bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quino[7,8‐h]quinoline, a New Type of “Proton Sponge”
Authors: Zirnstein Michael A. , Staab Heinz A. 
Published: December 2003

AbstractSo far, “proton sponges” have been defined as bis(dialkylamino)arenes whose dialkylamino groups are in close spatial proximity.[1] The unusual basicity of these compounds is ascribed to the destabilizing overlap of the lone electron pairs on the nitrogen atoms, to the formation of especially strong hydrogen bonds in the monoprotonated diamines, and to the hydrophobic shielding of these hydrogen bonds. In order to differentiate and assess the relative importance of these factors, we were interested in quino[7,8‐h]quinoline 1, whose nitrogen atoms exhibit a mutual orientation similar to that in 1,8‐bis(dimethylamino)naphthalene 2 (“proton sponge”). In contrast to 2, however, 1 lacks the hydrophobic shielding of the hydrogen bonds of its monoprotonated derivative. This shielding is considered to be responsible for the low rates of proton transfer, which make the “proton sponges” reported so far unsuitable as auxiliary bases in chemical reactions.
Amavadin, an Example for Selective Binding of Vanadium in Nature: Studies of Its Complexation Chemistry and a New Structural Proposal
Authors: Bayer Ernst , Koch Eckhard , Anderegg Giorgio 
Published: December 2003

AbstractThe selective enrichment of metals in nature is of great interest.[1] For example, the concentration of vanadium in the blood of tunicates is a millionfold higher than in sea water.[2–4] Fly agaric (Amanita muscaria) also concentrates vanadium.[5,6]The structure of the vanadium compound of tunicates has not yet been elucidated.[7,8] However, a structure (3) has been proposed[9] for the vanadium compound of fly agaric, amavadin.[9‐11] The structural elucidation and synthesis of the organic ligand 1b[9b] allows, for the first time, the specificity of a naturally occurring vanadium ligand to be investigated. Amavadin can be prepared from VIVO salts and synthetic 1b.[9b] Since the ligand 1b was previously unknown and since all attempts to obtain crystals of amavadin suitable for an X‐ray structure analysis failed, a detailed investigation of the complex formation of 1b should also provide an answer to the question of the arrangement of the ligands around the central atom in amavadin.
Cs3Mo5P6O25: a Novel Molybdenum Phosphate Containing a Cubane‐like Mo4O4 Cluster with Six Molybdenum‐Molybdenum Bonds
Authors: Lii Kwang‐Hwa , Haushalter Robert C. , O'Connor Charles J. 
Published: December 2003

AbstractFor corrigendum see DOI:10.1002/anie.198708041
Modern NMR Spectroscopy of Organolithium Compounds
Authors: Günther Harald , Moskau Detlef , Bast Peter , Schmalz Dietmar 
Published: December 2003

AbstractModern methods of NMR spectroscopy, in particular the two‐dimensional techniques, offer new chances for structure determinations in the field of organolithium compounds, where the combination of 1H‐, 13C‐, and 6(7)Li‐NMR spectroscopy is an especially useful feature. Chemical shift correlations which also include the lithium nuclei allow a complete assignment of the 1H‐, 13C‐, and 6Li‐NMR spectra and thereby a better characterization of the various aggregates and complexes present in solution. Spatial proximities of 6Li and 1H can be detected by nuclear Overhauser experiments, and 6(7)Li‐NMR exchange spectroscopy can provide new information with regard to the mechanisms and energetics of dynamic processes like aggregate interchange and complexation. After a short resumé of the experimental aspects of the NMR spectroscopy of organolithium compounds and a discussion of the NMR parameters of these systems, new experimental techniques are presented. Areas of application of these newly conceived one‐ and two‐dimensional NMR experiments are illustrated with selected examples. The results show that even more detailed information about the structure and reactivity of organolithium compounds, which are so important for organic synthesis, can be expected in the future.
Clusters of Valence Electron Poor Metals—Structure, Bonding, and Properties
Authors: Simon Arndt 
Published: December 2003

AbstractMetals in low oxidation states are capable of forming metal–metal bonds. An attempt has been made to classify the numerous phases and structures occurring in such metal‐rich systems of valence electron poor metals in some sort of order from a rather general point of view. With this purpose in mind, clusters of these elements, their different types of interconnections, and their condensation via shared metal atoms, which finally leads to extended MM bonded structures, are described. Interstitial atoms play an important role in stabilizing electron deficient clusters, and can actually lead to the loss of all MM bonds. Surprising similarities emerge between apparently very different systems as the metal‐rich oxides of alkali metals, the oxides, halides, and chalcogenides of d transition metals, and the halides and carbide halides of the lanthanoids.
Tetrahedrane and Cyclobutadiene
Authors: Maier Günther 
Published: December 2003

AbstractTetra‐tert‐butyltetrahedrane—a useless molecule? In the first part of this progress report an attempt is made to answer this provoking question in order that the reader can evaluate why it is worthwhile to track such an esoteric molecule. An account of the unusual properties of tetra‐tert‐butyltetrahedrane, the only tetrahedrane unequivocally identified so far, and the most highly strained saturated hydrocarbon of all, is followed by a summary of attempts to synthesize the parent compound. The emphasis in these two sections is on our own research achievements. Since any discussion of the tetrahedranes must necessarily include the corresponding cyclobutadienes, it was found appropriate to organize the article at the same time as a final account of the cyclobutadiene problem. Masamune[8c] characterized his excellent report in 1980 in the journal Tetrahedron as a comma in the history of cyclobutadiene. Meanwhile, the significant questions have been answered. The time is now ripe to replace the comma by a full stop.
Oxo‐Bridged Polyiron Centers in Biology and Chemistry
Authors: Lippard Stephen J. 
Published: December 2003

AbstractThe functions of iron in biology are often associated with heme or iron‐sulfur proteins. But dioxygen transport, reduction of ribo‐ to deoxyribonucleotides, acid phosphatase activity, oxidation of methane to methanol, and iron storage are amont the growing list of biological phenomena known or believed to be associated with a newly emerging class of proteins having oxo‐bridged di‐ or polyiron aggregates at their metallic cores. The recognition of these iron–oxo proteins as a separate class has stimulated efforts on the part of inorganic chemists to prepare and characterize model compounds that replicate the physical properties and functions of the polymetallic protein cores. As a consequence, a variety of new oxo‐bridged di‐, tri‐, tetra‐, hexa‐, octa‐, and undecairon aggregates has been synthesized. These novel molecules promise not only to provide insight into the detailed characteristics of the metal centers in iron–oxo proteins, but also to serve as a focal point for preparing new materials, including oxidation catalysts and corrosion inhibitors, for the evolution of new theories to describe their physical properties, for developing new strategies for the treatment of iron‐related disease, and for building links between the chemistry of the biosphere and the geosphere.
Two‐Dimensional NMR Spectroscopy: Background and Overview of the Experiments [New Analytical Methods (36)]
Authors: Kessler Horst , Gehrke Matthias , Griesinger Christian 
Published: December 2003

AbstractAn exact knowledge of the structure, dynamics, and reactions of molecules provides the key to understand their functions and properties. NMR spectroscopy has developed, through the introduction of two‐dimensional methods, into the most important method for the investigation of these questions in solution. A great variety of different techniques is available. However, for their successful application not only the appropriate equipment is required, but also the right choice of experiments and optimum measurement parameters, as well as a careful evaluation of the spectra. This contribution describes the necessary background for modern NMR spectroscopy. With the aid of the so‐called product operator formalism it is possible to understand pulsed Fourier transform NMR spectroscopy both qualitatively and quantitatively. Very few, readily understandable assumptions are sufficient for confident application of these methods. This article attempts to introduce in a simple manner this formalism as well as phase cycles necessary for the understanding of pulse sequences, and to train the reader through the discussion of several 2D NMR techniques. An overview of the most important techniques is given in the second part of this article.
Controlling the Microstructure of Monomolecular Layers
Authors: Möhwald Helmuth 
Published: December 2003

AbstractThis review reports on the range of lateral structures which have been detected in ultrathin organic films using the newly‐developed techniques of fluorescence microscopy and X‐ray scattering, together with more conventional methods. A number of unusual low‐dimensional ordering processes have been revealed. Hexagonal, lamellar or fractal domain structures are observed and shown to result from an interplay of electrostatic forces due to molecular alignment at interfaces, line energy between different phases and growth kinetics. On a molecular level the microstructure is distinguished by low coherence length of positional order but long coherence length concerning the orientation of crystallographic axes. It is demonstrated that the ideas presented are also relevant to adsorbate structures at interfaces and to the protein arrangement in monolayers.
Protein Phosphorylation in Bacteria—Regulation of Gene Expression, Transport Functions, and Metabolic Processes
Authors: Deutscher Josef , Saier Milton H. 
Published: December 2003

AbstractThe importance of protein phosphorylation to the regulation of physiological processes in eukaryotic cells has been known for almost half a century. In contrast, the first conclusive evidence for functionally relevant protein phosphorylation in bacteria was obtained less than a decade ago. To date, only five functionally well‐characterized bacterial proteins have been shown to be regulated by reversible phosphorylation. (1) Phosphorylation of isocitrate dehydrogenase in Escherichia coli controls the relative flux of carbon through the Krebs cycle and the glyoxylate shunt. (2) In gram‐positive bacteria, phosphorylation of HPr, a phosphoryl carrier protein of the phosphoenolypyruvate‐dependent sugar phosphotransferase system, regulates carbohydrate uptake via this system. (3) Phosphorylation of glycerol kinase in Streptococcus faecalis probably serves to regulate the degradation of glycerol. (4) Phosphorylation of the nitrogen regulatory protein I (NR1) in E. coli controls expression of genes encoding the enzymes which participate in nitrogen metabolism. (5) Phosphorylation of citrate lyase ligase in Clostridium sphenoides regulates the conversion of citrate lyase from the inactive sulfhydryl form to the active acetylated form. Recent investigations have shown that more than one hundred proteins in E. coli are phosphorylated. It can therefore be anticipated that protein phosphorylation will prove to be as important to the regulation of physiological processes in bacteria as it is in eukaryotes.
Laser Photochemistry at Surfaces—Laser‐Induced Chemical Vapor Deposition and Related Phenomena
Authors: Kompa Karl L. 
Published: December 2003

AbstractThe structural characterization of materials and the tailoring of their properties is an important area of chemical research. A new trend in this area is the recourse to lasers both for analytical as well as preparative purposes, exploiting the fact that lasers, by virtue of their properties (sharp energy, spatial and temporal resolution etc.), offer the most precise and selective interaction of energy and matter that we know. Furthermore, photochemical syntheses and material transformations can proceed “cold” and without causing damage to surface structures. Laser chemistry finds application in thin film deposition, in the formation of surface layers, as well as in (structuring) ablation or etching, and in the initiation and enforcing of reactions at surfaces. In the present paper an introduction to these new possibilities on the general basis of molecule‐surface interactions is followed by a brief characterization of lasers suitable for such purposes. Thereafter, four examples are discussed: gas phase deposition from volatile organometallic compounds with photoelectric activation at the surface or photochemical activation of the gaseous species (e.g. by employing molecular beams). In this way (noble) metal contacts can be deposited on various substrates. Instead of surface deposition, nucleation can occur in the gaseous medium, yielding highly disperse powders, e.g. of silicon carbide. Finally, an etching reaction is discussed where the laser does not act as an energy source but as an analytical instrument to provide diagnostic and mechanistic information.
Synthesis of the Heaviest Chemical Elements—Results and Perspectives
Authors: Herrmann Günter 
Published: December 2003

AbstractAll 17 artificially produced elements heavier than uranium were discovered by nuclear‐chemical synthesis. The three heaviest ones–elements 107, 108 and 109–were synthesized at the heavy‐ion accelerator UNILAC in Darmstadt by nuclear fusion from the heaviest stable nuclei, lead‐208 and bismuth‐209, and the most neutron‐rich stable isotopes of chromium and iron: element 107 from bismuth‐209 (atomic number Z = 83) and chromium‐54 (Z = 24), element 108 from lead‐208 (Z = 82) and iron‐58 (Z = 26), and element 109 from bismuth‐209 and iron‐58. The first isotopes detected were those with mass numbers 262 (Z = 107), 265 (Z = 108) and 266 (Z = 109); these nuclei are short‐lived α‐emitters with half‐lives of 8.2 ms, 1.8 ms and 3.4 ms, respectively. The yields of these reactions are extremely small; only three atoms of element 109 have ever been observed. Experiments to synthesize element 110 have given ambiguous results. All attempts to detect the “superheavy” elements with proton numbers near Z = 114 and neutron numbers near N = 184 have thus far failed. These elements have been predicted theoretically, and many attempts to synthesize them have been made, e.g. at UNILAC by fusion of calcium‐48 (Z = 20) with curium‐248 (Z = 96), or by transference of protons in the collision of two very heavy nuclei such as uranium‐238 (Z = 92). Surprisingly, the heaviest known nuclei are far more stable toward spontaneous fission into two fragments than was expected, but their synthesis is strongly hindered–much more than initially anticipated. It is this hindrance rather than the decreasing nuclear stability which seems to presently limit the extent of the periodic table: even heavier elements should be able to exist, but no way has yet been found to produce them.
Ketenimine Complexes from Carbene Complexes and Isocyanides: Versatile Building Blocks for Carbocycles and N‐Heterocycles [New Synthetic Methods (74)]
Authors: Aumann Rudolf 
Published: December 2003

AbstractKetenimine complexes are readily available in great variety by reaction of isocyanides with carbene complexes. They have proven to be useful building blocks in new synthetic approaches to carbocyclic and N‐heterocyclic four‐, five‐, and six‐membered rings. The reactions involve new metal‐induced bond formation patterns of the ketenimine ligands, which can be influenced across a wide range by varying the following five parameters: the metal, the ligands, and the three substituents on the NCC unit.
Uniform Heterogeneous Catalysts: The Role of Solid‐State Chemistry in their Development and Design
Authors: Thomas John M. 
Published: December 2003

AbstractHeterogeneous catalysts are generally assumed to be multiphasic and multicomponent; many of them are, and this is one of the resons why disentangling the factors that govern their mode of action is so difficult. But there is a large class of heterogeneous catalysts where the solid is monophasic and where the activity may be envisaged as being dispersed in a spatially uniform fashion throughout its bulk. This is true both of zeolites and many other microporous catalysts on the one hand, and of certain mixed metal oxides, where the non‐stoiohiometry is inextricably mingled with the catalysis, on the other. By recognizing this broad classification numerous operational advantages follow: the performance of existing catalysts and the design of those yet to be prepared can be placed on a rational footing; moreover, the myriad techniques of solid‐state chemistry and physics, often regarded as inapplicable to the subtle and special problems of surface chemistry, are seen to be of direct relevance as probes for the structure and properties of proven uniform heterogeneous catalysts as well as for the synthesis and development of new ones. This review, which draws analogies with and lessons from the chemistry of enzyme catalysts, focuses largely on the catalytic conversions of hydrocarbons over zeolites, clays, microporous AlPO4 and a wide range of metal oxides.
Chemical Modifications of Biopolymers by Quinones and Quinone Methides
Authors: Peter Martin G. 
Published: December 2003

AbstractMany quinones and their precursors, which are transformed oxidatively into quinones and/or quinone methides, are important natural products. As secondary metabolites, they frequently possess antibiotic and cytotoxic activities, in addition to acting sometimes as pathogens. Several plants and animals, especially insects, use quinonoid substances for defense, often with spectacular results. On the macromolecular level, quinone methides have a key role in the plant kingdom in lignin biosynthesis; the biosynthesis of melanoproteins exemplifies the reactions of o‐quinones in the animal kingdom. In insects, cross‐linking of structural proteins through quinones and quinone methides results in the construction of the sclerotized exoskeleton. For mankind, the reactivity of quinones in biological systems has far‐reaching consequences of pharmaceutical, toxicological, and technical relevance. The examples in this review show that a common principle underlies these reactions, namely, the chemical modification of biopolymers. As demonstrated particularly well in a more detailed discussion of the chemical principles of insect cuticle sclerotization, several major and important new results have emerged from the development and applications of modern methods of sample separation and from solid‐state NMR spectroscopy.
The Stereoselectivity of Intermolecular Free Radical Reactions [New Synthetic Methods (78)]
Authors: Giese Bernd 
Published: December 2003

AbstractThe regio‐ and chemoselectivities of free radical reactions are often high and largely predictable; systematic studies have now shown that the stereoselectivity of free radical reactions can also be directed. Examples involving five‐ and six‐membered cyclic radicals will be used to show how steric and stereoelectronic effects influence the diastereoselectivity of reactions of cyclic radicals with olefins. The temperature, the solvent, and the nature of the radical scavenger used also play a role, so that, if the correct reaction conditions are used, the stereoselectivity of reactions for cyclic reactants can be very high. Lower stereoselectivities are often observed for reactions between acyclic radicals and acyclic alkenes. However, preliminary experiments have indicated that under certain conditions such systems can also react in a stereoselective manner.
Catalytic Antibodies
Authors: Schultz Peter G. 
Published: December 2003

AbstractThe vast binding repertoire of the immune system has been exploited for the generation of tailor‐made biological catalysts. A number of strategies have been developed to generate catalytic antibodies that carry out a wide range of reactions with exquisite specificities. The generation and characterization of these novel catalysts is not only providing new insights into the nature of biological recognition and catalysis, but may also lead to novel catalysts for applications in chemistry, biology, and medicine.
Polymer Optical Fibers and Nonlinear Optical Device Principles
Authors: Groh W. , Lupo D. , Sixl H. 
Published: December 2003

AbstractBasic principles of the present optoelectronic research activities at Hoechst AG are presented in this contribution. Within the diversified broad band spectrum of optoelectronic materials and applications our interest is focused on new types of polymer optical fibers and nonlinear optical device principles using organic Langmuir–Blodgett films and electrically poled polymers. The basic principles and the limits of the present research and development activities which have a high market potential are outlined.
High‐Performance Silicon Nitride Materials
Authors: Boberski Cornelia , Hamminger Rainer , Peuckert Marcell , Aldinger Fritz , Dillinger Reinhard , Heinrich Jürgen , Huber Jürgen 
Published: December 2003

AbstractThe past decade has seen greatly increased interest in ceramic materials with special combinations of properties. Their hardness, high resistance to abrasive and erosive wear, and their good corrosion resistance when used under extreme mechanical and thermal conditions have made ceramics increasingly attractive for applications in mechanical and apparatus engineering. Silicon nitride (Si3N4) in particular, is suitable for the development of a high‐performance, high‐temperature material for vehicle engine construction. Although the properties of Si3N4 materials have been consistently improved in the last few years, they are still not adequate to allow these materials to be used in series production parts, for example as valves in automotive engines. One of the main reasons for this is the wide scatter and low reproducibility of the property parameters, which means that component reliability does not yet meet market requirements. Hence, one aim of Si3N4 materials development is to reduce the structural flaws resulting from physical and chemical inhomogeneity. This objective can only be achieved through a fundamental understanding of all the processing steps, from powder production to the final processing of the components.
Synthesis, Structure and Reactivity of Sulfur‐Rich Cyclopentadienyl‐Transition Metal Complexes: Sulfur Chemistry from an Organometallic Point of View
Authors: Wachter Joachim 
Published: December 2003

AbstractMetal‐sulfur centers play an important role in the activity of metalloproteins in enzymatic catalysis and the activity of metal sulfides as heterogeneous catalysts. The systematic search for MS model compounds led to the discovery of an interesting and novel structural chemistry, which stems from the numerous coordination possibilities of sulfur ligands. The intention of this review article is to present and outline new approaches to sulfur chemistry from the organometallic point of view. Reactive cyclopentadienyl‐transition metal fragments incorporate elemental sulfur to give polynuclear sulfur‐rich complexes, which can contain either mono‐, di‐ or polysulfido ligands or several such ligands in combined form. The versatile structural chemistry of the complexes formed and their reactivity towards organic, inorganic and organometallic compounds are discussed, and examples of some simple and rational procedures for their synthesis starting from cyclopentadienylcarbonyl‐ and cyclopentadienylhydrido‐complexes are outlined. Their reactivity is manifested in numerous metal‐ and ligandcentered reactions. Finally the, albeit far less extensive, complex chemistry of the other chalcogens (O, Se, Te) is also considered for comparison, thus providing a more detailed survey of particular aspects of this area of chemistry.