Showing papers on "Chemical binding published in 1992"

Departures from Bragg's rule of stopping power additivity for ions in dosimetric and related materials

Abstract: A survey of tests of Bragg's rule of stopping power additivity is updated, reviewing recent evidence on chemical binding and physical state effects on ion stopping power in dosimetric and related materials. A general failure of simple additivity of constituent element stopping powers is well established, depending on the atomic numbers of the compound components and the type of bonding involved. Magnitudes of discrepancies are subject to some uncertainty, but this is reducing. A modified additivity based on separation and summing of core and bond contributions can be used to fit experimental data reasonably well. Phase effects in low- Z dosimetric materials are ∼10%, near the stopping power maximum for light ions, decreasing as energy increases. ϵ(vapour)> ϵ(condensed phase). Lower energy trends still contain some uncertainties. The accuracy of stopping power measurements is often insufficient to detect and quantify the effects of interest unambiguously. In the absence of adequate measurements or theory for some materials of interest over the wide energy range of interest, some practical empirical guidelines are suggested to provide approximate corrections to simple additivity values for proton stopping power.

Countercurrent Concentration and Gas Secretion in the Fish Swim Bladder

Abstract: Many fish possess a gas-filled swim bladder, and the gas molecules enter the bladder by diffusion from its capillaries and epithelium. The high gas partial pressures necessary to drive diffusion are produced in two steps. First, in the blood perfusing the swim bladder capillaries the physical solubility of gases is reduced via the salting-out effect, and gases are released from a chemical binding site via blood acidification (e.g., Root effect for O₂; conversion of HCO₃⁻ to CO₂). These effects result in an increase in the gas partial pressure in blood. In a second step, this initial increase in partial pressure (the "single concentrating effect") is multiplied by back diffusion of gas molecules in the countercurrent system of the rete mirabile. For inert gases, the salting-out effect, induced by the release of lactic acid from the gas gland cells in the swim bladder tissue, results in a probably small reduction of solubility, while the acid-induced decrease in hemoglobin O₂-carrying capacity (Root effect)...

Effect Of The Temperature Of The Moderator on The Velocity Distribution of Neutrons With Numerical Calculations For H As Moderator

Abstract: In this paper we set up an integral equation governing the energy distribution of neutrons which are being slowed down uniformly thoughout the entire space by a uniformly distributed moderator whose atoms are in motion with a Maxwellian distribution of velocities. The effects of chemical binding and crystal reflection are ignored. When the moderator is hydrogen, the integral equation is reduced to a differential equation and solved by numerical methods. In this manner we obtain a refinement of the dv/v2 law.

Rydberg bonding in ammonium dimer ((NH4)2)

Abstract: Chemical binding of two monovalent Rydberg species to form a singlet-state Rydberg dimer molecule is predicted to be possible Ab initio electronic structure methods that include electron correlation (at levels up through QCISD(T)/6-31++G** MP2(full)/6-31++G** + ZPE) are shown to be essential to achieving a proper description of such bonding. The (NH{sub 4}) molecule, selected as the prototype for this study, is shown to be bound with respect to its Rydberg-species fragments, 2NH{sub 4} by 7.5-9.7 kcal/mol, depending on the level of treatment of electron correlation, and to be electronically stable (by ca.4 eV) with respect to (NH{sub 4}){sub 2}{sup +} at the neutral`s equilibrium geometry. The (NH{sub 4}){sub 2} Rydberg dimer is thermodynamically unstable with respect to 2NH{sub 3} + H{sub 2} by 86-89 kcal/mol mol yet possesses all real vibrational frequencies; it is thus a metastable molecular held together by a weak Rydberg bond. The dissociation energy of the (NH{sub 4}){sub 2}{sup +} cation to form NH{sub 4}{sup +} + NH{sub 4} is found to be larger than that of the neutral (NH{sub 4}){sub 2}. 12 refs., 4 figs., 9 tabs.

Morphology of cell-substratum adhesion. Influence of receptor heterogeneity and nonspecific forces.

Abstract: Many cell types modulate growth, differentiation, and motility through changes in cell substrate adhesion, including regulation of focal contact formation. Clustering of cell surface adhesion receptors is an essential early step in the development of focal contacts, and thus may influence cell physiology. In this paper, we present a theoretical framework to examine how cell surface chemistry affects receptor clustering. Our one-dimensional tape-peeling model couples the equations of mechanical equilibrium for a cell membrane with kinetic receptor-ligand binding relations. We considered two distinct model scenarios: Adhesion mediated by multiple receptor-ligand interactions of different length and specific binding of a single receptor type occurs in the presence of van der Waals attraction and nonspecific repulsion. In each case, nonuniform (wave-like) membrane morphologies are observed in certain parameter ranges that support the clustering of adhesion receptors. The formation of these morphologies is described in terms of a balance of membrane stresses; when cell-surface potential as a function of separation distance is symmetric between two potential energy minima, nonuniform morphologies are obtained. Increases in the chemical binding energy between receptor and ligand (e.g., increases in ligand density) or decreases in the membrane rigidity result in smaller wavelengths for nonuniform interfaces. Additionally, we show wave-like geometries appear only when the mechanical compliance of receptor-ligand bonds is within an intermediate range, and examine how the mobility of “repellers”—glycocalyx molecules that exert a nonspecific repulsive force—influences membrane morphology. We find fully mobile repellers always redistribute to prevent nonuniform morphologies.

Zum Verhalten des Spurenelementes Quecksilber in Steinkohlefeuerungen mit Rauchgasreinigungsanlagen

Abstract: In hard coal-fired power plants small amounts of heavy metal and heavy metal compounds are fed into the boiler via the fuel. In this connection mercury is of particular importance. Flue gas analysis turned out that the chemical binding form of mercury in flue gas cleaning devices is of enormous importance for its behaviour. The measuring technique as well as the influences of process technology on the whereabouts of mercury are described

Enzyme membrane based upon polyamide-6 for oil hydrolysis

Abstract: This work presents the preparation of the enzyme membrane based upon polyamide-6 (PA-6) destined for the hydrolysis of oils. The transport properties of the base PA-6 membranes, i.e., their porosity and permeability coefficients, as well as the mode of their modification, aimed at the chemical binding of lipase molecules have been estimated. © 1992 John Wiley & Sons, Inc.

Cell sorting using immunomagnetic beads.

Abstract: Immunomagnetic beads are uniform, polymer particles coated with a polystyrene shell that provides both a smooth hydrophobic surface to facilitate physical absorption of molecules, such as antibodies, and surface hydroxyl groups that allow covalent chemical binding of other bioreactive molecules, such as streptavidin, lectins, and peptides. Iron (III) oxide (Fe(2)O(3)) deposited in the core gives the beads superparamagnetic properties that lead to consistent and reproducible reactions to a magnetic field without permanent magnetization of the particles. These are the two qualities on which immunomagnetic separation (IMS) depends.

Xenon ion beam mixing in Ni3N/Al bilayers

Abstract: The effects of Xe irradiations on surface and interface modifications of Ni3N/Al bilayers deposited via de (reactive) sputtering on silicon substrates were investigated. The 250–1000 keV Xe irradiations were performed at RT and at fluences of 0.5-2×1016 ions/cm2. The samples were analyzed via Rutherford backscattering with 900 keV α-particles. The mixing rates k were found to depend strongly on the R p d ratio, where Rp denotes the mean ion range and d the film thickness; the maximum mixing observed was 2.9(2) nm4. This high k-value is compared with Monte Carlo calculations of ballistic mixing and experiments on Xe-mixed Cr2N and TiN films on several substrates. Effects of the chemical binding in the various compounds are discussed.

A new electronegativity-based approach to chemical binding

Abstract: A new theory of chemical binding modelled through the accumulation of electron density at the bond centre using the concepts of bond electronegativity and bond hardness is reported and shown to provide accurate prediction of bond energies for a number of diatomic and simple polyatomic (ABn) molecules.

Compositional Change of Silicon Carbide Surface due to Oxygen Adsorption and Heat Treatment

Abstract: The compositional change of the 6H-SiC(100) surface was studied under heat treatment and adsorption of oxygen. X-ray photoelectron spectroscopy (XPS) was used for the analysis of the depth composition profile and chemical binding state. After heat treatment, the carbon concentration of the surface was increased due to the surface segregation of the graphite type carbon. When the adsoption of oxygen was performed with heat treatment, the carbon concentration of the surface was increased, but the amount of segregated carbon was less than that of the sample after only heat treatment. The segregated carbon seemed to react with oxygen preferentialy and to desorb from the surface as a carbon monoxide or carbon dioxide.

Small‐angle electron scattering by formaldehyde and ketene: Effects of electron correlation and chemical binding

Abstract: The total (elastic plus inelastic) intensities of 51 keV electrons scattered by H2CO and H2CCO have been measured over a range of K = (4π/λ) sin(θ/2) = 1–9.5 A−1 and compared with the theoretical intensities calculated with SCF and CI wave functions. Significant discrepancies are found between the experimental intensities and the theoretical ones based on the SCF wave functions. Most of the chemical binding and electron correlation effects observed in the total scattered intensities are reproduced by the theoretical intensities based on the CI wave functions calculated with the basis set including polarization functions on all atoms. © 1992 John Wiley & Sons, Inc.

Theory of Surface Clusters in External Fields: Influence of Multipole Embedding on Local Adsorbate Binding

Abstract: From a theoretical point of view an adsorbate system represents a very complex quantum mechanical many-particle system which requires a fair number of severe approximations to allow a theoretical treatment[1,2]. The interaction of the adsorbate, atom or molecule, with the substrate surface is determined by local contributions such as chemical binding or local ionici-ties which involve substrate atoms close to the adsorption site. Further, there are environmental contributions which reflect the fact the adsorption site atoms are coupled electronically to the rest of the substrate.

Spectroscopy of Biliproteins and Energy Transfer in Photosynthetic Antenna Complexes

Abstract: In this contribution the various spectroscopic techniques which have been applied to elucidate the energy transfer in photosynthetic antenna pigments are described and some of the results of a large number of studies are presented. It is not intended to give a complete review on the investigations on different types of photosynthetic organisms but to demonstrate the fundamental principles which are important in the chain of processes (light absorption, energy transfer and charge separation) which is active during the process of converting light energy into chemical binding energy. Since the thermophilic cyanobacterium Mastigocladus laminosus is one which was subject to most intensive studies by all kinds of spectroscopic techniques and the models explaining the energy transfer are well developed, most of the data presented in this contribution will refer to this organism.

Optical Spectroscopy of Argon Cluster Ions

Abstract: Neutral rare gas clusters have one of the simplest kind of chemical binding. Every atom is bound to its neighbours by the same kind of weak undirectional van der Waals force. The first electronic excitation is far in the vacuum UV region [1]. An electron is removed from a strongly antibonding orbital if the duster is ionised. Consequently the bonding becomes much stronger around the charge, and differs in character, direction and strength. The optical absorption shifts to the visible. The absorption is due to the charge localisation on a molecular chromophore. It was originally deduced from energetic considerations and condensed state data [2], that the positive charge in a rare gas cluster ion localizes after about a picosecond on a dimer ion, e.g. Ar 2 + . An argon cluster ion Ar n + would thus have a Ar 2 + Ar n−2. structure. Later experimental [3, 4, 5, 6, 7] and theoretical [8, 9, 10] results pointed to charge localization on a larger unit: Ar n + with n = 3 or 4. This view is not universally shared: Stace and coworkers [11] deduce from their data that the charge does localize on a dimer ion. However Deluca and Johnson [12] and Bowers et al.[13] favour an Ar 2 + − Ar double minimum potential, a notion which is contested by Gadea and Malrieu [14]. Carnovale et al. [15] deduce from their photoelectron spectra n = 3, 7, or 13, depending on cluster size.

Studies on the compatibility of diethylpropion hydrochloride with carboxymethylcellulose and other dietary fibres.

Abstract: The combination of diethylpropion hydrochloride with carboxymethylcellulose induces decomposition of the drug. The principal decomposition products obtained were 1-phenyl-1,2-propanedione, benzoic acid and diethylamine as hydrochloride and benzoate salts. These phenomena have been explained with reference to basic decomposition scheme of the diethylpropion. Additionally, the behaviour of diethylpropion in mixtures with other dietary fibres such as methylcellulose, wheat bran and wheat germ was examined. Diethylpropion in combination with methylcellulose is rather stable while in mixtures with wheat bran and wheat germ suffers degradation. An increase in the weight of the excipient, probably due to chemical binding of the drug and other degradation products by the excipient, was observed in all these cases. The decomposition reactions were studied by applying a sequence of selective solvent extractions and using high-performance liquid chromatography (HPLC).

Method for promoting the chemical union between polyethyleneterephthalate fibers and polymer matrices through diazides or derivatives of azides

Abstract: Most textile organic fibers, and particularly polyethyleneterephthalate (PET) fibers, are used as reinforcements of polymers of minor rigidity in order to prepare polymer composites. The physical and mechnical characteristics of the composites may be enhanced by increasing the adhesion resistance at the reinforcement/polymer interface, the chemical binding being the mechanism which provides a stronger and more effective adhesion at the interface. Through treatment of PET fibers with azides or derivatives of azides, the chemical union is promoted between said fibers and polymer matrices such as polyolefines (polyethylene of high and low density, polypropylene, ...) and both saturated and unsaturated elastomers.

Polarized VBXES and XANES of GaSxSe1−x (x = 0, 0.6, 1)

Abstract: Polarized K-emission and absorption spectra of the layered compounds GaSxSe1−x (x = 0, 0.6, 1) are measured in the range of about 10 eV below and above the gap. The polarization dependence of the spectra enables the investigation of the local p-like DOS additionally resolved with respect to the m-quantum number symmetry. By alignment of the energy scales using XPS core-orbital energies the experimental spectra can be compared with calculated local partial (l, m-resolved) DOS after Robertson. The spectra are discussed with regard to the calculated DOS and to the structure of the compounds. The polarization dependence of the gap between the Ga K-emission and absorption spectra shows the anisotropic character of chemical binding. Polarisierte K-Emissions- und -Absorptionsspektren der Schichtverbindungen GaSxSe1−x (x = 0, 0.6, 1) wurden im Bereich von 10 eV unterhalb und oberhalb des Gaps gemessen. Die Polarisationsabhangigkeit der Spektren ermoglicht die Untersuchung der lokalen p-artigen und zusatzlich nach der Magnetquantenzahl m aufgelosten DOS. Durch die Korrelation der experimentellen Spektren in einer gemeinsamen Energieskala unter Verwendung der betreffenden XPS-Corelinienenergien konnen sie mit berechneten lokalen partiellen (l, m-aufgelosten) DOS nach Robertson verglichen werden. Die Spektren werden auf der Grundlage der berechneten DOS und im Zusammenhang mit der Kristallstruktur diskutiert. Die Polarisationsabhangigkeit des Gaps zwischen den Ga K-Emissions- und -Absorptionsspektren weist auf den anisotropen Charakter der chemischen Bindung hin.

Bond Enthalpy Transferability - is it Achievable?

Abstract: The provision of experimental data for compiling a table of values from which the heat of every possible chemical reaction can be calculated was declared as the aim of thermochemistry by Rossini,1 many years ago. Subsequently, this purpose was broadened2 to include the derivation of the heats of formation of compounds from their constituent elements and the relationship of these results to the chemical binding energies in the molecules. The “classical” examples of chemical binding energies come close to the beginning of anyone’s serious study of chemistry. Much understanding of deep significance has followed from these data: electronegativity, ideas about aromaticity, bond polarity and ionic character, are examples. Coherent with these has been the development of a relation between bond length and bond enthalpy (strength). This leads to the still widely held perception that a “long” bond is a “weak” bond. This, in turn, is related to bond order: an interatomic bond of high order is both shorter and stronger than a bond of lower order between the same two elements. There is a lot of reliable experimental evidence to support these principles when elements of the first short period (n = 2) are considered. In the second short period (n=3; l =0,1) the position is more confused. The lack of a substantial base of data for enthalpies of formation must be held responsible for this, in part at least.