bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synchrotron X‐ray and Neutron Diffraction Studies in Solid‐State Chemistry
Authors: Cheetham Anthony Kevin , Wilkinson Angus Paul 
Published: December 2003

AbstractNeutron diffraction studies, especially with powders, play an important role in structural solid‐state chemistry, making possible the precise determination of the location of light atoms, particularly hydrogen, and enabling a distinction to be made between certain neighboring elements in the periodic table that are difficult to distinguish in experiments with X‐rays. Neutron diffraction investigations also make a unique contribution in the area of magnetic structure determination. The availability of intense synchrotron X‐rays sources, however, is opening up new opportunities to the structural chemist, many of them complementary to the “traditional” strengths of neutron methods. The key features of synchrotron radiation in relation to structural studies are the wavelength tunability, which facilitates the use of resonant diffraction methods, and the high brightness and excellent vertical collimation of the source, which make possible the construction of diffractometers with unparalleled angular and spatial resolution. The following types of experiments are now possible with synchrotron X‐ray diffraction: (1) The ab initio determination of structures from powder diffraction data. (2) The differentiation between different oxidation states of an element (valence contrast experiments) based upon the sensitivity of an absorption edge to the valence of the element in question. (3) The differentiation of elements adjacent to each other in the periodic table, which is now feasible with synchrotron X‐rays for all elements beyond chromium. (4) Site‐selective X‐ray absorption spectroscopy. (5) The study of cation occupancies in materials where more than one element occupies a site that is, or may be, partially occupied. (Such problems are important in zeolite chemistry and high‐temperature superconductors.) (6) The determination of crystal structures from microcrystals. (7) In situ and rapid, time‐resolved diffraction studies. This review examines the roles played by X‐ray and neutron diffraction studies in modern solid‐state chemistry, and describes some recent examples in which the use of neutron radiation or synchrotron X‐rays has been advantageous.
Marine Metabolites and Metal Ion Chelation: The Facts and the Fantasies
Authors: Michael Joseph P. , Pattenden Gerald 
Published: December 2003

AbstractThe marine environment is a seemingly inexhaustible treasury of organisms whose secondary metabolites bear witness to the lavishness and inventiveness with which nature is able to manipulate molecular architecture. But to what purpose are these diverse and often grotesque compounds produced? This review is founded on the premise that some of them may be involved in the uptake and transport of metal ions present in the aquatic milieu. Many metabolites produced by terrestrial organisms are known to act as ionophores, but the case for similar behavior by their marine counterparts is far hazier. Notwithstanding the relative abundance of certain metal ions in the oceans, and of metabolite structures possessing features that should facilitate the chelation of metal ions, few attempts to establish a connection between these two phenomena have been reported. We have whittled down the voluminous literature of natural products derived from marine sources to expose a core of observations and speculations germane to our premise. These facts and fantasies are evaluated in this review. A mere handful of metal‐containing complexes has actually been isolated; furthermore, attempts to prepare such complexes in vitro are rare, and spectroscopic evidence for metal–metabolite interactions, whether in vivo or in vitro, is not common. Only with the vanadium‐sequestering tunichromes does a logical (but by no means complete) picture begin to emerge. In several other cases, the plausibility of metal chelation, though mooted by authors, remains unsupported by experimental evidence. However, continuing efforts to obtain structural, and particularly conformational, information on the metabolites by means of X‐ray crystallography, nuclear magnetic resonance spectroscopy, and molecular mechanics calculations would seem to provide the key to a rational approach to this neglected topic. On the basis of recent studies dealing with such structural aspects, we present a selection of candidate compounds, some of which are the targets of our own synthetic attentions, whose potential for binding to metal cations merits further research.
Defect Chemistry: Composition, Transport, and Reactions in the Solid State; Part I: Thermodynamics
Authors: Maier Joachim 
Published: December 2003

AbstractWithin the last few decades—though its foundations were laid over 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 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 synthesis and sintering of solid materials, problems of rock formation during the earth's evolution, the mechanisms of gas sensors and high temperature fuel cells, the performance of photosensitive 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 departure from the Dalton composition in the equilibrium state, but also, together with the mobility as the kinetic parameter, is the key parameter concerning the rates of physicochemical processes. Accordingly, in this first part of the review the emphasis will be on the equilibrium thermodynamics of point defects, whereas the second part will be specifically concerned with the kinetic aspects. Both parts will emphasize the fact that the defect chemistry of solids, as well as being the counterpart of solution chemistry in the liquid state, also provides a unifying approach in which electronic and ionic charge carriers are treated by analogous methods, both in the bulk and in boundary layers, and allows diffusion to be incorporated naturally into the overall kinetics of reactions as an elementary chemical process.
Dynamic Solvent Effects on Electron‐Transfer Reactions
Authors: Heitele Hans 
Published: December 2003

AbstractElectron‐transfer processes in solution are among the most important reactions in chemistry and biology. The huge number of redox reactions of transition metal ions and complexes, many preparatively important oxidations and reductions of organic compounds, photosynthesis, and metabolism are only a few examples where electron‐transfer reactions play a pivotal role. This ubiquity, as well as their relative simplicity, makes them excellent models for the study on a molecular level of chemical reactions in solution. A particularly important question in chemical reaction dynamics in solution is the influence of the solvent on the reaction rate. In this context one distinguishes between static and dynamic solvent effects. Static effects refer to the stabilization of reactants, transition state, and products, that is, how the solvent affects the free energies of these species and the energy of activation. This interpretation of solvent effects on all kinds of chemical reactions is well established. A more recent development is the investigation of the influence of solvent dynamics on the rate of a reaction. The transfer of an electron is usually thought to be triggered by a fluctuation of the dielectric polarization in the surrounding solvent. The dynamics of such fluctuations is determined by the finite response time of the orientational polarization of the solvent. Under certain conditions this dielectric response time can become the rate‐determining factor of the reaction. In this article I intend to give a review of these modern developments in the theory and experimental study of electron‐transfer processes. We shall see that solvent dynamics may lead to a whole plethora of phenomena in reaction dynamics. The concepts needed for their description are not limited to electron transfer but bear relevance to many other chemical reactions in solution.
Optical Spectroscopy of Single Impurity Molecules in Solids
Authors: Moerner W. E. , Basché Thomas 
Published: December 2003

AbstractOptical spectroscopy of a single impurity molecule provides the first truly local probe of host–guest interactions in doped solids. In conventional optical high‐resolution experiments with many molecules only ensemble averages of the microscopic parameters can be obtained. In the single‐molecule regime, the exquisite sensitivity of an individual dopant molecule to both the local environment and to external perturbations has been exploited in recent experiments to reveal a wealth of fascinating novel phenomena such as spectral diffusion in crystals and polymers, optical modification of a single impurity molecule (spectral hole‐burning), local field measurements at the site of a single impurity molecule, and quantum optics in a solid.
Poly(hydroxyalkanoates): A Fifth Class of Physiologically Important Organic Biopolymers?
Authors: Müller Hans‐Martin , Seebach Dieter 
Published: December 2003

AbstractAlong with polyisoprenoids, polypeptides, polysaccharides, and polynucleotides, Nature contains a further group of biopolymers, the poly(hydroxyalkanoates). The commonest member of this group, poly[(R)‐3‐hydroxybutyrate] P(3‐HB), had been identified by Lemoigne as early as the 1920s, as a storage substance in the microorganismBacillus megateriummade up of more than 12000 (3‐HB) units. However, the widespread distribution and significance of these biopolymers has only become clear recently. The work of Reusch, in particular, has shown that low molecular weight P(3‐HB) (100–200 3‐HB units) occurs in the cell membranes of prokaryotic and eukaryotic organisms. The function of P(3‐HB) in the latter sources is largely unknown; it has been proposed that a complex of P(3‐HB) and calcium polyphosphate acts as an ion channel through the membrane. Indeed, it has even been speculated that P(3‐HB) plays a role in transport of DNA through the cell wall. In the present article, the following subjects will be discussed: metabolism of P(3‐HB) and analogous polyesters in the synthesis and degradation of storage materials; P(3‐HB) as a starting material for chiral synthetic building blocks; synthesis of cyclic oligomers (oligolides) of up to ten 3‐HB units, and their crystal structure; high molecular weight bio‐copolymers of hydroxybutyrate and hydroxyvalerate (BIOPOL) as biologically degradable plastics; nonbiological production of polyhydroxyalkanoates from 3‐hydroxy carboxylic acids and the corresponding β‐lactones; specific synthesis of linear oligomers with a narrow molecular weight distribution, consisting of about 100 (R)‐3‐hydroxybutyrate units, by using an exponential coupling procedure; structure of the polyesters, and a comparison with other polymers; the experimental results which led to the postulation of a P(3‐HB) ion channel through the cell wall; modeling of P(3‐HB) helices of various diameters, by using the parameters obtained from the crystal structures of oligolides; formation of a crown ester complex and ion transport experiments with the triolide of 3‐HB. The article describes one example of the contributions that synthetic organic chemists can make to important biological problems in an interdisciplinary framework.
Capillary Electrophoresis: Methods and Scope
Authors: Engelhardt Heinz , Beck Wolfgang , Kohr Jörg , Schmitt Thomas 
Published: December 2003

AbstractCapillary electrophoresis is a new analytical technique that has seen a great upswing in recent years, because this adaptation of electrophoresis may be automated and simple, direct quantification is possible. As in conventional electrophoresis, samples with a large range of molecular weights, from inorganic ions to biopolymers such as DNA and proteins, can be separated. Uncharged molecules can also be separated when micelle‐forming detergents are added to the buffer; the distribution mechanism that then comes into play increases the separation efficiency of this micellar electrokinetic chromatography. Chiral additives such as cyclodextrins make the separation of enantiomers possible. The rapid increase in the popularity of capillary electrophoresis is reflected in instrument sales and the growing number of scientific publications dealing with the method, and it seems that the future of this technique is assured.
The Coordination Chemistry of Multidentate Isocyanide Ligands
Authors: Hahn F. Ekkehardt 
Published: December 2003

AbstractIn spite of the excellent ligation properties of isocyanides, until a few years ago there was only a small number of known multidentate ligands of this type. One of the reasons for this lack of interest, when compared to monodentate isocyanides, was the linear arrangement of the MCNR group, which usually inhibits the formation of mononuclear chelate complexes and leads to the formation of multinuclear or polymeric metal complexes. In these, the multidentate ligand acts in a monodentate fashion towards each metal atom. Only recently has a series of polyisocyanides with large ligand backbones been synthesized successfully. Bidentate isocyanides can bridge two metal atoms or react to give chelates with only one metal center. Tripodal ligands form mono‐ or binuclear complexes, in which the largest organometallic rings observed to date occur (up to 36 atoms). This class of ligands promises to be interesting for the synthesis of stable, diagnostically important technetium complexes of the type [Tc(CNR)6]+. There also appear to be applications for tripodal isocyanides in catalysis. A facial, chiral Cr(CNR*)3 unit might be able to catalyze the hydrogenation or isomerization of prochiral double bonds. It is even possible to bind triisocyanides with suitable backbones to carbonyl trimetal clusters, thereby stabilizing them, or making selective cluster formation possible. Coordinated isocyanides can be transformed readily into carbene ligands, which, in the future, could lead to complexes with polycarbene ligation.
Superelectrophiles
Authors: Olah George A. 
Published: December 2003

AbstractElectrophiles that are capable of further interaction (coordination, solvation) with strong Brønsted or Lewis acids can be activated in this way. The resulting enhancement of their reactivity is great compared to that of their parent compounds under conventional conditions and indicates superelectrophile formation (i.e., electrophiles with doubly electrodeficient (dipositive) nature whose reactivity substantially exceeds that of their parent electrophiles). As representative examples the protolytic or electrophilic activation of the following electrophiles in superacidic and related strongly electrophilic media is discussed: oxonium, carboxonium, sulfonium, selenonium, and telluronium ions; acyl cations; protonated CO, CO2, COS, and carbonic and thiocarbonic acids; heteroatom‐substituted carbocations (and some of their boron analogs); halonium ions; azonium (including nitronium) and carbazonium ions. Emphasis is placed on both experimental chemical investigations and theoretical treatment of the involved systems. As prototypes for protolytically activated onium ions, the protohydronium dication (diprotonated water) and its sulfur analogue, the protosulfonium dication, are of particular significance. The protoacetyl and protoformyl dication (diprotonated carbon monoxide), diprotonated carbonic acid and carbon dioxide, diprotonated hydrogen cyanide and nitriles, as well as the protonitronium dication among others—and their emerging chemistry are discussed as examples of superelectrophiles.
Solid‐State Chemistry with Nonmetal Nitrides
Authors: Schnick Wolfgang 
Published: December 2003

AbstractAmong the nonmetal nitrides, the polymeric binary compounds BN and Si3N4are of particular interest for the development of materials for high‐performance applications. The outstanding features of both substances are their thermal, mechanical, and chemical stability, coupled with their low density. Because of their extremely low reactivity, boron and silicon nitride are hardly ever used as starting materials for the preparation of ternary nitrides, but are used primarily in the manufacture of crucibles or other vessels or as insulation materials. The chemistry of ternary and higher nonmetal nitrides that contain electropositive elements and are thus analogous with the oxo compounds such as borates, silicates, phosphates, or sulfates was neglected for many years. Starting from the recent successful preparation of pure P3N5, a further binary nonmetal nitride which shows similarities with Si3N4 with regard to both its structure and properties, this review deals systematically with the solid‐state chemistry of ternary and higher phosphorus(V) nitrides and the relationship between the various types of structure found in this class of substance and the resulting properties and possible applications. From the point of view of preparative solid‐state chemistry the syntheses, structures, and properties of the binary nonmetal nitrides BN, Si3N4, and P3N5 will be compared and contrasted. The chemistry of the phosphorus(V) nitrides leads us to expect that other nonmetals such as boron, silicon, sulfur, and carbon will also participate in a rich nitride chemistry, as initial reports indeed indicate.
The Role of Chemistry in the Development of Boron Neutron Capture Therapy of Cancer
Authors: Hawthorne M. Frederick 
Published: December 2003

AbstractA therapeutic method that selectively destroys malignant cells in the presence of normal cells is a highly valued goal of oncologists and the possible salvation of cancer patients afflicted with some incurable forms of the disease. Selective cell destruction is, in principle, possible with a binary therapeutic strategy based upon the neutron capture reaction observed with the 10B nucleus and a neutron of low kinetic energy (thermal neutron). This nuclear fission reaction produces both 4He and 7Li+ nuclei along with about 2.4 MeV of kinetic energy and weak γ‐radiation. Since the energetic and cytotoxic product ions travel only about one cell diameter in tissue one may specify the cell type to be destroyed by placing innocent 10B nuclei on or within only the doomed cells. This article describes the current status of chemical research aimed at the eventual adoption of this therapeutic method (boron neutron capture therapy or BNCT). The multidisciplinary nature of this research effort involves chemistry, biology, nuclear physics, medicine, and related specialties. Methods devised for bringing 10B nuclei to tumor cells in therapeutic amounts are correlated with the structure of a generalized cell and the various cellular compartments available for boron localization. The synthesis methods employed for the creation of boron‐containing biomolecules and drugs are presented along with representative data concerning their efficacy in tumor localization. The outlook for BNCT is especially bright at this time because of rapid developments in the fields of bioorganometallic chemistry, microbiology, immunology, and nuclear science, to name but a few. Very effective boron delivery vehicles have been demonstrated, and through the interaction of chemistry and biology these species are undergoing further improvement and evaluation of their suitability for BNCT.
Classical and Nonclassical Methyleneboranes
Authors: Berndt Armin 
Published: December 2003

AbstractMethyleneboranes are stable compounds when their reactive CB bonds are shielded by sterically demanding substituents and the electron deficiency at the dicoordinate boron atom is reduced through delocalization of adjacent electron pairs. In the classical amino(methylene)boranes of Nöth and Paetzold, the electron deficiency at the dicoordinate boron atom is relieved through π‐π delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two‐center, two‐electron (2c–2e) bond. In nonclassical methyleneboranes, which are the focus of this review, the electrondeficient center at the dicoordinate boron atom forms nonclassical three‐center, two‐electron (3c2e) bonds with neighboring σ bonds. These methyleneboranes exhibit structural and spectroscopic properties similar to carbenium ions having strong σ–π delocalization (hyperconjugation, bridging) and to transition metal complexes having strong agostic interactions.
Electron Transfer Reactions in Chemistry: Theory and Experiment (Nobel Lecture)
Authors: Marcus Rudolph A. 
Published: December 2003

AbstractSince the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of other kinds of chemical reactions, presents to us an intriguing history, one in which many threads have been brought together. In this lecture, some history, recent trends, and my own involvement in this research are described.
How Nature Synthesizes Vitamin B12A Survey of the Last Four Billion Years
Authors: Scott A. Ian 
Published: December 2003

AbstractThis review contains an account of recent experimental results in the continuing saga of vitamin B12 biosynthesis (largely from the author's laboratory) as well as a personal view of future directions of research in natural product biosynthesis. The emphasis is on the powerful combination of molecular biology in the search for, and the expression of, the genes encoding the biosynthetic enzymes and state‐of‐the‐art spectroscopic techniques, in order to “view” the biochemical events as they take place in the NMR tube. As a logical development of these approaches, the feasibility of one‐flask, multienzyme synthesis of natural products is addressed.[Based on a lecture given at the 27th Euchem Conference on Stereochemistry, 26th April to 4th May, 1992. Bürgenstock, Switzerland. A more explicit account of this new field has been given in “Genetically Engineered Synthesis of Complex Natural Products” (A. I. Scott Terruhedron, 1992, 48, 2559).
] Implicit throughout is the profound change in the “tools of the trade” of the bioorganic chemist, thanks to the harnessing and exploitation of cloning techniques and the resultant availability of enzymes which can make carbon‐carbon bonds. It is also our desire to alert organic chemists, who may wish to take advantage of these technical developments, to the fact that they will be rewarded by a new world of natural catalysts capable of high yielding, synthetic chemistry often with a surprising, but welcome, lack of substrate specificity. Finally we hope to convey our enthusiasm for the methods now at the disposal of present and future generations of chemists for studying Nature's synthetic routes to complex natural products.