bluuebunny/crossref_metadata_2025_split huggingface dataset:
Advances in Acoustical Holography
Authors: Preston K. 
Published: December 2003

AbstractThe most recent branch of holography, acoustical holography employing acoustical radiation, presents a variety of new and highly interesting possible applications: Thus objects can be imaged in a turbid liquid medium (marine research); the technique can be used for non‐destructive investigations of the internal structure of objects that are opaque to light (testing of materials) and is possibly also a potential tool for the three‐dimensional imaging of biological structures (biomedical diagnosis).
Genetic Aspects of Hemoglobin Synthesis
Authors: Schmidt Berthold 
Published: December 2003

AbstractThe blood pigment hemoglobin (Hb) occurs in red blood cells in a higher concentration than the other proteins which are present. It is therefore particularly suitable for the investigation of genetically induced changes in protein molecules. Unlike the monomeric myoglobin (Mb) of muscle, the blood pigment is tetrameric. Several types of subunits are known that can tetramerize to hemoglobins having different properties. The information for all hemoproteins probably originated from the same primary gene. Doublets of this primary gene have developed differently, and now code different protein species. It is possible to establish from the Hb molecule the possible consequences of a mutation to the conformation and function of a protein molecule.
Decomposition of Organometallic Complexes in the Mass Spectrometer
Authors: Müller Jörn 
Published: December 2003

AbstractThe reliable interpretation of mass spectra for the determination of molecular constitutions requires systematic studies on the fragmentation behavior of classes of compounds, since the large number of kinetic and energy parameters that determine the decomposition of excited polyatomic ions makes ab‐initio predictions of spectra almost impossible. In this progress report, a number of general rules for the decomposition of organometallic complexes upon electron impact are discussed; a classification into decomposition types is also presented, and is illustrated by selected examples.
Cooperative Conformational Changes in Globular Proteins
Authors: Pohl Fritz M. 
Published: December 2003

AbstractIn globular proteins, the complicated steric arrangement of the polypeptide chain is determined by several interdependent cooperative interactions. These macromolecules are capable of reacting to changes in the environmental conditions such as temperature and pressure or the concentration of a wide range of compounds by changing their conformation and hence their biological and chemical properties. They are thus suitable for regulation processes or information storage in solution with a wide range of time constants. Environmental effects of this kind can be followed and explained in part at the molecular level by investigation of the time‐dependent reversible unfolding of a number of proteins that can be described to a good approximation by a strongly cooperative “all‐or‐none” transition between two states.
Crystal Chemistry of Complex Carbides and Related Compounds
Authors: Nowotny Hans 
Published: December 2003

AbstractNumerous metal carbides can be discussed in a rather uniform way by means of structural features, mainly characterized by the mode of linking of octahedral and occasionally trigonal prismatic [M6C] groups (M = transition element). From this point of view perovskite carbides (M3M′C, M′ = another transition or A‐group element) and derivatives, β‐Mn carbides (M3M′2C), κ‐carbides, carbides with V3AsC‐ and Cr2AlC type structures and derivatives, η‐carbides (M3M′3C) and carbides having the filled Mn5Si3 type structure will be treated. The high stability of these complex carbides is due to the strong bonding MC and additional bonding of MM′ atoms forming an ordered parent lattice. Besides the interstitial principle of filling of lattice holes (by isolated carbon atoms), substitution with A‐group elements may also take place. Thus in borocarbides extended structural elements occur.
Growth Processes due to Reactions in Swellable Cellular Matrixes
Authors: Wagner Kuno , Dahm M. 
Published: December 2003

AbstractIn matrix reactions in swellable cellular matrixes, the solids formed are oriented in space and arranged in the form of cells in strong three‐dimensional growth processes. Open‐cell, flexible, elastic polyurethanes have a particularly good matrix action. The principle is of general validity, and can be realized with a wide variety of matrixes and reaction types. The dimensional changes, which are always symmetrical, are largely determined by the chemical constitution of the matrix, segment structure, degree of crosslinking, and solvation and hydration behavior. One‐component reactions having high rates give sigmoid growth curves and exhibit parallels with growth curves of plant cell walls containing cellulose.
Semiempirical Use of Chirality Functions for the Description of the Optical Activity of Methane Derivatives in the Transparent Region
Authors: Richter Wolf Jürgen , Richter Bernd , Ruch Ernst 
Published: December 2003

AbstractThe optical activity can provide detailed information about the configuration and conformation of methane derivatives if the chirality function is resolved, according to the theory of chirality functions[3] and the quantum mechanical theory of the optical activity of methane derivatives[4], into its components which are compared with experimental data. A first step in this direction is undertaken in the present article and it will be shown that the second approximation method for determining the conformationally independent contribution to the optical activity yields quantitatively good results. Description of the overall optical rotation by this component is expectedly poor where interactions between ligands, e. g. hydrogen bonding, tend to favor specific conformations. A quantitative understanding of such additional effects appears promising in conjunction with analyses of the kind described.
Structure and Function of Ferritin
Authors: Crichton Robert R. 
Published: December 2003

AbstractFerritin is the major iron storage protein of mammals and consists of up to 4500 atoms of ferric iron surrounded by a shell of protein subunits. The protein component, apoferritin, consists of 24 identical polypeptide chains each of molecular weight 18500. The function of ferritin is to store iron in a soluble form from which it can be readily mobilized. Recent results concerning the structure of the protein are reported, and progress in the elucidation of the mechanisms whereby iron is introduced into apoferritin and released from ferritin is reviewed.
Displacement Reactions of Phosphorus(V) Compounds and Their Pentacoordinate Intermediates
Authors: Gillespie Paul , Ramirez Fausto , Ugi Ivar , Marquarding Dieter 
Published: December 2003

AbstractReactions of phosphorus(v) compounds involving the mutual interconversion of tetra‐ and pentacoordinate species are discussed in a critical review emphasizing stereochemical implications of the reaction mechanism. This discussion includes the formation and decomposition of the stable oxyphosphoranes, the Michaelis‐Arbusov, Perkov, and Wittig reactions, interconversions of phosphines and their oxides, and the nucleophilic displacements on phosphonium compounds. Reactions of phosphate esters and related compounds receive particular attention. All chemical arguments are derived by considering the effect of factors determining the relative stabilities of phosphorane derivatives, their rates of formation, decomposition and rearrangement by bond deformation or rupture and recombination processes, considerations which are uniformly applied on the basis of concepts developed in a preceding communication[2]. It is shown that a comprehensive mechanistic interpretation of the foregoing reactions requires substantial addition to available conceptual foundations such that, in many cases, present concepts and mechanisms must be revised.
Toxicological Problems Relating to Changes in the Environment
Authors: Henschler D. 
Published: December 2003

AbstractAssessment of the toxicological risk from foreign substances in the environment is based on the determination of action thresholds below which effects injurious to health no longer occur, even with life‐long intake. According to the latest theories it is likely that such thresholds exist even for “irreversible” (carcinogenic, mutagenic) effects. However, the difficulties besetting the determination of threshold values by experimental or epidemiological methods still appear to be virtually insuperable. The problems are both quantitative (excessive numbers of animals and observations required) and qualitative (poor predictability of effects owing to substantial differences in biological reactivity between species). Neither animal experiments nor experience with human subjects can guarantee total security from toxic effects. Legislative measures designed to restrict the introduction of chemical substances into the environment must therefore be based on compromise. Scientists must be consulted, but ultimately the decisions must be political, and politicians have to assume the responsibility for them.
Molecular Hysteresis and Its Cybernetic Significance
Authors: Neumann Eberhard 
Published: December 2003

AbstractThe general foundations for a thermodynamic analysis of hysteresis phenomena in solutions and suspensions of polyelectrolyte systems are presented using examples of molecular hysteresis in biopolymers and membranes. The fundamental cybernetic significance of metastable states and molecular hysteresis for a physical interpretation of phenomena of life such as memory recording and biological rhythms is discussed.
Very Low‐Pressure Pyrolysis (VLPP); A Versatile Kinetic Tool
Authors: Golden D. M. , Spokes G. N. , Benson S. W. 
Published: December 2003

AbstractThe theory and applications of the very low‐pressure pyrolysis technique to the study of the rates and mechanisms of unimolecular reactions, radical‐radical, and radical‐molecule bimolecular reactions, surface recombination of atoms, heterogeneous reactions, and vibrational energy transfer are reviewed.
The Physical Mechanism of the Chemical Bond
Authors: Kutzelnigg Werner 
Published: December 2003

AbstractFirst the interplay of kinetic and potential energy via the uncertainty relation is described with the aid of a variational function for the ground state of the H atom. The H ion is used to illustrate the physical mechanism of the occurrence of the chemical bond. The formation of the chemical bond can be divided into three steps: 1. the quasi‐classical (electrostatic) interaction of the unchanged electronic charges of the separate atoms; 2. the interference of the atomic orbitals, which (in the case of positive interference) leads to a displacement of charge into the bonding region and to a decrease in the kinetic energy; 3. deformation of the molecular orbitals to restore the correct balance of kinetic and potential energy. In simple models, it is often sufficient to consider just the second step. A two‐electron bond is not fundamentally different from a one‐electron bond. In larger molecules it is possible to distinguish between long‐range and short‐range interatomic contributions to the chemical bond. If the former are small, i. e. in molecules with non‐polar bonds, a one‐electron molecular orbital theory can be justified. Finally, the possibility of describing molecules by localized bonds is discussed.
Structure and Function of Nucleosides and Nucleotides
Authors: Saenger Wolfram 
Published: December 2003

AbstractThe nucleosides participating in biological processes consist of a sugar and a heterocyclic nucleobase; the nucleotides, which occur as monomers and as building units of polymeric nucleic acids, contain an additional phosphoester group. The complexity of the molecules leads to a complex stereochemistry with which the present progress report is concerned. Particular attention will be devoted to conformational considerations at the sugar groups, the syn‐anti conformation, the position of the C(5′)O(5′) bond relative to the sugar group, and the conformation of the phosphoester bonds. The article touches upon base pairing and base stacking, as well as forces stabilizing the syn conformation, and also deals with the reaction mechanism of the enzyme pancreatic ribonuclease as established from the stereochemistry of nucleotides and the mechanisms of action of the antileukemia drug 6‐azauridine and the antibiotic actinomycin D. Views on the effects of the unusual structures of the “rare” nucleosides 4‐thiouridine, isopentenyladenosine, and dihydrouridine on the structure of transfer ribonucleic acid are also presented.
Zintl Phases: Transitions between Metallic and Ionic Bonding
Authors: Schäfer Herbert , Eisenmann Brigitte , Müller Wiking 
Published: December 2003

AbstractExperimental studies on compounds of alkali and alkaline earth metals with semi‐ and metametals have considerably broadened the basis for a discussion of the transition from metallic to ionic bonding. Current interest is focused mainly upon the elucidation of the principles governing the structure of such compounds which are subject to a wide range of variation within this class of materials. A new definition of the term Zintl phase is proposed after consideration of available findings.
α‐Peroxyamines
Authors: Hawkins E. G. E. 
Published: December 2003

Abstractα‐Peroxyamines contain the group , which may also be incorporated into a ring. Numerous compounds of this type are accessible, inter alia, from ketones, hydrogen peroxide, or alkyl hydroperoxides and ammonia, as well as by autoxidation or photo‐oxidation of amines or imines. Apart from decomposition reactions, the chemistry of these compounds is characterized by interesting cyclizations to give lactams, imides, oxaziridines, tetroxanes, etc.
Electronic Transitions in Transition Metal Compounds at High Pressure
Authors: Drickamer H. G. 
Published: December 2003

AbstractVery high pressure is becoming increasingly important for investigating electronic structure. The relative shift in energy of electronic orbitals which is commonly observed at high pressure can frequently lead to a new ground state for the system. These electronic transitions may result in changes in electrical, optical, or magnetic properties as well as changes in chemical reactivity. Electronic transitions in metals and insulator‐metal transitions have been widely studied by physicists. Recently, it has been found that electronic transitions in aromatic hydrocarbons and their electron donor‐acceptor complexes can induce chemical reactivity and lead to the formation of new classes of hydrocarbons.Electronic transitions in transition metal complexes may lead to changes in spin state; both increase and decrease in multiplicity with increasing pressure have been observed. In addition, it has been shown that Fe(III) and Cu(II) reduce at high pressure in a variety of compounds. The behavior of these transition metal ions is described in some detail in relation to the general area of high pressure and electronic structure.
Phase‐Transfer Catalyzed Two‐Phase Reactions in Preparative Organic Chemistry
Authors: Dehmlow Eckehard V. 
Published: December 2003

AbstractQuaternary ammonium and phosphonium salts catalyze reactions between substances located partly in an aqueous and partly in an organic phase. Use of such phase‐transfer catalysts simplifies and accelerates numerous reactions traditionally conducted in nonaqueous media. These reactions include carbene reactions, nucleophilic substitutions, alkylations of ketones and nitriles, Wittig and Darzens reactions, formation of ethers and esters. Other reactions such as hydrolysis and oxidation can be accelerated.
Photochemistry with Circularly Polarized Light
Authors: Buchardt Ole 
Published: December 2003

AbstractThe reactions induced by circularly polarized light are reviewed and a general discussion of the induction of optical activity by circularly polarized light is presented. It is concluded that three different mechanisms, or their combinations, can cause asymmetric induction. In each case the optical yield is dependent on the optical anisotropy factor g, the ratio between the circular dichroism (Δϵ) and the extinction coefficient (ϵ). The results are discussed in terms of the presumed mechanisms, and particular attention is paid to the asymmetric synthesis of helicenes. It is concluded that this kind of photochemistry may have analytical importance.
Analysis of Herbicides
Authors: Thier Hans‐Peter 
Published: December 2003

AbstractHerbicides, like other pesticides, leave undesirable residues on the treated plants. To analyze them, the plant material is extracted and the herbicidal residues are separated from the bulk of the co‐extractives of natural origin, about which too little is known. Quantitative determinations at the ppm level are best performed by gas chromatography with highly sensitive, specific detectors. In food control, where the history of the sample is unknown, the herbicide residues are identified and determined quantitatively together with the residues of other pesticides.
X‐Ray Absorption Spectroscopy on Free Molecules
Authors: Schwarz W. H. Eugen 
Published: December 2003

AbstractThe most recent spectroscopic method that can be applied to chemical problems is X‐ray absorption spectroscopy, in which the excitation of individual core electrons of an atom is studied. The analysis of the spectra with the aid of simple theoretical models yields information both about the core orbitals of the atom and about the unoccupied molecular orbitals, from which various molecular properties can be deduced. These not only include charge distribution, electronic configuration, geometry, and spatial and energy characteristics of orbitals of the molecule under investigation; one can also obtain parameters of radicals (such as stability, bond length, ionization potential, and electronic excitation energy) that differ from the molecule under investigation in that the nuclear charge of one atom is one unit higher.
Halovinylene Carbonates in Organic Synthesis. New synthetic methods (2)
Authors: Scharf Hans‐Dieter 
Published: December 2003

AbstractMonohalo‐ and dihalovinylene carbonates constitute a new class of cyclophiles which permit simultaneous introduction of masked α‐hydroxyketo and α‐diketo functions, respectively, into the cycloadducts. Demasking can be performed by simple hydrolysis. Solvolytic opening of the carbonate ring leads to glycolic acid derivatives in the case of the monohalo compounds and to glyoxylic acid derivatives with the dihalo compounds. Preparation of the title compounds, their potential as synthetic reagents, and the chemistry of their simple reaction products are surveyed from a preparative viewpoint.
A Hundred Years of Stereochemistry—The Principal Development Phases in Retrospect
Authors: Weyer Jost 
Published: December 2003

AbstractThe theory of the arrangement of atoms in space was initiated in 1874 by van't Hoff and Le Bel. The starting points were mainly in the fields of structural chemistry and optical activity. After early difficulties, stereochemistry gained acceptance in the last quarter of the 19th century and led to many interesting investigations and discoveries: the investigation of geometrical isomerism, the Baeyer strain theory, the stereochemistry of nitrogen, the determination of the configurations of carbohydrates, Walden inversion, and the beginning of inorganic stereochemistry all fall in this period. Theoretical verification of the principal stereochemical hypotheses and the exact determination of the steric arrangement of atoms by physical methods were achieved only in the 20th century.
Modern Methods for the Study of Electrode Processes
Authors: Heitbaum Joachim , Vielstich Wolf 
Published: December 2003

AbstractThe investigation of electrode processes calls for the use of methods that yield information on the reaction mechanism and the reaction rate, on the nature and stability of intermediates, and on adsorption processes at the electrode. It is no longer enough to record current‐voltage curves or charging curves. In addition to the potentiostatic scan method, other methods that have proved useful for the investigation of electrode kinetics are the rotating disc electrode in particular, and to a smaller extent AC polarography. Intermediates can be identified with rotating double electrodes and by ESR spectroscopy or mass spectrometry. Adsorption processes on electrode surfaces can be studied by optical methods or by radioactive tracer methods.
Kinetics and Mechanism of Radical Polymerization
Authors: Mehnert Peter 
Published: December 2003

AbstractLow molecular reactions are frequently accompanied by side reactions producing a whole spectrum of by‐products whose removal in purification operations generally causes little difficulty because the major and side products usually differ sufficiently in their properties. Hence detailed kinetic studies in low molecular chemistry can claim industrial interest only insofar as they aim to reduce the rate of formation of side products. The situation is completely different in polymer chemistry, where the terms major and side product no longer apply. Instead, one obtains mixtures of similar and/or isomeric molecules which differ so little with regard to molecular weight and structure that resolution into pure substances or isolation of pure substances becomes practically impossible. However, since all partial reactions exert a direct influence on the product spread, i.e. the composition and properties of a product mixture are modified by any change in operating conditions, detailed kinetic analyses embracing all relevant partial reactions are an essential prerequisite for the production of defined polymeric products on any scale. This progress report illustrates the importance of kinetics for radical polymerization with the aid of selected examples.
High Pressure Kinetic Investigations in Organic and Macromolecular Chemistry
Authors: Jenner Gérard 
Published: December 2003

AbstractHigh pressure kinetics appears to be a valuable tool in investigating the mechanism of specific organic reactions. For instance, in pericyclic, Mentshutkin, cage, and polymerization reactions, such studies reveal various features of the transition state, in particular its localization along the reaction coordinate and its nature. However, precise conclusions require separation of the different effects (electrostatic, steric, orbital, etc.) which may contribute to the structure of the transition state.
Chemistry of Phosphorylcarbenes
Authors: Regits Manfred 
Published: December 2003

AbstractPhosphorylcarbenes, which have become known only recently during synthetic experiments in the field of phosphoryldiazoalkanes, have twofold preparative value. In the first place, they can be used to introduce phosphoryl groups into organic compounds, as in the phosphoryl‐cyclopropanation of alkenes or arenes and in the phosphorylcyclopropenation of alkynes. Secondly, phosphorylcarbenes readily undergo rearrangements; hydride, alkyl, aryl, or acyl shifts lead to phosphorylated alkenes. The phosphorylcarbene/methylenephosphane oxide rearrangement provides access to the short‐lived P analogs of the ketenes. Finally, the vinylcarbene/allene rearrangement was first discovered in this connection; it competes with ring closure to the cyclopropene.