On the encapsulation of nickel clusters by molecular nitrogen

The structures and energetic effects of molecular nitrogen adsorbates on nickel clusters are investigated using an extended Huckel model coupled with two models of the adsorbate-nickel interaction. The potential parameters for the adsorbates are chosen to mimic experimental information about the binding strength of nitrogen on both cluster and bulk surface phases of nickel. The first model potential is a simple Lennard-Jones interaction that leads to binding sites in holes defined by sets of near-neighbor nickel atoms. The second model potential has a simple three-body form that forces the model nitrogen adsorbates to bind directly to single nickel atoms. Significant rearrangement of the core nickel structures are found in both models. A disconnectivity graph analysis of the potential energy surfaces implies that the rearrangements arise from low transition state barriers and the small differences between available isomers in the nickel core.     

Polymerization of isobutylene by the adamantane bromide–diethylaluminum chloride catalyst system

Isobutylene has been polymerized in a continuous stirred tank reactor using a catalyst system comprised of 1-bromoadamantane and diethylaluminum chloride. The polymerization was carried out in hexane solvent at ?15 to ?27°C and gave 100% conversion to polyisobutylene (PIB) of 1000–3000 M_w. The results of pyrolysis–gas chromatography–mass spectrometry analyses are consistent with a mechanism involving the formation of the adamantyl cabocation and its addition to an isobutylene molecule to initiate polymerization. ¹³C-NMR analyses show that the PIB products contain R₂C? CR₂ and R₂C? CHR olefin types. Information on the nature of these olefins and the route to their formation has been developed.     

Cocatalysis by epoxides in the polymerization of tetrahydrofuran with trityl hexachloroantimonate initiator

The polymerization of tetrahydrofuran (THF) with (C₆H₅)₃C⁺SbCl₆^- initiator is markedly accelerated by small concentrations of propylene oxide or other epoxides. Molar concentrations of propylene oxide 4 to 10 times those of the carbonium-ion salt showed increasing conversion to polymer. The equilibrium conversion level at different temperatures with epoxides is the same as in their absence; the approach to equilibrium is first-order in THF. NMR experiments in the presence of propylene oxide indicate the formation of a trityl ether intermediate. The cocatalysis effect is interpreted on the basis of an acceleration in the initiation process in the system.     

Convenient synthesis of NCS-chromophore metabolite isosteres: binding agents for bulged DNA microenvironments

A designed molecule with capacity to bind DNA bulges (20) has been prepared from readily available starting materials. The spirocyclic template was modeled on a metabolite of neocarzinostatin chromophore (NCSi-gb) and is equipped with functionality to enable convenient bioassay. Preliminary studies confirm binding at specific bulged sequences and induction of polymerase-mediated slippage events. The target compound offers a convenient means to study affinity for unique bulged motifs and for use as a molecular biology reagent.     

A study of some problems in determining the stoicheiometric proton dissociation constants of complexes by potentiometric titrations using a glass electrode

Possible errors in the measurement of acid dissociation constants by potentiometric titration techniques have been considered, with particular references to nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA). Unknown junction potentials can arise when pH measurements are carried out using a glass electrode with saturated calomel reference electrode which have been previously calibrated with a standard buffer solution. The magnitude of the influence of these unknown potentials has been demonstrated and an experimental procedure recommended which gives meaningful results.The precision of calculated acid dissociation constants will also be influenced by the presence of cationic species (e.g. H₄L⁴), the total acid strength, the absolute values of the constants and the value accepted for the auto-dissociation constant of water (K_w). All these factors have been considered quantitatively and their effect on metal complex formation constants, calculated from these acid dissociation constants, noted. The proton dissociation constant of the cationic species of NTA (i.e. H₄L⁺) has been found to have the value of pK₀=0.80 at 20° and μ=0.10 M.     

An absorptiometric determination of perchlorate by means of a novel coloured liquid anion exchanger

A novel liquid anion exchanger can be prepared from tetra-n-hexylammonium iodide and Erdmann's salt, (NH₄)[Co(NH₃)₂(NO₂)₄]. When a solution of this quaternary erdmannate in a suitable organic solvent (e_max = 15,700 at 355 mμ) is equilibrated with an aqueous solution of a salt, the coloured ermannate ion is displaced to a greater or less extent; the effect decreases in the order ClO₄^- > ClO₃^- > NO₃^- > Cl^- > SO₄^2- etc. The effects of changes in the organic solvent composition, of ionic strength, and of other variables on the position of equilibrium have been studied with a view to defining the optimum conditions for the absorptiometric determination of perchlorate ion (≧25 μ) in the presence of chlorate and other ions. Methods are proposed for eliminating those ions that form insoluble silver salts and to allow for the presence of substantial proportions of chlorate or nitrate ions.     

Spurenanreicherungen in Ackerb?den mit selektiver Abtrennung von Eisen

Zusammenfassung Die bisher bekannten Verfahren zur Spurenanreicherung in Ackerböden wurden auf die Entfernung des Eisens erweitert. Zwei Anreicherungsverfahren werden angegeben; das eine beruht auf der Entfernung des Eisens aus salzsaurer Lösung durch Ausschütten mit Isobutylmethylketon mit nachfolgender Reduktion von Eisen(lIII) zu Eisen(II) mittels Ascorbinsäure und nochmaligem Ausschütteln der mitextrahierten Elemente mit Isobutylmethylketon. Dabei bleibt das Eisen(II) in der wäßrigen Phase zurück. Aus den vereinigten, die Spurenelemente enthaltenden Phasen beider Ausschüttelungen werden die Spurenelemente mit Pyrrolidindithiocarbaminat und Kupferron gefällt und mit Chloroform extrahiert. Das zweite Verfahren, die Extraktion des Eisen(III)-chlorids aus 7 n Lithiumchloridlösung, bietet Vorteile. Es bleiben mehr Elemente in der wäßrigen Phase zurück, und die Reduktion des Eisen(III)-ions durch Ascorbinsäure verläuft in der sehr schwach sauren Lösung stets quantitativ. Durch eine Ausschüttelung können von Eisen quantitativ die Alkalien, Erdalkalien, Al, Ti, Zr, Th, Pb, As^v, Bi, V^v, Cr^III, Cr^VI, Mo, Mn, Co, Ni, Ag, Cd, Zn (99,2%), Cu (99%) und U^VI (95,5%) abgetrennt werden. Daraus ergeben sich sehr einfach durchzuführende Trennungen von Eisen(III)/Chrom(VI), Eisen(III)/Vanadin(V) und Eisen(III)/Molybdän, die durch Extraktion aus salzsaurer Lösung nicht getrennt werden. Das Anreicherungsverfahren ist für alle Substanzen, die Eisen als Hauptbestandteil enthalten, anwendbar.     

Metal Cation Transport Studies Comparing Dibenzo-18-crown-6 (DB18C6) with N,N,N′,N′-Tetrakis (n-propyl)-2,3- naphthalenedioxydiacetamide (NPr)

The previously synthesized3,6-dioxa-4,5-disubstitutedoctanedicarboxamides including NPr bind cationsin methanol in the order: Ca²⁺ > Sr²⁺ >Ba²⁺ > Mg²⁺ > Na⁺,K⁺. These compounds also extract Group II cations better thanGroup I cations from water to dichloromethane. In contrast, thesediacetamides were found by W. Simon et al., to sense Na⁺ >Ca²⁺ when they were incorporated into ion-selective electrodesusing a low dielectric constant solvent. It was of interest to determine theorder of Group I and Group II cation transport rates of these compoundsusing a three-phase system with a liquid organic phase of low dielectricconstant. We now report that NPr transports thiocyanates in such a systemusing dichloromethane in the order K⁺ (7.2) >Ca²⁺ (6.6) > Ba²⁺ (5.8) > Na⁺(1). The transport rate for KSCN with DB18C6 is 20.5 times faster than withNPr.     

Generation of a dynamic combinatorial library using sialic acid aldolase and in situ screening against wheat germ agglutinin

This paper describes the generation of a dynamic combinatorial library of sialic acid analogues using sialic acid aldolase. Addition of wheat germ agglutinin to the equilibrating libraries results in selective amplification of one or more members.     

Quadrupole-based mass spectrometric evaluation of isotope ratios of carbon dioxide in expired air from mice and men following the administration of 13C-methyl methionine

A method is described for the measurement of the isotopic ratio of ¹³CO₂/¹²CO₂ in expired air from individual mice and from humans by means of a quadrupole-based mass spectrometer system. Following the administration of ¹³C-methyl methionine or another appropriately labeled substrate, the ¹³C portion of the molecule is converted to ¹³CO₂. The ¹³CO₂ enters the carbonate pool(s) and is ultimately eliminated in the expired air where it is available for analysis. The expired air is transported by a small pump from the subject to a digital valve which provides for the alternate influx of expired air and standard into the mass spectrometer for 30 or 60 seconds each, respectively. The inlet consists of a control valve connected to a microbore stainless steel tube, and can be adjusted manually to achieve a source pressure of 4 X 10^?5 torr. The correction factors for drift in sensitivity and in the mass axis are generated by repeated, automatic analysis of the running standard and relating those measurements to values generated for the standard during the first minutes of the experiment. Each measurement of an isotopic ratio in expired air is corrected by an amount determined by the standard immediately preceding it. Precision for the measurements of both sample and standard ratios is ±0.2%. The technique should prove useful in assessing the metabolism, of substrates that are converted to CO₂ and may find utility as a diagnostic tool for certain diseases and metabolic disorders.