bluuebunny/crossref_metadata_2025_split huggingface dataset:
Solid‐State Photochemistry—A Method of Generating Unusual Valence States
Authors: Lehmann Gerhard 
Published: December 2003

AbstractHigh‐energy radiation can give rise to pairs of complementary defects in nonmetallic solids by the transfer of electrons between various types of atoms. These “color centers” which are generally paramagnetic, can usually be described as unusual valence states of an element. They are destroyed by heating and in most cases regenerated by renewed irradiation. In a heteropolar solid the formation of color centers usually leads to cancellation of point charges due to foreign ions of other valence or to vacancies. This is shown by the examples of kunzite, brazilianite, smoky quartz, and citrine; the most important methods for the structural elucidation of color centers are also described. Application of the principle of charge balance opens up possibilities for the production of unusual valence states, e.g. Al2+, F2−, Fe4+, and O−. Moreover, the type of the color center often permits far‐reaching conclusions to be drawn about the defect structure of real crystals, which could hardly be clarified in other ways.
Interpenetrating Polymer Networks
Authors: Klempner Daniel 
Published: December 2003

AbstractBesides mechanical blending and copolymerization there is a third possible way in which two polymers can be combined. Each polymer forms its own network, while both networks interpenetrate each other. There are no covalent bonds between the polymers. Such interpenetrating networks have been synthesized sequentially (from polymer A and monomer B) and simultaneously (from monomer A and monomer B). It is preferable that the polymers be of different chemical type; usually, an elastomer and a glass are combined, e.g. a polyurethane and a polyacrylate. Depending upon the ratio of component polymers either a strengthened elastomer or a glass having impact strength is formed. So far, there are no direct methods for establishing the degree of interpenetration.
17O‐NMR Spectroscopy as a Structural Probe
Authors: Klemperer Walter G. 
Published: December 2003

Abstract17O‐NMR spectroscopy has found limited application as a structural probe due to the experimental problems associated with a quadrupolar nucleus having low natural abundance. A survey of data obtained from systematic studies of structurally related compounds shows, however, that large chemical shift differences are usually obtained for structurally nonequivalent nuclei, and that characteristic chemical shift values are observed for specific chemical environments. The present day availability of 17O enriched materials and Fourier transform NMR instrumentation should allow extensive application of the 17O‐NMR technique to structural problems in the immediate future.
Sodium‐23 Nuclear Magnetic Resonance Spectroscopy
Authors: Laszlo Pierre 
Published: December 2003

AbstractWith the availability of Fourier transform spectrometers,23Na‐NMR spectroscopy has become an important tool. It affords direct insight into solvation and ion pairing phenomena, both in organic and in bio‐inorganic systems. Monitoring the chemical shifts and linewidths of23Na signals gives access to binding constants, reorientational correlation times and the microdynamics of the sodium coordination shell. The binding of other cations can also be studied by23Na‐NMR spectroscopy, in competition experiments.
Skeletal Rearrangements of Organoalkali Metal Compounds
Authors: Grovenstein Erling 
Published: December 2003

AbstractOrganometallic compounds of the alkali metals undergo concerted [1,n] sigmatropic migration of aryl, vinyl, carbonyl, alkoxycarbonyl, silyl, germyl, and related groups. Sigmatropic migration of alkyl, benzyl, and hydrogen occur only under special circumstances and then may occur by an alternate fragmentation‐recombination pathway. Allyl groups may migrate by the latter process or by a concerted [2,3] sigmatropic reaction. These molecular rearrangements are discussed from the viewpoint of simple molecular orbital theory. By suitable choice of the alkali metal, temperature, ligands of the alkali cation and/or the solvent, considerable control may be exercised over the rate of rearrangement, the relative migratory aptitude of groups, and the mechanistic pathway.
Mechanistic and Preparative Aspects of Vinyl Cation Chemistry
Authors: Hanack Michael 
Published: December 2003

AbstractCompelling evidence for the intermediacy of vinyl cations has accumulated in recent years. Vinyl cations can be generated by electrophilic additions to alkynes and allenes, and on participation of such functions in solvolysis reactions. Heterolysis of vinyl compounds likewise affords vinyl cations. Syntheses of substituted indenes, cyclobutanones, and cyclopropyl ketones illustrate preparative applications.
Mouse Histocompatibility Antigens
Authors: Henning Roland 
Published: December 2003

AbstractIn this article experimental findings and new theories concerning the chemical structure and biological function of histocompatibility antigens in the immune surveillance of virus‐infected and malignant cells are discussed. Amongst these antigens, the H‐2 antigens of the mouse have been studied most extensively. They are membrane bound glycoproteins causing the rejection of transplants from foreign tissue. One future aim of this field of research is to utilize the immune surveillance system in the therapy of diseases which as yet cannot be treated.
Tetrahedral Carbonylcobalt Clusters
Authors: Schmid Günter 
Published: December 2003

AbstractA variety of general methods is available for the synthesis of tetrahedral heteronuclear cobalt clusters of type (CO)9Co3E(L)n (E=hetero element). The size of E has a limiting effect on the existence of such complexes. No compounds (CO)9Co3E(L)n have so far been prepared in which the covalent radius of E exceeds 1.30 Å;. Elements such as In, Sn, Pb, Sb, or Bi do not fit into the tetrahedral Co3E cluster unit and form open complexes E[Co(CO)4]n (n=3, 4). Structural variations among the heteronuclear clusters are encountered in the presence or absence of carbonyl bridges between the cobalt atoms: hetero atoms E bearing several ligands L favor bridging, whereas those bearing only one ligand or none at all permit terminal bonding of all the CO groups.
Cobalt‐Catalyzed Pyridine Syntheses from Alkynes and Nitriles
Authors: Bönnemann Helmut 
Published: December 2003

AbstractSoluble organocobalt catalysts permit selective formation of substituted pyridines under mild conditions in a single step: mono‐, di‐, and trisubstituted pyridines can be obtained at will in high conversions and high yields from nitriles and alkynes. Polynuclear derivatives such as bipyridines can also be prepared by this method.
A Coordination Chemist's View of Surface Science
Authors: Muetterties E. L. 
Published: December 2003

AbstractStructure, a necessary element for an understanding of chemistry, has not been precisely defined, in molecular detail, for the surface species formed at a solid‐gas interface. Dramatic advances in theory, instruments and experimental procedures have now provided the surface scientists with an impressive arsenal of structural and electronic probes. Chemistry in the form of the classic displacement reaction can also provide an insight to the structure of surface compounds. In fact, many of the experimental procedures and systematics of the coordination and organometallic chemist can be effectively utilized in a complementary fashion with surface physics techniques to gain a more definitive picture of chemisorption states and or reactions at a surface.
Physicochemical Principles of Extraction with Supercritical Gases
Authors: Schneider Gerhard M. 
Published: December 2003

AbstractThe present review considers some physicochemical properties of fluid mixtures that are of importance for fluid extraction and supercitical fluid chromatography (SFC). Firstly, the important types of phase diagrams are treated, the occurrence of solid phases also being considered in some simple cases. Specific examples are given of mixtures of a highly volatile component I (e.g.CO2, C2H6) with a relatively involatile component II (e.g.squalane) of very different molecular size, shape, structure, and/or polarity, and it is shown how the rather complicated types of phase diagrams can be calculated and correlated. The importance of fluid mixtures extends far beyond the fields of science and technology reviewed.
A Quick Method for the Microanalytical Evaluation of the Dissolving Power of Supercritical Gases
Authors: Stahl Egon , Schilz Winfried , Schütz Erwin , Willing Eberhard 
Published: December 2003

AbstractA new method of microextraction with supercritical gases involving direct coupling with thin‐layer chromatography enables the solubility behavior of a large number of natural products to be investigated in pressure gradients of up to 400 bar. The results are summarized in general rules that enable the extractability to be predicted on the basis of the chemical structure. The substances that can be satisfactorily extracted in supercritical carbon dioxide and nitrous oxide are preferably lipophilic. The extracting capacity of the two gases was compared using alkaloids. In the case of polar compounds such as sugars and amino acids the solubility is so low even at pressures of 2500 bar (less than 10 μg/Nl) that application is not practicable. Quantitative tests of the solubility behavior of model substances in supercritical gases provide a basis for the technical application of the method.
Large‐Scale Industrial Plant for Extraction with Supercritical Gases
Authors: Eggers R. 
Published: December 2003

AbstractCriteria for the design and construction of plant for obtaining carrier material or extracts are discussed for the example of the extraction of natural products with supercritical carbon dioxide. The parameters to be established and the course of the process in a large‐scale plant are indicated. Plant engineering and optimization must be attuned to specific process‐technological and construction problems.
High‐Resolution, Higher‐Order UV/VIS Derivative Spectrophotometry
Authors: Talsky Gerhard , Mayring Lothar , Kreuzer Hans 
Published: December 2003

AbstractDerivative spectrophotometry has gained increasing importance in the past two years and is currently experiencing vigorous development. Following and introduction the present article provides a review of this extremely effective method. In particular, the advantages ofhigher‐order derivative spectrophotometry (HODS method,n> 2) are discussed on the basis of practical examples from a wide range of analytical fields. The results are achieved with the aid of a newly developed analog computer unit, whereby for the first time readily reproducible, low‐noise, on‐line spectra can be obtained up to the 7th order and even, in favorable cases, up to the 9th order. In practice it has proved valuable to work with spectra of the 3rd to 5th order; but even higher derivations could be profitable for the separation of strongly superposed signals or for “fingerprinting”.
Superacid‐Catalyzed Oxygenation of Alkanes. [New synthetic methods (26)]
Authors: Olah Georg A. , Parker David G. , Yoneda Norihiko 
Published: December 2003

AbstractOwing to the ready availability of the starting materials, the functionalization of saturated hydrocarbons for preparing chemical intermediates and products represents an attractive goal. The possibility of converting alkane in a controlled way into alcohols, ketones etc., i.e. to oxyfunctionalize them, is particularly challenging. After a short account of previous methods used for the oxidation of alkanes, particular emphasis will be given to recently developed reactions of alkanes with oxygenating agents in superacidic media.
The Art of Titration. From Classical End Points to Modern Differential and Dynamic Analysis
Authors: Winkler‐Oswatitsch Ruthild , Eigen Manfred 
Published: December 2003

AbstractTitration represents the quantitative determination of a chemical change in response to the variation in concentration of a standrad. Information on reaction parameters is usually obtained from an analysis of the shape of the titration curve. In this paper the derivatives of titration functions with respect to concentration variables and with respect to equilibrium parameters are compared with one another. The latter can be determined directly from amplitudes and time constants of the dynamic responses to equilibrium perturbations, brought about by fast alterations of temperature, pressure or electirc field.Three general situations have to be distinguished: 1. If the interaction is very strong (i.e. the complex stability is high: K → ∞), the (average) number of sample praticles can be counted directly by means of their reaction with a calibrated number of standard particles.—2. If, on the contrary, the interaction is weak (i.e. the complex stability is low), the sample will react quantitatively only in the presence of a large excess of standard; the half‐way point of which provides a direct measure of the binding constant.—3. Only for moderate interactions can individual curves characteristic of the binding strength be observed. The limiting cases 1 and 2 yield titration curves which reflect only the general class to which the system belongs.The dynamical procedures are fairly insensitive in case 1, which represents the classical end‐point situation and is most suitable for quantitative anlaysis, i.e. for a determination of sample concentrations. For case 2, the classical and dynamical techniques yield comparable information: mass‐action parameters are obtained from the extrema of the curves, which occur only in the presence of an excess of the standard. In case 3 the dynamical treatment provides a more direct access to the equilibrium constants, and also to the enthalpies or volumes of reaction as well as to the rate constants. The advantage of the dynamic method is due to the fact that in the two derivatives of the titration function d Ti/d ln q and d Ti/d ln p (where q is the titration variable, i.e. the ratio of standard and sample concentration, and p the mass‐action parameter, i.e. a reduced binding constant) the terms resulting from a differentiation with respect to p are more closely related to the reaction parameters than those following from a variation of q.The dynamic analysis is generalized for applications to multiple‐step titration processes, where it allows for the measurement fo equilibrium and rate parameters of individual steps. A uniform representation utilizing trigonometric functions was chosen which expresses clearly the singualr character of the end‐point. The relations are summarized intabular form. They provide in conjunction with the illustrations the basis for a comparative discussion.
Plastics as Optical Materials
Authors: Dislich Helmut 
Published: December 2003

AbstractThe Principal material used in optics is glass. Plastics can only gain acceptance in optics if they have properties not encountered with glass or if a desired article can be produced more rationally from a plastic. An example of the first case is provided by UV light guides consisting of a quartz glass core surrounded by a plastic cladding; there is no glass with a sufficiently low refractive index. Examples of the second case are viewfinder optics for camerás and lenses for sunglasses and industrial safety spectacles.
Chemical Vapor Deposition of Silicon Carbide and Silicon Nitride—Chemistry's Contribution to Modern Silicon Ceramics
Authors: Fitzer Erich , Hegen Dieter 
Published: December 2003

AbstractPure silicon carbide and silicon nitride have valuable properties in bulk pore‐free form; however, their industrial exploitation has hardly been possible so far. Neither compound can be melted or sintered in pure form; hot pressing or sintering at normal pressure requires the presence of additives; and the reaction‐sintering process in which only Si and C or Si and N are employed as additives affords porous materials.–The novel process of chemical vapor deposition has partly overcome the drawbacks of the previous methods. In the new process SiC is produced, e.g., by pyrolysis of CH3SiCl3, and Si3N4 by reaction of SiCl4 with NH3. This technique can also be used for pore filling in objects made of SiC and Si3N4 (gas phase impregnation) and for producing extremely fine SiC and Si3N4 (gas phase impregnation) and for producing extremely fine SiC and Si3N4 powder and SiC monofilaments suitable as components for SiC composites. Moreover, gas phase impregnation can also give fiber composites.
Synthesis and Biosynthesis of Alkoxylipids
Authors: Mangold Helmut K. 
Published: December 2003

AbstractChromatographic methods have shown that lipids with long alkyl or 1‐alkenyl chains–called alkoxylipids or ether lipids–are widely distributed in human and animal tissues. These compounds comprise neutral alkoxylipids, i. e. 1‐O‐alkyl or 1‐O‐(1‐alkenyl)‐2,3‐di‐O‐acylglycerols, and alkoxylipids which are linked by a phosphate residue at C‐3 to aminoethanol, choline or serine. 1‐O‐Alkylglycerols, 1‐O‐(1‐alkenyl)glycerols and other natural alkoxylipids can be synthesized in high yields. 2‐Alkyl, 1,3‐dialkyl and trialkylglycerols which are not found in nature are also accessible by synthesis. The neutral alkoxylipids are employed in biomedical investigations, e.g. as substrates in acyl‐hydrolase systems or in fat absorption studies. The principal features of alkoxylipid biosynthesis have been elucidated. The alkyl residue is derived from a saturated or monounsaturated fatty acid via the corresponding alcohol. 1‐O‐(1‐alkenyl)‐2‐O‐acylglycerophosphoryl aminoethanols (“plasmalogens”) are apparently formed from the appropriate 1‐alkyl compounds; this is not possible in the case of the choline series. Both “aminoethanol” and “choline” plasmalogens are, nevertheless, present in most tissues.
Stereochemistry of [2,3]Sigmatropic Rearrangements
Authors: Hoffmann Reinhard W. 
Published: December 2003

AbstractStereoselective synthesis is attracting more and more attention. Successful stereoselective syntheses require the availability of reactions of known stereochemistry which give maximum yields of the desired products. This is particularly so with reactions proceeding via cyclic transition states. Of these, the stereochemistry of [3,3]sigmatropic processes has long been known. This article now summarizes the factors determining the stereoselectivity of [2,3]sigmatropic rearrangememts.
August Kekulé—The Architect of Chemistry Commemorating the 150th Anniversary of His Birth
Authors: Hafner Klaus 
Published: December 2003

AbstractImagination, daring, and critical understanding are the sources of the impulses still felt today which chemistry received from the theoretical studies ofKekulé.“Let us learn to dream, gentlemen, then perhaps we shall discover the truth; but let us beware of publishing our dreams abroad before they have been scrutenized by our vigilant intellect … Let us always allow the fruit to hang until it is ripe. Unripe fruit brings even the grower but little profit; it damages the health of those who consume it; it endangers particularly the youth which cannot yet distinguish between ripe and unripe.” (Kekulé1890).
New Synthetic Reactions Based on the Onium Salts of Aza‐Arenes [New synthetic methods (29)]
Authors: Mukaiyama Teruaki 
Published: December 2003

AbstractEffective syntheses employing 2‐halogenated pyridinium, benzoxazolium, benzothiazolium, and pyridinium salts have been accomplished in the absence of strong acids and bases. Activation of carboxylic acids or alcohols with these onium salts leads to 2‐acyloxy and 2‐alkoxy intermediates which can be transformed into esters, amides, thiol esters, (macrocyclic) lactones, acid fluorides, olefins, allenes, carbodiimides isocyanates, isothiocyanates, nitriles, and isocyanates. The possibility of performing stereospecific syntheses (involving configurational inversion) with onium salts deserves attention.
Multidentate Acyclic Neutral Ligands and Their Complexation
Authors: Vögtle Fritz , Weber Edwin 
Published: December 2003

AbstractThe properties of cyclic crown ethers are approximated by acyclic neutral ligands (popdands), which are compared and contrasted with open‐chain bioionophores and acidic chelating agents in this article. Variations of the endo‐polarophilicity/exo‐lipophilicity balance, complex stability, ion slectivity can often be accomplished more easily, with greater versatility, and at less expense with acyclic polyethers than with their cyclic counterparts; complexation and decomplexation are generally faster in acyclic systems; and the pseudocavity usually has greater conformational flexibility. Acyclic crown ethers and open‐chain cryptands stiffened by rigid “terminal groups” containing donor atoms readily form crystalline complexes of alkali and alkaline earth metals. Some oppen‐chain neutral ligands form helical conformations in their crycstalin complexes. The observed coordination numbers and geometries are of theoretical interst. Attractive terminal group interactions lead to pseudocyclic species occupying a position intermediate between cyclic and acyclic ligands. It has recently proved possible to isolate crystalline complexes of alkali and alkaline earth metal ions with weakly donating oligo(ethylene glycol ethers) and with glycols; such complexes have also been obtained with sugars. Acyclic neutral ligands can serve as simple models of nigericin‐type bioionophores and be used analytically in microelectrodes. The recently discoverd crystalline stoichiometric complexes formed by some acyclic neutral ligands with guest molecules such as urea, thiourea, and water provide a fresh insight into weak interactions between neutral molecules and for the development of urea receptors.
Bonding Properties of Cyclopropane and Their Chemical Consequences
Authors: de Meijere Armin 
Published: December 2003

AbstractAmong the cyclic compounds of carbon, cyclopropane and its derivatives are outstanding by virtue of their unusual structural, spectroscopic, and chemical properties. The cyclopropane ring more closely resembles the CC double bond than the cyclobutane ring: it is a small ring with “double bond character”. Cyclopropyl and vinyl groups interact with neighbouring π‐electron systems and p‐electron centers; both cyclopropane derivatives and olefins form metal complexes, and add strong acids, halogens, and ozone; they both undergo catalytic hydrogenation and cycloadditions. While distinct differences in reactivity do exist–the double bond usually being more reactive than the three‐membered ring–there are no fundamental differences in behavior.–Although cyclopropane derivatives have been known for more than 90 years, intensive studies have been limited to the past 25 years. The development of carbene chemistry has rendered cyclopropane derivatives far more readily accessible. In recent years, the synthetic potential of the small‐ring function has been increasingly exploited. A considerable number of newly developed methods utilizing this approach clearly demonstrates that the reactivity of the cyclopropene ring, like that of the CC double bond, qualify it as a “functional carbon group”. This development is in full swing; we may therefore justifiably devote considerable effort to the study of cyclopropane chemistry.
Neurophysins: Molecular and Cellular Aspects
Authors: Acher Roger 
Published: December 2003

AbstractNeurophysins are linear cystine‐rich proteins containing 93–95 amino acid residues which like the neurohypophysial hormones oxytocin and vasopressin are formed in the hypothalamus and travel from there to the hypophysial posterior lobe. A species usually contains two (or three) neurophysins which differ only slightly in chain length and/or sequence. Many observatios suggest that both oxytocin and one of the neurophysins as well as vasopressin and the other neurophysin have a common precursor whose long chain is split into neurophysin and hormone. It can be shown on rats having considerable diabetes insipidus that a single gene controls the biosynthesis of the vasopressin and one of the neurophysins.