bluuebunny/crossref_metadata_2025_split huggingface dataset:
Homoatomic d10–d10Interactions: Their Effects on Structure and Chemical and Physical Properties
Authors: Jansen Martin 
Published: December 2003

AbstractThe distinction between valence electrons and essentially inactive core electrons is the basis of many classifying concepts in chemistry. However, it has recently been recognized that this is an oversimplification and should, at least in some areas of chemistry, be modified. Many cases are known where cations with closed d10configurations are subject to homoatomic interactions that influence structure and properties. A characteristic and surprisingly uniform structural feature (e.g., of a number of compounds containing monovalent coinage metals) is a clusterlike assembly of d10cations that corresponds in geometry and bond lengths to fragments of the metal structures themselves. Further evidence for a special type of bonding in such compounds is provided by their physical properties; for example, the absorption in the UV/VIS region shows a drastic redshift and the compounds are often conductors or semiconductors. The d electrons in such cases have obviously lost their pure “core” nature. All bonding models so far proposed for such systems involve mixing of higher orbitals.
Molecular Architecture and Function of Polymeric Oriented Systems: Models for the Study of Organization, Surface Recognition, and Dynamics of Biomembranes
Authors: Ringsdorf Helmut , Schlarb Bernhard , Venzmer Joachim 
Published: December 2003

AbstractThe Part and the Whole. The principle of self‐organization for the creation of functional units is not an invention of modern natural sciences. It was already a basic idea of the ancient philosophies in Asia and Europe: only the mutuality of the parts creates the whole and its ability to function. Translated into the language of chemistry this means: the self‐organization of molecules leads to supramolecular systems and is responsible for their functions. Thermotropic and lyotropic liquid crystals are such functional units, formed by self‐organization. As highly oriented systems, they exhibit new properties. The importance of lyotropic liquid crystals for the life sciences has been known for a long time. They are a prerequisite for the development of life and the ability of cells to function. In materials sciences this concept offunction through organizationled to the development of new liquid‐crystalline materials. From the point of view of macromolecular chemistry, this review tries to combine these two different fields and especially hopes to stimulate their interaction and joint treatment. To exemplify this, the molecular architecture of polymeric organized systems will be discussed. Polymeric liquid crystals combine the ability to undergo spontaneous self‐organization–typical of liquid‐crystalline phases–with the polymer‐specific property of stabilizing these ordered states. As new materials, polymeric liquid crystals have already been investigated intensively. As model systems for biomembranes as well as for the simulation of biomembrane processes, they so far have been little discussed. The intention of this review article is to show that polymer science is able to contribute to the simulation of cellular processes such as the stabilization of biomembranes, specific surface recognition, or even the “uncorking” of cells. Polymer science, having an old tradition as an inter‐disciplinary field, can no longer restrict itself to common plastics. Attempts to reach new horizons have already begun. The borderland between liquid crystals and cells will certainly play an important role. Basic requirements to work in this frontier area between organic chemistry, membrane biology, life science, and materials science will be the delight in scientific adventures as well as the courage to go ahead. The most important prerequisite will be the willingness to cooperate with disciplines which so far have not really accepted each other. From this point of view, this review does not aim at giving defined answers. It wants instead to encourage the scientific venture: too often we cling to painfully acquired knowledge, fearing adventures.
Contributions to Organo‐Nickel Chemistry
Authors: Wilke Günther 
Published: December 2003

AbstractOne hundred years of organonickel chemistry have generated a wealth of new knowledge and a history of examples of accidental discoveries which have finally led to industrial applications. The historical development is associated with the names of Mond, Sabatier, Reppe and Ziegler and, with the methods and techniques available today, many of the original discoveries and unsolved problems are attracting renewed attention. For example, 70 years were to go by before a synthesis first conceived by Sabatier could finally be realized. The path leading from nickel‐catalyzed hydrogenation to highly enantioselective homogeneous catalysts is one of the contributions to organonickel chemistry which is described here.
Zeolites: Catalysts for Organic Syntheses
Authors: Hölderich Wolfgang , Hesse Michael , Näumann Fritz 
Published: December 2003

AbstractZeolites have proved to be valuable technical catalysts in petrochemistry and in oil processing. The characteristic properties of zeolites, such as acidity, shape‐selectivity and thermal stability also enable them to be used for highly selective synthesis in the fields of chemical intermediates and fine chemicals. This interesting area of application has grown continuously in recent years. The present article summarizes the various standard types of reaction involved in organic syntheses which can be catalyzed by zeolites; these include, inter alia, electrophilic and nucleophilic substitution reactions, isomerization of double bonds and carbon skeletons, as well as addition, elimination and hydrogenation reactions.
Multiphoton‐Ionization‐Mass Spectrometry (MUPI‐MS) [New Analytical Methods (34)]
Authors: Grotemeyer Jürgen , Schlag Edward W. 
Published: December 2003

AbstractMass spectrometry is one of the most important analytical tools in chemistry, biology, medicine and related areas. During the past 30 years, methods have been developed, both for the qualitative as well as the quantitative analysis of a wide variety of substances. The introduction of lasers into chemistry has also profited mass spectrometry, since its nonlinear properties and its tunability open up fundamentally new frontiers. Multiphoton ionization mass spectrometry combines UV‐spectroscopy and mass spectrometry, thus providing a two‐dimensional method that enables substance‐specific and even state‐specific analyses. This progress report presents the fundamentals and possibilities of MUPI mass spectrometry and discusses investigations on amino acids, peptides, chlorophylls and sugars. For each substance, multiphoton ionization can be tuned in such a way that only the molecular ion is formed. An increase of laser intensity induces substance‐specific fragmentations of the molecule, thus contributing to a quick and easy identification of the substance.
Microbial and Enzymatic Processes for the Production of Biologically and Chemically Useful Compounds [New Synthetic Methods (69)]
Authors: Yamada Hideaki , Shimizu Sakayu 
Published: December 2003

AbstractIn recent years, the most significant development in the field of synthetic organic chemistry has been the application of biological systems to chemical reactions. Reactions catalyzed by enzymes and enzyme systems display far greater specificities than more conventional organic reactions. Biological and/or enzymatic syntheses and transformations, that is, “microbial transformations,” have great potential. Some of these reactions have already been shown to have useful applications in the fields of synthetic organic chemistry and biotechnology. This article reviews the current status of the rapidly developing field of microbial transformation, the methodology, available technological procedures, and fields of application being described especially in relation to conventional organic synthesis methods.
Reduction and Hydrogenation with the System Hydrocarbon/Carbon [New Synthetic Methods (70)]
Authors: Dockner Toni 
Published: December 2003

AbstractFor the synthesis of organic compounds, reduction is an indispensible reaction type which is also widely used on an industrial scale. In industrial processes hydrogen is usually used as reducing agent, since strong reducing agents like alkali metals and hydrides can only be used to a limited extent for safety and economic reasons. Very economical reducing agents that are convenient to handle and have high potential application are hydrocarbons in presence of carbon. Hydrocarbon/carbon systems can be readily used instead of molecular hydrogen and expensive metal catalysts for the hydrogenation of compounds containing, for example, CC‐, CO‐, or NO‐double bonds. Furthermore, these systems can be used for carrying out reductions which hitherto required strong reducing agents such as zinc, tin, alkali metals and hydrides. Especially suitable as economic sources of hydrogen are refinery products such as vacuum gas oil, fuel oil S or vacuum residue oil. Hydrocarbons are dehydrogenated to unsaturated systems and finally to carbon.
The Pentazadienide Ion as a Ligand in Metal Complexes
Authors: Beck Johannes , Strähle Joachim 
Published: December 2003

AbstractThe pentazadienide ion is a surprisingly versatile ligand. The planar, zigzag N5 chain can utilize its atoms N1, N3, and N5 to complex one, two, or three metal atoms in a monodentate or bidentate fashion. Of particular interest is the ligand's ability to promote short metal‐metal contacts in complexes with d10 metal ions.
Catalytic Activity of Enzymes Altered at Their Active Sites
Authors: Kaiser Emil Thomas 
Published: December 2003

AbstractProtein engineering has as its goals the design and construction of new peptides and proteins with novel binding and catalytic properties. In one approach to protein engineering, new active sites have been introduced into naturally occurring proteins either by site‐directed mutagenesis or by chemical modification. Providing that important changes in the tertiary structures do not result from such alterations, at least a portion of the binding site of the original protein should be available for the formation of complexes between the altered enzyme and its substrates. Many examples of active‐site mutations have been described, including the generation by us of a cysteine mutant of alkaline phosphatase. A fundamental limitation of the site‐directed mutagenesis methodology is that replacements of residues are restricted to the twenty naturally occurring amino acids. The alternative, chemical modification, is difficult to carry out for the specific replacement of one amino acid by another. However, we have shown that through such modification coenzyme analogues can be introduced covalently into appropriate positions in proteins, allowing us to produce semisynthetic enzymes with catalytic activities radically altered from those of their precursor proteins. In another approach to protein engineering efforts have focused on the construction of systems where, as a first approximation, folding can be neglected and the preparation of secondary structural units is the target. Examples of the successful design of biologically active peptides and proteins along such lines, taken from our own work, include molecules mimicking apolipoproteins, toxins, and many hormones. In recent studies we have progressed to the stage where we are starting to combine the two general approaches to protein engineering we have described and are able to construct small enzymes like ribonuclease T1 and its structural analogues.
Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides
Authors: Lamb H. Henry , Gates Bruce C. , Knözinger Helmut 
Published: December 2003

AbstractMetal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface‐bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid‐base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O2⊖ ions; ion‐pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid‐base pair sites such as [Mg2⊕O2⊖]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface‐bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.
High Oxidation State Organometallic Chemistry, A Challenge—the Example of Rhenium
Authors: Herrmann Wolfgang A. 
Published: December 2003

AbstractHomogeneous catalysis as the major industrial outlet of organometallic basic research has been enjoying great benefit from organotransition metal species that promote bond forming between hydrocarbon fragments. Most of the commercially important processes that serve to produce large‐volume organic feedstock chemicals such as linear α‐olefins (Shell Higher Olefins Process), linear aldehydes (hydroformylation), acetaldehyde (Wacker‐Hoechst), acetic acid (Monsanto), adiponitrile (DuPont hydrocyanation of butadiene) operate at low‐valent metal centers. It is thus hardly surprising that by far the most part of organometallic research during the past few decades has been directed towards an understanding and the improvement of these catalytic reactions as well as towards the related stoichiometric chemistry. As a matter of consequence, our present knowledge on high‐valent organotransition metal compound is comparatively shallow, nor do we know much about the chemical relationship and interconvertability of high and low oxidation states within a given class of compounds. In this article I want to point out some ostensibly challenging perspectives of future organometallic research by describing a novel class of high oxidation state organorhenium compounds as well as by speculating on possible generalizations for other transition metals.
Donor‐Acceptor‐Substituted Cyclic π‐Electron Systems—Probes for Theories and Building Blocks for New Materials
Authors: Gompper Rudolf , Wagner Hans‐Ulrich 
Published: December 2003

AbstractDonor and acceptor substituents stabilize (4n)π‐electron systems and destabilize those with (4n+2)π electrons. The same is true for the transition states of pericyclic reactions, which explains the appearance of dipolar intermediates in symmetry‐allowed cycloadditions and sigmatropic rearrangements. Donor‐acceptor‐substituted semibullvalenes undergo rapid Cope rearrangement, as do tetraazabarbaralanes. In contrast, tetraazasemibullvalenes can not be isolated, since the isomeric tetrazocines always result. The usefulness of the donor‐acceptor concept in preparative chemistry is demonstrated by numerous stable cyclic (4n)π‐electron systems, like donor‐acceptor‐substituted cyclobutadienes, tetraaminobenzene, and p‐benzoquinone dications, benzodiazepinyl anions, and donor‐acceptor‐substituted cyclopentadienyl cations and their heteroatom‐containing analogues. The new compounds are of interest in the fields of organic metals and ferromagnets, nonlinear optics, and dyestuffs, among others.
Reaction Mechanisms in Heterogeneous Catalysis; CH Activation as a Case Study
Authors: Maier Wilhelm F. 
Published: December 2003

AbstractHeterogeneous catalysis is changing from an empirical art to an exact science. The various methods for the analysis of solids and surfaces, constantly refined by materials science and surface science, seem to be almost unlimited. The increasing availability of atomic resolution microscopy as well as synchrotron radiation allows the characterization of catalyst particles, surface structures, surface processes and surface intermediates. We have learned to determine the surface structure sensitivity of catalytic reactions. Thermodynamic and kinetic data of catalytic reactions are now determined routinely. Isotopic exchange and labeling experiments provide information about reactant‐catalyst interactions. How much have we learned through these techniques about the nature or mechanism of heterogeneously catalyzed reactions? The following article attempts to summarize the progress and the problems encountered in mechanistic studies of CH bond formation and activation in a hydrogen atmosphere as an example for the present state of the understanding of reaction mechanisms in heterogeneous catalysis.
The Structure of Lithium Compounds of Sulfones, Sulfoximides, Sulfoxides, Thioethers and 1,3‐Dithianes, Nitriles, Nitro Compounds and Hydrazones
Authors: Boche Gernot 
Published: December 2003

AbstractAcceptor‐substituted lithio compounds LiACR1R2, in which the acceptor A is an RC(0), NC, RSO2, RS(O)NR, RSO, RS, O2N, or RC(NNR2) group, have long played an important role in organic synthesis. Their significance has grown still further in the last fifteen years, as one has increasingly learnt to employ them successfully in chemo‐, regio‐, diastereo‐and enantioselective reactions. Remarkably, little if anything was known of the structures of these compounds. It is therefore not surprising that interest in the structures of this class of compounds has greatly increased in recent years. In the following review, we shall summarize recent research into the structures of the lithio compounds of sulfones, sulfoximides, sulfoxides, thioethers and 1,3‐dithianes, nitriles, nitro compounds, and hydrazones. Crystal structure determinations from recent years are central to this study. They are supplemented by solution studies and by calculations of structures. Structural similarities and differences between the individual states of aggregation are pointed out wherever possible, as is also the relationship between structure and reactivity.
Perspectives in Photochromism: A Novel System Based on the 1,5‐Electrocyclization of Heteroanalogous Pentadienyl Anions
Authors: Dürr Heinz 
Published: December 2003

AbstractThe molecular process whereby light induces color changes in certain substances, namely photochromism, is currently a topic of active research. Many reactions—mainly monomolecular—have been reexamined in greater detail from the standpoint of photochromism. Those found to be especially suitable for such studies includecis‐transisomerizations, pericyclic reactions, tautomerizations and dissociations. The most important in this group are undoubtedly the electrocyclization reactions. In this paper we present spiro[fluorene‐9,1′‐[1,8a]‐dihydroindolizine] and its bis‐ and tris‐aza analogues as examples of a promising new system with a wide range of variability. Not only the classical uses, but also those in new fields such as frequency doubling of laser radiation, holography or molecular microcircuit chips can now be considered.
A Chemical Approach to Protein Design—Template‐Assembled Synthetic Proteins (TASP)
Authors: Mutter Manfred , Vuilleumier Stéphane 
Published: December 2003

AbstractAdvances in methodology in both chemistry and molecular biology allow us to take a fresh look at protein science. Chemical synthesis of peptides and site‐directed mutagenesis are now standard research tools, paving the way for the construction of new proteins with tailor‐made structural and functional properties. The decisive hurdle on the way lies not in the synthesis of the molecules proper but rather in a better understanding of the complex folding pathways of polypeptide chains into spatially well‐defined structures. Can the chemist use his synthetic tools to bypass the notorious “folding problem?” In this article, we present a new approach developed in our laboratory, which opens a chemical route to artificial proteins with predetermined three‐dimensional structures, allowing a first step towards the synthesis of new proteins with functional properties.
Structure and Dynamics of Chiral Molecules
Authors: Quack Martin 
Published: December 2003

AbstractCould there be chiral methane? What is the characteristic structural feature (in a physicochemical sense) of a molecule? This question dates back to Louis Pasteur, the discoverer of molecular chirality, and since the work of van't Hoff and Le Bel is generally considered by chemists as solved. In the present article it is pointed out that there exist fundamentally conflicting theoretical views of the physical origin of molecular chirality. These views predict consequences that could, in principle, be distinguished experimentally, but at present there is no conclusive experimental evidence available. Possible experiments are suggested that test different hypotheses. The importance of the magnitude of the parity‐violating energy difference δEPV in molecules due to the weak nuclear force for both the structure and spectra of chiral molecules and for the kinetics of racemization is discussed. The chemical relaxation rate coefficient of chiral molecules with some appreciable energy of excitation is derived for several limiting cases of a simple statistical mechanical model, which takes ΔEPV into account.
Challenges in Materials for Health Care Applications
Authors: Williams David F. 
Published: December 2003

AbstractImportant issues face the use of advanced materials in medical applications. It is now possible to replace or augment many tissues and parts of the body by implanted devices, but there are still severe limitations to functions they are able to perform and problems associated with their compatibility with the tissues. Biomaterials of the future need to simulate more closely the tissues they are replacing and both the current position and future outlook are reviewed in this light.
Solid Ion Conductors in Heterogeneous Catalysis
Authors: Lintz Hans‐Günther , Vayenas Costas Georgios 
Published: December 2003

AbstractThe electrochemical measurement of oxygen activity using ion‐conducting solid electrolytes (λ‐sensors) has become widely known, at least since the application of three‐way catalysts in the postcombustion of exhaust gases from spark‐ignition engines. However, the use of solid ion conductors is not limited to control devices. There are various other potential applications and numerous problems which can be studied: the formation of oxides in the course of catalytic reactions on metal surfaces, the improvement of selectivity and yield of catalytic reactions, such as the epoxidation of ethylene on silver catalysts and, finally, the cogeneration of electrical energy during oxidation reactions, such as the partial oxidation of methanol to formaldehyde.