bluuebunny/crossref_metadata_2025_split huggingface dataset:
“Bicarburet of Hydrogen”. Reappraisal of the Discovery of Benzene in 1825 with the Analytical Methods of 1968
Authors: Kaiser R. 
Published: December 2003

AbstractThe historical background and the admirable analytical procedure that led Michael Faraday to the discovery of benzene in 1825 are discussed. Using modern techniques of instrumental analysis, the author retraces the isolation and analysis of benzene, and shows that the starting material used by Faraday must have been a very complex mixture of hydrocarbons, which would make severe demands even on present‐day preparative analytical techniques, since it contains more than 300 components at concentrations greater than 100 ppm. The magnitude of the changes in analytical techniques can be seen from the fact that it is no longer necessary to isolate individual compounds, and that a great deal of information can be obtained with a much smaller quantity of material and in a much shorter time. Nevertheless, Faraday's persistence, experimental skill, and accuracy of measurement remain an example to the analyst of today.
The Biosynthesis of Aromatic Amino Acids and its Regulation
Authors: Lingens F. 
Published: December 2003

AbstractMicroorganisms synthesize the aromatic amino acids phenylalanine, tyrosine, and tryptophan, as well as the related compounds p‐aminobenzoic acid, tetrahydrofolic acid, p‐hydroxybenzoic acid, ubiquinone, vitamin K, and nicotinic acid, by a highly branched route passing through shikimic acid. The biosynthesis is not regulated in the same way in all the organisms studied. The regulation is strongly dependent on the ability of the enzymes involved to be resolved into isozymes, their ability to be inhibited, and their activation, repression, and induction.
Soot Formation in Premixed Hydrocarbon Flames
Authors: Homann K. H. 
Published: December 2003

AbstractThe critical fuel‐oxygen ratio for soot formation and its dependence on temperature and pressure are discussed for various hydrocarbons. It is shown in the case of the Bunsen flame that the shape and structure of the flame front also have a decisive effect on soot formation. In the reaction zone of fuel‐rich hydrocarbon flames, unsaturated compounds are formed, and the concentration profiles of these products can provide quantitative information about their importance in soot formation. The “mechanism” of soot formation is discussed on the basis of the concentrations of higher hydrocarbons and the variation of the number and size of the soot particles. The effects of electric fields and foreign additives on soot formation have been given consideration.
Antibiotics as Inhibitors of Nucleic Acid and Protein Synthesis
Authors: Hartmann G. , Behr W. , Beissner K.‐A. , Honikel K. , Sippel A. 
Published: December 2003

AbstractThe antibiotics of the rifamycin, actinomycin, chromomycin, and anthracycline groups have been found to be specific inhibitors for the DNA‐controlled synthesis of RNA in vitro. Streptomycin, chloramphenicol, and puromycin can specifically suppress certain steps in the biosynthesis of proteins. The investigation of the mode of action of such substances may help us to gain a better insight into the transmission of hereditary information.
Memory Effects and Stereochemistry in Multiple Carbonium Ion Rearrangements
Authors: Berson J. A. 
Published: December 2003

AbstractThe stereochemical configuration at the initial site of heterolysis in a carbonium ion process in solution controls the specificity of migration two steps later in many sequential multiple rearrangements. These “memory effects” are attributable to the presence on the potential energy surface of small, hitherto undetectable minima. A variety of structural, stereochemical, and isotopic marking techniques are used to demonstrate that memory effects are a general feature of the ring‐expansion route to bicyclic carbonium ions. The intermediates formed in these reactions, although necessarily symmetrical or quasi‐symmetrical at equilibrium, act unsymmetrically. The memory effect uses the rate of an intramolecular carbonium ion rearrangement as a calibration with which to compare the rate of symmetrization. In this way, the system functions as a sensor of sub‐species.
Dehydration of Alcohols on Aluminum Oxide
Authors: Knözinger H. 
Published: December 2003

AbstractThe dehydration of alcohols on γ‐aluminum oxide, which yields water, olefins, and/or ethers, was studied with the aid of kinetic methods and IR spectroscopy. The unimolecular olefin formation probably proceeds via a surface compound, in which an alcohol molecule is joined by two angular H‐bonds to an OH group and an oxygen ion on the surface. The ether formation (bimolecular reaction), on the other hand, requires OH groups and oxygen and aluminum ions on the surface. The ether is formed from a surface alkoxide group and molecularly adsorbed alcohol.
Olefin Alkylation in Biosynthesis
Authors: Cornforth J. W. 
Published: December 2003

AbstractStereochemical studies, made possible by asymmetric labeling with hydrogen isotopes, have led to the hypothesis that enzymic association of C5 units in polysoprenoid synthesis is not a concerted process but proceeds in two steps: a trans 1,2‐addition to an olefin followed by a trans 1,2‐elimination. It is shown that similar mechanisms can also explain the cyclization of squalene epoxide to tetracyclic and pentacyclic triterpenoids.
Planar Complexes Containing Metal‐Metal Bonds
Authors: Krogmann K. 
Published: December 2003

AbstractPlanar complexes, and particularly those of platinum, can form structures containing linear chains of heavy metal atoms with metal‐metal distances as short as 3.1 Å. The bonding in these chains can be strengthened by partial oxidation, and the bond lengths can be reduced to 2.8 Å in this way. Model structures for the novel nonstoichiometric products, such as K2[Pt(CN)4]Cl0·32 · 2.6 H2O, are considered. The bonding is also discussed on the basis of a one‐dimensional band model.
Catalase and Peroxidase Activity of Cu2+ Complexes
Authors: Sigel H. 
Published: December 2003

AbstractThe relation between the structure of Cu2+ complexes and their catalytic activity in the decomposition of H2O2 can be used as a “molecular probe”, since it allows the deduction of the number of coordinated ligand groups in a Cu2+ complex from its catalytic activity. Since only coordinated ligand groups are oxidized by H2O2, further information about the coordination sphere of the metal ion can be obtained from the oxidation of certain parts of a ligand, which can be followed spectrophotometrically. The Cu2+ complexes of amides, polyamino acids, nucleotides, RNA, and native and denatured DNA have been investigated in this way. Kinetic studies provide an insight into the mechanisms of catalase and peroxidase reactions.
Electrochromism and Solvatochromism
Authors: Liptay W. 
Published: December 2003

AbstractThe position and the intensity of electronic bands are influenced by an electric field. Pronounced changes in the position of absorption bands are mainly due to the dipole moment of the molecule in the ground state and the change in the dipole moment during the excitation process, and pronounced changes in intensity are due to the field dependence of the transition moment, which can be described by the transition polarizability. The effect of an external electric field on the optical absorption (electrochromism) of suitable molecules can be used to determine the dipole moment in the ground state, the change in dipole moment during the excitation process, the direction of the transition moment of the electronic band, and certain components of the transition polarizability tensor. These data largely determine the strong solvatochromism (solvent‐dependence of the position and intensity of electronic bands), which is observed in particular with molecules having large dipole moments. Smaller contributions to solvatochromism result from dispersion interactions, which predominate in the case of nonpolar molecules. The models developed have been experimentally checked and verified by a combination of electro‐optical absorption measurements (influence of an external electric field on absorption) and investigation of the solvent‐dependence of the electronic bands.
The Synthesis of Rare Sugars
Authors: Brimacombe J. S. 
Published: December 2003

AbstractThe use of new oxidation reagents, such as the Pfitzner‐Moffatt reagent, dimethyl sulfoxide‐acid anhydrides, and ruthenium tetroxide, has significantly improved the yields obtained on oxidation of a secondary hydroxyl group to a carbonyl group in suitably protected sugars. The application of these new oxidants in the carbohydrate field is briefly surveyed. The product ketones are convenient intermediates for syntheses of amino, branched‐chain, deoxy, and other rare sugars. Some representative syntheses are illustrated.
Sigma‐Pi Rearrangements of Organotransition Metal Compounds
Authors: Tsutsui M. , Hancock M. , Ariyoshi J. , Levy M. N. 
Published: December 2003

Abstractσ‐Bonded organic groups in transition‐metal complexes can rearrange to π‐bonding ligands and vice versa: the classic example is the formation of π‐arenechromium complexes from σ‐arylchromium compounds. Rearrangements of this kind can be induced, inter alia, by reactions of the metal or of the ligand, as well as by migration or insertion of a ligand. Rearrangements play a role in many industrial processes, including the polymerization and isomerization of olefins and the hydroformylation of olefins.
Synthesis of Alkynes from Enol Esters
Authors: Craig J. Cymerman , Bergenthal M. D. , Fleming I. , Harley‐Mason J. 
Published: December 2003

AbstractEliminations leading from enol sulfonates, enol phosphates, and enol phosphonium salts to alkynes are reviewed. Olefin eliminations are usually the rate‐determining step in the formation of the triple bond. Enol esters may also play a role in the biosynthesis of alkynes.
Hydration Structure and Intermolecular Interaction in Polyelectrolytes
Authors: Zundel G. 
Published: December 2003

AbstractIn hydration, the intermolecular interaction is of decisive importance, since the ions modify the properties of the hydration water molecules, and the water molecules in turn have a strong influence on the interaction between the ions. These processes can be studied in detail with the aid of IR spectroscopy, which in addition provides a picture of the hydration structure. IR spectroscopy also shows that the excess protons are in energy bands and allows the detection of the proton dispersion forces acting between tunneling protons. The results of IR studies on polyelectrolytes contribute to our understanding of the many biological processes that depend on the behavior of ions.
Determination of the Structures of Organic Molecules and Quantitative Analyses with the Field Ionization Mass Spectrometer
Authors: Beckey H. D. 
Published: December 2003

AbstractField ionization and electron impact mass spectrometry complement each other well. Whereas bombardment of organic molecules with 70 eV electrons yields many characteristic fragment ions, which give valuable information about the structure of the molecules, the “soft” field ionization even of very unstable substances leads to the formation of relatively intense molecular ions which are often undetectable on ionization by electron bombardment. Field ionization also gives fragment ions that, though not very intense, are often very characteristic, and yield further information about the structure. The field desorption technique greatly reduces the degree of thermal decomposition of solid organic samples.New activation methods for field ion emitters lead to more intense ion beams and prolong the life of the emitters. Quantitative analyses using the field ionization mass spectrometer are of interest for hydrocarbon mixtures containing a very large number of components (e.g. several hundred). Simplified analyses are possible in particular for high‐boiling petroleum fractions.
Phosphorus. From Elemental Light to Chemical Element
Authors: Krafft F. 
Published: December 2003

AbstractExactly 300 years ago in the city of Hamburg, a certain Hennig Brand, self‐styled doctor medicinae, and chymist, discovered a strange substance in human urine, which was later called phosphorus (light bearer), a name then common to various luminous substances, and which created much excitement in the latter years of the 17th century on account of its properties. However, it was not Brand who profited from the discovery but others: Johann Daniel Krafft, Johann Kunckel, and Gottfried Wilhelm Leibniz, men who knew only too well how to exploit the weaknesses of the discoverer. “Cold fire”, Brand's own name for the new substance, was originally regarded as elemental light or fire, and it was not until the conception of the antiphlogistic theory by Antoine Laurent Lavoisier that the proper position of phosphorus among the chemical elements was recognized. In fact, the element played a decisive role in the overthrow of the phlogiston doctrine, a little over one hundred years after its discovery and almost two hundred years ago.
Classification of Partition Reactions of Inorganic Cations
Authors: Specker H. , Kettrup A. 
Published: December 2003

AbstractIn partition reactions of inorganic cations, it is necessary to establish the composition of the extracted compounds. The methods used and the limits of their application are described and illustrated for the extraction of cobalt thiocyanates. The results obtained allow new analytical applications, e.g. the fractional extraction of Hg, Bi, Cd, In, and Zn.
Organogermyl, Organostannyl, and Organoplumbyl Phosphines, Arsines, Stibines, and Bismuthines
Authors: Schumann H. 
Published: December 2003

AbstractThe preparation, properties, and reactions of the compounds named in the title are described, with particular reference to the possible participation of (p→d)π components in the bonding between the group IVB and the group VB elements.
Crystal Structure Analysis by Neutron Diffraction II
Authors: Will G. 
Published: December 2003

AbstractThis second part of the article “Crystal Structure Analysis by Neutron Diffraction” deals with the diffraction of neutrons by magnetically ordered crystals. Neutron diffraction is at present the only reliable method for the determination of the magnitude, direction, and spatial distribution of magnetic moments in crystalline substances. Since the magnetic moments are essentially due to the unpaired electrons, the distribution of these electrons in the crystal can be measured in this way.
The Defensive Chemistry of Land and Water Beetles
Authors: Schildknecht H. 
Published: December 2003

AbstractWherever life forms have arisen, they have had to defend themselves, even on the lowest levels, against their own kind. Darwin called this the struggle for life. The following discussion shows that this struggle can be carried on chemically with compounds of widely differing classes. Examples of defensive substances found are hot quinones in bombardier beetles, and weak and strong carboxylic acids, phenols, terpenes, steroids, and alkaloids in other Adephaga.
From Planck to Bohr
Authors: Hermann Armin 
Published: December 2003

AbstractThe quantum theory, which is nowadays a fundamental basis of chemistry that is regarded as self‐evident, initially presented immense difficulties to human thought. An insight into many particulars of its fascinating evolution is provided by the correspondence of those involved in the development of the theory between 1899 and 1913, which has now been thoroughly sifted for the first time by the author.
Energy Production in Anaerobic Organisms
Authors: Decker K. , Jungermann K. , Thauer R. K. 
Published: December 2003

AbstractBiological redox processes are required for the synthesis of “energy‐rich” compounds which are in an enzyme‐controlled equilibrium with the general energy carrier adenosine triphosphate (ATP). The basic mechanisms of biological energy transformation are substrate level (SLP) and electron transport phosphorylations (ETP). In anaerobic catabolism the numerous nutrients are channelled into only a few substrate level phosphorylations; the occurrence in chemotrophic anaerobes of energy conservation coupled to electron transport has not yet been demonstrated unequivocally. At present, the determination of the ATP turnover (YATP) in growing cells appears to be a promising method of approaching this problem. The existing knowledge of oxygen dependent enzyme reactions and their molecular evolution provides the basis for a biochemical definition of aerobic and anaerobic organisms.
Detection of Hindered Rotation and Inversion by NMR Spectroscopy
Authors: Kessler Horst 
Published: December 2003

AbstractRotations and inversions in organic molecules are readily recognizable from the temperature dependence of the NMR spectra. The study of the effects of substituents on the activation barriers of such processes permits the elucidation of reaction mechanisms, and the stability limits of isomers can also be determined. The knowledge of the stability limits is necessary for the specific synthesis of stable rotamers and invertomers.
Cooperative Conformational Transitions of Linear Biopolymers
Authors: Engel Jürgen , Schwarz Gerhard 
Published: December 2003

AbstractConformational transitions in proteins, nucleic acids, and other biopolymers evidently play a decisive role in many biological processes, particularly in control processes. They often proceed cooperatively, i.e. the elementary process of the transition of an individual segment of these macromolecules in influenced by the state of other segments via intramolecular interactions. In general, the segments favor the same state as their neighbours. The resulting equilibrium properties of cooperative systems, e.g. the sharpness of the transitions and their dependence on the chain length, can be quantitatively explained for linear systems by the linear Ising model. The molecular causes of the cooperativity can be explained for simple model polymers.
New Forms of Carbon
Authors: Vohler Otto , Reiser Peter‐Ludwig , Martina Renato , Overhoff Dieter 
Published: December 2003

AbstractA number of novel carbon materials whose unique properties fit them for many uses have recently been developed. Pyrolytic graphites are excellent conductors of heat and electricity parallel to the surface, whereas they are semiconductors perpendicular to the surface. A similar anisotropy is found in graphite foils, which are impermeable, but also very flexible. Glasslike carbon, which is also impermeable, is, however, completely isotropic. Carbon foams and felts are extremely light and exhibit very good thermal insulation up to high temperatures. In addition to very high strength, carbon fibers have values of Young's modulus greater than that of any other fibers or wires. Carbon fiber/resin composites are therefore more rigid than any other known materials; their specific Young's modulus is five times that of steel.
Ion‐Selective Sensors
Authors: Simon Wilhelm , Wuhrmann Hans‐Rudolf , Vašák Milan , Pioda Lavinia A. R. , Dohner René , Štefanac Zlata 
Published: December 2003

AbstractNumerous advances have recently been made in the development of ion‐selective (glass, solid, liquid) membrane electrode systems. The theoretical principles and practical requirements of such “sensors” are outlined in this article, and an attempt is made to review their possible applications.
Polarity of Covalent Bonds
Authors: Klessinger Martin 
Published: December 2003

AbstractThe concept of “partial ionic character” has proved exceedingly valuable and fruitful as a qualitative description of the electronic structure of molecules; however, the theoretical ideas underlying the empirical relations for the determination of the ionic character of a bond do not bear critical examination. Even Mulliken's population analysis fails in many cases as a means of interpreting quantum‐chemical calculations. On the other hand, a graphical representation of the spatial charge density distribution allows an intuitive interpretation even of complicated calculations, and (possibly in combination with an analysis of the forces acting on the atomic nuclei) yields very much farther‐reaching information on the polarity of a bond than can be expressed by the values of some parameters on the basis of excessively simplified models.
Recent Results on the Biochemistry of the Cell Wall Lipopolysaccharides of Salmonella Bacteria
Authors: Lüderitz Otto 
Published: December 2003

AbstractLipopolysaccharides, which are located in the cell wall of gram‐negative bacteria, are characterized by their biological versatility. They represent the O antigens of the bacteria, they are potent endotoxins, and they often function as the receptor sites for bacteriophages. The study of the mode of action of lipopolysaccharides and the search for structures in the macromolecules that are responsible for biological activity became promising when principles of the chemical fine structure of lipopolysaccharides were identified. The following review summarizes the results of recent investigations regarding the structure of lipopolysaccharides, their biosynthesis and its genetic determination.
Chemistry and Logical Structures
Authors: Ugi Ivar , Marquarding Dieter , Klusacek Hans , Gokel George , Gillespie Paul 
Published: December 2003

AbstractThe utility of the basic structures of modern mathematics for chemistry is discussed; general set theory, topology, and group theory are shown to pervade almost all static and dynamic aspects of chemistry. Chemical analogy, the systematic classification of molecules and a corresponding nomenclature system, conformational transformations, polyhedral rearrangements, and the relations between starting materials, transition complexes, and final products of chemical reactions are examples of where we apply the elements of modern mathematics to the solution of chemical problems.
Degenerate Carbonium Ions
Authors: Leone Ronald E. , von R. Schleyer Paul 
Published: December 2003

AbstractCarbonium ions are called “degenerate” when they rearrange through finite energy barriers to give products whose gross structures are identical with those of the starting ions. In such degenerate rearrangements (“automerizations”), individual atoms and groups become mixed but the overall structure is conserved. Automerizations can be observed by noting the dispersal of isotopic labels, the loss of optical activity, or the temperature dependence of NMR spectra. In certain cases, a combination of well‐known rearrangement processes results in complete mixing of all atoms, C or H. The 2‐norbornyl and 9‐barbaralyl cations are examples of fully degenerate systems; conditions can be found where both give but single‐line NMR spectra.
Valence Isomerism in Selected Heterocyclic Systems
Authors: Paquette Leo A. 
Published: December 2003

AbstractValence isomerization and the phenomenologically related principle of valence tautomerism, as they apply to several nitrogen‐containing heterocyclic systems, are discussed herein. Attention is called first to 1H‐azepines and the reluctance of such substances to isomerize to azanorcaradienes. The role of homo‐1H‐azepines in this context is then briefly outlined. Azocines (azacyclooctatetraenes) are seen to occupy an interesting position in the sense that pronounced shifts in the position of equilibrium (from almost entirely monocyclic to virtually completely bicyclic) occur with substitutional alterations of various types. The capability of azabullvalenes for degenerate valence isomerization is the next presented, and contrast is made with the properties of benzazabullvalenes and an azasemibullvalene derivative.