Showing papers in "ChemInform in 2003"

Handbook of metathesis

Abstract: Preface.CATALYST DEVELOPMENTS.Introduction.The Role of the "Tebbe Complex" in Olefin Metathesis.The Discovery and Development of High Oxidation State Mo and W Imido Alkylidene Complexes for Alkene Metathesis.From ill-defined to well-defined W alkylidene complexes.Discovery of Well-defined Ruthenium Olefin Metathesis Catalysts.Synthesis of Ruthenium Carbene Complexes.ynthesis of Rhodium and Ruthenium Carbene Complexes with a 16-Electron Count.Mechanism of Ruthenium-Catalyzed Olefin Metathesis Reactions.Intrinsic Reactivity of Ruthenium Carbenes.The Discovery and Development of High Oxidation State Alkylidyne Complexes for Alkyne Metathesis.Well-defined Metallocarbenes and Metallocarbynes Supported on Oxide Support prepared via Surface Organometallic Chemistry.APPLICATIONS IN ORGANIC SYNTHESIS.Introduction.General Ring-Closing Metathesis.Catalytic Asymmetric Olefin Metathesis.Tandem RCM.Ene-Yne Metathesis.Ring Opening Cross Metathesis.Ring Expansion Metathesis Reactions.Olefin Cross-Metathesis.The Olefin Metathesis Reaction in Complex Molecule Construction.Applications of Ring Closing Metathesis to Alkaloid Synthesis.Radicicol and the Epothilones: Total Synthesis of Novel Anti Cancer Agents Using Ring Closing Metathesis.The Use of Olefin Metathesis in Combinatorial Chemistry: Supported and Chromatography-Free Syntheses.Metal-Catalyzed Olefin Metathesis in Metal Coordination Spheres.Alkyne Metathesis.Metathesis of silicon-containing olefins.Commercial Applications of Ruthenium Metathesis Processes.POLYMER SYNTHESIS.Introduction.Living Ring-Opening Olefin Metathesis Polymerization.Synthesis of Copolymers.Conjugated polymers.Stereochemistry of ROMP.Syntheses and Applications of Bioactive Polymers Generated by the Ring-Opening Metathesis Polymerization (ROMP).Metathesis Polymerization: A Versatile Tool for the Synthesis of Surface-Functionalized Supports and Monolithic Materials.Telechelic Polymers from Olefin Metathesis Methodologies.ADMET Polymerization.Acyclic Diyne Metathesis Utilizing in Situ Transition Metal Catalysts: An Efficient Access to Alkyne-Bridged Polymers.Polymerization of Substituted Acetylenes.Commercial Applications.

The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

Abstract: The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...

Shape‐Controlled Synthesis of Gold and Silver Nanoparticles.

Abstract: Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-crystalline nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technologically important because of the strong correlation between these parameters and optical, electrical, and catalytic properties.

Super‐Hydrophobic Surfaces: From Natural to Artificial

Abstract: Super-hydrophobic surfaces, with a water contact angle (CA) greater than 150degreesC, have attracted much interest for both fundamental research and practical applications. Recent studies on lotus and rice leaves reveal that a super-hydrophobic surface with both a large CA and small sliding angle (alpha) needs the cooperation of micro- and nanostructure, and the arrangement of the microstructures on this surface can influence the way a water droplet tends to move. These results form the natural world provide a guide for constructing artificial super-hydrophobic surfaces and designing surfaces with controllable wettability. Accordingly, super-hydrophobic surfaces of polymer nanofibers and differently patterned aligned carbon nanotube (ACNT) films have been fabricated.

The Ecology of Arsenic

Abstract: Arsenic is a metalloid whose name conjures up images of murder. Nonetheless, certain prokaryotes use arsenic oxyanions for energy generation, either by oxidizing arsenite or by respiring arsenate. These microbes are phylogenetically diverse and occur in a wide range of habitats. Arsenic cycling may take place in the absence of oxygen and can contribute to organic matter oxidation. In aquifers, these microbial reactions may mobilize arsenic from the solid to the aqueous phase, resulting in contaminated drinking water. Here we review what is known about arsenic-metabolizing bacteria and their potential impact on speciation and mobilization of arsenic in nature.

Solid state gas sensors: state of the art and future activities

Abstract: This paper gives an overview about the principles and the technologies used in solid-state gas sensors. These devices work by measuring a physical property changed by adsorption/desorption processes and chemical reactions on the surface of a sensing element, i.e. a solid-state film of a gas-sensitive material. Some of the most used types of solid state gas sensors are here described together with novel sensor technologies in development for commercial exploitation in the future.

Flavonoids: Promising Anticancer Agents

Abstract: Flavonoids are polyphenolic compounds that are ubiquitously in plants. They have been shown to possess a variety of biological activities at nontoxic concentrations in organisms. The role of dietary flavonoids in cancer prevention is widely discussed. Compelling data from laboratory studies, epidemiological investigations, and human clinical trials indicate that flavonoids have important effects on cancer chemoprevention and chemotherapy. Many mechanisms of action have been identified, including carcinogen inactivation, antiproliferation, cell cycle arrest, induction of apoptosis and differentiation, inhibition of angiogenesis, antioxidation and reversal of multidrug resistance or a combination of these mechanisms. Based on these results, flavonoids may be promising anticancer agents.

Plant Metabolomics: Large-Scale Phytochemistry in the Functional Genomics Era.

Abstract: Metabolomics or the large-scale phytochemical analysis of plants is reviewed in relation to functional genomics and systems biology. A historical account of the introduction and evolution of metabolite profiling into today's modern comprehensive metabolomics approach is provided. Many of the technologies used in metabolomics, including optical spectroscopy, nuclear magnetic resonance, and mass spectrometry are surveyed. The critical role of bioinformatics and various methods of data visualization are summarized and the future role of metabolomics in plant science assessed.

Taking Advantage of Disorder: Amorphous Calcium Carbonate and Its Roles in Biomineralization

Abstract: Amorphous calcium carbonate (ACC) in its pure form is highly unstable, yet some organisms produce stable ACC, and cases are known in which ACC functions as a transient precursor of more stable crystalline aragonite or calcite. Studies of biogenic ACC show that there are significant structural differences, including the observation that the stable forms are hydrated whereas the transient forms are not. The many different ways in which ACC can be formed in vitro shed light on the possible mechanisms involved in stabilization, destabilization, and transformation of ACC into crystalline forms of calcium carbonate. We show here that ACC is a fascinating form of calcium carbonate that may well be of much interest to materials science and biomineralization.

Carbon Nanotubes: Synthesis, Integration, and Properties

Abstract: Synthesis of carbon nanotubes by chemical vapor deposition over patterned catalyst arrays leads to nanotubes grown from specific sites on surfaces. The growth directions of the nanotubes can be controlled by van der Waals self-assembly forces and applied electric fields. The patterned growth approach is feasible with discrete catalytic nanoparticles and scalable on large wafers for massive arrays of novel nanowires. Controlled synthesis of nanotubes opens up exciting opportunities in nanoscience and nanotechnology, including electrical, mechanical, and electromechanical properties and devices, chemical functionalization, surface chemistry and photochemistry, molecular sensors, and interfacing with soft biological systems.

NMR Spectroscopy Techniques for Screening and Identifying Ligand Binding to Protein Receptors.

Abstract: Binding events of ligands to receptors are the key for an understanding of biological processes. Gaining insight into protein-protein and protein-ligand interactions in solution has recently become possible on an atomic level by new NMR spectroscopic techniques. These experiments identify binding events either by looking at the resonance signals of the ligand or the protein. Ideally, both techniques together deliver a complete picture of ligand binding to a receptor. The approaches discussed in this review allow screening of compound libraries as well as a detailed identification of the groups involved in the binding events. Also, characterization of the binding strength and kinetics is possible, competitive binding as well as allosteric effects can be identified, and it has even been possible to identify ligand binding to intact viruses and membrane-bound proteins.

Bond Dissociation Energies of Organic Molecules

Abstract: In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

Soluble Carbon Nanotubes

Abstract: Carbon nanotubes have attracted great interdisciplinary interest because of their unique structure and properties. However, carbon-nanotube research is challenged by several problems, such as: i) mass production of material, ii) control of length, diameter, and chirality, and iii) manipulation for use in diverse technological fields. Issues regarding the synthesis and purification as well as the functionalization and solubilization of carbon nanotubes are relevant topics in this rapidly growing field. In this paper, covalent and noncovalent approaches to functionalized and solubilized nanotubes are examined in detail, with particular emphasis on the change of properties that accompany the chemical modification.

Low‐Temperature Wafer‐Scale Production of ZnO Nanowire Arrays.

Abstract: Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.

Metal‐Organic Frameworks

Abstract: Metal-organic frameworks are a recently-identified class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands, and are a new development on the interface between molecular coordination chemistry and materials science. A range of novel structures has been prepared which feature amongst the largest pores known for crystalline compounds, very high sorption capacities and complex sorption behaviour not seen in aluminosilicate zeolites. The development of synthetic approaches to these materials and investigations of their properties are reviewed.

The Chalcone Synthase Superfamily of Type III Polyketide Synthases

Abstract: This review covers the functionally diverse type III polyketide synthase (PKS) superfamily of plant and bacterial biosynthetic enzymes. from the discovery of chalcone synthase (CHS) in the 1970s through the end of 2001. A broader perspective is achieved by a comparison of these CHS-like enzymes to mechanistically and evolutionarily related families of enzymes, including the type I and type II PKSs, as well as the thiolases and beta-ketoacyl synthases of fatty acid metabolism. As CHS is both the most frequently occurring and best studied type III PKS, this enzyme's structure and mechanism is examined in detail. The in vivo functions and biological activities of several classes of plant natural products derived from chalcones are also discussed. Evolutionary mechanisms of type III PKS divergence are considered, as are the biological functions and activities of each of the known and functionally divergent type III PKS enzymc families (currently twelve in plants and three in bacteria). A major focus of this review is the integration of information from genetic and biochemical studies with the unique insights gained from protein X-ray crystallography and homology modeling. This structural approach has generated a number of new predictions regarding both the importance and mechanistic role of various amino acid substitutions observed among functionally diverse type III PKS enzymes.

Property Distributions: Differences Between Drugs, Natural Products, and Molecules from Combinatorial Chemistry.

Abstract: The differences between three different compound classes, natural products, molecules from combinatorial synthesis, and drug molecules, were investigated. The major structural differences between natural and combinatorial compounds originate mainly from properties introduced to make combinatorial synthesis more efficient. These include the number of chiral centers, the prevalence of aromatic rings, the introduction of complex ring systems, and the degree of the saturation of the molecule as well as the number and ratios of different heteroatoms. As drug molecules derive from both natural and synthetic sources, they cover a joint area in property space of natural and combinatorial compounds. A PCA-based scheme is presented that differentiates the three classes of compounds. It is suggested that by mimicking certain distribution properties of natural compounds, combinatorial products might be made that are substantially more diverse and have greater biological relevance.

Intercalation Chemistry of Layered Double Hydroxides: Recent Developments and Applications

Abstract: Layered double hydroxides (LDHs) have been investigated for many years as host materials for a range of anion exchange intercalation reactions. In this role they have been used extensively as ion-exchange materials, catalysts, sorbents and halogen absorbers. More recently, there have been a tremendous number of new developments using these materials to store and deliver biologically active materials in vivo. Significant advances have been made recently on the characterisation of these materials, including structural studies and on the mechanism of intercalation using in situ techniques.

Chemical Synthesis of Magnetic Nanoparticles

Abstract: Recent advances in the synthesis of various magnetic nanoparticles using colloidal chemical approaches are reviewed. Typically, these approaches involve either rapid injection of reagents into hot surfactant solution followed by aging at high temperature, or the mixing of reagents at a low temperature and slow heating under controlled conditions. Spherical cobalt nanoparticles with various crystal structures have been synthesized by thermally decomposing dicobalt octacarbonyl or by reducing cobalt salts. Nanoparticles of Fe–Pt and other related iron or cobalt containing alloys have been made by simultaneously reacting their constituent precursors. Many different ferrite nanoparticles have been synthesized by the thermal decomposition of organometallic precursors followed by oxidation or by low-temperature reactions inside reverse micelles. Rod-shaped iron nanoparticles have been synthesized from the oriented growth of spherical nanoparticles, and cobalt nanodisks were synthesized from the thermal decomposition of dicobalt octacarbonyl in the presence of a mixture of two surfactants.

Miniaturized Gas Ionization Sensors Using Carbon Nanotubes.

Abstract: Gas sensors operate by a variety of fundamentally different mechanisms1,2,3,4,5,6,7,8,9,10,11,12,13,14. Ionization sensors13,14 work by fingerprinting the ionization characteristics of distinct gases, but they are limited by their huge, bulky architecture, high power consumption and risky high-voltage operation. Here we report the fabrication and successful testing of ionization microsensors featuring the electrical breakdown of a range of gases and gas mixtures at carbon nanotube tips. The sharp tips of nanotubes generate very high electric fields at relatively low voltages, lowering breakdown voltages several-fold in comparison to traditional electrodes, and thereby enabling compact, battery-powered and safe operation of such sensors. The sensors show good sensitivity and selectivity, and are unaffected by extraneous factors such as temperature, humidity, and gas flow. As such, the devices offer several practical advantages over previously reported nanotube sensor systems15,16,17. The simple, low-cost, sensors described here could be deployed for a variety of applications, such as environmental monitoring, sensing in chemical processing plants, and gas detection for counter-terrorism.