Showing papers in "ChemInform in 1994"

Multidimensional solid-state NMR and polymers

Abstract: Introduction. Principles of NMR of Organic Solids. High-Resolution NMR Techniques for Solids. Fourier Theory for 1D and 2D NMR. Short Review of Structure and Dynamics of Solid Polymers. Multidimensional Separation and Correlation. Polymer Dynamics: Multidimensional Exchange Experiments. Multidimensional Exchange NMR Above the Glass Transition. Multidimensional Spectra, Correlation Functions, and Stochastic Processes. Time-Domain Signals for Multidimensional Spectra. Multidimensional Exchange Spectra: Simulations, Models, Angle Distributions. Determination of Order in Polymers by Multidimensional NMR. Domain Sizes and Internuclear Distances from Spin Diffusion and Dipolar Couplings. Appendices. Chapter References. Index.

Characteristics of C-F Systems

Abstract: The physical properties and chemical reactivities of organic molecules can be dramatically affected by fluorination. Today’s diverse commercial applications of organo-fluorine materials clearly manifest the possible beneficial effects of fluorination, and the numerous ways in which industry has taken practical advantage of these effects are the subjects of the following chapters in this book. This chapter focuses on the characteristic substituent effects that underlie the physicochemical properties of organofluorine compounds, especially those important to the design of commercial products. Fortunately, advances in both experimental and theoretical physical organofluorine chemistry over the past two decades have made the “unusual” behavior of fluorinated compounds much more understandable and predictable. General principles that govern the characteristic effects of fluorination will be presented, but important exceptions also will be noted. Misleading generalizations like “fluorination increases lipophilicity” and myths about fluorine steric effects will be dispelled, for example.

Modeling the hydrogen bond

Abstract: Introduction: a brief history of the hydrogen bond, Douglas A. Smith. Part 1 Solvent and molecular electrostatic potential effects: computation of intermolecular interactions with a combined quantum mechanical and classical approach, Jiali Gao inclusion of explicit solvent molecules in a self-consistent-reaction field model of solvation, T.A. Keith and M.J. Frisch effect of hydrogen bonding on molecular electrostatic potential, M. Dominic Ryan competing intra- and intermolecular hydrogen bonds for organic solutes in aqueous solution, Peter I. Nagy et al. Part 2 Theoretical methods and graphical analysis: energetics and structure in model neutral, anionic and cationic hydrogen-bonded complexes - combined ab initio SCF/MP2 supermolecular, density functional and molecular mechanics investigation, Nohad Gresh et al SAM1 - general description and performance evaluation for hydrogen bonds, Steve Scheiner and Earl M. Evleth search for analytical functions to simulate proton transfers in hydrogen bonds, Steve Scheiner and Xiaofeng Duan effective fragment method for modeling intermolecular hydrogen-bonding effects on quantum mechanical calculations, Jan H. Jensen et al modeling the hydrogen bond with transferable atom equivalents, Curt M. Breneman et al analysis of hydrogen bonding and stability of protein secondary structures in molecular dynamics simulation, S. Vijayakumar et al unusual cross-strand hydrogen bonds in oligopurine.oligopyrimidine duplexes - computer graphics presentations of hydrogen bonds in DNA molecular dynamics simulation, Masayuki Shibata and Theresa Julia Zielinski STRIPS - an algorithm for generating two-dimensional hydrogen-bond topology diagrams for proteins, G. Ravishanker et al. Part 3 Applications to molecules and polymers. (Part contents)

Fungal Metabolites. Part 11. A Potent Immunosuppressive Activity Found in Isaria sinclairii Metabolite.

Abstract: A potent immunosuppressive activity was found in the culture broth of the fungus Isaria sinclairii (ATCC 24400). The metabolite, ISP-I ((2S,3R,4R)-(E)-2-amino-3,4-dihydroxy-2- hydroxymethyl-14-oxoeicos-6-enoic acid, myriocin = thermozymocidin) suppressed the proliferation of lymphocytes in mouse allogeneic mixed lymphocyte reaction, but had no effect on the growth of human tumor cell lines. It also suppressed the appearance of plaque-forming cells in response to sheep red blood cells and the generation of allo-reactive cytotoxic T lymphocytes in mice after intraperitoneal or oral administration. The metabolite was 10- to 100-fold more potent than cyclosporin A as an immunosuppressive agent of the immune response in vitro and in vivo, and appears to be a candidate for clinical application as a powerful immunosuppressant.

Competition between ligand centered and charge transfer lowest excited states in bis cyclometalated Rh 3+ and Ir 3+ complexes

Abstract: In this article the results of a detailed optical spectroscopic investigation of a series of related bis-cyclometalated Rh3+ and Ir3+ complexes of the general formula [M(C∩N)2N∩N]+, M=Rh3+, Ir3+ (HC∩N=2-phenylpyridine or 2-(2-thienyl) pyridine; N∩N-2,2′-bipyridine or ethylenediamine) are summarized. The nature of the lowest excited states of the compounds is discussed on the basis of their absorption, luminescence and luminescence line narrowing spectra in solutions and glasses at temperatures from 10 K to 300 K, whereas for the characterization of higher excited states single crystal absorption spectra and excitation spectra of polycrystalline samples are used. In the Rh3+ complexes the lowest excited states correspond to a 3π-π* transition localized on the cyclometalating ligands, whereas in the Ir3+ complexes, depending on the environment, either a 3π-π* or a metal to ligand charge transfer (3MLCT) excited state is lowest in energy. The pronounced solvatochromic and rigidochromic effects of the Ir3+ compounds are responsible for the reversal of the order of the lowest excited states. Mixing between the π-π* and MLCT excited states is reflected in the oscillator strengths, luminescence lifetimes, vibrational structure and zero field splittings. The phenomenon of dual luminescence is attributed to a large inhomogeneous distribution of sites in solution and glasses.

The Fenton Reagents

Abstract: Numerous transition metal ions and their complexes in their lower oxidation states (LmMn+) were found to have the oxidative features of the Fenton reagent, and, therefore, the mixtures of these metal compounds with H2O2 were named “Fenton-like” reagents. Using the Marcus theory and the experimental data in the literature, it is shown that in most cases the reaction of these metal complexes with H2O2 is unlikely to occur via an outer-sphere electron-tranfer mechanism. It is suggested that the first step in this process is the formation of a transient complex LmMH2O2n+, which may decompose to an OH radical or a higher oxidation state of the metal, LmM(n+2)+, or it may yield an organic free radical in the presence of organic substrates. Thus, the question whether free .OH radicals are being formed or not via the Fenton reaction depends on the relative rates of the decomposition reactions of the metal-peroxide complex and that of its reaction with organic substrates. Contradictory conclusions described from the study of different systems might only indicate that these relative rates are different in these systems.

Chemical Analysis of Inorganic and Organic Surfaces and Thin Films by Static Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS)

Abstract: By using mass spectrometry to analyze the atomic and molecular secondary ions that are emitted from a solid surface when bombarded with ions, one obtains detailed information about the chemical composition of the surface. A time-of-flight mass spectrometer is especially suitable for the analysis of secondary ions because of its high transmission, high mass resolution, and ability to detect ions of different masses simultaneously. By using a finely focused primary ion beam it is also possible to analyze microareas and generate surface images with a lateral resolution of 0.1 μm or less. Static time-of-flight secondary ion mass spectrometry (TOF-SIMS) allows monolayer imaging and local analysis of monolayers with high sensitivity, a wide mass range, high mass resolution, and high lateral resolution. Besides information on elements and isotopes, the technique yields direct information on the molecular level and can also be used to analyze surface species of high molecular mass that are thermally unstable and cannot be vaporized. The method can be applied to practically all types of materials and sample forms, including insulators in particular. In this article the basic principles of TOF-SIMS are explained, and its analytical capabilities for both large area and imaging applications are illustrated by examples. These include silicon surfaces (both uniform and structured), thermally unstable organic molecules on surfaces, synthetic polymers, and synthetically prepared molecular surface films, particles, and fibers. Emitted neutral particles can also be analyzed by postionization with a laser, and the possibilities of this technique are discussed.

Introduction to Solid-State NMR

Abstract: Solid-state NMR spectra generally reflect the sum of several independent interactions such as Zeeman, dipolar and quadrupolar coupling as well as magnetic shielding effects. To obtain the desired information, separation of these effects and their decoding is essential. Within the first part, as a basis of the successful separation of the NMR effects, the common and also the different (tensorial) properties of each interaction are discussed in detail. The second part mainly deals with the experimental design of solid-state high-resolution NMR experiments to suppress certain interactions and with wide-line experiments. Finally the principles of NMR imaging and NMR microscopy are described as an alternative method for investigating solids by NMR.

Flavor Compounds Formed During the Maillard Reaction

Abstract: The Maillard reaction is one of the most important routes to flavor compounds in cooked foods. The initial stages of the reaction involve the condensation of the carbonyl group of a reducing sugar with an amino compound, followed by the degradation of the condensation products to give a number of different oxygenated compounds. The subsequent stages of the Maillard reaction involve the interaction of these compounds with other reactive components such as amines, amino acids, aldehydes, hydrogen sulfide and ammonia. These additional reactions lead to many important classes of flavor compounds including furans, pyrazines, pyrroles, oxazoles, thiophenes, thiazoles and other heterocyclic compounds. The large number of different reactive intermediates that can be generated in the Maillard reaction gives rise to an extremely complex array of volatile products. This review discusses some of the reaction pathways by which the important aroma compounds of different cooked foods may be formed.

Conducting Tin Halides with a Layered Organic-based Perovskite Structure

Abstract: THE discovery1 of high-temperature superconductivity in layered copper oxide perovskites has generated considerable fundamental and technological interest in this class of materials. Only a few other examples of conducting layered perovskites are known; these are also oxides such as (La1-xSrx)n+1 MnnO3n+1 (ref. 2), Lan+1NinO3n+1 (ref. 3) and Ban+1PbnO3n+1 (ref. 4), all of which exhibit a trend from semiconducting to metallic behaviour with increasing number of perovskite layers (n). We report here the synthesis of a family of organic-based layered halide perovskites, (C4H9NH3)2(CH3NH3)n-1Snnl3n+1 which show a similar transition from semiconducting to metallic behaviour with increasing n. The incorporation of an organic modulation layer between the conducting tin iodide sheets potentially provides greater flexibility for tuning the electrical properties of the perovskite sheets, and we suggest that such an approach will prove valuable for exploring the range of transport properties possible with layered perovskites.

The Synthesis and Structure‐Activity Relationships of 5‐(3′‐Indolal)‐2‐ thiohydantoin Derivatives as Aldose Reductase Enzyme Inhibitors.

Abstract: A new series of 5-(3'-indolal)-2-thiohydantoin derivatives was synthesized and tested for the ability to inhibit bovine lens aldose reductase (AR) enzyme. The compounds were prepared by condensation of substituted indole-3-aldehyde derivatives with 2-thiohydantoin. The capacity of inhibiting the semi-purified bovine lens enzyme in vitro was observed for several of the compounds tested. One of them was found to be effective in reducing the enzyme activity compared with a corresponding well-known AR inhibitor.

Estimation of Heats of Formation of Organic Compounds by Additivity Methods

Abstract: In the present manuscript describing traditional, macroscopic thermochemical properties, the authors` language will be that of molecular structure. Enthalpies (or heats of formation) are the subject of this article, and since the most important practical application of enthalpies is to explore reactivities and/or equilibria, they take a kineticist`s perspective in answering the question, what is a ``sufficiently accurate`` prediction of an enthalpy of formation. In a general reaction, A + B {yields} C + D, a shift in {Delta}{sub r}H (enthalpy of reaction) of 1 kcal/mol will generally result in a change in the equilibrium constant, K{sub eq}, of exp({minus}500/T) where T is the temperature in Kelvins. At room temperature, this means a factor of over 5 in K{sub eq}; the difference between, say, 90% and 64% reaction yield. Or, in terms of the time required for reaction completion, it could also mean an increase of a factor of 5. This factor of 5 is the same whether the total enthalpy of reaction is 5 kcal/mol or 500 kcal/mol. Thus, while theoreticians have struggled to attain the stage where they can with pride calculate enthalpy quantities with 2--4 kcal/mol uncertainty, they are not solving the practical problems at hand.

The Biological Properties of Carotenoids.

Abstract: The biological properties of carotenoids can be divided into functions, which are essential to the well-being of organisms, and actions or associations, which are either responses to the administration of carotenoids, or are phenomena which still lack evidence of a causal relationship to the presence of carotenoids. Regardless of the strength of the evidence for these various biological properties, there must be either a physical or chemical basis that will serve to explain these phenomena. Many of the reported effects still require further investigations to help us understand the relationship between chemical properties and biological activity.

Nanophase Materials: Synthesis, Structure, and Properties

Abstract: In the past few years, atom clusters with average diameters in the range of 5–50 nm of a variety of materials, including metals and ceramics, have been synthesized by evaporation and condensation in high-purity gases followed by consolidation in situ under ultrahigh vacuum conditions to create nanophase materials. These new ultrafine-grained materials have properties that are often significantly different and considerably improved relative to those of their coarser-grained counterparts. The property changes result from their small grain sizes, the large percentage of their atoms in grain boundary environments, and the interactions between grains. Since their properties can be engineered during the synthesis and processing steps, cluster-assembled nanophase materials appear to have great technological potential beyond the current scientific interest in their grain-size dependent properties. Recent research on nanophase materials is reviewed and their future is considered.

Chemical Catalysis by Colloids and Clusters

Abstract: This review covers catalysis by metal clusters and colloids. Any system that is a continuous phase is considered. Specifically not covered will be supported catalysts or heterogeneous catalysts except as they serve as comparative examples. Polymer-stabilized clusters and colloids are considered if they are soluble. The review will roughly cover the period from 1980 to mid-1992. Essential to this review is the establishment of definitions for the terms cluster and colloid. There are certain intuitive definitions for cluster and colloid. Recent work discussed below shows that the terms cluster and colloid are not clearly distinguishable. This review uses cluster and colloid interchangeably. However, several studies describe methods for distinguishing homogeneous from heterogeneous catalysts where heterogeneous catalysts may include colloids. For purposes of this review, a heterogeneous catalyst is defined as one where the catalyst is a separate phase, usually a filterable solid from a liquid phase. The distinction between mononuclear or low nuclearity clusters and large clusters or colloids does not necessarily distinguish homogeneous from heterogeneous catalysts. Methods for distinguishing whether a reaction is catalyzed by a mononuclear species or one where metal-metal bonds are present are described. The paper discusses the following: structure of large clusters and small colloids;more » onset to metallic properties; mononuclear vs. cluster catalysis; hydrogenation; CO activation; hydroformylation; H-H and C-H activation; hydrosilylation; isomerization; oligomerization; redox reactions; photocatalysis; water activation; oxidation; electrochemistry and electroless metal deposition; and ammonia synthesis and related nitrogen chemistry. 402 refs.« less

Total synthesis of taxol.

Abstract: Taxol, a substance originally isolated from the Pacific yew tree (Taxus brevifolia) more than two decades ago, has recently been approved for the clinical treatment of cancer patients. Hailed as having provided one of the most significant advances in cancer therapy, this molecule exerts its anticancer activity by inhibiting mitosis through enhancement of the polymerization of tubulin and consequent stabilization of microtubules. The scarcity of taxol and the ecological impact of harvesting it have prompted extension searches for alternative sources including semisynthesis, cellular culture production and chemical synthesis. The latter has been attempted for almost two decades, but these attempts have been thwarted by the magnitude of the synthetic challenge. Here we report the total synthesis of taxol by a convergent strategy, which opens a chemical pathway for the production of both the natural product itself and a variety of designed taxoids.

Low‐Barrier Hydrogen Bonds and Enzymic Catalysis.

Abstract: Formation of a short (less than 2.5 angstroms), very strong, low-barrier hydrogen bond in the transition state, or in an enzyme-intermediate complex, can be an important contribution to enzymic catalysis. Formation of such a bond can supply 10 to 20 kilocalories per mole and thus facilitate difficult reactions such as enolization of carboxylate groups. Because low-barrier hydrogen bonds form only when the pKa's (negative logarithm of the acid constant) of the oxygens or nitrogens sharing the hydrogen are similar, a weak hydrogen bond in the enzyme-substrate complex in which the pKa's do not match can become a strong, low-barrier one if the pKa's become matched in the transition state or enzyme-intermediate complex. Several examples of enzymatic reactions that appear to use this principle are presented.