bluuebunny/crossref_metadata_2025_split huggingface dataset:
Emission of Microparticles from Automotive Sources—X‐ray Photoelectron Spectroscopy in Environmental Analysis
Authors: Schlögl R. , Indlekofer G. , Oelhafen P. 
Published: December 2003

AbstractIn the last fifteen years the necessity for efficient environmental protection has become generally accepted. However, before practical measures can be taken, a comprehensive assessment of the present environmental situation should be made, taking into consideration the risks involved in as well as the positive effects of any planned environmental changes—whether natural or man made. This clearly calls for close cooperation between the various scientific disciplines. For example, the treatment of exhaust fumes from motor vehicles has been of primary importance in measures taken to restrict the recent damage caused to woodland areas. The introduction of 3‐Way‐Catalyst technology has been regarded as a viable countermeasure. The following report is intended to describe a method for examining some side effects of catalyst usage. Accordingly, the chemical composition of microparticles emitted from combustion motors through the use of catalysts will be examined and some possible changes in the environment caused by such particles will be briefly considered. As in the area of research into catalysts for industrial synthesis a knowledge of the surface structure of such particles, i.e. the boundary layer between emission product and environment, is necessary in the investigation into their effects. Consequently, a surface sensitive technique from solid state physics, X‐ray photoelectron spectroscopy (XPS or ESCA) has found novel use in the area of environmental analysis. In the following report emphasis will be placed on the description of measurement techniques and the interpretation of the results obtained. After a critical consideration of the experimental methods, examples from two different series of measurements on diesel and Otto motors equipped with soot filters or 3‐way‐catalysts respectively, will be given. Hopefully, this report will also stimulate establishment of the use of EPS in environmental research techniques.
The Importance of Chemistry in the Development of High‐Performance Ceramics
Authors: Aldinger Fritz , Kalz Hans‐Jürgen 
Published: December 2003

AbstractDue to their mechanical, thermal and chemical stability, high‐performance ceramics have a range of properties which makes it possible in many fields to go beyond the limits of use of the various classes of materials nowadays available. In spite of this favorable starting situation, the worldwide endeavors to introduce this new group of materials have suffered some setbacks in both conventional applications and in new technologies. The ultimate reason is that these materials are very brittle. After the introduction of fracture mechanics, designers in many fields have learned how to handle brittle materials and how to exploit the potential of their properties in components subject to extreme loadings. Materials scientists have also been very successful in developing ceramics with high strength and fracture toughness characteristics. Nevertheless, the field of use has hitherto been restricted, since, in many cases, the reliability of the components is inadequate. Small and very small defects, such as micropores and cracks, as well as any other chemical and physical inhomogeneity in the structure can cause catastrophic failure of the components due to the high brittleness. Since, on the one hand, these defects are as a rule caused by inadequate control of the process sequences and, on the other, the production processes comprise a large number of chemical process steps, it is to be hoped that consistent application of the way of thinking and procedures of a chemist will make an important contribution in overcoming these difficulties.
Computer‐Assisted Molecular Design (CAMD)—An Overview
Authors: Frühbeis Horst , Klein Robert , Wallmeier Holger 
Published: December 2003

AbstractA new instrument, long established as CAD in engineering, is beginning to make its presence felt in chemical research laboratories: Computer‐Assisted Molecular Design (CAMD). The combined use of computer graphics and theoretical chemistry is opening up new perspectives in molecular research. Structures and properties of molecules such as spacefilling, charge distribution, or dynamic behavior can be determined and used for comparison. For research on complex systems like biomolecules (protein engineering), this new approach turns out to be indispensable.
Stereoselective Syntheses of Building Blocks with Three Consecutive Stereogenic Centers: Important Precursors of Polyketide Natural Products [New Synthetic Methods (68)]
Authors: Hoffmann Reinhard W. 
Published: December 2003

AbstractSequences of directly adjacent stereogenic centers were first discovered for the monosaccharides, which therefore constituted the first focus of interest in Stereoselective synthesis. The structures of many polyketide‐derived natural products that have been elucidated in recent years often exhibit longer sequences of stereogenic centers in which hydroxy‐substituted secondary carbon atoms alternate with tertiary carbon atoms. This finding inspired the development of methods allowing the specific construction of first two and then—more importantly–three neighboring stereogenic centers. The diverse methodologies that have thereby emerged reveal common principles and stress the variety of approaches possible in stereoselective synthesis.
The Development of Bioglass Ceramics for Medical Applications
Authors: Vogel Werner , Höland Wolfram 
Published: December 2003

AbstractThe enormous progress made in the field of medicine over the past few decades has been partly due to the introduction of new instruments but also a result of the use of new materials. It is impossible to imagine modern medicine without metals, alloys, sintered corundum, organic high polymers (also as composite materials), glassy carbon, etc. Bioglass ceramics open up new possibilities for medical treatment and constitute a new area of research in the natural sciences and medicine. Owing to their widely variable combinations of properties, bioglass ceramics can be more easily adapted to suit medical requirements than can customary implants. Two properties of bioglass ceramics are of primary importance: their biocompatibility, i.e., acceptance of the material by the tissues of the human body without irritation, rejection reactions, or toxic effects; and their bioactivity, i.e., the ability to establish firm intergrowths with tissues of the human body. This property is not shared by any of the classical biomaterials. A wide range of applications is envisaged for the bioglass ceramics that have so far been developed; some are still undergoing animal tests while others are being clinically tested in humans. Possible applications are the replacement of vertebrae and use in the middle ear, throat, nose, and eye, in the entire head region, in the shoulder and leg, and in dental prosthetics, in particular the replacement of dental roots (a hard tissue substitute in the broadest sense of the word). The question as to the behavior of a bone/bioglass ceramic contact or bond on a long‐term scale and on being exposed to varying mechanical stress has still not been satisfactorily answered, because interdisciplinary research in this field is still immature. All observations made so far indicate, however, that the materials do not cause any adverse effects.
Diene, Alkyne, Alkene, and Alkyl Complexes of Early Transition Metals: Structures and Synthetic Applications in Organic and Polymer Chemistry
Authors: Yasuda Hajime , Nakamura Akira 
Published: December 2003

AbstractAn unprecedented series of highly reactive alkene‐and diene‐complexes of the early transition metals (Groups 3A–5A of the periodic system) have been isolated recently. Diene complexes of this sort (M  Ti, Zr, Hf, Nb, Ta) prefer, besides the (η4‐s‐cis‐diene)metal structure, either a novel bent η4‐metallacyclo‐3‐pentene structure or the unique (η4‐s‐trans‐diene)metal structure. In bis(diene)metal complexes of Nb and Ta the η4‐s‐cis‐dienes assume an unusual exo‐endo (supine‐prone) geometry. The MC bonds in these diene‐metal complexes generally exhibit highly polarized σ‐bonding along with π‐bonding character. The complexes therefore undergo a variety of regio‐ and stereoselective carbometalations with substrates containing CC, CO, or CN multiple bonds. Examples of the products that can be obtained include ketones, vinyl ketones, unsaturated primary, secondary, and tertiary alcohols, as well as diols and unsaturated acids. Mechanistic studies on the stoichiometric and catalytic conversions of unsaturated hydrocarbons provides, inter alia, some insights into the course of polymerization reactions.
RP‐Bridged Metal Carbonyl Clusters: Synthesis, Properties, and Reactions
Authors: Huttner Gottfried , Knoll Konrad 
Published: December 2003

AbstractMetal carbonyl clusters possess a complicated chemistry that is only beginning to be understood. One of the main current goals in this area is thus an understanding of their reactivity. This article describes the syntheses and reactions of clusters that contain metal carbonyl fragments bridged by a main‐group element. But what is the sense of making such clusters still more complicated by the incorporation of main‐group elements? The example of μ3‐bridged carbonyl clusters will serve to show that the main‐group element plays an important role in the study of reaction paths; it holds the metal carbonyl fragments together even when the bonds between them are broken in the course of a reaction. Trinuclear μ3‐bridged clusters prove to be small enough to allow the analysis of typical cluster reactions (such as the reversible breaking of metal‐metal bonds) in terms of single reaction steps. They are also large enough to provide surprises by their multifaceted reactivity. It will be shown that a detailed study of trinuclear RX‐bridged metal carbonyl clusters (X  N, P, As, Sb, Bi)—a very small part of carbonyl cluster chemistry—can lead to a better understanding of the general reaction principles involved.
Liquid Crystalline Polymers
Authors: Finkelmann Heino 
Published: December 2003

AbstractThe liquid crystalline behavior of low molecular weight compounds has been known for more than a century; synthetic polymers have been manufactured on a large scale for several decades, but just recently it was found possible to produce polymers using the structural principles of liquid crystalline compounds. The resulting materials have, as expected, unusual properties. Numerous applications, not only in opto‐electronics, are already anticipated for such materials.
How Chemistry and Physics Meet in the Solid State
Authors: Hoffmann Roald 
Published: December 2003

AbstractTo make sense of the marvelous electronic properties of the solid state, chemists must learn the language of solid‐state physics, of band structures. An attempt is made here to demystify that language, drawing explicit parallels to well‐known concepts in theoretical chemistry To the joint search of physicists and chemists for understanding of the bonding in extended systems, the chemist brings a great deal of intuition and some simple but powerful notions. Most important among these is the idea of a bond, and the use of frontier‐orbital arguments. How to find localized bonds among all those maximally delocalized bands? Interpretative constructs, such as the density of states, the decomposition of these densities, and crystal orbital overlap populations, allow a recovery of bonds, a finding of the frontier orbitals that control structure and reactivity in extended systems as well as discrete molecules.
Some Concepts in Reaction Dynamics (Nobel Lecture)
Authors: Polanyi John C. 
Published: December 2003

AbstractThe objective in this work has been one which I have shared with the two other 1986 Nobel lecturers, D. R. Herschbach and Y. T. Lee, as well as with a wide group of colleagues and co‐workers who have been responsible for bringing this field to its current state. That state is summarized in the title; we now have some concepts relevant to the motions of atoms and molecules in simple reactions, and some examples of the application of these concepts. We are, however, richer in vocabulary than in literature. The great epics of reaction dynamics remain to be written. I shall confine myself to some simple stories.For corrigendum see DOI:10.1002/anie.198712982
Carbosilanes
Authors: Fritz Gerhard 
Published: December 2003

AbstractCarbosilanes are compounds in which the elements carbon and silicon occupy alternate positions in the molecular framework. Formal replacement of every second carbon atom in the diamond lattice by a silicon atom gives silicon carbide, which exists, however, in several modifications characterized by different stacking orders. The SiC lattice is the basis of most carbosilanes.[1] These are divided on the basis of structural differences into carborundanes, silascaphanes, and molecules that no longer contain elements of the silicon carbide structure. In the carborundanes the Si–C six‐membered rings are arranged in the chair form, in the silascaphanes in the boat form. The molecular framework of the third group is derived from barrelane and asterane. The reactivity of the carbosilanes is determined mainly by the substituents on the skeletal atoms. The presence of SiH or Si–halogen groups leads to a high reactivity, while derivatives with organic groups on the silicon atoms and CH2 or CH groups in the skeleton are much less reactive. However, even such CH2 and CH groups permit certain characteristic reactions. Because of their structural features and the range of substituents that can be introduced, the carbosilanes are presently of considerable interest with respect to technological developments as precursors for ceramics.
Chemistry of the Silicon‐Silicon Double Bond
Authors: West Robert 
Published: December 2003

AbstractDisilenes, compounds containing silicon‐silicon double bonds, can be isolated as thermally stable, yellow‐ or orange‐colored crystalline compounds; the substituent groups at silicon must be large (mesityl, tert ‐butyl, etc.) to prevent polymerization. Structural and spectro‐scopic studies indicate many similarities between the 3p–3p π bonding in disilenes and the 2p‐2p π bonding in olefins, but disilenes are much more reactive than alkenes. The reactions of disilenes lead to many new classes of silicon compounds, including three‐ and four‐membered rings which present novel problems in chemical bonding.
Vitamin B12: Experiments Concerning the Origin of Its Molecular Structure
Authors: Eschenmoser Albert 
Published: December 2003

AbstractFollowing the chemical synthesis of vitamin B12, a search was begun for a potentially biomimetic “dark” variant of the photochemical A/D‐secocorrin → corrin cycloisomerization, the central ring‐closure step in one of the two cobyric acid syntheses. Significantly, not just one but a whole family of such variants was discovered. According to what is currently known, one of these variants can indeed be regarded as a chemical model for the reaction path followed by Nature in the biosynthetic construction of the corrin ring. These chemical studies of vitamin B12 biosynthesis had revealed that the A/D‐ring junction, regarded as the main obstacle to a chemical vitamin B12 synthesis at the outset, is in fact a structural element that is formed readily and in a variety of ways from structurally appropriate precursors. More recent investigations have shown that the same holds for other specific structural elements of the vitamin B12 molecule, including the characteristic arrangement of double bonds in the corrin chromophore, the special dimension of the macrocyclic ring of the corrin ligand, the specific attachment of the nucleotide loop to the propionic acid side chain of ring D, and the characteristic constitutional arrangement of the side chains around the ligand periphery (which vitamin B12 shares with all uroporphinoid cofactors). All these outwardly complex structural elements are found to “self‐assemble” with surprising ease under structurally appropriate preconditions; the amount of “external instruction” required for their formation turns out to be surprisingly small in view of the complexity and specificity of these structural elements. We view these findings as steps on the way toward a chemical rationalization of the vitamin B12 structure. The goal is to arrive experimentally at a perception of the biomolecule's intrinsic potential for structural self‐assembly. This potential, together with the specific type of reactivity related to the biological function, is considered to be responsible for the biomolecule having been chosen by natural selection. The chemical rationalization of the structure of biomolecules is an objective of organic natural product chemistry. The field of natural product synthesis provides appropriate conceptual and methodological tools to approach this objective experimentally.
Supramolecular Chemistry—Scope and Perspectives Molecules, Supermolecules, and Molecular Devices (Nobel Lecture)
Authors: Lehn Jean‐Marie 
Published: December 2003

AbstractSupramolecular chemistry is the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by association of two or more chemical species. Molecular recognition in the supermolecules formed by receptor‐substrate binding rests on the principles of molecular complementarity, as found in spherical and tetrahedral recognition, linear recognition by coreceptors, metalloreceptors, amphiphilic receptors, and anion coordination. Supramolecular catalysis by receptors bearing reactive groups effects bond cleavage reactions as well as synthetic bond formation via cocatalysis. Lipophilic receptor molecules act as selective carriers for various substrates and make it possible to set up coupled transport processes linked to electron and proton gradients or to light. Whereas endoreceptors bind substrates in molecular cavities by convergent interactions, exoreceptors rely on interactions between the surfaces of the receptor and the substrate; thus new types of receptors, such as the metallonucleates, may be designed. In combination with polymolecular assemblies, receptors, carriers, and catalysts may lead to molecular and supramolecular devices, defined as structurally organized and functionally integrated chemical systems built on supramolecular architectures. Their recognition, transfer, and transformation features are analyzed specifically from the point of view of molecular devices that would operate via photons, electrons, or ions, thus defining fields of molecular photonics, electronics, and ionics. Introduction of photosensitive groups yields photoactive receptors for the design of light‐conversion and charge‐separation centers. Redox‐active polyolefinic chains represent molecular wires for electron transfer through membranes. Tubular mesophases formed by stacking of suitable macrocyclic receptors may lead to ion channels. Molecular self‐assembling occurs with acyclic ligands that form complexes of double‐helical structure. Such developments in molecular and supramolecular design and engineering open perspectives towards the realization of molecular photonic, electronic, and ionic devices that would perform highly selective recognition, reaction, and transfer operations for signal and information processing at the molecular level.
Pharmaceutical Proteins
Authors: Blohm Dietmar , Bollschweiler Claus , Hillen Heinz 
Published: December 2003

AbstractThere is currently great interest in pharmaceutical proteins, and work in this area, from basic research to drug marketing, is expanding vigorously. The starting points of these developments are genetic engineering and biotechnology, which contribute to the discovery of new proteins and allow them to be produced on a worthwhile scale. Ever since the commercial introduction of insulin in 1923, of thyroid hormone in 1934, of factor VIII in 1948, and of calcitonin in 1970, hormones, serum proteins, and enzymes have been firmly established as therapeutic agents. Immunomodulators and various tissue proteins are in a turbulent phase of development as a new approach to the therapy of tumors. Increasingly, diagnostic aids now take the form of enzymes and monoclonal antibodies, and recently there has even been development of vaccines based on defined single proteins and oligopeptides. Of the more than 200 pharmaceutical proteins which have been investigated to date, more than half are undergoing research and development, about 100 are in clinical trials, and a dozen or so have already been marketed. The most important indications for them are cardiovascular disorders, tumors, autoimmune diseases, and infections. Genetically engineered proteins can be used as active substances, which are chemically and biologically exactly defined, for new drug products and in research to acquire new information on therapeutic use, based on the concept of therapy with endogenous proteins. Very recent developments have shown that proteins, either in their natural form or modified, are becoming of increasing importance for research into active substances.
Complexation of Neutral Molecules by Cyclophane Hosts
Authors: Diederich François 
Published: December 2003

AbstractSince the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme‐substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor‐acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force‐field calculations together with computer graphics are powerful tools in the design of water‐soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates in a rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.