Showing papers in "ChemInform in 1996"

Rare-earth iron permanent magnets

Abstract: Introduction 1. Intrinsic magnetic properties 2. Phase diagrams 3. Gas-solid reactions 4. Coercivity 5. Microstructure and magnetic domains 6. Processing 7. Static applications 8. Magneto-mechanical devices 9. Permanent magnet motors 10. Actuators Index

Heats of Formation of Organic Free Radicals by Kinetic Methods

Abstract: It has long been recognized that the key intermediates for the majority of the reactions of organic molecules in the gas phase, and frequently in the liquid phase as well, are free radicals. An important prerequisite for the description of the behavior of such systems is their thermodynamic properties. At first glance it may be surprising that thermodynamic properties should have any applicability to transient species that are present in trace quantities and disappear in short times. Indeed, if the kinetic properties of such systems are known, then thermodynamic properties are not really necessary and can in fact be derived from the kinetics. In reality, however, kinetic properties may be unavailable or difficult to measure. Thermodynamic properties serve as limits for kinetics and more generally as a basis for the estimation and evaluation of kinetic information [1]. More directly, through the equilibrium constant, rate constants for the reverse direction can be directly calculated from that in the forward direction. There are other physical situations where local thermodynamic equilibrium turns out to be a satisfactory approximation and kinetic information is not important.

Quantum-Chemical Descriptors in QSAR/QSPR Studies

Abstract: Quantitative structure-activity and structureproperty relationship (QSAR/QSPR) studies are unquestionably of great importance in modern chemistry and biochemistry. The concept of QSAR/QSPR is to transform searches for compounds with desired properties using chemical intuition and experience into a mathematically quantified and computerized form. Once a correlation between structure and activity/property is found, any number of compounds, including those not yet synthesized, can be readily screened on the computer in order to select structures with the properties desired. It is then possible to select the most promising compounds to synthesize and test in the laboratory. Thus, the QSAR/QSPR approach conserves resources and accelerates the process of development of new molecules for use as drugs, materials, additives, or for any other purpose. While it is not easy to find successful structureactivity/property correlations, the recent exponential growth in the number of papers dealing with QSAR/ QSPR studies clearly demonstrates the rapid progress in this area. To obtain a significant correlation, it is crucial that appropriate descriptors be employed, whether they are theoretical, empirical, or derived from readily available experimental characteristics of the structures. Many descriptors reflect simple molecular properties and thus can provide insight into the physicochemical nature of the activity/ property under consideration. Recent progress in computational hardware and the development of efficient algorithms has assisted the routine development of molecular quantummechanical calculations. New semiempirical methods supply realistic quantum-chemical molecular quantities in a relatively short computational time frame. Quantum chemical calculations are thus an attractive source of new molecular descriptors, which can, in principle, express all of the electronic and geometric properties of molecules and their interactions. Indeed, many recent QSAR/QSPR studies have employed quantum chemical descriptors alone or in combination with conventional descriptors. Quantum chemistry provides a more accurate and detailed description of electronic effects than empirical methods.1 Quantum chemical methods can be applied to quantitative structure-activity relationships by direct derivation of electronic descriptors from the molecular wave function. In many cases it has been established that errors due to the approximate nature of quantum-chemical methods and the neglect of the solvation effects are largely transferable within structurally related series; thus, relative values of calculated descriptors can be meaningful even though their absolute values are not directly applicable.2 Moreover, electronic descriptors derived from the molecular wave function can be also partitioned on the basis of atoms or groups, allowing the description of various molecular regions separately. Most work employing quantum chemical descriptors has been carried out in the field of QSAR rather than QSPR, i.e. the descriptors have been correlated with biological activities such as enzyme inhibition activity, hallucinogenic activity, etc.3-6 In part this has been because, historically, the search for quantitative relationships with chemical structure started with the development of theoretical drug design methods. Quantum-chemical descriptors have also been reported to correlate the reactivity of organic compounds, octanol/water partition coefficients, chromatographic retention indices, and various physical properties of molecules.7-11 The present article reviews applications of quantum chemical descriptors in the development of QSAR/QSPR dealing with the chemical, physical, biochemical, and pharmacological properties of compounds.

A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery.

Abstract: A novel rechargeable Li/O{sub 2} battery is reported. It comprises a Li{sup +} conductive organic polymer electrolyte membrane sandwiched by a thin Li metal foil anode, and a thin carbon composite electrode on which oxygen, the electroactive cathode material, accessed from the environment, is reduced during discharge to generate electric power. It features an all solid state design in which electrode and electrolyte layers are laminated to form a 200 to 300 {micro}m thick battery cell. The overall cell reaction during discharge appears to be 2Li + O{sub 2} {yields} Li{sub 2}O{sub 2}. It has an open-circuit voltage of about 3 V, and a load voltage that spans between 2 and 2.8 V depending upon the load resistance. The cell can be recharged with good coulombic efficiency using a cobalt phthalocyanine catalyzed carbon electrode.

Domino Reactions in Organic Synthesis

Abstract: Introduction Cationic domino reactions Anionic domino reactions Radical domino reactions Pericyclic domino reactions Photochemically induced domino processes Transition metal catalysis Domino reactions initiated by oxidation or reduction Enzymes in domino reactions Multicomponent reactions Special techniques in domino reactions

Supramolecular Synthons in Crystal Engineering ‐ A New Organic Synthesis

Abstract: A crystal of an organic compound is the ultimate supermolecule, and its assembly, governed by chemical and geometrical factors, from individual molecules is the perfect example of solid-state molecular recognition. Implicit in the supramolecular description of a crystal structure is the fact that molecules in a crystal are held together by noncovalent interactions. The need for rational approaches towards solid-state structures of fundamental and practical importance has led to the emergence of crystal engineering, which seeks to understand intermolecular interactions and recognition phenomena in the context of crystal packing. The aim of crystal engineering is to establish reliable connections between molecular and supramolecular structure on the basis of intermolecular interactions. Ideally one would like to identify substructural units in a target supermolecule that can be assembled from logically chosen precursor molecules. Indeed, crystal engineering is a new organic synthesis, and the aim of this article is to show that rather than being only nominally relevant to organic chemistry, this subject is well within the mainstream, being surprisingly similar to traditional organic synthesis in concept. The details vary because one is dealing here with intermolecular interactions rather than with covalent bonds; so this article is divided into two parts. The first is concerned with strategy, highlighting the conceptual relationship between crystal engineering and organic synthesis and introduces the term supramolecular synthon. The second part emphasizes methodology, that is, the chemical and geometrical properties of specific intermolecular interactions.

The Interpretation of Collision‐Induced Dissociation Tandem Mass Spectra of Peptides

Abstract: The introduction of fast atom bombardment (FABJ ionization mass spectrometry (MSJ ( 1 ) in the early 1980s enabled mass spectrometrists to obtain molecular weights of many nonvolatile molecules, including p e p tides comprising 10-20 amino acids, and, by the mid-1980s. small proteins could be desorbed and their molecular weights measured (2-7). FAB (here used generically to include fast atom as well as ion bombardment of the sample dissolved in a liquid matrix such as glycerol), being a ”soft ionization” technique, produces little or no fragmentation of protonated peptide molecules. This characteristic is, certainly, desirable, because it allows for the unambiguous assignment of molecular weights, even in multicomponent mixtures. Nevertheless, fragmentation of a molecule, after ionization in the mass spectrometer, can be used to obtain structural information, often completely and uniquely, and for decades mass spectra of thousands of compounds have been acquired, interpreted, and cataloged. Peptides consisting of as few as several amino acids are nonvolatile and, often, thermally labile and cannot be introduced into the mass spectrometer in the gas phase without prior derivatization. Derivatized peptides have been successfully fragmented, primarily using electron impact (El) ionization, and their amino acid sequence determined (8-1 3). sometimes with the assistance of computer programs ( 1 4). Complex mixtures of protein enzyme digests, often followed by partial acid hydrolysis, were derivatized and introduced into the mass spectrometer after fractionation with a gas chromatograph (GC). The El spectra were used to obtain amino acid sequences for the peptides, which were subsequently combined to arrive at the sequence of the protein. Peptide fragments, which sometimes are produced during FAB ionization, can be correlated to the peptide amino acid sequence ( 15-20). The abundance of such fragment ions, however, is quite low compared with the ions due to the protonated molecule. In addition, in order to obtain reasonably abundant fragment ions, a fairly concentrated sample is required, and even then large peptides (e.9.. longer than 10 amino acids) do not fragment efficiently. Also, at lower mass (e.g., below m/z 500). fragment ions are usually difficult to identify, due to the FAB matrix background, and, of course, in mixtures more than one peptide usually fragments, thereby increasing the difficulty of fragment ion assignments to particular precursor ions. The advent of tandem mass spectrometry (MS/MS) (2 1-23) made it possible to analyze complex peptide mixtures (24-34) without the need to separate the individual components, because a mass window narrow enough to exclude all but one of the mixture components can be specified

Chemical Redox Agents for Organometallic Chemistry

Abstract: 1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

Filled Skutterudite Antimonides: A New Class of Thermoelectric Materials.

Abstract: A class of thermoelectric materials has been synthesized with a thermoelectric figure of merit ZT (where T is temperature and Z is a function of thermopower, electrical resistivity, and thermal conductivity) near 1 at 800 kelvin. Although these materials have not been optimized, this value is comparable to the best ZT values obtained for any previously studied thermoelectric material. Calculations indicate that the optimized material should have ZT values of 1.4. These ternary semiconductors have the general formula RM4X12 (where R is lanthanum, cerium, praseodymium, neodymium, or europium; M is iron, ruthenium, or osmium; and X is phosphorus, arsenic, or antimony) and represent a new approach to creating improved thermoelectric materials. Several alloys in the composition range CeFe4-xCoxSb12 or LaFe4-xCoxSb12 (0 < x < 4) have large values of ZT.

The Electrochemical Activation of Catalytic Reactions

Abstract: The use of electrochemistry to activate and precisely tune heterogeneous catalytic processes is a new development1-7 which originally emerged due to the existence of solid electrolytes. Depending on their composition, these specific anionic or cationic conductor materials exhibit substantial electrical conductivity at temperatures between 25 and 1000°C. Within this broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used as reversible in situ promoter donors or poison acceptors to affect the catalytic activity and product selectivity of metals deposited on solid electrolytes in a very pronounced, reversible, and, to some extent, predictable manner.

Synthesis and Antimicrobial Activity of Some 2,5-Disubstituted 1,3,4- Thiadiazole Derivatives.

Abstract: A series of 5-substituted 2-arylamino-1,3,4-thiadiazole derivatives was prepared. The antimicrobial activity of these compounds against some strains of bacteria and a strain of Candida albicans was determined, together with that of the corresponding thiosemicarbazone derivatives, which are intermediates in the synthetical procedure.

Biosurfactants: Recent Advances

Abstract: Surfactants find applications in a wide variety of industrial processes. Biomolecules that are amphiphilic and partition preferentially at interfaces are classified as biosurfactants. In terms of surface activity, heat and pH stability, many biosurfactants are comparable to synthetic surfactants. Therefore, as the environmental compatibility is becoming an increasingly important factor in selecting industrial chemicals, the commercialization of biosurfactant is gaining much attention. In this paper, the general properties and functions of biosurfactants are introduced. Strategies for development of biosurfactant assay, enhanced biosurfactant production, large scale fermentation, and product recovery are discussed. Also discussed are recent advances in the genetic engineering of biosurfactant production. The potential applications of biosurfactants in industrial processes and bioremediation are presented. Finally, comments on the application of enzymes for the production of surfactants are also made.

Synthesis and Applications of Small Molecule Libraries

Abstract: One of the initial steps in the development of therapeutic agents is the identification of lead compounds that bind to the receptor or enzyme target of interest. Many analogs of these lead compounds are then synthesized to define the key recognition elements for maximal activity. In general, many compounds must be evaluated in both the lead identification and optimization steps. Increasing burdens have been placed on these efforts due to the large number of new therapeutic targets that continue to be identified thorough modern molecular biology methods.1 To address this demand, very powerful chemical and biological methods have been developed for the generation of large combinatorial libraries of peptides2 and oligonucleotides3 that are then screened against a receptor or enzyme to identify high-affinity ligands or potent inhibitors, respectively. While these studies have clearly demonstrated the power of library synthesis and screening strategies, peptides and oligonucleotides generally have poor oral activities and rapid in vivo clearance;4 therefore their utility as bioavailable therapeutic agents is often limited. Due to the favorable pharmacokinetic properties of many small organic molecules (<600-700 molecular weight),5 the design, synthesis, and evaluation of libraries of these compounds6 has rapidly become a major frontier in organic chemistry. Lorin A. Thompson was born in Lexington, KY, in 1970. He received the Bachelor of Science degree from the University of North Carolina, Chapel Hill, in 1992 where he worked under the guidance of Joseph Desimone. He is currently pursuing his doctorate in the laboratory of Jonathan Ellman at UC Berkeley where he is the 1994 Glaxo-Wellcome fellow. His research interests include the development of synthetic methodology for organic library construction.

Preparation and Characterization of TiO2 Photocatalysts Supported on Various Rigid Supports (Glass, Quartz, and Stainless Steel). Comparative Studies of Photocatalytic Activity in Water Purification.

Abstract: In the present study TiO 2 has been supported on several rigid substrates. Deposition on glass and quartz was carried out by a dip coating procedure and the deposition on stainless steel by an electrophoretic deposition process. The resulting materials have been characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy. The materials were then tested for the photocatalytic degradation of malic acid. For this reaction, the sample supported on quartz showed the highest catalytic activity. The photocatalytic activity pattern as a function of the nature of the support (fused silica, glass, stainless steel) followed the decreasing order: TiO 2 /quartz > TiO 2 /steel ≈ TiO 2 /glassaphotolysis. This decline in activity has been correlated with the presence of cationic impurities (Si 4+ , Na + , Cr 3+ , Fe 3+ ) in the layer as a consequence of the necessary thermal treatments to improve the cohesion of the titania layer and its adhesion onto the support.