Showing papers on "Chemical binding published in 1975"

Bandstruktur des Festkörpers – Interpretation der chemischen Bindung in einigen Übergangsmetallverbindungen aufgrund von Bandstrukturrechnungen

Abstract: Die chemischen Bindungsverhaltnisse einiger in der NaCl-Struktur kristallisierenden Ubergangsmetallverbindungen (ScN. ScO, TiC, TiN, TiO, VC, VN, VO) werden auf Grund von Bandstrukturrechnungen diskutiert. Grundlage der Betrachtungen bilden die nach der quasi-selbstkonsistenten APW-Methode berechneten Wellenfunktionen und Energieeigenwerte sowie die nach dem LCAO-TB-Inter-polationsverfahren von Slater und Koster ermittelten Energieeigenwerte und Eigenvektoren. Die partiellen LCAO-TB l-Zustandsdichten erlauben eine Analyse der Valenzbander, aus der hervorgeht, das das energetisch tiefstliegende Valenzband nahezu ausschlieslich s-Charakter besitzt. In den folgenden Bandern, die sich im wesentlichen von den Nichtmetall 2p-Atomzustanden herleiten, besitzt die LCAO-TB-Zustandsdichte neben einem hohen Anteil an partieller p-Zustandsdichte auch einen deutlichen Beitrag an partieller d-Zustandsdichte, der sich vom Karbid zum Oxid verringert. In der LCAO-TB-Zustandsdichte der funf Bander, die sich von den Metall-3d-Atomzustanden herleiten, findet man neben einem uberwiegenden Anteil an partieller d-Zustandsdichte auch einen kleineren Beitrag an partieller p-Zustandsdichte. Die partielle d-Zustandsdichte wird in den eg- und t2g-Anteil zerlegt und deren Verhalten diskutiert. Eine Analyse der APW-Kristallwellenfunktionen gibt Aufschlus uber die l-artigen Ladungen in den Atomspharen. Auf Grund eines Vergleichs der APW-Gesamtladungen in den Atmospharen mit den Ladungen, die sich durch die Uberlagerung der Ladungsdichten eines hypothetischen Kristalls neutraler Atome ergeben, wird auf eine Uberfuhrung von Elektronen geschlossen, die in allen Verbindungen aus der Metallatomsphare in die Nichtmetallatomsphare erfolgt. The nature of chemical binding for some transition metal compounds (ScN, ScO, TiC, TiN, TiO, VC, VN, VO) with NaCl structure is discussed in terms of energy band structure calculations. The discussion is based on the wave functions and energy eigenvalues, as calculated by the “Quasi self consistent APW method” as well as on the energy eigen values and eigen vectors resulting from the Slater-Koster LCAO-Tight binding interpolation scheme. The LCAO-TB l-like partial density of states can be used to analyse the valence bands. This analysis shows, that the deepest valence band has predominantly s-character. The following bands, which can be derived in essence from the atomic 2p states of the non metal are characterized not only by the partial p-like density of states but also by a substantial contribution from the d-like partial density of states, which is decreasing going from a particular Carbide to the corresponding Oxide. For the valence bands, which can be associated with the atomic 3d states of the metal, the density of states exhibits besides the partial d-density of states small contributions from the partial p-like density of states. The decomposition of the d-like partial density of states into an eg-like and a t2g-like contribution is discussed. By analyzing the APW crystal wave functions the partial l-like charges inside each atomic sphere can be obtained. If the APW total charges within the atomic spheres are compared with the charges of a hypothetical crystal, which correspond to a superposition of the charge densities of the neutral atoms, a transfer of electrons from the metal to the non-metal sphere is found for all compounds under investigation.

An electron microscopic study of the location of teichoic acid and its contribution to staining reactions in walls of Streptococcus faecalis 8191.

Abstract: SUMMARY: The location of the glucosylated teichoic acid in whole cells and isolated walls of Streptococcus faecalis 8191 has been investigated using ruthenium red, gold-labelled concanavalin A and concanavalin A-peroxidase-diaminobenzidine. Dense laminae were revealed in sections of osmium-fixed walls stained with ruthenium red which corresponded to similar regions stained by uranyl and lead. Such regions were not seen after teichoic acid had been extracted, suggesting that the uptake of stain was by teichoic acid. However, these regions were not labelled on exposure to gold concanavalin A or concanavalin A-peroxidase-diaminobenzidine; these stains indicated that teichoic acid was situated between the dense laminae, although the distribution of stain could have been due to the inability of the concanavalin A stains to penetrate deeply. Chemical binding studies showed that the teichoic acid was the major uranyl binding component in isolated walls, from which it might be inferred that teichoic acid was located in the densely staining regions. However, since osmification significantly increased the binding of uranyl (and lead stains) to non-teichoic acid material, such an inference was not necessarily valid. It is concluded that the presence of teichoic acid can be demonstrated in certain regions of the wall by concanavalin A, but its presence in densely staining regions has not been established. These experiments therefore suggest that teichoic acid may not be intimately associated with the mechanisms that generate contrast patterns in stained sections of cell walls of Streptococcus faecalis.

Role of amino groups in formation of human lymphocyte-xenogeneic erythrocyte rosettes; a proposed mechanism for antigen recognition

Abstract: Formation of coulombic and possible hydrogen bonds between amino groups on human lymphocytes and negatively charged sites on sheep erythrocytes is involved in rosette formation. Supportive evidence includes rosette inhibition by chemical binding of lymphocyte membrane amino groups, and the results of changing the pH, ionic concentration, and temperature of the reaction. Although the possibility has not been excluded that the amino group dependence of this reaction is related to the property of certain proteins attached to the T-cell (thymus processed) surface, it is suggested that this dependence may be related to a charge pattern recognition in the form of "codes" present on the T-cell membrane. It is speculated that this type of recognition may be a contributory mechanism in the initiation of the T-cell-dependent immune response.

Excited states and photochemistry of saturated molecules. I. General approach and application to propane

Abstract: Following Ruedenberg, we suggest that a useful measure of chemical binding is the two-center, one-electron interference contribution to the energy in the electronic state of interest. Using this conceptual framework, the vertical valence excited states of propane in the optimal ground state geometry are examined using INDO in its original parametrization. The results are compared with available experimental information. The calculations yield bond energies which are consistent with observed decomposition modes for ground states of small alkanes and excited states of propane. Configuration interaction calculations are discussed in the case of propane. Inclusion of all single excitations from the ground state results in considerable rearrangement of the order of excited states: however, the relative bond energies within each state are unaffected. With the exception of ethane, absorption or electron immum of linear alkanes decreases with increasing chainpact spectra of small (n I 6 ) acyclic alkanes contain little length and increases with branching. Based on these results discernible information. The broad, essentially featureless and energy transfer studies, he concluded that these spectra',2 give little in the way of structural information frequencies are related to the extent of nuclear distortion in about the excited state. Recently Lipsky3 observed that the the excited states. Numerous photochemical studies of separation between absorption onset and fluorescence maxismall acyclic alkanes have established the striking predomiJournal of the American Chemical Society / 97.18 / September 3 ,1975