bluuebunny/crossref_metadata_2025_split huggingface dataset:
Kink‐Block and Gauche‐Block Structures of Bimolecular Films
Authors: Lagaly Gerhard 
Published: December 2003

AbstractLayer silicates and some other layer compounds can take up organic compounds such as long‐chain alkanols and alkylammonium ions with formation of ordered bimolecular films. This system enables new ideas to be developed concerning the structure of monomolecular and bimolecular films: the films form reversible kink‐block and gauche‐block structures. Conclusions can be derived about the structure of the lipid layers in membranes.
Dual‐Wavelength Spectrophotometry
Authors: Shibata Shozo 
Published: December 2003

AbstractAt present, dual‐wavelength spectrophotometry is probably the least well known of the techniques of absorption spectrophotometry. However, recent improvements in application techniques have shown that for various organic and inorganic materials dual‐wavelength spectrophotometry can provide higher sensitivity and selectivity than conventional spectrophotometry. This article reviews the principles and scope of dual‐wavelength spectrophotometry, including derivative absorption spectrophotometry, in the hope that these techniques may be adopted more frequently in the future for resolving many analytical problems.
Cyclometalation Reactions
Authors: Bruce Michael I. 
Published: December 2003

AbstractReactions of transition metal complexes in which an organic ligand undergoes intramolecular metalation with formation of a metal‐carbon σ bond are termed cyclometalation reactions. This article provides a general summary of our knowledge of these reactions, and discusses them in terms of their scope, mechanisms, and utility in organic synthesis, particularly of heterocyclic molecules.
Early History of the Quantum Mechanical Treatment of the Chemical Bond
Authors: Hund Friedrich 
Published: December 2003

AbstractFormulation of the Schrödinger equation (1926) laid the foundation for a theory of the electronic structure of molecules, and thus for a physically sound theory of the homopolar chemical bond. Physicists reached a qualitative understanding of the bond in diatomic molecules during the ensuing years 1927 to 1929, and of stereochemistry by 1931.
The Surface Chemical Bond
Authors: Somorjai Gabor A. 
Published: December 2003

AbstractAdsorbed particles can be bound chemically to the surface of a solid (chemisorption). Modern variants of electron diffraction and electron spectroscopy now enable us to gain an insight into the nature of such surface chemical bonds, which play a key role in heterogenous catalysis and many other technically important phenomena such as adhesion, lubrication etc. Predominantly localized surface bonds can be compared to the covalent bonds in cluster compounds.
Utilization of Microbes—Modern Developments in Biological Technology
Authors: Präve Paul 
Published: December 2003

AbstractUntil recently microbiology had found use only in the foodstuffs sector and in the production of antibiotics; however, the production of biomass should be regarded as the key development of the new science of biotechnology. The biomass consists of fatty and protein‐rich microorganisms that can be used for example in animal feeds. The microorganisms grow not only on cellulose waste and other biological substrates but also, for example, on petroleum and methanol. Their energy demand is comparatively low. It is being realized more and more that inflexible technologies pose problems for our civilization and that biotechnology, which utilizes the cyclic metabolic pathways of microorganisms and often recycles waste materials, shows great promise for the future.
New Developments in the Field of Vitamin B12: Enzymatic Reactions Dependent upon Corrins and Coenzyme B12
Authors: Schrauzer G. N. 
Published: December 2003

AbstractSimple corrins such as vitamin B12 and vitamin B12 coenzyme catalyze a variety of unusual enzymatic reactions of which some are still without analogy in organic or organometallic chemistry. The mechanisms of these reactions are currently the subject of lively discussion. The present review focuses attention on new ideas about the mode of action of vitamin B12 coenzymes in enzymatic reactions.
The Biochemistry of Sphingolipid Storage Diseases
Authors: Sandhoff Konrad 
Published: December 2003

AbstractSphingolipids are components of animal membranes that are made up of a hydrophobic ceramide and a hydrophilic moiety (phosphorylcholine, oligosaccharides or their derivatives). Several enzymes are required for the step‐by‐step degradation of the sphingolipids. If one of these enzymes is deficient or totally lacking, then its substrates accumulate in the body and cause severe damage, especially when storage occurs in the nervous system. The enzyme defects are inherited in a recessive manner.
Recent Applications of α‐Metalated Isocyanides in Organic Synthesis
Authors: Schöllkopf Ulrich 
Published: December 2003

AbstractBeing both nucleophilic and electrophilic, α‐metalated isocyanides can add to polar double bonds, forming heterocycles. They are also synthons for α‐metalated primary amines. This article describes recent or improved procedures for their use in organic synthesis: (1) In heterocyclic syntheses to give 2‐oxazolines, 2‐imidazolines, 2‐thiazolines, oxazoles and oligooxazoles, thiazoles, triazoles, imidazolinones, pyrroles, 5,6‐dihydro‐1,3‐oxazines and ‐thiazines, and (via cycloaddition with nitrones) 2‐imidazolidinones. (2) In the field of formylaminomethylenation, for example transformation of estrone methyl ether and a keto sugar into the corresponding α‐formylaminoacrylic esters, and the conversion of aldehydes and ketones by 3‐ and 4‐pyridyl‐methyl isocyanides into N‐(1‐pyridyl‐1‐alkenyl)formamides and their hydrolysis to 3‐ and 4‐acylpyridines. (3) In connection with the use of α‐metalated isocyanides as synthons for α‐metalated primary amines, the author demonstrates how they may be used for preparation of 1,2‐ and 1,3‐amino alcohols, 1,2‐diamines, 2,3‐diaminoalkanoic acids and for synthesis of higher amino acids starting from simple amino acids.
Electrophotography
Authors: Weigl John W. 
Published: December 2003

AbstractElectrophotographic images are formed by the displacement of matter in electric fields: the deposition of charged particles or the deformation of a plastic surface is controlled by a geometric field pattern, configured by the photo‐response of a photoconductive insulating material. Image amplification up to 105may be achieved by the application of external potentials and the judicious selection of imaging particles having a high ratio of absorbance per unit charge. These particles may be ions, colloids, photoresponsive pigments, or (most commonly) colored plastic “toner” composites. Of the many processes which have been invented to date, transfer xerography remains the most useful. This paper presents a concise review of electrophotographic imaging processes and materials, as well as of some of their applications.
(A→D)‐Cyclization of a Nickel(II) Δ18‐Dehydro‐1‐methylidene‐1,19‐secocorrinate
Authors: Rasetti Vittorio , Kräutler Bernhard , Pfaltz Andreas , Eschenmoser Albert 
Published: December 2003

AbstractCommunications are brief preliminary reports of research work in all areas of chemistry which, on account of its fundamental significance, novelty, or general applicability, should be of interest to a broad spectrum of chemists. Authors of communications are requested to state reasons of this kind justifying publication on submission of their manuscript. The same reasons should be clearly apparent from the manuscript. In cases where the editorial staff decide, after due consultation with independent referees, that these conditions are not met, manuscripts will be returned to the authors with the request to submit them for publication in a specialist journal catering for scientists working in the field concerned.
Advances in Phase‐Transfer Catalysis [New synthetic methods (20)]
Authors: Dehmlow Eckehard V. 
Published: December 2003

AbstractPhase‐transfer catalysis (PTC) has grown into a versatile preparative method in the last few years. The most notable new developments include the use of crown ethers as phase‐transfer catalysts and the introduction of solid‐liquid PTC. Some representative examples have been selected from the large number of new PTC reactions and some of them are summarized in tables.
Molecular States and Molecular Orbitals
Authors: Bock Hans 
Published: December 2003

AbstractMolecules change their properties on acquisition or loss of energy. The state of a molecule can be characterized by the difference between its energy and that of the preceding initial state or that of the subsequent final state, as well as by the respective charge distribution. Molecules in their various energy states each display a particular structure and characteristic properties, and it is these entities which the chemist uses as synthetic building blocks.—The best model hitherto available for describing molecular states is the molecular orbital representation. Molecular orbitals are especially suitable for comparing mutually corresponding molecular states of similar compounds, and thus provide a manifold and stimulating overview of large areas of chemistry.—An attempt will now be made to point out important characteristic parameters of molecular states and to explain the rules governing their description by molecular orbitals, while circumventing mathematical barriers. A principal objective is the comparison of states of “chemically related” molecules with regard to the connection, the arrangement, and the potential of their atoms. The simple models employed for this purpose, whose inherent limitations and potential expansions are discussed, have meanwhile become valuable tools in the hand of the chemist.
Some Aspects of Antibiotics Research
Authors: Zähner Hans 
Published: December 2003

AbstractThe era of spectacular developments in the field of antibiotics appears to be over. Ever greater efforts are needed to find ever fewer new antibiotics. The view that well‐considered hypotheses can lead as successfully as proven results to further research is supported by the establishment of five theses. “Non‐classical” screening methods and the search for secondary metabolites with unusual properties can provide a rich harvest.
Entanglement of Thread Molecules and Its Influence on the Properties of Polymers
Authors: Hoffmann Martin 
Published: December 2003

AbstractThis article shows the extent to which the entanglement of threadlike molecules can influence many of the properties of deformable polymers and of solutions of macromolecular substances. Thus, the molecular concept of entanglement leads to quantitative predictions of the dependence of the entanglement number on the nature of the polymer, its concentration, and extension. Experimental values of the relaxation modulus confirm these predictions for melts and solutions. The influence of entanglemnt on relaxation and flow processes leads to very simple relationships for the magnitudes of the structural viscosity and the shear stress at which the structural viscosity starts to become measurable. Osmotic pressures, light scattering intensities, and diffusion coefficients of concentrated solutions can be derived quantitatively from the conformational constraints due to entanglement. Entanglement is effective above a concentration at which the three‐dimensional coils fill the solution volume, and probably also at lower concentrations. At very high concentrations entangled structures may be formed which drastically reduce the number of possible conformations. The swelling and extension behavior of cross‐linked deformable polymers can be understood only if entanglement is taken into account.
Use of Electron‐Density Plots in Applied Quantum Chemistry
Authors: Absar Ilyas , Van Wazer John R. 
Published: December 2003

AbstractThe uses in applied quantum chemistry of computer‐produced three‐dimensional diagrams of electron‐density distributions are discussed. The two major catagories of such diagrams are contrasted; and advantages are described for plots showing as the third dimension the point‐by‐point electron density in a cross‐sectional cut (corresponding to the other two dimensions) through a molecule. Examples are presented to show how these plots may be used (1) to assess the adequacies of a given mathematical representation employed in the calculation of a wavefunction, (2) to clarify the interrelationship between an individual molecular orbital and the atomic orbitals of the constituent atoms, as well as (3) to explain the canonical set of molecular orbitals of any selected molecule. Furthermore these plots serve(4) to demonstrate clearly a replication of key characteristics between certain molecular orbitals in different molecules. These examples are accompanied by others which indicate how the electron‐density plots may be used(5) to understand and clarify accepted chemical dogma. Finally, the possibility is discussed of employing quantum calculations on a wider scale so as to be of value in the more practical aspects of chemistry.
Production and Utilization of Amino Acids
Authors: Izumi Yoshiharu , Chibata Ichiro , Itoh Tamio 
Published: December 2003

AbstractThe amino acid industry has been steadily expanding since monosodium glutamate was first marketed as a flavoring material in 1909. Its production has recently reached almost the 1 billion dollar level. Amino acids are produced by extraction from protein hydrolyzates, by fermentation with the aid of microorganisms, by enzymatic processes, and by chemical synthesis. To obtain natural L‐amino acid, chemical synthesis generally requires two additional steps, i.e. optical resolution and racemization of the D isomer. The most important applications of amino acids include the fortification of plant food and feeds by supplementation of the deficient essential amino acid(s). Apart from their uses in the food industry, medical applications of amino acids (nutritional preparations and therapeutic agents) are becoming increasingly important.
The Versatility of Tertiary Phosphane Ligands in Coordination and Organometallic Chemistry
Authors: Mason Ronald , Meek Devon W. 
Published: December 2003

AbstractTertiary phosphanes form kinetically stable complexes with formal oxidation states of the metal ranging from + IV to −I (“electronic versatility”). Variously substituted phosphanes and polydentate, chelate forming phosphanes and polyphosphanes can be synthesized (“steric versatility”). Studies on complexes containing such ligands provide, inter alia, useful information about the metal‐phosphorus bond, about the steric effects of the phosphane ligand, and about reactions of coordinated phosphanes.
Phase Equilibria and Critical Curves of Binary Ammonia–Hydrocarbon Mixtures
Authors: Lentz Harro , Franck Ernst Ulrich 
Published: December 2003

AbstractLiquid or dense supercritical ammonia has been suggested as an extraction fluid. It is indeed good solvent for very different classes of compounds, as can be seen from phase diagrams. Such diagrams for binary systems of ammonia and hydrocarbons are presented and discussed on the basis of their critical curves. Apparatus and methods for the measurement of phase equilibria and equation of state data of fluid mixtures at high pressure are described.
Activation Analysis—Its Present State of Development and Its Importance as an Analytical Tool
Authors: Krivan Viliam 
Published: December 2003

AbstractActivation analysis is one of the most important methods available for the determination of traces of elements and of isotopes. One special advantage of activation analysis is its ability to cope with the simultaneous determination of many elements with very high detection sensitivity–for many elements lying below the ppb level. In some cases as little as 10−14—10−15g of an element can be detected. A number of sources of systematic errors common to other analytical procedures, mainly those caused by the blank, are eliminated in activation methods. The disadvantages may be seen as the inconvenience of handling radioactive materials, the dependence on large irradiation facilities, and sometimes the unavoidably long irradiation and cooling periods. On account of its unique capabilities activation analysis finds applications in all fields of endeavor where minute amounts of elements are of significance, from materials research through medicine to archaeology. Further improvements in performance and increases in scope are seen to lie in the development of new radiation sources, activation techniques, measurement systems, and separation procedures.
Annulenoannulenes
Authors: Nakagawa Masazumi 
Published: December 2003

AbstractAnnulenoannulenes formally consist of two annulenes fused together. Hitherto, almost all the known representatives of this class of compounds were the strongly dehydrogenated forms, containing a few triple bonds and several cumulated double bonds. Besides ortho‐annelated dehydroannulenoannulenes also such compounds containing bridges made up of two or four carbon atoms (“acetylene‐cumulene” annulenoannulenes) were obtained by multistep syntheses. Most dehydroannulenoannulenes are stable and form colored crystals. In addition to dehydro[4n+2]annuleno[4n+2] annulenes with 14‐, 18‐, and 22‐membered individual rings, two dehydro[4n]annuleno[4n+2]annulenes with 16‐ and 14‐, and 16‐ and 18‐membered rings have also been synthesized. Planar combinations of two [4n]‐rings of this type and size are not known.—The dehydrogentated[14]annuleno[14]annulene containing a bridge of two carbon atoms has been studied in detail; however, the problem of the induction of a diamagnetic ring current in compounds of this type has still not been completely clarified.
A Model of Simple Liquids
Authors: Luck Werner A. P. 
Published: December 2003

AbstractThe intuitively clear hole defect model can be used for quantitative estimates and for understanding the properties of liquids. An ideal liquid is defined in analogy with an ideal gas and corrected for real liquids with hole defects. The good agreement permits an application of this approximation process to the understanding of some important material properties. The effective model fills a gap in the area of applied chemistry and of chemical teaching. It also leads to a correction factor for the failure of the theorem of corresponding states. The reasons for the applicability of such a simple model are demonstrated on the basis of the Lennard‐Jones potential.
A Basis for Orbital Symmetry Rules
Authors: Ross John , Whitesides George M. , Metiu Horia 
Published: December 2003

AbstractThe influence of molecular symmetry on reaction rates is examined with an approach in which reactions are viewed as electronic transitions between states of reacants and products (described, in turn by quasiadiabatic potential surface). The moleculer Hamiltonian is used to derive selection rules for these transitions. The complete Hamilatonian has no useful symmetery. Neglect of non‐Born‐Oppenheimer and spin‐orbit terms (and of other angular momentum coupling terms) leads to an apporixmate Hamiltonian and to selection rules which from the basis of the Woodward‐Hoffmann rules. This apporch provides an alternative to the adiabatic potantial surfaces, reaction coordinates, and transition state theory used in more familiar discussions of the Woodward‐Hoffmann rules. Further, it provides a particulary clear method for discussing violations of these symmetry rules, and for differentiating concerted and nonconcerted reactions.
Triphase Catalysis [New synthetic methods (27)]
Authors: Regen Steven L. 
Published: December 2003

AbstractTriphase catalysis (TC) has recently been introduced as a unique form of heterogeneous catalysis in which the catalyst and each of a pair of reactants are located in separate phases. Based on this concept, new synthetic methods have been developed for aqueous phase–organic phase reactions using a solid phase catalyst. Although it is only at an early stage of development, TC shows considerable potential for practical use. Our mechanistic understanding of these highly complex catalytic systems is at present very limited and detailed examination will be required before their relationship to phase‐transfer, micellar, and interfacial catalysis becomes clear.
Key Building Blocks of Natural Product Biosynthesis and Their Significance in Chemistry and Medicine
Authors: Franck Burchard 
Published: December 2003

AbstractThe principle features of the biosynthesis of natural products have been elucidated during the past thirty years by the use of isotopic methods. It was discovered that large groups of natural products originate from the same biosynthetic precursor—the key building block. The conversion of key building blocks into biologically active natural products serves as a model for the development of more efficient syntheses in chemistry. In medicine, information about key building blocks permits the elucidation and therapy of metabolic diseases.
The Chemistry of Isolated Cations
Authors: Bowen Richard D. , Williams Dudley H. , Schwarz Helmut 
Published: December 2003

AbstractOrganic ions decompose in the field‐free regions of a mass spectrometer after a relatively well‐defined lifetime (t ≈ 10−5s). This lifetime is very long compared with the time‐scale of molecular bond vibrations and consequently the excess internal energy (“non fixed” energy) present in the transition state for dissociation is comparable with that found in solution experiments. In general, only the energetically most favorable decay routes are able to compete. Moreover, information concerning kinetic energy release (T) accompanying unimolecular decomposition may be obtained from a consideration of the metastable peak for the process; these extremely valuable data are inaccessible to solution experiments because of the effects of collisions. The chemistry of ions may therefore be investigated conveniently by generating the ions of interest in a mass spectrometer and analyzing the reactions which occur in metastable transitions.
Compounds of Alkali Metal Anions
Authors: Dye James L. 
Published: December 2003

AbstractAnions of sodium, potassium, rubidium, and cesium are stable both in suitable solvents and in crystalline solids. The latter can be prepared either by cooling a saturated solution or by rapid solvent evaporation. Thermodynamic arguments show that alkali metal anions can probably exist in saturated solutions of the alkali metals in any compatible solvent, but that below saturation, dissociation into the cation and solvated electrons is favored in highly polar solvents such as ammonia. The key to solvent‐free salts of the alkali metal anions is stabilization of the cation by incorporation into a suitable crown or cryptand complex. By using such complexes it also appears possible to produce “electride” salts in which the charge of the complexed cation is balanced by a trapped electron. The chemical, electrical, and optical properties of salts of the alkali metal anions and “electrides” could provide useful applications.
Rates of Reactive Removal of Anthropogenic Emissions in the Atmosphere
Authors: Wagner Heinz Georg , Zellner Reinhard 
Published: December 2003

AbstractAnthropogenic emissions such as SO2, CO, NOx, and hydrocarbons constitute a perturbation of the natural equilibrium of the atmosphere. On a global scale, the extent of this perturbation does not significantly exceed the natural background levels as a result of direct or indirect photochemical “clean‐up” processes in the atmosphere. Regionally, however, considerably increased concentrations of these trace components may occur. They are degraded by free radical reactions having widely differing yet substance‐specific rates.