Showing papers on "Chemical binding published in 1980"

Effective-medium theory of chemical binding: Application to chemisorption

Abstract: An approximate theory of the total energy change connected with the embedding of an atom in an inhomogeneous host is developed. The primary effect of the inhomogeneous environment is included by replacing it with a homogeneous electron gas of a density equal to that of the host at the atom site. The lowest-order corrections to this simple picture are derived. The scheme, which is computationally very simple, is tested against first-principles calculations for several chemisorption systems. It is found that including a simple first-order correction gives excellent agreement for H and O adsorbates. For less electronegative atoms like Si and Li, it seems that a second-order term involving the polarizability of the atom in a homogeneous electron gas must be included.

A systems approach to immunology

Abstract: A tutorial overview of system analysis in the rapidly changing field of immunology is presented here. Many concepts which are well entrenched in engineering practice are shown to be relevant to immunology. For example, state-space analysis and a network topology arise naturally for the generation of lymphocytes (a class of white-blood cells) and in turn antibodies according to the clonal selection theory. Engineers embroiled in large-scale network problems may well appreciate immunological complexes whereby the humoral subsystem alone includes some 1012lymphocytes and 1020antibody molecules of 107different chemical affinities in the immunologically unstimulated human body. Unfortunately, linear system theory, so significant in electrical networks, is shown to have limited application to immunology. On the other hand, coupled bilinear control systems and rate-limited processes arise due to the nature of cell division, differentiation, and chemical binding. Similarly, a chemical cascade of nonlinear amplifiers is generated as the complement subsystem for such purposes as tumor cell lysis. This controlled use of instability by means of parametric manipulation, which is so effective in disease control, may very well provide a clue for new design methodologies in engineering practice.

The concept of pressure in density functional theory

Abstract: Using density functional theory, a prescription is given for determining the internal scalar pressure at a point within an atom or molecule. From this expression for the pressure, a thermodynamic expression for the energy is obtained in terms of the chemical potential of the reference system, forces acting on the system, the internal pressure and an additional component X[ρ]; namely, Here μ0 is the chemical potential in the absence of fields, v is the potential due to nuclei, φ is the potential due to electrons, P is the scalar pressure, and X[ρ] is defined by the formula where T[ρ] and K[ρ] are the exact kinetic energy and exchange–correlation functionals. The functional X[ρ] vanishes as N approaches infinity, or in Thomas–Fermi or Thomas–Fermi–Dirac approximation, yet it is necessary to account for chemical binding.

Chemical binding energies of point defects in palladium doped with hydrogen and d impurities

Abstract: 2014 Using an extra-orbital model within a generalized « spd » tight-binding approximation, we calculate the chemical binding energy of a pair of hydrogen atoms in palladium within the infinite dilution limit; we find that clusters of hydrogen cannot form in accordance with inelastic neutron scattering data in 03B1-palladium hydride. The strong repulsive interaction between a pair of hydrogen impurities at distance a, where a is the host lattice parameter, seems to be related to the presence of an intermediate metallic site between the two impurities. Also we determine the chemical binding energies between a hydrogen atom and d substitutional impurities and we discuss our results in relation with recent Mössbauer experiments. Finally we calculate the chemical binding energy between two d impurities in palladium which we compare with experimental data on the enthalpy of formation. J. Physique 41 (1980) 1001-1007 SEPTEMBRE 1980, 1 Classification Pltysics Abstracts 71.55

Stable silicate glasses containing up to 10 weight percent of water

Abstract: Alkali silicate glasses containing other metal oxides such as PbO, ZnO, Al2O3, BaO, MgO, P2O5, B2O3, and ZrO2 in amounts of 0–20 mol% total are hydrated in a steam atmosphere at elevated temperatures resulting in a water content of 3–30% by weight. Various species of water are shown to exist in the hydrated glasses. They consist of the free silanol groups, hydrogen-bonded silanol groups, monomeric molecular water and polymeric species of molecular water up to hexamer. These species of water are in true solution in the homogeneously hydrated silicate glasses. Four types of hydration kinetics are distinguished among the silicate glasses of various compositions. The properties of the resulting products are compared and the origin of the differences is discussed. The properties of hydrated glasses are studied with measurements of infrared absorption spectra, thermogravimetric analysis, durability, viscosity and stress-strain behavior. The relative polarizing power of cations in silicate glasses are calculable from infrared spectra. The overall polarizing power of all the cations in a silicate glass is proportional to the fraction of free silanol groups in the glass and is a function of the glass composition. If reflects the extent of the chemical reaction and chemical binding of the various species of water in the glass, and offers a clue in synthesizing hydrated glasses with no loosely bonded molecular water. Theories are proposed to explain various observed phenomena and properties of the hydrated glasses. The structure cohesion of hydrated glasses is distinguished from those of the anhydrous glasses, invert glasses and organic polymers. A flow mechanism is proposed to explain the lowering of viscosity due to the presence of water in glass. A dynamic structure of the bonding of water in glass is deduced from the combined data (infrared and others) of the present study. With this model, the unusual combination of properties may be understood.

Electron distribution in water by high-energy electron scattering

Abstract: The differential cross-sections for high-energy electrons scattered from water have been measured over a wide range of momentum transfers. The effect of chemical binding was seen from the comparison between the experiment and the calculation for an independent-atom model. The ab initio calculation using SCF MO was carried out with respect to the elastic scattering. It was in a good agreement with the experiment and thus a reliable electron distribution in water was obtained.

Atomic electronegativity from density functional theory

Abstract: The atomic electronegativity, identified with the chemical potential of energy density functional theory, has been studied by using approximate functionals containing gradient corrections to the kinetic energy. The electronegativity has been found to be related to the asymptotic behavior of the electron density in the free atom. By requiring good agreement between the density functional electronegativity and the Mulliken scale we suggest that the Weizsacker coefficient (λ=1/8) in the gradient term is more accurate than the Kirzhnits coefficient (λ=1/72) for the valence electrons and then for chemical binding studies.

Molecular charge distribution and chemical binding in five‐membered heterocycles. I

Abstract: Electron density maps for the heterocycles thiophene, furan, and pyrrole are determined from ab initio 4‐31G wavefunctions. The charge distributions in these molecules are analyzed in terms of the total molecular density and difference density maps and their profiles. The atomiclike core, especially the L core of sulfur, is found to play an important role, via its polarization and interaction, in determining the extent and direction of valence density transfer from the carbon to the heteroatom. The changes in the charge distributions that occur in the immediate vicinity of the heteroatoms and the relation of density quantities to binding and antibinding characteristics are discussed. The quantum topological features of the molecular charge distributions of the three heterocycles are analyzed and discussed and the different bonding situations, e.g., ring strain, ionic and covalent binding, etc., are compared in a model‐independent way.

Impaired permeability of the vitelline membrane to macromolecules in small eggs of hens poisoned with ethylene dibromide

Abstract: 1. 1. The rate of passage of proteins and polyvinylpyrrolidone (PVP) was measured in vitelline membranes from control and ethylene dibromide (EDB)-treated hens. 2. 2. The rate constant of the EDB group was lower than that of the control with all proteins tested. The effect of EDB on passage rate increased with the molecular weight of proteins. In contrast, the transfer of PVP was faster in the EDB group than in the control. 3. 3. The free amino groups of membranes from the EDB-group showed a decrease from the control when tested by fluorodinitrobenzene, but did not differ from the control by glutaraldehyde. No effect on SH groups was found. 4. 4. It is assumed that EDB changes the permeability by altering the structure of the membrane rather than by chemical binding to its constituents.

Localized molecular orbitals and chemical binding in five-membered heterocycles. II

Abstract: The localized molecular orbitals (LMOs) of thiophene, furan, and pyrrole are derived from ab initio 4-31G wavefunctions using Boys' criteria for localization. From the transferability point of view, these LMOs are classified as (i) completely different and nontransferable LMOs (these are the lone-pair orbitals on O and N on one hand and those on S on the other hand), (ii) chemically similar lone pairs and inner shells on O and N (of furan and pyrrole, respectively), and (iii) chemically equivalent CC, CC, and CH LMOs in the three heterocycles. The sp3 hybridization of the L core of sulfur, its appreciable polarization, and considerable involvement in bonding in the CS bond region have been discussed. The present investigation indicates the limitation of the application of semiempirical MO methods to molecules that contain second-row atoms due to both the appreciable core—valence and π-σ interactions involving such atoms. Qualitative investigation of aromaticity and reactivity of the studied heterocycles agrees satisfactorily with experimental observations and shows that conclusions drawn based solely on static factor considerations (charge distribution in the noninteracting molecules) might very well be misleading and such factors determine the ease rather than the final orientation of the substituent.

Transport of Thermal Neutrons in D2O in the Temperature Range 5 to 60°C Based on a New Scattering Kernel

Abstract: A new scattering kernel for heavy water has been proposed. The kernel takes into account the chemical binding energy effects and also includes the rotational and intramolecular vibrational modes. Using this scattering kernel, various neutron transport processes in the temperature range 5 to 60/sup 0/C have been studied and compared with the corresponding experimental results. The calculated results include total neutron scattering cross section at 20/sup 0/C; asymptotic decay of neutron pulses in the temperature range 5 to 60/sup 0/C and temperature variation of the diffusion coefficient and diffusion cooling coefficient; timedependent spectra inside finite-sized assemblies of heavy water at 20 and 43.3/sup 0/C thermalization time; and diffusion length and space-dependent study in pure and poisoned assemblies of heavy water. The calculated results are in good agreement with the experimental results. At some places notable differences are observed between the results obtained using our scattering kernel and those based on the Honeck kernel.

An Electrostatic Description of Chemical Binding

Abstract: The reasons for our interest in charge-density maps are probably extremely varied But surely one important reason is the hope of learning something about chemical binding For this purpose it is useful to think of the chemical bond in electrostatic terms

Oxygen transporting agent for artificial blood

Abstract: PURPOSE: To oxygen transporting agent for artificial blood, containing a combination product of hemoglobin with inultin, having a long residence time in the blood vessel and the oxygen transporting ability almost equal to that of the hemoglobin, and nontoxic to the human body. CONSTITUTION: Amino groups in hemoglobin derived from an animal, e.g. man, cattle or pig, having the hemoglobin is combined with hydroxyl groups in inulin having a molecular weight of 4,000W6,000 by using a condensing agent, e.g. cyanogen bromide, or with chemical binding agent, e.g. cyanuric chloride or 2,2'-dichlorobenzidine. The number of bonds per molecule of the hemoglobin is suitably 15W50 to such an extent as not to increase the viscosity excessively. The combination product of the hemoglobin with the inulin prolongs the half-life to twice in blood plasma in tests on rats. The results obtained by measuring the 50% oxygen dissociation pressure showed that the product had the affinity for oxygen almost equal to that of the hemoglobin. COPYRIGHT: (C)1982,JPO&Japio