bluuebunny/crossref_metadata_2025 huggingface dataset:
Dependence of the Reactivity of Ag and Ni Clusters Deposited on Solid Substrates on the Cluster Size
Authors: Rao C. N. Ramachandra , Vijayakrishnan Venugopal , Santra Asok Kumar , Prins Menno W. J.
Published: December 2003

Bulk silver and nickel are less reactive than small clusters of these elements, as reactivity studies with O2 as well as with H2S and CO prove. The dissociation of O2 on small Ag clusters is favored, and the proportion of O2 molecules cleaved does not depend upon temperature, as is the case for larger clusters and silver metal. The change in reactivity is greatest at the metal–insulator transition—a finding that has implications for heterogeneous catalysis.
Interactions Between Acyclic and Cyclic Peralkylammonium Compounds and DNA
Authors: Schneider Hans‐Jörg , Blatter Thomas
Published: December 2003

Increments of 5 ± 1 kJmol−1 per salt bridge satisfactorily yield the energy of ionic interactions between polyamines and calf thymus DNA. This value lies in the same range as those of numerous inorganic and organic ion pairs. Peralkylated polyamines have almost the same affinity for DNA as protonated polyamines; hydrogen bonds thus have little effect on the binding. Relatively rigid azoniacyclophanes sometimes have a higher affinity although the arene units are not intercalated.
Scanning Tunneling Microscopy and Atomic Force Microscopy in Organic Chemistry
Authors: Frommer Jane
Published: December 2003

When Kekulé awoke from dreams of snakes biting their own tails, he didn't have the benefit of a scanning tunneling microscope (STM) image to confirm that his vision of benzene as a cyclic molecule was accurate. References to STM in the chemical literature increase steadily, although the technique was perhaps oversold in its early days of the 1980s, with such promises as DNA sequencing and tailored bi‐molecular chemical reactions (literally, two molecules). Publications alternate between attempting to explain the process by which images of traditional insulators are obtained and simply presenting the end images themselves as stunning views of atoms and molecules. While imaging mechanisms are still being debated, these instruments' ability to “see” single molecules has been established, albeit at the fringes of our expectations. For example, whereas STM studies at present might not be able to answer the question of why adsorption of CO doubles the density of platinum atoms on the surface of a single crystal of the metal, the images go far in illustrating that this is a process which platinum undergoes. As with any emerging analytical tool, these scanning, very localized microscopic methods are undergoing the growing pains of irreproducible results and mis‐marketed artifacts. Nonetheless, we assemble here, primarily for the uninitiated, a collection of careful and credible studies to mark the progress of scanning tunneling microscopy and atomic force microscopy into chemistry, and to encourage a healthy blend of idealism and skepticism toward future work.
On the Reaction of Pentamethylcyclopentadiene with Cobalt Atoms: A Reexamination
Authors: Schneider Jörg J.
Published: December 2003

[CpCo2] is still unknown. The compound reported a year ago as [Cp Co2] (Angew. Chem. Int. Ed. Engl. 1991, 103, 1124) is, in fact, [CpCo2H3]. [CpCo3H4], the major component of the eluate of the chromatography after removal of a mononuclear sandwich complex, was not identified at that time.
The Photochemistry of Stilbenoid Compounds and Their Role in Materials Technology
Authors: Meier Herbert
Published: December 2003

Stilbenes and compounds containing stilbene units in their structures form the material basis for numerous research projects in photophysics and photochemistry. Moreover, because these compounds are easy to synthesize and are thermally and chemically stable, they are taking on an increasingly prominent role in the area of materials science investigations into optical, electrical, and optoelectronic properties. In accordance with the interdisciplinary nature of such studies, this article aims to provide a bridge extending from molecular theory and photophysical measurements, through preparative applications, to material effects and their potential technical applications.
Cubanes: Starting Materials for the Chemistry of the 1990s and the New Century
Authors: Eaton Philip E.
Published: December 2003

The study of non‐natural products has led to a broad understanding of bonding and reactivity in organic chemistry. Many times, compounds thought impossible have been realized in the course of such studies. Cubane, a landmark in the world of “impossible” compounds, has been found to have a rich chemistry, full of the unexpected. The recent renaissance of cubane chemistry, triggered by potential applications of the system to the production of high‐energy fuels and the like, has led to many discoveries including the first methods for systematic substitution on strained, saturated systems and a new process for the metalation of arenes, ortho magnesiation. Reactive intermediates with exceptional bonding parameters have been uncovered and characterized including 1(9)‐homocubene, the most twisted olefin; cubene, the most pyramidalized olefin; cubyl cation, once the “least likely” cation; cubylmethyl radical, a saturated radical that rearranges on the picosecond timescale; and many other extraordinary species. There is certainly good reason to believe that future work in the cubane arena will be at least as productive (probably more so), and that it will help develop a deeper understanding of chemistry.
Ether‐Soluble Ti0 and Bis(η6‐arene)titanium(0) Complexes from the Reduction of TiCl4 with Triethylhydroborate
Authors: Bönnemann Helmut , Korall Barbara
Published: December 2003

The first preparation of bis (η6‐arene)titanium(0) complexes was achieved by reduction of TiCl4 with the borate [BEt3H]− in the appropriate arene solvent. In contrast the same reduction in THF leads to soluble “TiH2”, which loses H2 when dried to give a substance which can be formulated as Ti · 0.5 THF with various amounts of residual hydrogen. X‐ray photoelectron spectroscopy and EXAFS analysis support the assumed oxidation state of zero for the titanium.
Distortional Isomerism in Oxomolybdenum Systems; the Evidence Re‐evaluated
Authors: Bashall Alan P. , Bligh S. W. Annie , Edwards Andrew J. , McPartlin Mary , Gibson Vernon C. , Robinson Oliver B.
Published: December 2003

Two blue forms of the oxomolybdenum complex [Mo(O)Cl2(PMe2Ph)3] exist, and not one blue and one green or even only one blue form! The molecules of the two blue forms can be characterized and are deformation isomers in which the phosphane ligands are clearly differently oriented, and the MoO bonds have different lengths, which lead to stretching frequencies differing by about 12 cm−1 in the IR spectra of the solids. The lack of change in the IR spectrum for the isomer absorbing at ṽ = 943 cm−1 on cocrystallization with [MoCl3(PMe2Ph)3] suggests that a crystal packing effect cannot explain the presence of two forms.
Mössbauer Spectroscopy and Electronic Structure Calculations of the Nitridoferrates(III): Li3[FeN2] and Ba3[FeN3]
Authors: Jansen Nils , Spiering Hartmut , Gütlich Philipp , Schmidt Peter C. , Eyert Volker , Stahl Dagmar , Kniep Rüdiger , Kübler Jürgen
Published: December 2003

An unusually negative isomer shift δ is found in the Mössbauer spectrum of the nitridoferrate(III), Ba3[FeN3]. A quantitative analysis of the δ values and the quadrupole splittings for Ba3[FeN3], Li3[FeN2], and a number of other iron compounds by band structure calculations gave a remarkably good linear correlation between δ and the calculated valence electron density. The weakest nuclear shielding by the valence electrons was found for the 3s electrons in Ba3[FeN3], which explains the δ value of −0.55(2) mms−1 relative to α‐Fe.
Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules
Authors: Weller Horst
Published: December 2003

In 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

In 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

The 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

Carbohydrates 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

The 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

Within 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.
Reply: Standards of Demonstration for the Heterogeneous Photoreactions of N2 with H2O
Authors: Davies Julian A. , Edwards Jimmie G.
Published: December 2003

The synthesis of ammonia from nitrogen and water by irradiation in the presence of certain metal oxide powders as catalysts has been published a number of times since 1977 but was called into question last year by Edwards et al. (Angew. Chem. Int. Ed. Engl. 1992, 31, 480). Criticism of those arguments and the authors' reply should allow our readers to draw their own conclusions. This discussion is welcome in view of the importance of the reaction, whose apparently simple stoichiometry is even open to debate.
Aza‐ versus Phospha‐Substitution in Cyclobutadiene: A Theoretical Evaluation of Bonding Properties
Authors: Schoeller Wolfgang W. , Busch Thilo
Published: December 2003

Completely different effects on the stability of cyclobutadiene are found upon replacement of C with N or P. Whereas in the phospha derivatives the cyclobutadiene structure is less stable than the tetrahedrane, and the barrier to valence isomerization is reduced, the exchange of N for C increases this barrier, and the tetrahedranes are prone to fragmentation. According to the results of the ab initio calculations described here, not only the monoaza‐ and monophosphacyclobutadienes but also their analogues with two heteroatoms should, in principle, be experimentally accessible.
Transition Metal Complexation of σ Bonds
Authors: Crabtree Robert H.
Published: December 2003

The 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

Certain 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.
The Importance of the Rigidity of the Peptide Backbone for the Inhibitory Properties of BPTI Demonstrated by Semisynthetic Structural Variants
Authors: Groeger Christian , Wenzel Herbert R. , Tschesche Harald
Published: December 2003

“Backbone mutations” otherwise possible only in model peptides or small peptide hormones by total synthesis—drastically change the biological properties of the trypsin‐kallikrein inhibitor (BPTI) and can be selectively introduced into the contact region of the inhibitor by a simple semisynthesis. The derivatives produced in this way show impressively the difference between a substrateanalogous trypsin inhibitor and a trypsin substrate.
Methods for the Synthesis of μ‐Hydrocarbon Transition Metal Complexes without Metal–Metal Bonds
Authors: Beck Wolfgang , Niemer Burkhard , Wieser Michael
Published: December 2003

Transition 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. Chisholm

M. 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”.
Reply:
Authors: Daniel Hans‐Dieter
Published: December 2003

Is peer review working—that is, is it fair and does it identify significant papers? H. D. Daniel (Angew. Chem. Int. Ed. Engl. 1993, 32, 234) looked into this question in a study of the referee system of Angewandte Chemie that threw light on important aspects of this process. This further discussion between Ross and Daniel will therefore interest every reader.
[Cp*Ru(acac)] Is Not a Coordinatively Unsaturated, Stable 16‐Valence‐Electron Complex
Authors: Kölle U. , Raabe G.
Published: December 2003

The bent structure of [C5Me5Ru(acac)] in the solid state (Hacac = acetylacetone) arises from the interaction of the ruthenium center with the y‐carbon atom of the acac ligand of a second, symmetry‐equivalent molecule of [C5Me5Ru(acac)] and is not an intrinsic characteristic of the complex as claimed by Kölle et al. in 1990 in a communication in this journal. Reinterpretation of the X‐ray data provided these results.
The Solution of the Sherman Paradox or the Successful Search for the Huntington Gene
Authors: Winnacker Ernst‐L.
Published: December 2003

With the help of reverse genetics the gene whose changes cause Huntington chorea, also known as St. Vitus's dance, was identified from a 500 000 base‐pair sequence of the short arm of chromosome 4 several months ago. This rare hereditary disease usually breaks out between the ages of 35 and 50, and is characterized by disturbances of the motor and neural systems, and inevitably leads to death. The genetic reason appears to be repetitions of a certain base triplet; the greater their number, the earlier the disease breaks out. How this “molecular disease” was tracked down is described in an intelligible and absorbing way.
β‐Na3Hg: A Solid with a Quasi‐Fluid Sodium Substructure between 36 and 60°C
Authors: Deiseroth Hans‐Jörg , Rochnia Matthias
Published: December 2003

The higher melting component of β‐Na3Hg, namely the Na atoms, are mobile in the solid at relatively low temperature (36–60 °C) throughout the range of existence of this structural modification. In contrast, the Hg atoms show no unusual mobility. β‐Na3Hg crystallizes in a distorted rhombohedral Li3Bi‐type structure.
Water‐Soluble Ligands, Metal Complexes, and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis
Authors: Herrmann Wolfgang A. , Kohlpaintner Christian W.
Published: December 2003

Rapid 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

Although 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.
Homoleptic Noble Metal Carbonyl Cations**
Authors: Weber Lothar
Published: December 2003

Novelties and surprises from metal carbonyl chemistry: salts thermally stable up to 200 °C with [M(CO)2]+ (M = Ag, Au) and [M(CO)4]2+ ions (M = Pt, Pd), which show only negligible π‐backbonding, indicate that we need to revise our understanding of transition metal carbonyl interactions. Further investigations should reveal whether these salts with their unusual bonding also react differently from classical metal carbonyls.
Stable Highly Concentrated Fluorocarbon Gels
Authors: Krafft Marie‐Pierre , Riess J. G.
Published: December 2003

Highly viscous, transparent, and unchanged after one year of storage, concentrated gels with potential medical applications can be prepared from fluorocarbons, small amounts of water, and a fluorinated surfactant. These gels form polyhedral domains of fluorocarbon surrounded by thin shells of hydrated surfactant.
[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

For corrigendum see DOI:10.1002/anie.199508261
Synthetic DNA and Biology (Nobel Lecture)
Authors: Smith Michael
Published: December 2003

Most of the significant work has been summarized in a number of reviews and articles. In these there was, of necessity, a good deal of simplification and omission of detail…. With the passage of time, even I find myself accepting such simplified accounts. F Sanger[1]
Metalation of Tris(trimethylsilyl)‐ and Tris(dimethylphenylsilyl)methane with Methylsodium: The First Dialkylsodate
Authors: Al‐Juaid Salih S. , Eaborn Colin , Hitchcock Peter B. , Izod Keith , Mallien Michael , Smith J. David
Published: December 2003

A two‐coordinate sodium atom is present in the linear diorganosodate anion in 1, obtained by metalation of the relevant trisilylmethane with sodium in the presence of tmen (N,N,N′,N′‐tetramethylethylenediamine); the corresponding reaction with tris(dimethylphenylsilyl)methane in the absence of the chelating ligand yields 2, which has a distorted tetrahedral environment about the sodium atom and an unprecedented chain structure.
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Fluorinated Isocyanides—More than Ligands with Unusual Properties
Authors: Lentz Dieter
Published: December 2003

The 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

Base‐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.
Attempts to Carry Out Enantioselective Reactions in a Static Magnetic Field
Authors: Feringa Ben L. , Kellogg Richard M. , Hulst Ron , Zondervan Charon , Kruizinga Wim H.
Published: December 2003

They tried everything, but it still didn't work. Two research groups report here on their attempts to reproduce the results reported by Breitmaier et al. (Angew. Chem. Int. Ed. Engl. 1994, 33, 454). According to the subsequent statement by E. Breitmaier (see p. 1414 this issue), the claim of absolute asymmetric synthesis in a static homogeneous magnetic field is fraudulent.
Absolute Asymmetric Synthesis Solely under the Influence of a Static Homogeneous Magnetic Field?
Authors: Kaupp Gerd , Marquardt Thorsten
Published: December 2003

They tried everything, but it still didn't work. Two research groups report here on their attempts to reproduce the results reported by Breitmaier et al. (Angew. Chem. Int. Ed. Engl. 1994, 33, 454). According to the subsequent statement by E. Breitmaier (see p. 1414 this issue), the claim of absolute asymmetric synthesis in a static homogeneous magnetic field is fraudulent.
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

Organotinsulfides 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.
Crystallographic report: Aquadicylcohexyltin(IV) bis(2‐picolinate) ethanol solvate
Authors: Dakternieks Dainis , Duthie Andrew , Smyth Douglas R. , Stapleton Clynton P. D. , Tiekink Edward R. T.
Published: December 2003

The tin atom in the title compound is in a distorted pentagonal bipyramidal geometry defined by two sets of nitrogen and oxygen donors derived from the carboxylate ligands, two carbon atoms from the cyclohexyl substituents and an oxygen atom from the coordinated water molecule; CSnC 170.85(15)°. Extensive hydrogen bonding occurs in the lattice. Copyright © 2004 John Wiley & Sons, Ltd.
The effect of cholesterol on the adsorption of phenyltin compounds onto phosphatidylcholine and sphingomyelin liposome membranes
Authors: Gabrielska Janina , Przestalski Stanisław , Miszta Adam , Soczyńska‐Kordala Monika , Langner Marek
Published: December 2003

Organometallic compounds are widely spread in the human environment sometimes, causing a substantial health risk. Their amphiphilic character enables them to intercalate and penetrate cell membranes, potentially affecting various vital cell functions. Compound adsorption onto the membrane depends on the compound properties, as well as on the membrane composition and state. When adsorbing onto the lipidic surface, phenyltins localize at areas where lipid bilayer organization is compatible with compound spatial requirements. The lipid bilayer is a dynamic and laterally nonuniform structure with complex local and global architecture correlated with a variety of cell functions. The selective binding of a toxic compound to selected membrane areas may, therefore, interfere with some types of cellular process. We present experimental results concerning phenyltin adsorption onto the lipid bilayer surface measured with the fluorescent probe fluorescein‐PE. Model lipid bilayers were formed from lipid mixtures mimicking various plasma membrane regions. The adsorption of Ph3SnCl and P2SnCl2 onto the phosphatidylcholine–cholesterol bilayer was qualitatively different from sphingomyelin–cholesterol. The results presented indicate that phenyltins are likely to accumulate in areas containing phosphatidylcholine, outside of lipid rafts. Copyright © 2004 John Wiley & Sons, Ltd.
Crystallographic report: Dimeric tetraphenyl‐1‐hydroxo‐3‐trifluoromethanesulfonatodistannoxane, [Ph2(HO)SnOSn(O3SCF3)Ph2]2
Authors: Beckmann Jens , Dakternieks Dainis , Duthie Andrew , Mitchell Cassandra
Published: December 2003

The molecular structure of the title compound, obtained by an adventitious phenyl group cleavage of Ph3SnOSnPh3 with triflic acid, reveals discrete centrosymmetric units of [Ph2(HO)SnOSn(O3SCF3)Ph2]2 that are loosely associated via hydrogen bonding. Copyright © 2004 John Wiley & Sons, Ltd.
Characterization of synthetic and commercial trisiloxane surfactant materials
Authors: Bonnington L. S. , Henderson W. , Zabkiewicz J. A.
Published: December 2003

The organosilicone surfactant Silwet L‐77® (L‐77), used as an agrochemical adjuvant, is a mixture comprised predominantly of [(CH3)3SiO]2(CH3)Si(CH2)3(OCH2CH2)nOCH3 oligomers (n = 3–16, average n ≈ 7.5). The commercially available L‐77 mixture was purified by reversed‐phase high‐performance liquid chromatography (HPLC) to obtain individual trisiloxane surfactant components. Pure oligomers (n = 3, 6 and 9) were also synthesized. Synthesis was achieved by hydrosilylation of monomeric ethoxylate monomethyl ether starting reagents. Pure hexa‐ and nona‐ethylene glycols were produced by condensation of smaller oligomers.

Atmospheric‐pressure ionization mass spectrometry (MS) methods were used to characterize fully the commercial L‐77 product and synthesized or isolated components. The application of Fourier‐transform ion cyclotron resonance MS and online HPLC–electrospray ionization MS techniques to the analysis of this surfactant are described here. The application of these analytical techniques also enabled elucidation of the synthetic by‐products present in the commercial formulation.

In addition, physico‐chemical properties specific to agrochemical uses, such as droplet spread areas on plant foliage and surface tension for the different oligomer solutions, are also reported. Copyright © 2004 John Wiley & Sons, Ltd.
Synthesis of calix[4]arenes modified with germanium‐containing side chains on the upper rim
Authors: Sakurai Takayuki , Takeuchi Yoshito
Published: December 2003

With the expectation to prepare a new type of heteroditopic host, a series of calix[4]arenes modified with germanium‐containing side chains at the upper rim was prepared. Compounds 1 and 2 bear two or four triethylgermyl moieties respectively, and compounds 3 and 4 bear two or four trimethylgermyl moieties respectively. For 1 and 2, the hydroxy groups at the lower rim are either tosylated (series a) or kept free (series b). For reference, a carbon analog of 3, compound 5, was also prepared. Unfortunately, all calixarenes prepared failed to show any appreciable cation transport property, indicating that the structure designed, i.e. a calixarene with germanium‐containing side chains at the upper rim, cannot meet the requirement for an appropriate heteroditopic host. Copyright © 2004 John Wiley & Sons, Ltd.
111In–L–LDL and 153Gd–L–LDL as radiotracers for detection of AR4‐2J rat pancreatic tumors
Authors: Ratovo Ginette , Souchard Jean‐Pierre , Urizzi Pascale , Coulais Yvon , Nepveu Françoise , Hollande Etienne
Published: December 2003

Pancreatic cancer has an extremely poor prognosis, due, in part, to lack of methods for early diagnosis. The present study was designed to evaluate the potential of labeling low‐density lipoprotein (LDL) with a radionuclide using a lipid chelating agent, bis(stearylamide) of diethylenetriaminepentaacetic acid (L), to detect pancreatic tumors by gamma‐scintigraphy. Previous studies indicated that the difficulty of visualization of pancreatic tumors was due to their poor vascularization. This study compares the ability of two radiotracers, 111In–L–LDL and 153Gd–L–LDL to target highly vascularized rat pancreatic tumors (AR4‐2J) implanted in nude mice. Biodistribution studies showed that the tumor uptake of 111In–L–LDL and 153Gd–L–LDL tracers was twofold and fivefold higher respectively than with the controls (111In citrate and 153Gd citrate respectively). These tracers would thus be suitable for scintigraphic imaging.

We show here that LDL could be employed as a delivery system for tracers such as 111In or 153Gd when these two radionuclides are complexed by a lipid‐chelating anchor, and that 111In–L–LDL and 153Gd–L–LDL enabled better visualization of the pancreatic tumor tissues, with a better result with 153Gd–L–LDL. Copyright © 2004 John Wiley & Sons, Ltd.
Electrochemically reduced tungsten‐based active species as catalysts for cross‐metathesis reactions: cross‐metathesis of erucic acid with 2‐octene
Authors: Çetinkaya Sevil , Düz Bülent , İmamoǧlu Yavuz
Published: December 2003

The cross‐metathesis of erucic acid, (CH3(CH2)7CHCH(CH2)11COOH), with an excess of 2‐octene in the presence of an electrochemically produced tungsten‐based catalyst has been studied. Cross‐ and self‐hydrocarbon products, viz. 2‐undecene (C11), 6‐dodecene (C12) and 6‐pentadecene (C15), were detected. The influence of several parameters, such as the 2‐octene/erucic acid and 2‐octene/catalyst ratios and the reaction time, on the yield of the cross‐metathesis product, 6‐pentadecene, was studied. The cross‐metathesis of functionalized olefins in the presence of an Al–e−–WCl6–CH2Cl2 system has not been reported in the literature so far. The cross‐metathesis products in the presence of this catalyst system can be obtained with high yield and high specificity. Copyright © 2004 John Wiley & Sons, Ltd.
Synthesis and catalytic properties of 1‐alkyl‐2‐imidazolineruthenium(II) complexes
Authors: Özdemir İsmail , Çetinkaya Bekir , Demir Serpil , Çetinkaya Engin , Gürbüz Nevin , Çiçek Metin
Published: December 2003

New [RuCl2(imidazoline)(arene)] complexes have been prepared. The complexes were characterized by conventional spectroscopic methods and elemental analyses. Upon reaction with 1,1‐diphenylprop‐2‐ynol they generate catalyst precursors that can perform the cycloisomerization of diallyltosylamide into N‐tosyl‐α‐methylenepyrrolidine. Copyright © 2004 John Wiley & Sons, Ltd.