideas are applied to correlate a number of equilibrium properties of ionic fluids. Consideration is given to the thermodynamic excess functions used to characterize dilute ionic fluids, to ion pairing and higher association, and to the characteristic coexistence curve and critical point found in many ionic fluids. The correlation applies to simple ionic fluids such as liquid or gaseous salts, to plasmas, and to ionic solutions as well as to the primitive model to the degree that the properties of all these systems are known. New HNC calculations are reported that were made to elucidate the structural changes in an ionic system as it is compressed along an isotherm corresponding to 2–2 aqueous electrolytes. Finally an evaluation is given of the usefulness of a number of approximation methods for calculating the equilibrium properties of the primitive model, and presumeably other Hamiltonian models, in various regions of the corresponding states diagram. Among these methods is the theory of Bjerrum (1926) which is surprisingly accurate, although some of its limitations are illustrated in the present work.

Molecular theory of solvated ion dynamics. II. Fluid structure and ionic mobilities

Abstract: Calculations of ionic mobility based on a molecular theory of solvated ion dynamics [J. Chem. Phys. 68, 473 (1978)] are presented. We develop approximation schemes for the equilibrium averages needed in the theory. The results for ions in water are in remarkably good agreement with experiment. The importance of fluid structure, arising from the finite size of solvent molecules, is clearly illustrated.

Electrostatic lattice sums for semi-infinite lattices

Abstract: The techniques for the rapid computation of energies of three-dimensional neutral periodic assemblies of charged particles are extended to semi-infinite arrays and assemblies of ions in infinite filsm. The results will be useful for simulation of ionic movements in fast-ion conductors and dense colloidal dispersions.

Electrical properties of unusual ionic melts

Abstract: The paper reviews the advances that have been made in recent years in the understanding of electrical transport in fully ionized molten salts, partly dissociated molecular liquids and liquid ionic stoichiometric alloys like CsAu. Special emphasis is placed on the recently observed temperature and pressure induced gradual transition between the limiting cases of molecular insulators and ionic melts. At supercritical temperatures salts undergo a continuous transition from an insulating vapour to a highly conducting ionic fluid if the density is increased sufficiently. This transition is due to a shift of the ionization equilibrium between molecules and ions, in favour of the ions, with increasing density. Poorly conducting molten salts and polar substances like water and ammonia also become more ionic, and consequently better conductors, at very high pressures. Recent thermodynamic, magnetic and electrical measurements on liquid alloys which are composed of two metallic elements and which are non-m...

X‐ray diffraction study of MgCl2 aqueous solutions

Abstract: X-ray diffraction data for three aqueous solutions of MgCl2 were analyzed. It is shown that in a very concentrated solution the structure and correlation functions can be reproduced almost completely by using a nearest-neighbor model of ionic hydration, whereas in the more dilute solutions a certain number of interactions between the hydrated cation and external water molecules must also be considered. The mean coordination geometry for the close hydration was found to be octahedral both for cation and anion with mean cation–water distances (2.10–2.12 A) and mean anion–water distances (3.11–3.14 A) in accordance with results found in other cases.

Spectroscopy of Molecular Ions in Noble Gas Matrices

Abstract: Molecular ions are of considerable chemical and physical interest for comparing the spectroscopic and bonding properties of neutral mole­ cules with their positive and negative molecular ions. The study of molecular ions in the gas phase by photoelectron, photoionization mass, and ion cyclotron resonance spectroscopies can be complemented by infrared and optical absorption spectra of the molecular ion trapped in a solid inert gas host. Further ion studies with tunable infrared lasers will be greatly aided by the vibrational data obtained for molecular ions in noble gas solids. The ion-matrix interaction is of fundamental and practical interest as matrix. spectra of ions are related to the gas phase. Charged species in matrices form two general classes described in the literature as "isolated" and "chemically bound" with respect to the counterion. The first ionic species characterized in matrices, Li + 0;, is of the latter type where the lithium cation and the superoxide anion are Coulombically bound together (1, 2), and charge transfer occurs be­ cause this electrostatic attraction more than makes up for the difference between the ionization energy of lithium and the electron affinity of oxygen. The next molecular ions identified in matrices, B2H; and �-, are of the isolated type where the cation is separated by an unde­ termined number of matrix atoms from the anion (3, 4). These ionic systems have been characterized as "Coulomb ion pairs," which exist because of essentially zero overlap between the wavefunctions for the electron on the recipient molecule and the cation that provided the electron (5). Clearly, the formation of ions of the isolated type requires

Defect structure of the ionic conductor lithium nitride (Li3N)

Abstract: The defect structure of Li3N (space group P6/mmm, a ≃ 3.65, c ≃ 3.88 A) has been investigated with data sets measured at seven temperatures in the range 153 to 678 K. The data have been used for normal and high-order refinements and for the calculation of the corresponding difference electron densities. The final Rw ranged between 0.7 and 1.3%. The N atom position at z = 0 and the Li(1) positions at z = ½ are completely occupied and remain ordered within the whole temperature range. The Li(2) sites at z = 0 already show a vacancy concentration of 1-2% at low temperatures, which does not change significantly with increasing temperature. The Li(2) ions exhibit strong anharmonic thermal vibrations within the Li2N layers at z = 0 along the Li(2)-Li(2) connection lines. The high Li ionic conduction is caused by jumps of Li(2) ions from an occupied Li(2) site into an unoccupied one. The anharmonic thermal vibrations are the precursors of these jumps. Interstitial sites are not involved in this mechanism.

Calculations on ionic solvation. III. The electrostatic free energy of solvation of ions, using a multilayered continuum model

Abstract: For the calculation of the electrostatic free energy (and also the entropy) of solvation of an ion, a model is set up in which an ion of given crystallographic radius is surrounded by a series of concentric spherical layers, each with a different relative permittivity, immersed in the bulk liquid. A complete general solution is given for any number of such layers, both for the electrostatic free energy of solvation and the corresponding entropy term. The dielectric saturation effect is taken into account through the different relative permittivities of the layers. The first layer, next to the ion, is considered to have a special role and to have a relative permittivity e=n2, independent of the dielectric saturation effect of the ion.

The role of metal‐ion binding in modifying the toxic effects of sulphur dioxide on the lichen umbilicaria muhlenbergii

Abstract: Summary K+ efflux from Umbilicaria muhlenbergii was not affected following the uptake of known amounts (< 20 μmol g-1) of Ca2+, Mg2+, Sr2+ and Zn2+, but was increased in the cases of Cu2+ and Pb2+. In a number of samples that had taken up increasing amounts of Cu2+, a discontinuity in K+ release during metal incubation correlated with a decrease in subsequent 14C fixation. Some of the binding sites associated with the first phase of Cu2+ uptake (up to 12 μmol g-1) are interpreted to occur near, on or within the algal cells. The larger K+ loss associated with higher Cu2+ uptake levels indicated that a component of the second Cu2+ uptake phase involved binding at or penetration of the fungal membranes. The uptake of Sr2+, Ni2+ and Zn2+ conferred some protection to samples subsequently exposed to 75 p.p.m. aqueous SO2 for 1 h, while Mg2+, and Ca2+ had no effect. The combined effects of Cu2+ uptake and SOS exposure (and also Pb2+ and SO8) were approximately cumulative. Finally, the results are explained by reference to a classification that separates metal ions into three chemically and biologically significant categories. It is concluded that class A metal ions (those with a preference for ionic interactions) and borderline metal ions with class A character tend to protect lichens against SO2 damage, while borderline metal ions with class B character (a preference for covalent interactions) have the opposite effect.

The internal friction of glasses

Abstract: Basic fundamentals of the elastic behaviour of a standard linear solid and the internal friction technique based on this model are briefly described. The internal friction (Q−1) caused by various thermally activated processes in vitreous solids are reviewed for several glass compositions in relation to ionic mobility and other relevant properties. Similar relaxation mechanisms characterize many vitreous solids. The relaxation of alkali ions is essentially independent of the glass network former and has an activation energy of 15 to 25 kcal mol−1. Other processes like the interaction of oxygen ions and protons, alkali ions and protons, mixed-alkali ion interaction and the motion of single bonded oxygen ions have an activation energy of 30 to 40 kcal mol−1. The activation energy for relaxation of the glass network is 100 to 120 kcal mol−1.