bluuebunny/crossref_metadata_2025_split huggingface dataset:
Surfactants, Micelles and Fascinating Phenomena
Authors: Hoffmann Heinz , Ebert Gerlinde 
Published: December 2003

AbstractIn this article we shall describe processes and phenomena which may appear startling and unusual for someone who is familiar with the behavior of solutions of low molecular weight solutes. Some of these phenomena might even be surprising for someone who works with surfactants. The authors still remember their astonishment when they first became aware of a viscoelastic surfactant solution. This was many years after our group had started investigations on surfactants. But all of the solutions we had encountered until that time had behaved normally. Some of the phenomena which will be discussed are also encountered in solutions of polymers. Due to their high molecular weight one expects to find properties which are different from the properties of low molecular weight solutions and does not find them so surprising. Surfactant solutions, however, are aqueous solutions of small molecules with typical molecular weights of 200 to 400. For some of our experiments even dilute solutions were used which contained only up to one percent of surfactant.
Two‐Dimensional Solid‐State NMR Spectroscopy: New Possibilities for the Investigation of the Structure and Dynamics of Solid Polymers [New Analytical Methods (38)]
Authors: Blümich Bernhard , Spiess Hans Wolfgang 
Published: December 2003

AbstractNMR spectroscopy is an effective method not only for examining liquid samples but also for characterizing molecular sturcture, order and dynamics in amorphous and ordered solids. Recent developments in the area of solid‐state NMR spectroscopy span from model‐dependent studies of conventional one‐dimensional spectra to the more definitive two‐dimensional (2D) spectra which provide more specific information. For example, with 2D‐NMR spectroscopy it is possible to determine the orientational distribution functions of molecular segments in drawn polymers and to distinguish different mechanisms of complex molecular motions. Following an introduction to basic NMR spectroscopy, an overview of the current state‐of‐the‐art of 2D methods in solid‐state NMR spectroscopy is presented and demonstrated with selected examples.
Main‐Chain Chirality and Optical Activity in Polymers Consisting of CC Chains
Authors: Wulff Günter 
Published: December 2003

AbstractMain‐chain chirality is the optical activity resulting from the configurational or conformational arrangement in the main chain of a polymer. The chirality of the most important types of structures has been investigated on the basis of systematic considerations of symmetry. This has led to the surprising result that even in polymers derived from 1‐substituted or nonsymmetric 1,1‐disubstituted olefins (the technologically most important polymers) several types of chiral structures exist, which are expected to result in optical activity if a particular enantiomer is favorably formed. By carrying out an asymmetric cyclopolymerization, it has been possible to obtain certain structural types in the form of optically active copolymers or homopolymers (e.g., copolymers of styrene with methyl methacrylate, or even the homopolymer of styrene). Another new group of optically active polymers consists of the atropisomeric helical polyisocyanides, poly(trityl methacrylates), and polychlorals. Optically active polymers are already used as adsorbents for the chromatographic separation of racemic mixtures. Further applications are likely to emerge.
Biosynthetic Studies on Antibiotics
Authors: Floss Heinz G. , Beale John M. 
Published: December 2003

AbstractThe intricacies of the synthetic chemistry executed by living cells in the assembly of complex organic natural products can be probed in great detail by stable‐isotope labeling in conjunction with sophisticated 2D and multiple‐quantum NMR methods of analysis. Mechanistic details are revealed by unraveling the “cryptic” stereochemistry of biosynthetic reactions with the aid of stereospecific isotope labeling. Analysis of blocked mutants and cloning of biosynthetic genes provides new tools for the study of biosynthetic pathways and for the exploitation of the chemical capabilities of living organisms.
Metallocene Carbene Complexes and Related Compounds of Titanium, Zirconium, and Hafnium
Authors: Erker Gerhard 
Published: December 2003

AbstractThe structure and creativity spectrum of transition‐metal carbene complexes has been widened significantly by the bent metallocenes of titanium, zirconium, and hafnium. Proceeding from metal carbonyls and reactive (butadiene)‐, (aryne)‐, or (olefin) MCp2 complexes, many new Fischer‐type metaloxycarbene complexes of Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Ni, and Rh have been synthesized. The incorporation of titanocene, zirconocene, or hafnocene fragments allows new types of carbene complexes to be prepared. For the (alkylidenamido)metallocene complexes
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, metallocene ylides
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, and binuclear (μ‐alkynyl)metallocene compounds, some metalligand π bonding is indicated. Metallocene complexes with metallaoxirane units,
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, show similar chemical behavior to that of binuclear (μ‐methylene)complexes
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. The methylene groups of zirconaoxirane complexes, which are derived from carbon monoxide, may be expelled as ethylene by thermally induced CC coupling. With metal hydrides, CH2 transfer with insertion into the metal–hydride bond occurs. In one case, methylene insertion into a metal–carbon bond can even be observed. These reactions of Ti‐group metallaoxiranes could be models for postulated intermediates in the Fischer–Tropsch synthesis.
Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements
Authors: Chisholm Malcolm H. , Clark David L. , Hampden‐Smith Mark J. , Hoffman David H. 
Published: December 2003

AbstractAlkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant dπ orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled dπ orbitals. Both ligands readily form bridges that span MM bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)3], [CpW(CO)2], [Co(CO)3], and CH are related by the isolobal analogy. Thus the compounds [(RO)3W  W(OR)3], [Cp(CO)2WW(CO)2Cp], hypothetical [(CO)3CoCo(CO)3], and HCCH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W3(μ3‐CR)(OR′)9] versus [Co3(μ3‐CR)(CO)9] and [W4(C)(NMe)(OiPr)12] versus [Fe4(C)(CO)13]—but differ with respect to MM bonding. The carbonyl clusters use eg‐type orbitals for MM bonding whereas the alkoxide clusters employ t2g‐type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18‐electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or MM bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand‐centered and the LUMO metal‐centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M2(OR)6] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.
The Search for Highly Colored Organic Compounds
Authors: Fabian Jürgen , Zahradník Rudolf 
Published: December 2003

AbstractUntil now the study of organic compounds in which the π‐electron system is excited by absorbed light has been mainly concentrated on the ultraviolet and visible regions of the electromagnetic spectrum. Various new applications, such as the use of conjugated organic compounds as dye lasers or as materitals for storing information with the help of diode lasers, led to the synthesis of new compounds which absorb light in the near in infrared (NIR). It is possible to use structure‐color relationships to predict the properties of such new compounds when they belong to dyestuff classes which have already been studied in detail; in this case the approach involves decreasing the energy difference between the ground state and the first excited state. A less conventional starting point is provided by molecular structures in which from the outset there is only a very small energy difference between the lowest‐energy electronic states; such diradicaloid molecules occupy a special position among the various types of organic compounds. It is possible by means of suitable structural modification to stabilize such molecules in a singlet from which absorbs light at very long wavelengths (i.e. at small wave numbers).
Gene Synthesis [New Synthetic Methods (77)]
Authors: Engels Joachim W. , Uhlmann Eugen 
Published: December 2003

AbstractOligonucleotide synthesis, until a few years ago the rather exotic preserve of a few experts, has become an integral part of the arsenal of molecular‐biological techniques. The last decade, in particular, has seen unbelievably rapid development in this area. DNA synthesis has been automated and can now produce genes greater than 1000 base pairs in length. Tailor‐made synthetic genes also permit the synthesis of altered or even novel proteins (de novo protein design) by gene‐technological methods. Together with modern methods of gene isolation, sequencing, and expression, gene synthesis has played a major part in the enormous advances achieved in gene technology.
Generation of Complex Metal Oxides by Aerosol Processes: Superconducting Ceramic Particles and Films
Authors: Kodas Toivo T. 
Published: December 2003

AbstractComplex metal oxides, including ferroelectric, ferrimagnetic and superconducting ceramics, have a variety of technologically useful properties that can be exploited for a number of applications. Fabrication of complex metal oxides ceramics with specific properties reqires high‐purity powders with controlled chemical compositions, size distributions, and morphologies. Powders with these characteristics can be produced by aerosol processes in which fine particles are generated in gaseous flow systems. The particles can also be deposited from the gas phase onto surfaces to form films. This paper will discuss the use of aerosol processes for the generation of complex metal oxide powders. A review of aerosol processes will be presented first, followed by a discussion of the methods used for the generation of superconducting ceramic powders. Examples include the production of YBa2Cu3O7, La1.85Sr0.15CuO4, and Tl‐Ca‐Ba‐Cu‐O powders and films. Emphasis will be placed on defining the conditions required for the generation of chemically homogeneous particles with controlled morphologies.
A Structural Basis of Light Energy and Electron Transfer in Biology (Nobel Lecture)
Authors: Huber Robert 
Published: December 2003

AbstractAspects of intramolecular light energy and electron transfer are discussed for three protein cofactor complexes whose three‐dimensional structures have been elucidated by X‐ray crystallography: the light harvesting phycobilisomes of cyanobacteria, the reaction center of purple bacteria, and the blue multi‐copper oxidases. A wealth of functional data is available for these systems which allows specific correlations to be made between structure and function and general conclusions to be drawn about light energy and electron transfer in biological materials.
Lipophilic Tartaric Acid Esters as Enantioselective Ionophores
Authors: Prelog Vladimir , Kovačević Miće , Egli Martin 
Published: December 2003

AbstractTartaric acid esters with lipophilic alcohol moieties can discriminate between enantiomeric ammonium ions such as norephedrinium ions. Quantitative results were obtained by partitioning the components between water and 1, 2‐dichloroethane. The stereoselectivity was characterized by the free energy difference of the partition process (ΔΔG). Diamond‐lattice sectors were used to construct models of the (unstable) lipophilic ester/ammonium salt complexes from X‐ray structures of the individual components (esters and ammonium salts). These models can be used to interpret the effect of structure and configuration of the alcohol moiety on the stereoselectivity and enantioselectivity towards 1‐phenyl‐2‐amino‐1‐propanol (norephedrine) salts.
Liquid Crystalline Elastomers
Authors: Zentel Rudolf 
Published: December 2003

AbstractToday, material science is directed towards the development of multifunctional and oriented structures. One example of such supramolecular systems are liquid crystalline (LC) elastomers which combine the properties of LC phases (the combination order and mobility) with rubber elasticity, one of the most typical polymer properties. Their most outstanding characteristic is their mechanical orientability; strains as small as 20% are enough to obtain a perfectly oriented LC monodomain. This orientability, if LC elastomers with chiral phases are used, leads, for example, to elastomers with chirals smectic C*phases which are likely to show piezo‐electric behavior.
Materials for Optical Data Storage
Authors: Emmelius Michael , Pawlowski Georg , Vollmann Hansjörg W. 
Published: December 2003

AbstractThe outstanding feature of materials research in the eighties has been the convergence of basic research and practical application, leading to ever shorter cycles of innovation. This is especially true of materials which form the basis of key technologies. The mass storage units of the next generations of computers will be based on optical processes having a storage density which exceeds that of all hitherto known storage techniques that are practicable from the technical standpoint. In view of the fact that since 1982 read‐only memories in the form of compact discs (CD‐ROM) have become very successful in the field of audio electronics, research and development are now concentrated on materials for write‐once (WORM) and erasable (EDRAW) storage systems. Suitable materials for optical data storage are substances in which data markings can be recorded and deleted respectively using semiconductor lasers. Materials development is centered on the synthesis of infrared‐absorbing dyes for WORM memories and the production of rare earth/transition metal alloys for magneto‐optical data recording. An introduction to CD‐ROM technology will be followed by an overview of the state of development of the most important storage materials which are currently available commercially, and then the properties of these materials will be discussed with reference to selected examples.
Image Enhancement—Chemical, Digital, Visual
Authors: Lohmann Joachim 
Published: December 2003

AbstractDigital image processing has become well known through the high quality of some spectacular satellite images. It has developed into a discipline in its own right within information science and now finds application, not only in space travel, but also in such diverse fields as, for example, robotics, medical imaging, and television technology. Image processing also occurs, however, in photographic films and biological visual systems. In these systems, too, recorded images are manipulated for better recognition. This poses the question as to whether similar principles of image processing are operative in these different image handling systems.
Fractals and Related Hierarchical Models in Polymer Science
Authors: Blumen Alexander , Schnörer Horst 
Published: December 2003

AbstractPolymer science, an interdisciplinary science well‐rooted in organic chemistry and in materials science, encompasses an inordinate number and diversity of substance classes and thus has far‐reaching applications. Interestingly, polymers also represent a great challenge to the theoreticians, since their theoretical treatment often necessitates appropriate extensions of the classical methods from solid state physics and from statistical physics. Thus, new concepts often have to be invoked when considering the special properties of polymers. In this review we concentrate on one of the modern concepts in the theory of polymers, namely on scaling. Scaling is closely associated with new developments in the field of fractals and of hierarchical structures. Such concepts are invaluable for the modeling of complex geometries and for describing dynamical processes in polymeric materials. Here, we focus on a presentation of these ideas and we outline examples of recent research in which these concepts have been successfully applied.
Molecular Recognition with Model Systems
Authors: Rebek Julius 
Published: December 2003

AbstractHow structures fit together is the principal domain of molecular recognition, and current studies are evolving from the host–guest chemistry of ions to interactions between two molecules. Recent advances in the synthesis of sizable concave molecules, especially those featuring convergent functional groups, make it possible to bind smaller convex structures with considerable selectivity. One result is that hydrogen bonding can be addressed in model systems. The present review emphasizes the use of cleftlike structures as a means of probing the forces involved in nucleic acid recognition. The application of such molecules to the catalysis of chemical reactions, particularly those involved in self‐replicating systems, is also described. Some implications for future pharmaceutical agents are suggested as a result of access to synthetic receptors for biologically relevant targets.
Aspartic and Glutamic Acid as Ligands to Alkali and Alkaline‐Earth Metals: Structural Chemistry as Related to Magnesium Therapy
Authors: Schmidbaur Hubert , Classen Hans Georg , Helbig Joachim 
Published: December 2003

AbstractMagnesium and other alkaline‐earth and alkali metals play an important role in the biochemistry of virtually all living organisms. In contrast to “trace elements” these “common metals” are distributed in most cells and tissues, often in considerable concentrations, and a constant supply is indispensable for unrestricted performance of biological functions. In contemporary medicine, various forms of magnesium therapy are recommended in order to compensate for magnesium deficiency due to certain dysfunctions as well as to loss from excessive transpiration. In addition, there are numerous indications for prophylaxis. Magnesium is usually administered orally in the form of “complexes” with “natural” anions derived from I‐aspartic, L‐glutamic, L‐pyroglutamic (pidolic), citric, and orotic acid. Analytical, electrochemical, and spectroscopic investigations of aqueous solutions of these Pharmaceuticals, together with X‐ray diffraction studies of crystalline samples, have given a detailed knowledge of these coordination compounds. Only certain combinations can be addressed as magnesium complexes, whereas others are only present as aquo complexes both in aqueous solutions and in the solid state. Thus, a delicate balance of metal complexation and hydrogen bonding exists for the components of the drugs, which is relevant for the metal carrier functions of the organic ligands. Compared with the more flexible stereochemistry of Ca, Sr, Ba, MnII, Zn, Li, Na, and K complexes, which have coordination numbers between 4 and 9, octahedral hexacoordination is found exclusively for magnesium, the most stereoselective of these metals.
Organic Synthesis in the Age of Computers
Authors: Hendrickson James B. 
Published: December 2003

AbstractThe computer affords four areas of interest to synthetic chemists: (1) storage and retrieval of information; (2) complex calculations of shape and reaction dynamics; (3) artificial intelligence (AI) to solve synthetic and analytical problems; (4) overall direction of multiple laboratory experiments. Our work began 30 years ago initiating molecular geometry calculations to match experimental results. The explosive growth of computer power since will be illustrated. Synthesis design (as AI) requires an efficient protocol to search huge trees. The SYNGEN program is based on simplification and systematization, first of skeletal dissection, then of structure and reaction characterization, for generation of optimal synthesis routes. The program will be be described and its future development outlined. Finally, a perspective on the future of all four areas will be sketched, noting the necessary attendant growth in systematic use by chemists and in sympathetic interfaces by software designers.
CC Bond Formation by Addition of Carbenium Ions to Alkenes: Kinetics and Mechanism
Authors: Mayr Herbert 
Published: December 2003

AbstractThe addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product‐like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity‐selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.
3‐Amino‐2H‐Azirines. Synthons for α,α‐Disubstituted α‐Amino Acids in Heterocycle and Peptide Synthesis [New Analytical Methods (43)]
Authors: Heimgartner Heinz 
Published: December 2003

AbstractThe recent upswing in peptide chemistry has been accompanied by an increasing interest in nonproteinogenic amino acids. These include the α,α‐disubstituted glycines, the best known of which is Aib (2‐aminoisobutyric acid, 2‐methylalanine). These α‐amino acids occur in natural oligopeptides such as the peptaibols, a class of membrane‐active ionophores that has been isolated from fungal cultures. The twofold substitution at the α‐C atom of the amino acids severely restricts the conformational freedom of the peptides and causes particular secondary structures to be favored; thus, α, α‐disubstituted α‐amino acids induce the formation of β turns or helices. 3‐Amino‐2H‐azirines are ideal synthons for the construction of oligopeptides, cyclic peptides and depsipeptides (peptolides) containing such α,α‐disubstituted α‐amino acids. The presence of the ring strain in these molecules means that they can be used in peptide coupling without the need for additional activating reagents. Using 3‐amino‐2H‐azirines a large array of heterocycles containing α, α‐disubstituted α‐amino acids as structural elements within their skeleton can be synthesized. The driving force in these reactions is the release of the strain on the three‐membered ring, which usually takes place in a ring‐expansion reaction. The mechanistic elucidation of these reactions, which can be quite complex, contains some surprises.