bluuebunny/crossref_metadata_2025_split huggingface dataset:
Biogenic Substances in Sediments and Fossils
Authors: Albrecht Pierre , Ourisson Guy 
Published: December 2003

AbstractMost sedimentary rocks contain small concentrations of finely divided organic material. With the aid of modern analytical methods, samples of such rocks can be examined in detail, and conclusions can be drawn concerning the origin of the organic substances. Intact or modified biogenic compounds such as paraffins, isoprenoids, alcohols, ketones, carboxylic acids, steroids, triterpenes, and porphyrins have been isolated from the soluble fractions of the organic material of many sediments and from identified fossils of various geological ages. Even Precambrian rocks contain biogenic substances, so that life forms must have existed more than three billion years ago. The main component of the organic material is “kerogen”, the complicated polymeric structure of which has not yet been established.
Sulfinato Complexes
Authors: Vitzthum Günther , Lindner Ekkehard 
Published: December 2003

AbstractSulfinatometal complexes exhibit unusual features in their structure, preparation, and use, and have attracted rapidly growing interest. Their structures are determined by the ability of the RSOgroup both to act as a monodentate ligand (sulfinato‐S or sulfinato‐O) and to add to coordination centers as a bidentate ligand, either intramolecularly or intermolecularly, via the two oxygens atoms (sulfinato‐O,O′) or via the sulfur atom and one of the oxygens (sulfinato‐O,S).
Gamma Resonance Spectroscopy and Chemical Bonding
Authors: Mössbauer Rudolf L. 
Published: December 2003

AbstractGamma resonance spectroscopy makes it possible to observe directly hyperfine interactions in solids. In the field of chemistry the feature of greatest interest is the measurement of isomeric shifts of gamma lines because these shifts are proportional to the electron density at the nucleus and can thus give detailed information about the electronic structure of the chemical bond. After a brief introduction to gamma resonance spectroscopy, this paper deals with the fundamental problem of the interpretation of measured isomeric shifts in terms of electron densities and then gives representative examples of chemical information that can be obtained for transition elements from the shifts and the splitting of the gamma lines.
Pulse Fourier Transform 13C‐NMR Spectroscopy, Principles and Applications
Authors: Breitmaier Eberhard , Jung Günther , Voelters Wolfgang 
Published: December 2003

AbstractAs a result of the development of pulse Fourier transform NMR spectroscopy 13C resonance has become accepted as a means of elucidating the structures of organic compounds having the natural 13C abundance. Assignment of the signals is facilitated by broad‐band and off resonance proton decoupling and by knowledge of longitudinal relaxation times. The pulse Fourier technique, which is more sensitive and faster than conventional NMR spectroscopy because of multi‐channel excitation, allows 13C measurements within a short time even on dilute samples of high molecular weight compounds without prior enrichment in 13C.
Vanadium and Chromium Catalysts for Polymerization of Ethylene
Authors: Henrici‐Olivé G. , Olivé S. 
Published: December 2003

AbstractIn the present progress report literature data concerning homogeneous catalyst systems based on vanadium and chromium are reviewed and complemented by a combined study of the magnetic properties and the polymerization activity of such systems. The results show that vanadium forms active species as V (III) and as V (II), but chromium only as Cr (II); models are suggested for the polymerization process. — A comparison with heterogeneous systems (Phillips catalyst) allows the establishment of some general principles concerning the activity of the catalyst and the properties of the polymers.
Application of Ligand Field Spectroscopy to Problems of Chemical Bonding in Solids
Authors: Reinen Dirk 
Published: December 2003

AbstractSelected examples (dn systems) are presented to illustrate the interpretation of ligand field spectra (remission spectra) of solid inorganic compounds. In particular, this article demonstrates possible means for the investigation of weak cooperative interactions between transition metal coordination polyhedra and for the determination of the cation distribution in crystal lattices. The information value of the spectroscopic method to the chemist is critically discussed.
Vibration‐Spectroscopic Studies on Solid Compounds
Authors: Becher Hermann J. 
Published: December 2003

AbstractConsiderably improved possibilities are now available for the investigation of the optical vibrations in lattice compounds by Raman and IR spectroscopy. These spectra can be interpreted on the basis of the models and findings of molecular vibration spectroscopy. For simple lattice types simple frequency equations are found for the optical lattice vibrations with in‐phase vibration motions in all unit cells (wave vector k = 0). As an approximation, the force constants that occur in these equations may be interpreted as stretching force constants. For typical ionic lattices, the influence of Coulomb forces due to the lattice polarization that occurs during the vibrational motion must be taken into account. As is shown by the example of some spinels, the assignment of lattice vibrations can be facilitated by model calculations.
X‐Ray Spectroscopic Investigation of Chemical Bonding in Solids
Authors: Faessler Alfred 
Published: December 2003

AbstractWhen an atom is incorporated into a molecule or a crystal, its X‐ray spectrum undergoes characteristic changes, the study of which leads to important information on the nature of the chemical bonding and on the electronic structure in a substance. A number of examples are given to illustrate the possibilities of the X‐ray spectroscopy of bonded atoms. Special attention is given to the displacement of the Kα lines, from which conclusions can be drawn regarding the charge on a bonded atom, as well as to the investigation of the emission bands resulting from valence electron transitions, which yields information on the energy band structure of the solid. X‐ray spectroscopic studies on free molecules and theoretical work on the calculation of the molecular orbitals of simple molecules are finally reported.
Structure of Ion Pair Solvation Complexes
Authors: Smid Johannes 
Published: December 2003

AbstractIn solvents of low polarity, salts containing a carbanion, e.g. fluorenylsodium, or a radical anion, e.g. coronenesodium, can exist as contact ion pairs or as solvent separated ion pairs. Particular interest attaches to the complexes of ion pairs with the strongly solvating, polydentate polyglycol dimethyl ethers (glymes) and with crown compounds. These systems can be studied by optical spectroscopy, NMR spectroscopy, and other methods.
Chemistry and Biochemistry of Branched‐Chain Sugars
Authors: Grisebach Hans , Schmid Rolf 
Published: December 2003

AbstractBranched‐chain sugars have recently been isolated in larger numbers from microorganisms and higher plants as glycoside components of antibiotics or phenolic compounds. They also occur in plants in cell wall polysaccharides. Sugars with methyl, hydroxymethyl,formyl, hydroxyethyl, and glycoloyl branches have so far been found in nature. Many new methods have been developed for the synthesis of these sugars. With one exception, the branched sugars known at present can be divided biogenetically into three groups. Sugars with methyl branches are formed by C1 transfer, sugars with hydroxyethyl branches by C2 transfer to a hexose chain, and sugars with hydroxymethyl or formyl branches by rearrangement of a hexose or pentose chain.
Chemistry of the Positron and of Positronium
Authors: Ache Hans J. 
Published: December 2003

AbstractIn this progress report, the properties and behavior of the positron (“positive electron”, anti‐electron) and of the positronium, a “hydrogen atom” containing a positron instead of a proton, are considered from the chemist's viewpoint. Examples are given to demonstrate the development of positronium chemistry, in aqueous solution and in the gaseous, liquid, and solid phases, with its problems and possibilities.
Carrier‐Bound Biologically Active Substances and Their Applications
Authors: Orth Hans Dieter , Brümmer Wolfgang 
Published: December 2003

AbstractStable catalysts can be obtained by fixation of enzymes on suitable supports. The principal methods for their preparation and their properties are described. The use of supported enzymes in routine biochemical analysis as “insoluble reagents” for automatic analyzers and for enzyme electrodes appears particularly interesting. For preparative transformations and syntheses, the supported enzymes offer the advantage that many processes that were formerly operated batch‐wise can be run continuously to give protein‐free products. The use of supported proteinases as specific adsorbents provides a simple method for the isolation of various naturally occurring inhibitors. This principle, which is the basis of affinity chromatography, has now also been adapted to the purification of enzymes and other biologically active substances on suitable supported effectors.
Structure of Porous Adsorbents
Authors: Unger Klaus 
Published: December 2003

AbstractPorous solids are gaining in importance as adsorbents and catalysts both for scientific investigations and for industrial applications. Adsorbents having predetermined pore structure can be prepared by specific synthesis. This pore structure can be described in detail and comprehensively by specific parameters, which can be determined by gas adsorption measurements and by other means. Information concerning the bonding state and the concentration of the functional groups on the surface is obtainable by the use of chemical and physical methods of investigation. Other functional groups can be attached to the surface by chemical reactions. It is possible in this way to prepare selective adsorbents.
Perspectives in Nonsteroidal Anti‐inflammatory Agents
Authors: Shen T. Y. 
Published: December 2003

AbstractAmong numerous nonsteroidal anti‐imflammatory agents synthesized in the past few years, various analogs of indomethacin, phenylacetic acid and heteroarylacetic acid have reached the stage of clinical evaluation. Their biochemical mechanisms of action are exemplified by the broad activity profile of indomethacin which includes inhibition of mediators and enzymes, effects on cell membranes, and, most recently, inhibition of prostaglandin biosynthesis. The importance of pharmacodynamic properties to clinical efficacy was clearly demonstrated in some cases. Several candidates were eliminated because of their side‐effects. A group of α‐methylarylacetic acids showed a high degree of stereospecificity in their potency and metabolisms in vivo, as well as inhibition of prostaglandin synthetase and albumin binding in vitro. Extrinsic Cotton effect provides a sensitive technique in the study of interactions of these drugs with biopolymers. Competitive binding and antagonistic interactions between nonsteroidal drugs, particularly salicylate, were observed in vitro and in vivo. Progress in salicylate research was marked by the synthesis of flufenisal as a new derivative with enhanced potency and longer duration of action. Several fenamate analogs and new chemical types have shown promise in preliminary clinical trials. Various immunological approaches are under investigation for the treatment of rheumatoid arthritis. Newer concepts are still needed to achieve more effective control of arthritic disorders.
Fluorinated β‐Sultones
Authors: Knunyants I. L. , Sokolski G. A. 
Published: December 2003

AbstractFluorinated (β)‐sultones are formed on addition of sulfur trioxide to fluorinated olefins. Tetrafluoroethanesultone has been studied particularly thoroughly. The title compounds are characterized by the ease with which they undergo ring cleavage to give, e.g., derivatives of α‐sulfo carboxylic acids, of sulfonic acids, of carboxylic acids, and of sulfuric acid. Fluorinated compounds of this type containing an α‐hydrogen atom are especially valuable in preparative work.
Basic Behavior of Epoxides in the Presence of Halide Ions
Authors: Buddrus J. 
Published: December 2003

AbstractEpoxides behave as bases in the presence of catalytic quantities of halide ions. They convert halogenated hydrocarbons into olefins, suitable phosphonium salts into ylides, and haloforms into dihalocarbenes. Kinetic studies with the aid of JR and NMR spectroscopy show that the species that functions as the proton acceptor is neither the epoxide ion nor the halide ion, but the 2‐haloethoxide ion.
Biochemistry of the Pteridines
Authors: Rembold Heinz , Gyure William L. 
Published: December 2003

AbstractBiochemical information about the ubiquitous pteridines has become generally available only within the last fifteen years. This delay can be traced to the chemical lability of these compounds and their small concentrations in organisms. New methods of isolation and isotopic techniques have provided data on the biosynthesis, anabolism, and catabolism of this class of compounds. Hydrogenated pteridines are recognized today as cofactors for various mixed function oxygenases and are involved in cellular electron transport. Further unknown catalytic functions for pteridines in cellular metabolism are indicated by their physiological activity, negative redox potentials, and histoautoradiographic data.
Hereditary Diseases—Causes, Cures, and Problems
Authors: Brady Roscoe O. 
Published: December 2003

AbstractThe nature of the biochemical abnormalities in over a hundred inherited diseases of humans is now convincingly established. This impressive achievement has provided numerous practical benefits, amongst which are: 1. accurate methods for the diagnosis of patients with these diseases; 2. identification of symptom‐free heterozygous carriers of the defective genes; and 3. procedures for the antenatal detection of a large number of these disorders in fetuses. It is anticipated that equally impressive advances will be made in the future regarding the therapy for patients with these diseases who escape prenatal detection, or in cases when an affected fetus is identified but is carried to term in compliance with the decision of the parents. The current concepts regarding the therapy of heritable metabolic diseases are outlined and potential innovations for future corrective procedures are indicated in this report.
Chemistry and Biochemistry of Azaquinones
Authors: Knackmuss Hans‐Joachim 
Published: December 2003

AbstractThe azaquinones are a new class of bacterial secondary metabolites. Investigations were necessary on the chemistry of the azaquinones, their imides, and 2,3,6‐pyridinetriols and 3‐aminopyridine‐2,6‐diols in order to obtain information about the structure and biogenesis of these natural products.
The Origin of Life
Authors: Paecht‐Horowitz Mella 
Published: December 2003

AbstractThe theory of the evolution of the species, which is today widely accepted, requires a starting point. It is postulated that the biological starting point could have emerged only if chemical evolution had preceded it. Experiments are described which show the formation of organic substances from inorganic gases under conditions which prevailed on earth in the prebiotic era; also demonstrated is the polymerization of such organic substances into biological macromolecules. Hypotheses are discussed whereby these macromolecules could be transformed into dynamic structures which might finally lead to self‐reproduction.
Anomalies in the Structural Chemistry of Silicon
Authors: Bürger Hans 
Published: December 2003

AbstractIn contrast to carbon, silicon fails to form multiple bonds that are stable at room temperature. Consequently molecules in which silicon exhibits coordination numbers (CN) of 1, 2, and 3 may only be obtained at very high or low temperatures. Under these conditions their structural features, including multiple bonds, resemble those of carbon. On the other hand, silicon is capable of forming various hexacoordinated compounds making use of its d orbitals. Nitrogen and oxygen bonded to silicon develop an unusual stereochemistry: planar nitrogen, nearly or completely linear oxygen, and considerable shortening of SiN, SiO, and SiF bonds are specific examples. N(SiR3)2 and CH2SiR3 ligands permit stabilization of unusually low CNs of many metals and give rise to amino and alkyl derivatives of unexpectedly high stability due to the particular electronic, the R3Si group.
Thermodynamics of Addition Polymerization Processes
Authors: Ivin K. J. 
Published: December 2003

AbstractThe polymerizability of compounds such as 1,l‐disubstituted ethylenes, aldehydes, ketones, isocyanates, and five‐ and six‐membered ring compounds is determined largely by thermodynamic considerations. The transition from non‐polymerizability to polymerizability, correspondlng to a change in the sign of ΔG, is often quite sharp. Factors which generally make the free energy of polymerization more negative, and which therefore favor polymerization, are low temperature, high pressure, and high monomer concentration. Additional driving force is sometimes available if the monomer is in the supercooled (glassy) state rather than the crystalline state, or if the polymer crystallizes on formation. Alkyl substituents have an unfavorable effect on polymerizability; halogen substituents have a favorable effect. Many monomers which do not homopolymerize for thermodynamic reasons will copolymerize with a second monomer to the extent of forming copolymers containing 50 or even 66 mol per cent of the first monomer.
Sesquiterpenes
Authors: Rücker Gerhard 
Published: December 2003

AbstractOf the 4000 or so terpenoid natural products known today, whose carbon skeleton can be divided up into isopentane units by the “isoprene rule” (C10, monoterpenes; C15, sesquiterpenes; C20, diterpenes; C25, sesterterpenes; C30, triterpenes; steroids; carotenoids; polyprenes), the sesquiterpenes, numbering about 1000, represent the largest single class. According to the well founded concepts developed in recent years for the biogenesis of their highly diverse carbon skeletons, the present report is divided into nine sections dealing respectively with farnesanes, bicyclofarnesols (drimanes, iresanes), bisabolanes, cadinanes, humulanes and caryophyllanes, germacranes, “hydroazulenes”, selinanes and eremophilanes, and maalianes and aristolanes. Further groups of sesquiterpenes can be derived from each of these structural types.
Syntheses of Seven‐ and Five‐Membered Rings from Allyl Cations
Authors: Hoffmann H. M. R. 
Published: December 2003

AbstractThe cycloaddition of allyl cations to conjugated dienes offers an efficient and general tool for the construction of seven‐membered carbocycles. The silver salt route is highly suitable for generating allyl cations and can be used even in most difficult cases, i. e. for elusive and intractable allyl cations such as 2‐methylallyl, 2‐methoxyallyl, and the parent allyl cation. Cycloadditions to the 2‐methoxyallyl cation come very close to the prototype of a 6π, 7C reaction, since the carbonyl group introduced into the new ring can either be used for further transformations or be removed altogether. The combination of an allyl cation with π reactants, especially conjugated dienes, is a powerful and strongly convergent synthetic method since it permits one‐step assembly, from a C3 and a C4 unit, of a cluster of up to four chiral centers in a ring which is larger than six‐membered. Allyl cations combine with electron‐rich 2π systems to form novel five‐membered rings. The nature of the organometallic intermediates has been elucidated in many cases.
Methylchlorophosphanes and Their Reaction Products
Authors: Staendeke Horst , Kleiner Hanss‐Jerg 
Published: December 2003

AbstractMethyldichlorophosphane and dimethylchlorophosphane can be prepared only with considerable difficulty. The ease with which they yield numerous reaction products is an indication of the pronounced reactivity of these organohalophosphanes. Possible applications of such organophosphorus compounds exist inter alia in the fields of plant protection, corrosion prevention, and flame retardants, as well as in the catalysis of gas phase reactions, for instance the desulfurization of gas mixtures.
Possibilities and Limitations of Delegated Literature Searches by Computer
Authors: Fugmann Robert , Ploss Gottfried 
Published: December 2003

AbstractThe present‐day chemist is constantly on the lookout for information which may be useful to his work. If he is able to define his interests precisely enough then he can avail himself of information scientists and of specialized literature stores. However, retaining access to such information, which only he himself can subjectively judge for relevance to his work, and which most often constitutes a powerful incentive to his creative thinking, will always require his own “browsing” literature studies or personal contacts. These are the limitations of any delegated literature search.
Chemical Applications of Raman Spectroscopy
Authors: Schrader Bernhard 
Published: December 2003

AbstractRaman spectorscopy is—like infrared spectroscopy—a method for the study of vibrations of molecules and crystals. The two methods are complementary: if a vibration results in a change of the polarizability of a molecule, it is Raman active; if a change in the molecular dipole moment results, it is infrared active Vibrations of nonpolar groups and totally symmetrical vibrations of molecules are often only Raman active. IR and Raman spectra together give information about the symmetries and structures of molecules and crystals and about the properties of chemical bonds and intermolecular interactions. Until about 10 years ago Raman spectra could only be recorded on relatively large amounts of essentially colorless substances. After the advent of laser light sources the situation changed completely. The amount of sample substance required is now in the region of milli‐ and micrograms. Gases, liquids and solid samples, especially air‐sensitive and reactive substances, single crystals, crystal needles and filaments as well as aqueous solutions can be readily investigated. The identification of molecules and the elucidation of molecular structures, biochemical analysis, and control of evnivornmental pollution are important aplications of Raman spectroscopy. Raman spectroscopy now constitutes an additional powerful tool in instrumental analysis
Structures of the Intermediate Complexes in Friedel‐Crafts Acylations
Authors: Chevrier Bernard , Weiss Raymond 
Published: December 2003

AbstractAddition compounds of Lewis acids MXn and acyl halides RCOX occur as intermediates in Friedel‐Crafts acylations. IR and NMR studies on these intermediates have indicated the probable existence of structural isomers. In X‐ray structural analysis, it is possible to distinguish two forms, i.e. the molecular form, in which the compounds are present as donor‐acceptor complexes RCXO→MXn, and the ionic form, in which they can be formulated as oxocarbenium salts [RCO]+[MXn+1]−. The compounds of the donor‐acceptor type RCXO→MXn are characterized by the formation of a coordinate oxygen‐metal bond; the transfer of electrons from the oxygen to the metal of the acceptor is always due to a weak donor‐acceptor interaction. The positive charge of the aryloxocarbenium ions is partly delocalized over the aromatic nucleus. The positive charge in alkyloxocarbenium ions, on the other hand, is essentially localized on the carbon atom of the carbonyl group, as is confirmed by electron density distribution calculations.