Universität Stuttgart
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Item Open Access Metallophospha-alkenes: a simple access and some reactivities involving the phosphorus atom(1985) Gudat, Dietrich; Niecke, Edgar; Malisch, Wolfgang; Hofmockel, Ullrich; Quashie, Sapé; Cowley, Alan H.; Arif, Atta M.; Krebs, Bernt; Dartmann, MechtildThe synthesis and studies on the reactive behaviour of metallophospha-alkenes are reported.Item Open Access Fe-C and Fe-N compound layers : growth kinetics and microstructure(2007) Greßmann, Thomas; Mittemeijer, Eric (Prof. Dr. Ir.)Improvement of surface’s properties of iron and steel work pieces by gas nitriding/nitrocarburising plays an important role in metallurgy. In particular the fatigue, tribological and corrosion properties are enhanced by these processes without changing the properties of the bulk. Gas nitriding is mainly performed in NH3/H2 gas mixtures, whereas by nitrocarburising additionally a carbon delivering species (mostly CO) is present in the gas atmosphere. Typical treatment temperatures range from 773 K to 863 K. If sufficient nitrogen (and carbon) is supplied by the gas phase to the workpiece an iron-(carbo-) nitride compound layer will develop at the surface and the matrix becomes enriched with nitrogen (and carbon). Such compound layers consist in general of a hcp (with respect to Fe) epsilon-Fe3(N,C)1+x layer adjacent to the surface and a fcc-type (with respect to Fe) gamma’-Fe4N layer at the layer/substrate interface. This work addresses (i) growth kinetics studies of Fe-C compound layers produced on pure iron sheets by nitrocarburising as well as (ii) microstructural investigations on iron-nitride compound layers obtained by nitriding of iron. Nitrocarburising of iron usually leads to the formation of epsilon/gamma’ compound layers, where the presence of carbon promotes the epsilon phase which can dissolve considerable amounts of carbon what is not the case for the gamma’ phase. It has been observed previously that additionally to the epsilon and gamma’ phases also some cementite (Fe3C) can form within the compound layer leading to complex microstructures. However, it was found for the first time in this work that it is possible to grow massive Fe3C layers using a certain composition of the gas mixture consisting of CO, H2, NH3 and N2. With this new developed treatment procedure one can also prevent the often observed sooting/graphite formation at the surface, leading in some cases to disintegration of the metastable Fe3C in alpha-Fe and graphite, which is associated with “metal dusting”. The growth kinetics of such Fe3C surface layer is evaluated and discussed. During nitriding a N concentration gradient due to the inwards diffusion of N from the surface to the bulk builds up within the compound layer. Since, especially, the epsilon phase has a wide homogeneity range for N, this concentration gradient leads to a considerable variation of the lattice parameters with depth. Furthermore, macrostresses may build up within the compound layer during growth due to the concentration gradient and after growth during cooling due to different coefficients of thermal expansion of the layer phases and the substrate. High-resolution X-ray diffraction measurements at different sample tilting angles using synchrotron radiation revealed a pronounced anisotropic diffraction-line broadening of the epsilon reflections. The obtained diffraction patterns are successfully described by a newly developed model with which it is possible to fit the evolution of the (strain-free) lattice parameters with depth as well as a stress-depth profile simultaneously. Analysis of gamma’ layers by X-ray stress measurements using several reflections simultaneously revealed a for fcc-type metals unusual elastic anisotropy of gamma’-Fe4N with <100> as stiffest and <111> as most compliant direction. These results are compared with single-crystal elastic constants obtained by ab-initio calculations and are related to the crystal structure of gamma’-Fe4N in order to get a better understanding of the behaviour of the elastic properties of gamma’. The stresses determined on both layers, epsilon and gamma’, can be understood as thermally induced, whereas the stresses in the gamma’ layer (compressive stresses) are much larger than those present in the epsilon layer, which change from tensile at the surface to compressive at the epsilon/gamma’ interface.Item Open Access Thin film growth and structural investigation of DyBa2Cu3O7-δ(2020) Putzky, Daniel; Keimer, Bernhard (Prof. Dr.)Item Open Access Campherchinoniminliganden und ihr Elektronentransferverhalten unter Metallkoordination an Rhenium und Mangan(2021) Blickle, Svenja; Kaim, Wolfgang (Prof. Dr. phil. nat.)Die vorliegende Arbeit beschäftigt sich mit der Synthese und dem Koordinations- und Elektronentransferverhalten von neuen redoxaktiven Campherchinoniminliganden und deren Komplexe mit dem Metallkomplexfragment M(CO)3Cl mit M = Re, Mn. Der Fokus liegt dabei auf der Untersuchung der redoxchemischen Eigenschaften von dinuklearen Komplexen, bei welchen die Metalle über einen Bischelatliganden verbrückt sind. So konnten Einblicke in die elektronische Struktur der Verbindungen in neutraler, radikalanionischer und dianionischer Form gewonnen werden. Zur Charakterisierung dieser neuen Verbindungen kommen zum Einsatz: CHN-Elementaranalyse, Massenspektrometrie, Kristallstrukturanalyse, Cyclovoltammetrie, 1H-NMR-, IR-, UV/Vis/NIR- und EPR-Spektroskopie und Spektroelektrochemie. Die experimentellen Ergebnisse werden mit Hilfe von quantenchemischen Methoden (DFT) unterstützt. So konnte für diese Art von Verbindungen eine neue Art von gemischtvalenten Metallkomplexen etabliert werden: Die Reduktionen finden ligandenzentriert statt und die Ladungsverteilung der radikalanionischen Spezies konnte als asymmetrisch lokalisiert nachgewiesen werden. Somit handelt es sich bei den hier vorgestellten zweikernigen einfach reduzierten Komplexen um organisch-gemischtvalente Verbindungen unter homovalenter Metallkoordination.Item Open Access Stabile σ-Komplexe : ein Beitrag zum Mechanismus der Aromatenalkylierung(1968) Niess, Rolf; Nagel, Klaus; Effenberger, FranzDa man bei der Alkylierung von Aromaten nur einen unbedeutenden kinetischen Isotopeneffekt beobachtet, wird die Bildung des π- oder des σ-Komplexes als geschwindigkeitsbestimmend angenommen. Die einzigen bisher isolierten Zwischenverbindungen bei der Alkylierung sind nach neueren Untersuchungen wahrscheinlich keine echten σ-Komplexe, sondern liegen als orientierte π-Komplexe vor. Die Rildung eines σ-Komplexes, als Teilreaktion der Alkylierung, ist als nucleophiler Angriff des Aromaten an einem positivierten Kohlenstoff aufzufassen.Item Open Access Mechanistic insights into the allosteric regulation of the Clr4 protein lysine methyltransferase by autoinhibition and automethylation(2020) Khella, Mina S.; Bröhm, Alexander; Weirich, Sara; Jeltsch, AlbertClr4 is a histone H3 lysine 9 methyltransferase in Schizosaccharomyces pombe that is essential for heterochromatin formation. Previous biochemical and structural studies have shown that Clr4 is in an autoinhibited state in which an autoregulatory loop (ARL) blocks the active site. Automethylation of lysine residues in the ARL relieves autoinhibition. To investigate the mechanism of Clr4 regulation by autoinhibition and automethylation, we exchanged residues in the ARL by site-directed mutagenesis leading to stimulation or inhibition of automethylation and corresponding changes in Clr4 catalytic activity. Furthermore, we demonstrate that Clr4 prefers monomethylated (H3K9me1) over unmodified (H3K9me0) histone peptide substrates, similar to related human enzymes and, accordingly, H3K9me1 is more efficient in overcoming autoinhibition. Due to enzyme activation by automethylation, we observed a sigmoidal dependence of Clr4 activity on the AdoMet concentration, with stimulation at high AdoMet levels. In contrast, an automethylation-deficient mutant showed a hyperbolic Michaelis–Menten type relationship. These data suggest that automethylation of the ARL could act as a sensor for AdoMet levels in cells and regulate the generation and maintenance of heterochromatin accordingly. This process could connect epigenome modifications with the metabolic state of cells. As other human protein lysine methyltransferases (for example, PRC2) also use automethylation/autoinhibition mechanisms, our results may provide a model to describe their regulation as well.Item Open Access Interrelationships of microstructure, stress and diffusion(2008) Kuru, Yener; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Extensive research has been performed on thin metal films due to their interesting mechanical, electrical and magnetic properties. They can exhibit very high residual, internal stresses arising from the film growth and/or external effects. Apart from direct mechanical consequences, several processes such as grain growth and diffusion can be affected by these stresses and their gradients. As a result, it is of cardinal importance to measure and control the residual stresses in thin films. X-ray diffraction (XRD) is one of the most frequently used approaches for (residual) stress measurement. It is non-destructive, highly accurate (stress (variation) of some MPa can be detected) and the stress states of all crystalline phases in a layered structure can be obtained separately. Moreover, additional microstructural information, as the crystallographic texture, the density of crystalline defects, such as dislocations, and the crystal size can be acquired from the collected XRD data. This thesis is dedicated to the investigation of microstructural changes, residual stresses and interdiffusion in thin films by in-situ XRD. A focal point of interest is methodological aspects of in-situ measurements, which are discussed in detail in Chapter 2 and come to application in the following Chapters 3 and 4.Item Open Access Nucleophile Substitution von Nitrit in Nitrobenzolen, Nitrobiphenylen und Nitronaphthalinen(1991) Effenberger, Franz; Koch, Markus; Streicher, WilliAromatic compounds, accessible only by multistep procedures, can be synthesized easily by nucleophilic substitution of nitrite in nitrobenzenes, nitrobiphenyls, and nitronaphthalenes. Thus, meta-substituted phenols 3, 4, and 7 are obtained from 1,3-dinitrobenzene (1) and meta-substituted nitrobenzenes 6, as well as 3,5-disubstituted phenols 10 and 5-substituted resorcinol derivatives 11 from 3,5-disubstituted nitrobenzenes 9. The unsymmetrically substituted nitrobiphenyls 13, 15, 17, 19, 21, 23, 24, and 26 are also available by nitrite exchange from the corresponding easily accessible dinitrobiphenyls 16, 18, 20, 22, and 25. A nitrite exchange with nucleophiles is easily possible in the 1,5-disubstituted naphthalenes 29, 34, while in the case of the 1,8-disubstituted naphthalenes 31, 36 only the chloro derivative 36 undergoes this exchange under much stronger conditions in low yield.Item Open Access Multiscale modeling of fracture and deformation in interface controlled materials(2007) Brödling, Nils Christian; Arzt, Eduard (Prof. Dr. phil.)Many nanostructured metals are characterized by scale dependent mechanical properties and by size effects due to geometrical confinement. Dislocation activities, interface mediated plasticity, and macroscopic yielding are quite different from those in unconstrained metals. The role of interfaces for the material properties and for the governing deformation mechanisms remains unclear despite the large efforts made in experimental and theoretical investigations. Here we approach the effect of geometrical confinement on the atomic and on the mesoscopic scale. We elucidate size effects on failure mechanisms and on scale dependent plasticity of nanostructured dual phase composite materials with the aid of computer simulations. Cleavage failure of dual phase layered materials is simulated with a mesoscopic model to clarify the scaling behavior of the materials fracture toughness. The model accounts for the confinement effect that a layer geometry imposes on the collective dislocation behaviour near a moving crack tip. The critical layer thickness at which the bulk fracture toughness of the elastic-plastic material is reached as well as the bulk fracture toughness itself increase with the cohesive strength of the interface, but become smaller for higher yield strengths. The main conclusion drawn in this work is that fracture toughness as a function of layer thickness saturates gradually if dislocation activity is dispersed, dilute and not compact around the crack tip. It increases abruptly with the thickness when dislocation activity right at the crack tip is possible and a compact, shielding dislocation array forms near the crack tip. Furthermore this work provides preliminary understanding of the governing mechanisms that control the limiting length scale for the strengthening of bioinspired metallic nanocomposits. Large-scale molecular dynamics simulations are performed to investigate the plastic deformation behavior of a bioinspired metallic nanocomposite which consists of hard nanosized Ni platelets embedded in a soft Al matrix. The simulation results are analyzed with respect to the prevailing deformation mechanisms quantifying the contribution of dislocation-based plasticity and interface-mediated interfacial slip as a function of the nanostructural scaling. The results of the simulations show that interfacial sliding contributes significantly to the plastic deformation despite a strong bonding across the interface. Critical for the strength of the nanocomposite is the geometric confinement of dislocation processes in the plastic phase. The confinement effect strongly depends on the length scale and the morphology of the metallic nanostructure. The main conclusion drawn for this material is that below a critical length scale, the softening caused by interfacial sliding prevails, giving rise to a maximum strength at the optimum size.Item Open Access Synthese und Struktur des Tetra(tert-butyl)tetrarsetans und des Tetra(tert-butyl)tetrastibetans (Element-Element-Bindungen ; 1)(1982) Mundt, Otto; Becker, Gerd; Wessely, Hans-Jürgen; Breunig, Hans Joachim; Kischkel, HelmutDilithium-(tert-butyl)arsenid reagiert mit (tert-Butyl)dichlorarsan zum Tetra(tert-butyl)tetrarsetan 1; das homologe Tetrastibetan 2 bildet sich bei der Reduktion von (tert-Butyl)dichlorstiban mit Magnesium. Die isotypen Verbindungen kristallisieren monoklin in der Raumgruppe P21/c mit Z = 4. Die bei der Meßtemperatur von -45 ± 5°C bestimmten Gitterkonstanten von 1/2 sind: a = 957,4(8)/1 000,2(3); b = 1 399,1(14)/1 423,9(4); c = 1 697,4(9)/1 749,8(7) pm; β = 96,02(6)/96,77(3)°. Nach den Ergebnissen der Röntgenstrukturanalysen (3 531/3 232 symmetrieunabhängige Reflexe, Rg = 4,0/4,6%) sind die E4-Ringe (E = As, Sb) gefaltet; die voluminösen organischen Substituenten stehen pseudo-äquatorial in der all-trans-Konfiguration.