Cluster-assisted decomposition reactions of the molecular anions of SF6 and C7F14

The cluster ions formed by the attachment of dimethylsulfoxide (DMSO) and methanol to the molecular negative ions of C₇F₁₄ and SF₆ have been studied by a pulsed e-beam high pressure mass spectrometer (PHPMS) and by an atmospheric pressure ionization mass spectrometer (APIMS). The free energy change (ΔG°) for the clustering equilibria reaction, M⁻ + S M⁻S, at 35 °C are found to be −7.7 and −7.s kcal/mol for S = DMSO and M⁻ = C₇F⁻₁₄ and SF⁻₆, respectively, and −6.4 and −4.5 kcal/mol for S = methanol and M⁻ = C₇F⁻₁₄ and SF⁻₆, respectively. While the cluster ions formed by DMSO are found to be stable against side reactions, those formed by methanol undergo decomposition processes in which the central core ion is fragmented. At 35 °C, the rate law for the decomposition of the SF⁻₆ (CH₃OH)₁ ion is second-order, involving the M⁻ (CH₃OH)₁ cluster ion and another methanol molecule. While the C₇F⁻₁₄(CH₃OH)₁ ion also decomposes through this second-order process, a competing unimolecular mechanism is also operative at 35 °C. With increases in the PHPMS ion source temperature to 150 °C, the unimolecular decomposition process becomes progressively dominant for both of the M⁻(CH₃OH)₁ cluster ions of C₇F₁₄ and SF₆. Methanol cluster ions of the type M⁻S₂ are not observed under any of the conditions examined here. When methanol or water partial pressures of a few torr or higher are present in the buffer gas of the APIMS ion source, the decomposition reactions are very fast and only the fragment ions produced by these reactions are observed in the electron-capture (EC)-APIMS spectra of C₇F₁₄ and SF₆. Also, in the methanol-containing APIMS ion source, the course of the SF⁻₆ decomposition reaction is altered so that fragment ions of the type F⁻(S)_n dominate the EC-APIMS spectrum of SF₆ at all ion source temperatures. For C₇F₁₄, fragment ions of the type F⁻(S)_n become dominant at lower ion source temperatures. These previously unknown reactions are expected to be important in the analysis of perfluorinated compounds by mass spectrometric methods that utilize ionization by electron capture or negative chemical ionization. The nature of the fragment ions produced in these cluster-assisted reactions may also provide a new source of information concerning the structures of the molecular negative ions of SF₆ and C₇F₁₄.     

Detection mass bias in atmospheric pressure ionization mass spectrometry

A previously uncharacterized source of detection mass bias is shown to be associated with atmospheric pressure ionization mass spectrometry (APIMS), and is attributed to a mass dependence in the sampling of ions from the supersonic free jet expansion of gas emerging from the ion source. The halide ions Cl ^?, Br^?, and I^? are shown to be transported from the ion source aperture to a quadrupole mass filter with efficiencies that increase linearly with increasing mass of the ion. While the polyatomic anions SF ₆ ^- and C₇F ₁₄ ^- are detected with even greater efficiencies than would be expected for monatomic anions of the same mass, this additional sensitivity to the polyatomic anions is thought to be related to ion loss processes occurring within the ion source. The experimental conditions under which these mass bias effects can be minimized or enhanced in APIMS are described.     

Reactions of Verbenone Dienolate with Aromatic Aldehydes

Lithium dienolate of verbenone reacts with aromatic aldehydes to give α- and γ-aldols. Unusual features of the reaction include: selective formation of the γ-aldols under kinetic conditions, absence of deconjugated α-products, high sensitivity to electronic effects and formation of bis-aldols 5.     

Determination of nanogram quantities of uranium by pulsed-laser fluorometry

Summary A versatile, direct method for the determination of trace amounts of uranium in solution has been developed utilizing pulsed-laser fluorometry and a pyrophosphate fluorescence enhancing reagent. Measurements with a 2–3% relative standard deviation and accurate to better than 1% have been obtained in the 0.01 to 4g U/g range. The detection limit is 0.005 ng uranium. Time required per determination is 6 min. A special feature of the method is the use of a standard addition technique to eliminate sample matrix effects. The effects of temperature, acidity, reagent impurity and anionic impurities have been studied.

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Bestimmung von Nanogramm-Mengen Uran mit Hilfe eines Laser-Fluorometers

Zusammenfassung Eine Direktmethode zur Bestimmung von Uranspuren in Lösungen mit Hilfe der Laser-Fluorometrie und eines fluoreszenz-anregenden Pyrophosphates wurde entwickelt. Die Messungen erfolgen mit einer rel. Standardabweichung von 2–3% und einer Genauigkeit besser als 1% bei Mengen von 0,01–4g U/g. Die Nachweisgrenze beträgt 0,005 ng U. Eine Bestimmung dauert 6 min. Um Matrix-Effekte zu vermeiden, verwendet man die Standard-Zusatz-Technik. Der Einfluß der Temperatur, der Acidität und etwaiger Verunreinigungen wurde untersucht.

     

Electro-optic and electroreflectance effects in perovskite-type crystals

A model is presented which relates the electro-optic (E-O) and electroreflectance properties of ABO₃ perovskite-type crystals to their electronic energy band structure. These properties are assumed to result from electric-field-induced ionic displacements within each unit cell. SrTiO₃ was selected as the model perovskite and the band structure was calculated using LCAO parameters. LCAO momentum matrix elements were used to derive the optical excitation spectrum ?₂(ω). Field-induced changes in the Madelung potentials are found to dominate overlap effects. The E-O effects are insensitive to detailed relative ionic displacements and effective charges, but are characterized by the sum of the products of these quantities, namely the polarization. The results are compared with E-O, reflectance, and electroreflectance experiments.     

Hollow-core dendrimers revisited

In revisiting the original argument of de Gennes and Hervet [J. Phys. (Paris) 44, L351 (1983)]] leading to "hollow-core" dendrimers, we show that a self-consistent application of their model leads to the "filled-core" model first elucidated by Lescanec and Muthukumar [Macromolecules 23, 2280 (1990)]]. The monomer density falls off parabolically from the center of the dendrimer, and the tips are distributed throughout the molecule.     

Measuring silver nanoparticle dissolution in complex biological and environmental matrices using UV-visible absorbance

Distinguishing the toxic effects of nanoparticles (NPs) themselves from the well-studied toxic effects of their ions is a critical but challenging measurement for nanotoxicity studies and regulation. This measurement is especially difficult for silver NPs (AgNPs) because in many relevant biological and environmental solutions, dissolved silver forms AgCl NPs or microparticles. Simulations predict that solid AgCl particles form at silver concentrations greater than 0.18 and 0.58 μg/mL in cell culture media and moderately hard reconstituted water (MHRW), respectively. The AgCl NPs are usually not easily separable from AgNPs. Therefore, common existing total silver techniques applied to measure AgNP dissolution, such as inductively coupled plasma mass spectrometry (ICP-MS) or atomic absorption, cannot accurately measure the amount of silver remaining in AgNP form, as they cannot distinguish Ag oxidation states. In this work, we introduce a simple localized surface plasmon resonance (LSPR) UV–visible absorbance measurement as a technique to measure the amount of silver remaining in AgNP form for AgNPs with constant agglomeration states. Unlike other existing methods, this absorbance method can be used to measure the amount of silver remaining in AgNP form even in biological and environmental solutions containing chloride because AgCl NPs do not have an associated LSPR absorbance. In addition, no separation step is required to measure the dissolution of the AgNPs. After using ICP-MS to show that the area under the absorbance curve is an accurate measure of silver in AgNP state for unagglomerating AgNPs in non-chloride-containing media, the absorbance is used to measure dissolution rates of AgNPs with different polymer coatings in biological and environmental solutions. We find that the dissolution rate decreases at high AgNP concentrations, 5 kDa polyethylene glycol thiol coatings increase the dissolution rate, and the rate is much higher in cell culture media than in MHRW.     

Application of [BoPhoz Rh] catalysts in the asymmetric hydrogenation of a pyridyl benzothiophene alkene

The asymmetric hydrogenation of a pyridyl benzothiophene alkene has been successfully accomplished using [BoPhoz Rh] catalysts. By tailoring the steric properties of the BoPhoz ligand, the ee of the product could be increased from just 12% to 90%. This research further expands the substrate scope of the conventional class of functionalized alkenes amenable to catalytic asymmetric hydrogenation.     

Experimental calibration and field investigation of the oxygen isotopic fractionation between biogenic aragonite and water

Marine molluscs have long been recognised as potential records of palaeoclimate change using the patterns and differences in the stable isotopic composition of the carbonate shells. The aim of this study is to improve the robustness of this approach for aragonitic molluscs by completing the first experimental calibration of the fractionation between water and biogenic aragonite. Fractionation factors were calibrated by growing specimens of the freshwater mollusc Lymnaea peregra under controlled conditions of water temperature and isotopic composition. Fifteen populations of L. peregra were maintained at constant temperature and isotopic conditions for five months (at five different temperatures and using three different water compositions). Water samples and temperature measurements were taken regularly throughout the experiment. The temperature dependence of the fractionation factor, between 8 and 24 degrees C, is given by: 1000 ln alpha=16.74x(1000T(-1))-26.39 (T in Kelvin) and the relationship between temperature (T), delta(18)O(carb) and delta(18)O(wat) is given by: T=21.36-4.83xdelta(+ degrees )O(carb)-delta(+ degrees )O(wat) (T is in degrees C, delta(18)O(carb) is with respect to Vienna Pee Dee Belemnite (PDB), the International Atomic Energy Agency (IAEA) replacement standard for PDB, and delta(18)O(wat) is with respect to Vienna standard mean ocean water (VSMOW)) The outcome of the controlled experiment is compared with previous studies on synthetic, and biogenic, calcite and aragonite from field and laboratory investigations. These comparisons suggest that although a vital offset exists between the fractionation of isotopes in synthetic and biogenic aragonite for molluscs in general, there is no vital effect that is specific either to freshwater, or to individual, genera. Therefore, the calibrated relationship may be used for any freshwater or marine mollusc to derive palaeotemperatures providing the isotopic composition of the environmental water can be reliably constrained. Copyright 1999 John Wiley & Sons, Ltd.