bluuebunny/crossref_metadata_2025_split huggingface dataset:
High‐Resolution Solid‐State 13C‐NMR Spectroscopy of Polymers [New Analytical Methods (37)]
Authors: Voelkel Rüdiger 
Published: December 2003

AbstractUntil a few years ago, solid‐state nuclear resonance yielded spectra containing broad lines only. Meanwhile, CP/MAS‐NMR spectroscopy has provided a method which gives narrow nuclear resonance lines from a solid‐state specimen as well. Using this technique, it is now possible to produce spectra of “rare” nuclei (13C, 29Si, 15N etc.) which are resolved in terms of chemical structure. The analytical capabilities of NMR spectroscopy can be applied to the solid state: it may be that it is necessary to identify compounds in the solid state because, for example, a solvent would alter the coordination sphere, or that it is desired to monitor chemical reactions in the solid state, for example the baking of an enamel. Where a substance in the solid state is concerned, high‐resolution 13C‐NMR spectroscopy provides not only information about the chemical structure, but also about the solid state itself. To mention just a few examples, information on the conformation, crystal structure and molecular dynamics, as well as molecular miscibility is given. This opens up a broad spectrum of applications, from a statement concerning the crystal modification of an active substance in ready‐to‐use pharmaceutical preparations, e.g. tablets, to the question of whether two polymers are miscible with one another at a molecular level.
Phosphaalkynes—Syntheses, Reactions, Coordination Behavior [New Synthetic Methods (73)]
Authors: Regitz Manfred , Binger Paul 
Published: December 2003

AbstractOrganophosphorus compounds have been applied in two ways in chemical synthesis. They can either be used as a reagent in a step of the synthesis (for example, in the Wittig reaction) or they can be incorporated directly into the target molecule. This second application, in particular, has expanded greatly in the last few years with the preparation of low‐coordination phosphorus compounds. These include the phosphaalkynes, which are of great interest to organic and inorganic chemists. Phosphaalkynes have been employed in the synthesis of heterocyclic compounds, phosphaarenes and their valence isomers, and polycyclic compounds. Further applications have been the use of phosphaalkynes as new ligand systems in complex chemistry and their cyclooligomerization with organometallic reagents. While the chemical properties of phosphaalkynes have little in common with those of nitriles, they are in many ways very similar to those of the isoelectronic acetylenes.
The Chemistry of Amino Imino Boranes
Authors: Nöth Heinrich 
Published: December 2003

AbstractDicoordinated boron compounds can no longer be considered as “exotic” species in boron chemistry: this is proved by the extensive chemistry of bis(dialkylamino)boron(1+) ions, alkylidene boranes, and imino boranes. The latter are characterized by high reactivity and chemical versatility. A special position is taken by amino imino boranes, whose chemistry is reported here. The presence of an amino group enhances the reactivity of imino boranes on the one hand and increases their kinetic stability on the other. New types of diamino boranes accessible via amino imino boranes often undergo intramolecular ring closure. These heterocycles are well suited to generate new types of cations of boron. Cycloaddition reactions using amino imino boranes open up a new heterocyclic chemistry of boron. The chemistry ofN‐functional amino imino boranes, which is still its infancy, shows much promise.
Stiff‐Chain Polymers—Structure, Phase Behavior, and Properties
Authors: Ballauff Matthias 
Published: December 2003

AbstractMost of the industrial plastics used up to now consist of flexible macromolecules in which the repeat units are joined together in a nonlinear fashion by rotating bonds. Stiff‐chain polymers, which in the ideal case have a rodlike shape, have received much less attention. The reason for this is that such polymers usually exhibit an exceedingly low solubility. Very often, they are absolutely insoluble and do not melt. However, in recent years these polymers have aroused technical interest because high‐modulus fibers and moldings can be fabricated from them. These favorable mechanical properties are directly related to the nearly parallel arrangement of the rodlike macromolecules in the solid state which is achieved through spinning or extrusion from a liquid crystalline (nematic) state. It is therefore evident that a thorough understanding of the phase behavior and the structure is required for a universal technical utilization of these materials. The great variety of methods used in polymer science today has led to a deeper understanding of stiff‐chain polymers. Experimental results together with theoretical modeling of the phase behavior have direct implications for the practical use of these macromolecular materials.
Functionalized Conducting Polymers—Towards Intelligent Materials
Authors: Garnier Francis 
Published: December 2003

AbstractOrganic conjugated polymers and oligomers constitute a three‐dimensional network of molecular wires, in which all monomeric units can be functionalized with various prosthetic groups. By varying the nature of these groups, specific interactions with external physical or chemical phenomena can be developed in these materials, leading to molecular devices such as sensors, transducers, memories and logic operators. Chemists have already mastered the realization of many of these functional elements, which mimic those existing in organized beings.The further assembly of these elements in multifunctionalized organic conducting polymers and oligomers will represent the next step towards intelligent materials.
General Aspects and Optimization of Enantioselective Biocatalysis in Organic Solvents: The Use of Lipases [New Synthetic Methods (76)]
Authors: Chen Ching‐Shih , Sih Charles J. 
Published: December 2003

AbstractEnantioselective biocatalysis in nonaqueous media is becoming increasingly important in preparative synthetic chemistry. This article discusses (1) the general catalytic properties of enzymes in nonaqueous environments, (2) the basic principles that govern lipase‐catalyzed enantioselective esterification and transesterification reactions in organic media for the preparation of optically active acids and alcohols, (3) the determination of kinetic and thermodynamic parameters, and (4) the quantitative analysis of published data.
The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas viridis (Nobel Lecture)
Authors: Deisenhofer Johann , Michel Hartmut 
Published: December 2003

AbstractIn our lecture we first describe the history and methods of membrane protein crystallization, before we show how the structure of the photosynthetic reaction center from the purple bacterium Rhodopseudomonas viridis was solved. The structure of this membrane protein complex is then correlated with its function as a light‐driven electron pump across the photosynthetic membrane. Finally, we draw conclusions on the structure of the photosystem II reaciton center of plants and discuss the aspects of membrane protein structure. Sections 1 (crystallization), 4 (conclusions on the structure of photosystem II reaction center and evolutionary aspects) and 5 (aspects of membrane protein structure) were presented and written by H. M., sections 2 (determination of the structure) and 3 (structure and function) by J. D. We have arranged the manuscript in this way in order to facilitate continuous reading.
The Active Sites in Manganese‐Containing Metalloproteins and Inorganic Model Complexes
Authors: Wieghardt Karl 
Published: December 2003

AbstractManganese is an essential trace element, forming the active sites of a number of metalloproteins. Several metalloproteins contain two or more manganese ions per subunit. The structural properties of these enzymes and the experimental evidence for their proposed structures are described. Parallel to the efforts of biochemists, who are seeking to understand the function of these enzymes on a molecular level, inorganic chemists have been investigating the coordination chemistry of bi‐ and polynuclear complexes of manganese which contain O, N donor atoms and a variety of bridging O, N ligands. A large number of such complexes have been synthesized, their X‐ray structures determined and their magnetic and spectroscopic properties studied in detail. In some instances the electronic and spectroscopic properties of these model compounds are amazingly similar to those of the biomolecules. This has led to a deeper understanding of the structure and sometimes of the function of the metalloproteins. Research on manganese metalloproteins with polynuclear active sites represents a fascinating example of interdisciplinary cooperation between physicists, biochemists and inorganic chemists.
The Beginnings of Organic Photochemistry
Authors: Roth Heinz D. 
Published: December 2003

AbstractAlthough sunlight induced photochemistry must have occurred on the planet Earth for billions of years, the chemical changes caused by light have attracted systematic scientific scrutiny only relatively recently. How did scientists first conceive the idea that the interaction of materials with light could not only cause physical phenomena, but could also alter their chemical nature? When sunlight began to be employed as a heat source for distillation, the eventual discovery of photochemical reactions was assured. One can envision three types of changes that would have aroused the curiosity of laboratory chemists: color changes; the evolution of gas bubbles (oxygen in photosynthesis); and the precipitation of a photoproduct less soluble than its precursor. Less predictable was the observation that sunlight caused crystalline santonin to burst because it is converted into a product with a different crystal lattice. In the course of the eighteenth and nineteenth centuries a variety of photochemical reactions, some observed by chance, others uncovered in carefully planned studies, ultimately led to a major systematic investigation that established photochemistry as a viable branch of chemistry.
Fundamentals of Silicon Chemistry: Molecular States of Silicon‐Containing Compounds
Authors: Bock Hans 
Published: December 2003

AbstractSilicon and its compounds have made possible the design of new materials, which, from computers to space travel, have helped to shape the technology of our 20th century. Conversely, the demands of new technology have stimulated the fast development of silicon chemistry as part of the “renaissance” of inorganic chemistry. This article uses selected examples of predominantly organosilicon compounds to discuss in simplified terms the measurement and assignment of suitable spectroscopic “molecular fingerprints” as well as the resulting benefit for the preparative chemist. The comparison of “equivalent” states of “chemically related” molecules is emphasized, based on perturbation arguments and supporting quantum‐chemical models. Special attention is given to the relation between structure and energy, which allows us to understand and to predict the connectivity between and the spatial arrangement of silicon “building blocks”, the energy‐dependent electron distribution over the effective nuclear potentials of a molecular framework, and, especially, the partly considerable effects of “silicon substituents” on molecular properties. Future‐directed extensions and applications include polysilane band structures, Rydberg states of chromophores containing silicon centers, redox reactions and ion‐pair formation of silicon‐substituted π systems, and molecular dynamic phenomena in solution or on thermal fragmentation in the gas phase. The main objective is a set of clear and practical rules for interpreting measurements and planning experiments.
NMR Microscopy—Fundamentals, Limits and Possible Applications
Authors: Kuhn Winfried 
Published: December 2003

AbstractAlongside the numerous applications of NMR spectroscopy to structural elucidation in analytical chemistry, and to biochemical and morphological studies by NMR tomography, NMR microscopy makes possible a whole new range of applications. These include imaging, the investigation of biological objects such as plants and small animals, and also the observation of microscopic structures and structural changes in polymers and ceramics. NMR spectroscopy can also be conducted combinationally as volume‐selective spectroscopy, whereby it is possible to spatially resolve the NMR‐specific parameters: spin density ϱ, chemical shifts δ, and the relaxation times T1 and T2. The numerous well developed methods available make it possible to study dynamic processes by fast imaging with a temporal resolution in milliseconds. This not only allows the imaging of moving objects without incurring movement artefacts but also the measurement of diffusion constants in isotropic and anisotropic diffusion—in the latter case allowing, in principle, the determination of the complete diffusion tensor. The spatially resolved measurement of the relaxation times yields information on molecular mobility and bonding, e. g. the bonding of water, or other solvents, to polymers, the mobility of fluids in polymers or ceramics, or the three‐dimensional evaluation of pore size in porous materials. In biomedicine, NMR microscopy allows the observation of growth on the cellular level, the study of embryos, and the development of therapeutic methods in animal experiments. It can lead to a drastic reduction in the number of animal experiments, and in combination with volume‐selective spectroscopy gives valuable information on in‐vivo metabolism.
Biologically Active Analogues of the Extracellular Matrix: Artificial Skin and Nerves
Authors: Yannas Ioannis V. 
Published: December 2003

AbstractAnimal development starts as a single cell which proliferates into several new cells; these differentiate into highly specialized tissues, organs, and limbs; and the small but functioning organism eventually grows into its full scale. Throughout development the extracellular matrices, which are complex macromolecular networks, also undergo dramatic changes. Matrix transformations occasionally control the much more well‐studied changes in number and type of differentiating cells. Extracellular matrix (ECM) networks are typically broken down enzymatically to oligopeptides and are then resynthesized (remodeled) to form insoluble and nondiffusible macromolecular structures which confer stability of shape to multicellular systems. Mature ECM, such as skin, tendon, cartilage, and blood vessels, provides stiffness and strength to tissues and organs. Remodeling of ECM also occurs in adult organisms, during wound healing. An understanding of the role that ECM plays during development or wound healing can be obtained by use of synthetic ECM analogues. Several simple chemical ECM analogues have been synthesized and a few have been found to possess remarkable biological activity. One of these analogues has induced the partial regeneration of skin in an adult guinea pig wound model as well as in man. Peripheral nerve has been regenerated in another animal model by use of a similar ECM analogue. In all these mammalian lesions it is well‐known that regeneration does not occur spontaneously. These analogues are graft copolymers of collagen and chondroitin 6‐sulfate (a glycosaminoglycan) in the state of highly hydrated and covalently cross‐linked gels. Procedures are summarized for synthesis of copolymers with adjusted physicochemical properties, such as the rate at which they degrade enzymatically when implanted, the elements of their pore structure, and the degree of collagen crystallinity. ECM analogues have provided a novel window into the complexities of morphogenesis and regeneration and they have pointed towards entirely new directions in the medical treatment of serious organ dysfunction and organ loss. An ECM analogue has already become the basis of a new clinical treatment for massively burned patients. An interpretation of the results leads to a hypothesis about the nature of ECM during development. Since biological activity appears only when the physicochemical parameters fall within very narrow limits, it is intriguing to speculate that these experiments describe a single insoluble growth factor which is specific for skin synthesis. Such an insoluble growth factor appears to be just as essential to skin development as are the much more well‐known soluble growth factors. A different ECM analogue appears to induce nerve regeneration, possibly because each tissue requires its own developmentally active ECM.
Active MgH2Mg Systems for Reversible Chemical Energy Storage
Authors: Bogdanović Borislav , Ritter Alfred , Spliethoff Bernd 
Published: December 2003

AbstractAn overview of the importance of and methods available for heat storage in the form of sensible and latent heat is followed by a discussion of the advantages and disadvantages of reversible thermochemical energy storage compared to conventional energy sources such as fuels, i.e. irreversible chemical energy carriers. Of the reversible metal‐hydride–metal systems, the MgH2Mg system is particularly attractive as a hydrogen and a high‐temperature heat storage material because of its high hydrogen content and the high energy content of the Mg–H bond. The advances made in this area over the past few years, namely in catalytic hydrogenation and the doping of magnesium powders, have led to the development of “active MgH2Mg systems” for energy storage. The first experimental results on high‐temperature heat storage (also with cooling) by coupling a MgH2Mg storage system with a low‐temperature metal hydride storage system are presented.
Nickel: An Element with Wide Application in Industrial Homogeneous Catalysis
Authors: Keim Wilhelm 
Published: December 2003

AbstractThe efficiency and future development of the chemical industry are closely linked to catalysis. It has been estimated, for example, that 60 to 70% of all industrial chemicals have involved the use of a catalyst at some point during their manufacture. In the past two decades the share of the market credited to homogeneous transition metal catalysis increasead to 10–15%. Besides cobalt, which is used mainly in hydroformylation reactions, nickel is the most frequently used metal. Many carbon–carbon bond formation reactions can be carried out with high selectivity if catalyzed by organonickel complexes. Such reactions include, inter alia, carbonylation reactions, cyclic and linear oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions. It was Reppe and Wilke who pioneered and shaped the field of homogeneous nickel catalysis. Great impetus was also given to the development of organonickel chemistry by Wilke and his students. Research in this area has contributed immensely towards an understanding of the reactions involved in catalysis.—This review is primarily concerned with nickel‐catalyzed reactions which are of interest both preparatively and industrially; some mechanistic aspects are also dealt with.
On the Conformation and Structure of Organometal Complexes in the Solid State: Two Studies Relevant to Chemical Synthesis
Authors: Shambayati Soroosh , Crowe William E. , Schreiber Stuart L. 
Published: December 2003

AbstractConformational preferences of two classes of organometal complexes have been surveyed by inspecting the Cambridge Structural Database (CSD). Lewis acid carbonyl complexes demonstrate a variety of coordination geometries, depending on the electronic and steric requirements of the carbonyl ligands and the nature of the Lewis acid. Similarly, the solid‐state conformation of various π‐bonding ligands in metal acyl, metal nitrosyl, metal acetylene, and metal imido complexes is revealed. These insights have stimulated the development of a conformational model that is based on considerations of π‐bond hybridization and frontier molecular orbital theory. The analyses are relevant to the mechanism and transition structures of many synthetically important transformations. A deeper understanding of the conformational properties of organometal complexes, based on accurate structural information, will likely expedite the design and improvement of metal‐mediated processes.
Carbonyl Oxides: Zwitterions or Diradicals?
Authors: Sander Wolfram 
Published: December 2003

AbstractOver the last few years new experimental and theoretical methods have made it possible to gain a more detailed insight into the chemistry of short‐lived reaction intermediates. In 1949 Criegee postulated the intermediacy of carbonyl oxides in the mechanism of ozonolysis, and since then these species have become the goal of much research effort. Even though the formation of “Criegee zwitterions” during ozonolysis and carbene oxidations was proven by scavenger experiments, the electronic structure—zwitterion or diradical—of this short‐lived species is still a subject of debate. To date no stable carbonyl oxide has been found to exist under “normal” laboratory conditions, although by using matrix isolation and laser spectroscopy, it has been possible to obtain highly resolved IR and UV/VIS spectra of carbonyl oxides as well as to determine their dipole moments experimentally. The influence of substituents, exact kinetic data on modes of formation, and the subsequent reactions of carbonyl oxides as well as their photochemistry complete the picture. In accordance with ab initio calculations carbonyl oxides, are best viewed as polar diradicals. The zwitterionic state lies at higher energies and should be stabilized by π donors.
Modern NMR Pulse Experiments: A Graphical Description of the Evolution of Spin Systems
Authors: Eggenberger Urs , Bodenhausen Geoffrey 
Published: December 2003

AbstractMany modern methods designed to enhance the power of NMR spectroscopy have been widely adopted by the scientific community. Some of the best known among these techniques are one‐dimensional methods for polarization enhancement and spectral editing (“INEPT” and “DEPT”) and various two‐dimensional experiments such as correlation spectroscopy (“COSY”) and nuclear Overhauser spectroscopy (“NOESY”). Most of these methods cannot be described in terms of classical magnetization vectors, and although they can be understood with the help of the density operator formalism, this approach remains difficult for many users, in part because of its cumbersome notation, which prevents one from recognizing the important features at a glance. In this paper, a set of graphical conventions is described which allows one to represent the effect of sequences of radio‐frequency pulses on spin systems in a variety of experiments. The graphs provide a concise description of the network of spins that are coupled through scalar and dipolar interactions, of the state of a spin system expressed in terms of products of operators, and, most importantly, of the transformations from one state to another due to the effect of radio‐frequency pulses, chemical shifts, and scalar couplings. The graphical approach also allows one to represent the effect of phase‐cycling and multiple‐quantum filtration, cross‐relaxation (Overhauser effects) and cross‐correlated relaxation.