bluuebunny/crossref_metadata_2025_split huggingface dataset:
Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules
Authors: Weller Horst 
Published: December 2003

AbstractIn semiconductor particles of nanometer size, a gradual transition from solid‐state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q‐particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q‐particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance‐specific quantity in macrocrystalline materials, increases by several electron volts in Q‐particles with decreasing particle size. In Q‐particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico‐chemical properties of the particle. Q‐particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q‐particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q‐particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q‐particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q‐particles, as well as with the spectroscopic, electrochemical, and electron‐microscopic investigation of these particles.
Aminoacyl‐tRNA Synthetases: The Division into Two Classes Predicted by the Chemistry of Substrates and Enzymes
Authors: Cramer Friedrich , Freist Wolfgang 
Published: December 2003

AbstractIn biological systems, almost all chemical reactions are catalyzed by enzymes. In order to understand the mode of action of these biocatalysts, we need to know precise details of their structures, their active sites, and their functional groups. Most important are those parts of the protein molecule that are responsible for precise recognition of the substrates, for the specific interaction between the enzymes and their reactants. Decades can pass between the isolation of an enzyme and the determination of its exact structure by X‐ray analysis. For the chemist, however, means are known by which initial information may be gathered relatively quickly: the synthesis of modified substrates and affinity labeling. During the last twenty five years these two methods have been put to the test on aminoacyl‐tRNA synthetases. Today, we know enough about the structures of these enzymes to evaluate the success of this “chemistry on macromolecules,” or their substrates. Chemists have had some successes, which provided the structural analysts with valuable preliminary results.
Computer‐Assisted Solution of Chemical Problems—The Historical Development and the Present State of the Art of a New Discipline of Chemistry
Authors: Ugi Ivar , Bauer Johannes , Bley Klemens , Dengler Alf , Dietz Andreas , Fontain Eric , Gruber Bernhard , Herges Rainer , Knauer Michael , Reitsam Klaus , Stein Natalie 
Published: December 2003

AbstractThe topic of this article is the development and the present state of the art of computer chemistry, the computer‐assisted solution of chemical problems. Initially the problems in computer chemistry were confined to structure elucidation on the basis of spectroscopic data, then programs for synthesis design based on libraries of reaction data for relatively narrow classes of target compounds were developed, and now computer programs for the solution of a great variety of chemical problems are available or are under development. Previously it was an achievement when any solution of a chemical problem could be generated by computer assistance. Today, the main task is the efficient, transparent, and non‐arbitrary selection of meaningful results from the immense set of potential solutions—that also may contain innovative proposals. Chemistry has two aspects, constitutional chemistry and stereochemistry, which are interrelated, but still require different approaches. As a result, about twenty years ago, an algebraic model of the logical structure of chemistry was presented that consisted of two parts: the constitution‐oriented algebra of be‐ and r‐matrices, and the theory of the stereochemistry of the chemical identity group. New chemical definitions, concepts, and perspectives are characteristic of this logic‐oriented model, as well as the direct mathematical representation of chemical processes. This model enables the implementation of formal reaction generators that can produce conceivable solutions to chemical problems—including unprecedented solutions—without detailed empirical chemical information. New formal selection procedures for computer‐generated chemical information are also possible through the above model. It is expedient to combine these with interactive methods of selection. In this review, the Munich project is presented and discussed in detail. It encompasses the further development and implementation of the mathematical model of the logical structure of chemistry as well as the experimental verification of the computer‐generated results. The article concludes with a review of new reactions, reagents, and reaction mechanisms that have been found with the PC‐programs IGOR and RAIN.
Carbohydrates as Chiral Auxiliaries in Stereoselective Synthesis. New Synthetic Methods (90)
Authors: Kunz Horst , Rück Karola 
Published: December 2003

AbstractCarbohydrates are inexpensive natural products in which numerous functional groups and stereogenic centers are combined in one molecule. By directed regio‐and stereoselective formation of derivatives they can be converted into efficient chiral auxiliaries for controlling asymmetric syntheses. Stereoelectronic effects and pre‐orientation of the reactive and shielding groups through formation of complexes can often be used for effective diastereofacial differentiation. In aldol reactions and alkylations on carbohydrate ester enolates intramolecular complexation promotes simultaneous elimination with formation of ketene. The steric, stereoelectronic, and coordinating properties of carbohydrate templates can also be used selectively to attain high levels of asymmetric induction in processes such as Diels–Alder reactions, hetero‐Diels–Alder reactions, [2 + 2] cycloadditions, cyclopropanations, and Michael additions. It was possible with bicyclic, strongly stereodifferentiating carbohydrate auxiliaries to achieve a diastereoselective synthesis of carboxylic acid derivatives branched in the β position by a new 1,4‐addition of alkylaluminum halides to α,β‐unsaturated N‐acylurethanes, in which methylaluminum halides and higher alkyl‐ or arylaluminum compounds behave mechanistically in a strikingly different manners. As complex ligands in chiral reagents and promoters, carbohydrates allow highly stereoselective reductions and aldol reactions that lead, amongst others, to chiral alcohols and β‐hydroxy‐α‐amino acids in excellent enantiomeric excesses. Glycosylamines offer the possibility of versatile stereoselective applications: in the presence of Lewis acids the corresponding aldimines permit high‐yielding syntheses of enantiomerically pure α‐amino acids by Strecker and Ugi reactions, controlled by steric and stereoelectronic effects and by complex formation. They can be used with equal efficiency for asymmetric syntheses of chiral homoallylamines and for asymmetric Mannich syntheses of β‐amino acids and chiral heterocycles, for example alkaloids.
Neural Networks in Chemistry
Authors: Gasteiger Johann , Zupan Jure 
Published: December 2003

AbstractThe capabilities of the human brain have always fascinated scientists and led them to investigate its inner workings. Over the past 50 years a number of models have been developed which have attempted to replicate the brain's various functions. At the same time the development of computers was taking a totally different direction. As a result, today's computer architectures, operating systems, and programming have very little in common with information processing as performed by the brain. Currently we are experiencing a reevaluation of the brain's abilities, and models of information processing in the brain have been translated into algorithms and made widely available. The basic building‐block of these brain models (neural networks) is an information processing unit that is a model of a neuron. An artificial neuron of this kind performs only rather simple mathematical operations; its effectiveness is derived solely from the way in which large numbers of neurons may be connected to form a network. Just as the various neural models replicate different abilities of the brain, they can be used to solve different types of problem: the classification of objects, the modeling of functional relationships, the storage and retrieval of information, and the representation of large amounts of data. This potential suggests many possibilities for the processing of chemical data, and already applications cover a wide area: spectroscopic analysis, prediction of reactions, chemical process control, and the analysis of electrostatic potentials. All these are just a small sample of the great many possibilities.
Defect Chemistry: Composition, Transport, and Reactions in the Solid State; Part II: Kinetics
Authors: Maier Joachim 
Published: December 2003

AbstractWithin the last few decades—though with its beginnings traceable to as early as 60 years ago—a direction of research has developed almost unnoticed by the classical chemical disciplines, such that one can now recognize a chemistry within the solid state that is analogous to the long‐familiar chemistry in the liquid state. It arises from those departures from the ideal structure that are thermodynamically unavoidable, the point defects, and is referred to as defect chemistry. It includes the description of ionic and electronic effects, and it considers diffusion as a special step of the overall reaction. This area of chemistry enables one to describe and treat in a unified way many apparently widely different phenomena such as ionic conduction in crystals, doping effects and p–n junctions in semiconductors, color centers in alkali metal halides, image development in photography, passivation and corrosion of metals, the kinetics of the synthesis and sintering of solid materials, the problems of rock formation during the earth's evolution, the mechanisms of gas sensors and high temperature fuel cells, the performance of photosentive electrodes, variations of the electron balance in high temperature superconductors, elementary processes of heterogeneous catalysis, nonequilibrium transitions and oscillations in semiconductors in electric fields, and many more. In such phenomena the equilibrium concentration of defects has an important double role: it not only determines the disorder and the departures from the stoichiometric composition in the equilibrium state, but also, together with the mobility as the kinetic parameter, is the key parameter concerning the rates of physical processes, which are the main subject of the present discussion. Following the account of the thermodynamic fundamentals in Part I, this second part of the review will be concerned with kinetic processes. These are essentially the equilibration processes (physical diffusion, electrical conduction, chemical diffusion, and reactions) that take place under the influence of chemical and electric driving forces. As in Part I, here again the general characteristics of defect chemistry will clearly emerge, including the coexistence of ionic and electronic charge carriers, and the importance of the spatial coordinates. The concluding section will discuss some special characteristics of nonlinear processes that are too often overlooked in solid‐state studies.
Transition Metal Complexation of σ Bonds
Authors: Crabtree Robert H. 
Published: December 2003

AbstractThe first σ complexes were found in the 1960s and 1970s, but they did not attract more than passing attention. Only now are we beginning to recognize their key role in the chemical reactions of σ bonds, and this has encouraged more detailed study. In contrast with the more familiar π‐donor complexes such as M(CH2CH2) and complexes like MNH3, in which the one pair of electrons on the N atom is bound to the metal atom, in a σ complex an XH group binds to the transition metal atom; the XH σ‐bonding electron pair acts as a 2e donor to give an (X‐H)‐M type complex. Dihydrogen complexes (X = H) are one important group of σ complexes. C‐H‐M complexes (X = R3C) with an agostic C‐H‐M interaction have not only been found in the ground state but also implicated in the transition states of many important organometallic transformations such as Ziegler–Natta catalysis and sigma bond metathesis. The importance of XH bond activation will encourage continued growth in this field.
Molecular Machines: How Motion and Other Functions of Living Organisms Can Result from Reversible Chemical Changes
Authors: Urry Dan W. 
Published: December 2003

AbstractCertain model proteins dramatically fold and become more ordered on raising the temperature. When the temperature is raised to drive folding and assembly, these model proteins can lift weights and perform work; they can produce motion. The temperature of warm‐blooded animals, however, is kept constant. Therefore, motion cannot result from a change in temperature. In this case, a free energy change, caused, for example, by an increase in the concentration of a chemical, can lower the temperature at which the protein folding and assembly transition occurs from above to below physiological temperature. Raising the concentration of a chemical isothermally has indeed been shown to result in motion and the efficient performance of work. These model proteins and the mechanism they reveal provide insight into the molecular basis for diverse biological functions; they are models for the molecular machines that comprise the living organism, and they provide a new class of materials for both medical and nonmedical applications.
Methods for the Synthesis of μ‐Hydrocarbon Transition Metal Complexes without Metal–Metal Bonds
Authors: Beck Wolfgang , Niemer Burkhard , Wieser Michael 
Published: December 2003

AbstractTransition metal complexes in which hydrocarbons serve as σ,σ‐, σ,π‐ or π,π‐bound bridging ligands are currently of great interest. This review presents efficient and directed syntheses for such compounds, which often have very aesthetic structures. These reactions are among the most important reaction types in modern organometallic chemistry. They can be a useful aid for the synthesis of tailor‐made compounds, for example, for models of catalytic processes and, specifically, for the construction of heterometallic compounds. We will discuss reactions of electrophilic complexes with nucleophilic ones, numerous transformations of (functionalized) hydrocarbons with metal complexes, the currently very topical complexes with bridging acetylide and carbide ligands, and organometallic polymers, which can be expected to have interesting and novel materials properties. ChisholmM. H. Chisholm, Polyhedron 1988, 7, 757–1077.
has described the importance of these complexes as follows: “Central to the development of polynuclear and cluster chemistry are bridging ligands and central to organometallic chemistry are metal–carbon bonds. Thus bridging ligands hold a pivotal role ins the development of Binuclear and polynuclear organometallic chemistry”.
Water‐Soluble Ligands, Metal Complexes, and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis
Authors: Herrmann Wolfgang A. , Kohlpaintner Christian W. 
Published: December 2003

AbstractRapid developments in the field of catalysis are leading to an increased demand for tailor‐made catalysts. Water‐soluble complex catalysts, which are being intensively investigated at the present time, combine the advantages of homogeneous and heterogeneous catalysis: simple and complete separation of the product from the catalyst, high activity, and high selectivity. From the large number of available water‐soluble ligands, the appropriate catalysts can be developed for many reactions. The industrial applications in the fields of hydrogenation and hydroformylation have already indicated the wide scope of this type of catalyst. In addition, the annual production of 300 000 tons of butyraldehyde through application of water‐soluble rhodium complexes at Hoechst AG in Oberhausen, Germany, has demonstrated the industrial importance of the concept of complex‐catalyzed reactions in aqueous two‐phase systems. The efficient operation of catalytic processes increasingly requires the loss‐free recycling of the noble metal catalyst, and this can be simply and economically realized in two‐phase systems. Special applications in biochemical problems open up developments in the field of water‐soluble transition metal complexes that far transcend the familiar kinds of homogeneous catalysis. In the near future, the investigation and application of metal complex catalysts that are compatible with the physiological, cheap, and environmentally friendly solvent, water, is likely to become a high priority in catalysis research.
X‐Ray Absorption Spectroscopy of Amorphous Solids, Liquids, and Catalytic and Biochemical Systems—Capabilities and Limitations
Authors: Bertagnolli Helmut , Ertel Teja S. 
Published: December 2003

AbstractAlthough the effect on which X‐ray absorption spectroscopy (EXAFS spectroscopy) is based has been known for about 70 years, up until about 20 years ago it had not found any application. The reasons for this were on the one hand the shortage of efficient X‐ray sources, and on the other hand the lack of a suitable theoretical description. With the advent of synchrotron radiation the situation has changed completely. Today, EXAFS spectroscopy is applied to a variety of very different problems and used more and more frequently to answer chemical questions. The extraordinary behavior of EXAFS spectroscopy that allows the determination of local structures in almost any mixture, independent of the sample's physical state as well as element‐specifically with high sensitivity, carries with it the danger that the amount of the obtainable structural information will be overestimated. A realistic estimate of the capability of EXAFS spectroscopy has in the meantime been achieved for crystalline materials, but not for amorphous materials. In this case the viewpoints range from complete rejection to overinterpretation of the EXAFS spectrum. In this article an attempt is made to show how EXAFS spectroscopy can be applied in the investigation of noncrystalline materials. First, the measurement techniques, data analysis procedures, and the simulation of EXAFS spectra are explained. Starting with the simplest systems, the amorphous metals and nonmetals, and using representative but not necessarily current examples, an explanation is given of the structural information provided by EXAFS spectroscopy, of how the local environment about the two components can be determined in binary systems, and of how even the crystallization process and structural changes can be investigated in chemical reactions. Thereafter, EXAFS studies on solutions, molecular liquids, and melts are presented. The final section is concerned with typical applications of EXAFS spectroscopy, the investigation of homogeneous and heterogeneous catalysts as well as the elucidation of active sites in enzymes.
[Zr3Cp2(O2CNiPr2)6(μ‐O)(μ‐CCO)], the First Crystallographically Established Ketenylidene Complex; A Model for CO Reductive Cleavage on Metal Surfaces
Authors: Calderazzo Fausto , Englert Ulli , Guarini Alessandro , Marchetti Fabio , Pampaloni Guido , Segre Annalaura 
Published: December 2003

AbstractFor corrigendum see DOI:10.1002/anie.199508261
Fluorinated Isocyanides—More than Ligands with Unusual Properties
Authors: Lentz Dieter 
Published: December 2003

AbstractThe substitution of hydrogen by fluorine in organic compounds usually results in drastic changes in their properties. For isocyanides, for which fluorinated examples have only recently become available in preparative quantities, this substitution leads to a significantly increased reactivity and a tendency to polymerize, which, on one hand, makes their handling more difficult. On the other hand, this high reactivity makes the fluorinated isocyanides useful building blocks for the synthesis of compounds like N‐trimethylformamide. Energetically favorable π* orbitals bestow excellent π‐acceptor properties towards low‐valent transition metal complexes, especially on the ligand trifluoromethyl isocyanide. The pronounced tendency of this ligand to bridge two metal atoms enables the formation of structural types that are not accessible with other π‐acceptor ligands. Thus it was possible to prepare [(Os3(CO)11(μ2‐CNCF3)2] (a) derivative of the hypothetical [Os(CO)13]) which may be considered as a model for an associative mechanism of ligand substitution at carbonyl clusters. In contrast to the well‐studied chemistry of trifluoromethyl isocyanide, that of the few other known fluorinated isocyanides is only now receiving attention. In particular the only recently synthesized trifluorovinyl isocyanide promises a rich chemistry as a result of its difunctionality.
Gas‐Phase Synthesis of Reactive Molecules Using Adsorbed Reagents
Authors: Billups W. E. , McCord Dianne J. 
Published: December 2003

AbstractBase‐induced elimination reactions have found wide application in the synthesis of strained alkenes. The method is often complicated by a competing process in which the incipient alkene is trapped by nucleophilic addition of the base. Recent studies have shown that the bi‐molecular side reactions can be avoided when bases such as potassiumtert‐but‐oxide are supported on an inert surface like silica gel and the reaction is carried out in vacuo. This vacuum gas‐solid reaction (VGSR) procedure has the additional advantage that very unstable species may be isolated under conditions that allow detailed studies of their physical and chemical properties. The scope of this procedure has been extended to include the use of fluoride salts supported on glass helices to effect the elimination of β‐halosilanes. The fluoride route to strained alkenes is particularly attractive since the reaction may be carried out at very moderate temperatures and the starting materials are readily available. This review describes the development of this vacuum gas‐phase procedure and its application in the synthesis and characterization of small‐ring alkenes and bicycloalkenes, including methylene‐cyclopropene, spiropentadiene, bicyclo‐propenyls, 1,2‐ and 1,3‐bridged cyclo‐propenes, and other simple cyclo‐propenes.
Organotin sulfides as precursors for nanometric γ‐Sn2S3 powders: a study by 119Sn Mössbauer spectroscopy and X‐ray powder diffraction
Authors: Porto A. O. , de Lima G. M. , Pereira A. G. , Batalha L. A. R. , Ardisson J. D. 
Published: December 2003

AbstractOrganotinsulfides Sn4S4R6 (R = methyl, n‐butyl and phenyl) were employed as single‐source precursors for nanometric tin sulfides. The residues obtained after pyrolysis in hydrogen, nitrogen and oxygen were characterized by X‐ray diffraction, 119Sn Mössbauer spectroscopy, scanning electron microscopy and X‐ray electron probe microanalysis. The results clearly showed the formation of tetragonal SnO2 (rutile‐type structure) in oxygen and pure phase orthorhombic γ‐Sn2S3 in nitrogen when Sn4Bu4S6 (2) was employed as precursor. The 119Sn Mössbauer spectroscopic results were very important in the elucidation of the decomposition process. Compound 2 is the best starting material for the process. Copyright © 2004 John Wiley & Sons, Ltd.