bluuebunny/crossref_metadata_2025_split huggingface dataset:
Chromatography with Supercritical Fluids
Authors: Klesper E. 
Published: December 2003

AbstractChromatography with supercritical fluids unites the features of both gas chromatography and liquid chromatography, yet retains special characteristics of its own. The diffusion coefficient and particularly the viscosity of fluid phases may approach values for low‐pressure gases, while the solvent power may be similar to that of liquids. However, with supercritical fluids it is possible to control chromatographic separations very effectively by pressure programming, since the solubility increases with increasing density. Temperature programming, on the other hand, can have the opposite effect to that in gas‐ or liquid‐chromatography since the density decreases with increasing temperature at a given pressure. Supercritical fluid chromatography is primarily of interest for the separation of higher molecular weight compounds. The efficiency of this method of separation is demonstrated on several homologous series. Thus, a styrene oligomer with nominalMw=2200 can be resolved by a pressure and temperature program into 40 species.
Electrostatics, the Chemical Bond and Molecular Stability
Authors: Benson Sidney W. 
Published: December 2003

AbstractElectrostatic models of the chemical bond are based on the Virial Theorem and hold promise for providing a reliable and accurate method for predicting heats of formation of molecules and free radicals. The Principle of Alternating Polarity which states that those compounds are most stable in which atoms of opposite polarity are bonded is shown to be quantitatively described by electrostatic models. Current fixed partial‐charge models account for ΔH of hydrocarbon molecules and radicals. With inclusion of polarization effects, whose energies are small, they also account for the dipole moments in hydrocarbons. A brief account is given of a more general model with significant polarization interaction which is under development and which appears to be able to account for both ΔH and dipole moments of polar molecules.
Lise Meitner: Her Life and Times—On the Centenary of the Great Scientist's Birth
Authors: Krafft Fritz 
Published: December 2003

AbstractCooperation between the three scientists Lise Meitner, Otto Hahn, and Fritz Strassmann in the study of the “transuranics” (1934—1938) prepared the way for the chemical identification of a first fission product of uranium by Otto Hahn and Fritz Strassmann at the end of 1938. Together with her nephew Otto Robert Frisch, she also gave the first theoretical interpretation of that experimental finding (barium instead of the expected radium) and suggested the inert gas krypton to be the second fission products, which Hahn and Strassmann subsequently identified (besides xenon) among the products. But she was kept from direct participation in the experiments conducted at the Kaiser Wilhelm Institute for Chemistry at Berlin—and therefore from the unrestricted claim to being co‐discoverer of uranium fission—because she was an Austrian “Jewess”. Thus after the “annexation” of Austria by the German Reich, the Nazi racial laws also applied to her and precluded her continuing to work in Germany. She therefore emigrated illegally via Holland to Sweden in the middle of 1938. There she continued to work on problems of nuclear physics under less favorable conditions untill 1960. Work on radioactivity already started at Vienna had also brought her together with Hahn at Berlin in 1907, and she continued the work at the Kaiser Wilhelm Institute studying α‐, β‐, and γ‐radiation and related nuclear processes. Not only because of her descent, but also because of her status of a woman in science, her fate was molded by the circumstances prevailing in Germany prior to World War II. On November 7, 1978 Lise Meitner would have celebrated her hundredth birthday, reason enough for recalling her lot as a woman scientist in Germany against the background of her times, which should stand as a warning to us. To that end contemporary documents could be quoted which till now were largely unknown.
Multistage Organic Redox Systems—A General Structural Principle
Authors: Deuchert Klaus , Hünig Siegfried 
Published: December 2003

AbstractA general structural principle for organic compounds that have the capacity for two‐stage electron transfer is based on the following reaction sequence:equation imageIn this scheme one or both of the entities X may be replaced by Y⊖. The radical partners in these systems often have very high thermodynamic stability. The choice of the end groups X and Y, (partial) inclusion of n‐systems in rings, alteration of the number of vinylene groups, and aza‐substitution make many variations possible. By means of this general structural principle numerous known compounds are brought together and new redox systems are simultaneously set up. The aim of the present review is to demonstrate the broad applicability of this principle, to indicate its significance for science and industry, and to describe some reactions.
Pore Sizes of Solids
Authors: Halász István , Martin Kornél 
Published: December 2003

AbstractA new, fast, and simple method based on size exclusion chromatography permits determination of the volumes of open and closed pores of rigid solids such as silica or porous glasses, together with the specific surface and the pore‐size distribution for pore diameters ranging from 10 to 4000Å. The results are in good agreement with those obtained by classical methods (capillary condensation, mercury porosimetry).
New Applications of Computers in Chemistry
Authors: Ugi Ivar , Bauer Johannes , Brandt Josef , Friedrich Josef , Gasteiger Johann , Jochum Clemens , Schubert Wolfgang 
Published: December 2003

AbstractThe mathematical model of constitutional chemistry described here is based on a concept of isomerism which has been extended from molecules to ensembles of molecules. A chemical reaction is the conversion of an ensemble of molecules into an isomeric ensemble. An ensemble of molecules is representable by an atomic vector and an associated bond and electron (BE)‐ matrix, and a reaction by a reaction (R)‐matrix. These BE‐and R‐matrices serve as a basis for computer programs for the deductive solution of chemical problems. We present here algorithms and computer programs based on the theory of BE‐ and R‐matrices. They enable the classification and documentation of structrues, substructures and reactions, the prognosis of reaction products,the design of syntheses, the construction of networks of mechanistic and synthetic pathways and the prediction of chemical reactions.
Anchimerically Accelerated Bond Homolyses
Authors: Reetz Manfred T. 
Published: December 2003

AbstractRadical reactions of the type R1abR2→•abR1 + •R2 are feasible if the new bond formed between b and R1 is considerably stronger than the old bond between a and R1. Furthermore, both the radical fragments formed must be resonance stabilized. An example of this reaction type is the thermolysis of benzyl trimethylsilylmethyl ethers, in which the trimethylsilyl group (with empty orbitals) migrates to the oxygen atom (with lone pairs). Assumption of a cyclic reactive intermediate with pentacoordinated silicon explains the observed intramolecular nature and the negative entropy of activation of such homolyses.
Alkenediazonium Salts: A New Chapter of Classical Organic Chemistry
Authors: Bott Kaspar 
Published: December 2003

AbstractWhereas the protonation of α‐diazo‐β‐diketones and α‐diazo‐β‐oxo esters with hydrogen chloride and antimony pentachloride yields the corresponding hydroxyalkenediazonium hexachloroantimonates, the “acid‐sensitive” α‐diazomonocarbonyl compounds can be transformed into resonance‐stabilized alkenediazonium salts by an analogous O‐alkylation with triethyloxonium hexachloroantimonate. A general route for synthesizing these compounds consists in the transformation of the p‐toluenesulfonylhydrazones of α‐halogenated aldehydes and ketones with Lewis acids. Alkenediazonium ions which are not resonance stabilized can be isolated at room temperature only if their decomposition at higher temperatures leads, via evolution of nitrogen, to vinyl cations in particularly high energy levels. Facile substitution reactions at the CC double bond with anisole and methanol demonstrate the strongly electrophilic nature of alkenediazonium salts. When electron‐donating substituments are attached to C2 of an ethylenediazonium ion, the bond order of the NN triple bond can be reduced to a level comparable with that in diazoacetic ester.
Polymer Alloys—Their Structure, Morphology, and Properties
Authors: Schmitt Burghard J. 
Published: December 2003

AbstractPolymeric materials with novel properties for new technological applications are increasingly obtained by combining existing polymers, while the synthesis of new monomers has receded into the background. These polymer combinations or “alloys” (polyblends) are characterized by their chemical composition, the conformation of the chain molecules, and the morphology, i.e. the state of order at supramolecular level. Multiphase constitution is a typical characteristic of these substances, with a decisive influence on their macroscopic properties. The morphology of multiphase polymer alloys can be controlled to a limited extent via the chemical composition of their components when homopolymers are mixed in the melt or as dispersions. Graft copolymerization, on the other hand, makes it possible to achieve the desired morphology at a given chemical composition. Furthermore, transprent two‐phase polymer alloys can be obtained under certain conditions. In multiphase polymers the reduction of stress without fracture, caused by mechanical loading will be treated using models. Certain combinations of properties such as hardness and toughness are connected with the coexistence of disperse and continuous phases. Equilibrium thermodynamical criteria for liquid mixtures wil be used to explain demixing phenomena in polymers. In the last few years it has become possible to determine the chain conformation experimentally using neutron scattering.
Photochemical Reactions of Surfactant Molecules in Condensed Monolayer Assemblies—Environmental Control and Modification of Reactivity
Authors: Whitten David G. 
Published: December 2003

AbstractAn investigation of the effects of a controlled microenvironment on reactivity has been carried out by the synthesis and study in organized monolayers of surfactant molecules containing photochemically reactive chromophores. Reactions where substantial environmental effects have been observed include the cis‐trans isomerization of olefins, ketone photoeliminations, ligand substitution processes, excimer and photodimer formation, and photoredox reactions. Both reactions occurring within the organized assemblies as well as interfacial phenomena have been investigated. Results obtained in these studies suggest possible extensions and applications for the development of new catalyst systems, biological models and controlled syntheses.
Pyrolysis of Sulfones as a Synthetic Method [New synthetic methods (28)]
Authors: Vögtle Fritz , Rossa Ludovica 
Published: December 2003

AbstractCyclic sulfoness containing structural elements such as aromtic rings, heteroatoms. functional groups, and further SO2‐groups as ring members decompose on heating with cleavage of SO2 and formation of a new CC bond. In the last decade this “sulfone pyrolysis” has been expanded into a generally applicable method even allowing the synthesis of sterically strianed medium‐membered and multi‐membered cyclic and polycyclic systems containing aromatic ring. By the pyrolysis of sulfones which are only unilateally activated by benzyl moieties, aromatic systms can be bridged by (CH2)n chains of any desired length. In addition, (CH2) chains can be split off together with two SO2 molecules, with recombination of the remaining centers, resulting in ring contraction by four to n atoms. However, sulfone pyrolysis is of importance not only as a ring‐contraction method but as a crucial final step in the synthesis of multi‐membered hydrocarbon cycles, e.g. of the phane type.
Spatial Structure Formation in Protein Gels
Authors: Bezrukov M. G. 
Published: December 2003

AbstractThe present article is concerned with the practical and theoretical aspects of gelation in protein systems. The processes involved are, inter alia, so complicated that several types of protein molecule assemblies and conformational changes during the association have to be considered. The gel network can be stabilized by five types of interaction: covalent crosslinking, polar interactions, hydrogen bonding, salt bridges and hydrophobic interactions. The microstructures of gels formed from fibrillated proteins (collagen) and from globular proteins (e.g. α‐ and β‐ casein) differ considerably. Of practical interest are methods of controlling the solubility of gels, including e.g. the use of cross‐linking agents and addition of salts.
Conformational Analysis by Photoelectron Spectroscopy
Authors: Klessinger Martin , Rademacher Paul 
Published: December 2003

AbstractThe conformation of organic molecules may be determined by means of photoelectron (PE) spectroscopy if there are orbital interactions present which depend on a dihedral angle. Straightforward application of the method requires that all ionization bands of interest can be assigned unambiguously and that inductive effects and secondary orbital interactions do not occur or can be accounted for appropriately. In many cases, this method complements conventional methods of conformational analysis. PE conformational analysis is particularly suited to compounds containing vicinal lone pairs or π systems.
Chromatographic Resolution of Racemates. New analytical methods (17)
Authors: Blaschke Gottfried 
Published: December 2003

AbstractA number of racemates can be resolved into optically pure enantiomers by chromatography on optically active adsorbents. Synthetic polymers with optically active amide, amino acid, and crown ether groups, natural products such as starch and cellulose, and also microcrystalline triacetylcellulose are suitable for this purpose. Racemates can also be resolved by gas chromatography on optically active stationary phases.
3‐Phenylpropylamines, A New Class of Systemic Fungicides
Authors: Himmele Walter , Pommer Ernst‐Heinrich 
Published: December 2003

AbstractThe biological activity of 3‐phenylpropylamines is not exclusively determined by their structure. It could be demonstrated that the fungicidal activity of partially hydrogenated compounds of this class of substances is also substantially influenced by the conformation. An optimization of the conformational criteria—illustrated by many examples, yet constituting only a small number of those actually examined—afforded compounds with excellent properties, e.g. N‐[3‐(4‐tert‐butyl‐1‐cyclohexenyl)‐2‐methylpropyl]‐cis‐2, 6‐dimethylmorpholine.
Photochemical Rearrangements and Fragmentations of Benzene Derivatives and Annelated Arenes
Authors: Kaupp Gerd 
Published: December 2003

AbstractIn this article the numerous intramolecular reactions of electronically excited benezene derivatives, which in many instances are only mentioned in original papers, are systematically analyzed and arranged according to reaction types. All known reaction types can be classified, and subdivided into reactions of the benzene ring (ionization, ring opening, ring alteration), reactions with participation of side chains (α‐, β‐, γ‐cleavage, homolysis, heterolysis), reactions of substituents with side chains (cyclization, dealkylation, cleavage of protective groups), and reactions of side chains with the aromatic ring (substitution, addition, dearomatization, cyclization). The selectivity of the energetically feasible competing reactions is primarily determined by geometric factors. Applications of the empirical effects are numerous and varied in preparative organic chemistry. Many of the reactions under discussion are already utilized industrially (e.g. in photochromism, UV stabilization, photography, information storage, printing, coating and polymer technology, and pharmacy).
Thermal Degradation of Polymers to Polymeric Carbon—An Approach to the Synthesis of New Materials
Authors: Fitzer Erich 
Published: December 2003

AbstractOne hundred years ago, Edison succeeded in preparing carbon fibers for his incandescent lamp bulb by thermal decomposition of natural polymeric fibers. Ten years ago, progress reports about “Novel Forms of Carbon” predicted outstanding properties and promising new applications for the carbonization products of synthetic polymers. Research and development in this field have been promoted by the problems of conventional technology (shortages of raw material and energy, pollution problems). Polymeric carbon materials—prepared by thermal degradation of synthetic polymers—exhibit a special ribbon‐like microstructure. They will provide the chemist with many challenges.
Alkali Metal‐Transition Metal π‐Complexes
Authors: Jonas Klaus , Krüger Carl 
Published: December 2003

AbstractIn this progress report a new principle is described for the synthesis of multimetal π‐complexes with hitherto unknown structural features. The nickel(0)‐olefin complexes 1,5,9‐cyclododecatrienenickel(0) and bis(1,5‐cyclooctadiene)nickel(0) react unexpectedly with main‐group metals, in particular alkali metals, their hydrides, and organometallic compounds, to give nickelligand species with a surplus charge. These species endeavor to transfer the excess charge onto π‐ligands such as olefins or dinitrogen. Multimetal complexes with electron rich transition metal π‐ligand units can, in addition, be prepared from metallocenes, alkali metals, and unsaturated compounds. The syntheses, structures, and reactions of this new class of substances will be summarized.
Physicochemical Principles and Applications of Supercritical Fluid Chromatography (SFC). New analytical methods (19)
Authors: van Wasen Ulrich , Swaid Issam , Schneider Gerhard M. 
Published: December 2003

AbstractIn supercritical fluid chromatography (SFC) compressed gases in the region of their critical temperature are used as mobile phases. SFC has important advantages over gas chromatography (GC) for the separation of low‐volatile or thermally unstable substances. Like high pressure liquid chromatography (HPLC) and gel chromatography, it is used for various special applications and preparative separations, e.g. in the petroleum industry and in the separation of oligomers. SFC is of great interest in fundamental research on fluid extraction and for the determination of the physicochemical properties of fluid systems. In this contribution the most important physicochemical, methodological, and instrumental principles of SFC are summarized; characteristic physicochemical applications are the determination of capacity ratios, partition coefficients, partial molar volumes, interaction second virial coefficients, and difusion coefficients.
Valence Changes at High Pressure in Systems Containing Lanthanoids
Authors: Jayaraman A. 
Published: December 2003

AbstractPressure‐induced valence change in samarium sulfide SmS is reminiscent of alchemy, for when the electronic transition has taken place, the originally dull looking substance glitters like gold. Although it is not the realization of the alchemist's dream in the true sense, the material in the new state exhibits extraordinarily interesting behavior. As a novel feature, the valence state of the samarium fluctuates between two electronic configurations at a very fast rate. The physical properties of systems containing lanthanoids can be profoundly altered by manipulating the valence state by pressure or by chemical substitution. These phenomena will be described and discussed in the present article.
Stereospecific Polymerization of Propylene: An Outlook 25 Years after Its Discovery
Authors: Pino Piero , Mülhaupt Rolf 
Published: December 2003

AbstractIn 1955Nattaand his co‐workers from the Istituto di Chimica Industriale del Politecnico di Milano reported the properties of highly crystalline polypropylene and other poly‐α‐olefins[1]which posses, at least in long sections of the main chain, asymmetric carbon atoms of the same absolute configuration (isotactic poly‐α‐olefins). The discovery of the new crystalline polymers was judged at that time “revolutionary in its significance”[2]and heralded a new era in polymer science and technology. Indeed, crystalline polypropylene, because of its relatively high melting point and its outstanding mechanical properties, has found very extensive application in the field of plastics films and fibers.—25 years after this publication, it is worthwhile examining the further developments following this discovery which has had such a great scientific and industrial impact, and the problems which are still open concerning the structure of the catalytic centers and the mechanisms by which practically completely stereoregular polypropylenes are formed.
Trinuclear Clusters of the Early Transition Elements
Authors: Müller Achim , Jostes Rainer , Cotton F. Albert 
Published: December 2003

AbstractTrinuclear clusters of the early transition elements, i.e. those elements situated on the left‐hand side of the transition series, represent the simplest types of clusters. They characteristically have a pronounced formation tendency and high stability; they are therefore produced under a wide variety of conditions and their triangular M3 skeleton is conserved in ligand exchange reactions. These clusters play a very important role in the chemistry of the respective elements, and especially those of the 4 d and 5 d series. Biochemical implications are of interest in connection with MoIV. This progress report presents a systematic account of the chemistry and the molecular and electronic structure of such compounds. A connection is also established with the crystal field treatment of mononuclear metal complexes.
Chemistry of Thiazyl Fluoride (NSF) and Thiazyl Trifluoride (NSF3): A Quarter Century of Sulfur‐Nitrogen‐Fluorine Chemistry
Authors: Glemser Oskar , Mews Rüdiger 
Published: December 2003

AbstractSince the first sulfur‐nitrogen‐fluorine compounds were synthesized 25 years ago[2], great progress has been made in this field of covalent fluorine chemistry[3–9]. The two small molecules thiazyl fluoride (NSF) and thiazyl trifluoride (NSF3), are key substances; nearly all sulfur‐nitrogen‐fluorine compounds can be derived from them.
Thermal and Photochemical Transformations of Hetero‐1,3,5‐hexatrienes into Five‐Membered Rings—Possible Pericyclic Reactions
Authors: George Manapurathu V. , Mitra Abijit , Sukumaran Kutikat B. 
Published: December 2003

AbstractHeterosubstituted 1,3,5‐hexatrienes undergo thermal and photochemical cyclizations, most of which can be viewed as pericyclic reactions. Numerous cyclizations of this type have been reported in the literature. This article presents a systematic account of such reactions, classified according to the number and nature of the heteroatoms.
Condensed Metal Clusters
Authors: Simon Arndt 
Published: December 2003

AbstractThe chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal‐rich compounds of transition metals with p‐elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal‐metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.