Molecular recognition for stable organic radicals--2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl

Stable radical 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl shows antiferromagnetic spin pairing with 2J/k= -14 K, attributable to a close contact between unpaired spin density on the imidazole-type nitrogen atoms; hydrogen bonds aid dimer formation, but do not appear to play an eletronic role in the magnetic behaviour.     

High accuracy analysis by isotope dilution mass spectrometry using an iterative exact matching technique

The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes.     

Evaluation of NMR spectroscopy for the quantitative characterization of urea-formaldehyde resins

A comparative evaluation has been made of both proton and (13)C nuclear magnetic resonance techniques in the quantitative characterization of commercial urea-formaldehyde resins. There is good agreement between data derived from (13)C NMR spectra and from (1)H high-field continuous wave, or low-field (Fourier transform) NMR spectra. Low-field continuous wave proton spectra exhibit inferior resolution and provide inaccurate quantitative data. Combination of (13)C and proton NMR with nitrogen analysis gives a quantitative characterization technique for these resins.     

Synthesis of a μ4-PPh mixed metal cluster: the x-ray structures of [Ru3Rh2(CO)13(PEt3)(μ4-PPh)] and [Ru3Au(μ2-H)(CO)9(PMe2Ph)(μ3-PPh)]

The synthesis of a (μ₄-PPh) and some related (μ₃-PPh) mixed metal clusters containing ruthenium is described together with the X-ray structures of [Ru₃Rh₂(CO)₁₃(PEt₃)(μ₄-PPh)] and [Ru₃Au(μ₂-H)(CO)₉(PMe₂Ph)(μ₃-PPh)].     

Stability studies and purification procedure for nitrite solutions in view of the preparation of isotopic reference materials

The lack of reference materials, accurately certified for nitrite, is a problem in view of the importance of this species for environmental and medical reasons. This work outlines a plan for the preparation of nitrite isotopic reference materials (IRMs) in the form of high purity solutions, certified for their nitrite-nitrogen isotopic composition and nitrite concentration. To achieve the desired accuracy (expanded uncertainty U with a coverage factor k=2 of 相似文献   

Modes of decomposition of silver,lead and mercuric toluenedithiolates. A synthetic and analytical study

The silver, lead and mercuric toluenedithiolates were synthesised and analysed by both conventional chemical methods and thermoanalytical methods. The thermal decomposition was studied by thermogravimetric analysis in air, nitrogen and vacuum. The formulae of the decomposition products were derived using the mole weights and i.r. absorption spectra. The activation energies for the first stage of decomposition were calculated. The volatile products contain mostly carbon and hydrogen while the residues contain the corresponding metals either free or combined with sulfur.

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Zusammenfassung Die Silber-, Blei- und Quecksilber-Toluoldithionate wurden synthetisiert und sowohl mit konventionellen chemischen Methoden als auch thermoanalytisch analysiert. Die thermische Zersetzung wurde durch thermogravimetrische Analyse in Luft, Stickstoff und Vakuum untersucht. Die Formeln der Zersetzungsprodukte wurden aus den Molekulargewichten und den Infrarot-Absorptionsspektren abgeleitet. Die Aktivierungsenergien für den ersten Zersetzungsschritt wurden berechnet. Die flüchtigen Produkte enthalten hauptsächlich Kohlenstoff und Wasserstoff, während die Rückstände die entsprechenden Metalle in freier Form oder in Form von Schwefelverbindungen enthalten.

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Abstracted in part from a thesis submitted by T. Ktenas to the Graduate School of Howard University, in partial fulfillment of the requirements for the degree of Master of Science.     

Elution of Cationic Species with/without Ion Pair Reagents from Polar Stationary Phases via SFC

Traditionally, the application of supercritical fluid chromatography has been limited to compounds of relatively low polarity. Thus, we report here that one secondary amine and two quaternary amine salts were successfully eluted from a Deltabond Cyano-bonded silica column with the addition of sodium alkylsulfonate to the methanol-modified, carbon dioxide-based mobile phase. A possible ion-pairing interaction between the positively charged analytes and the anionic part of the sulfonate additive has been proposed. In another set of experiments, the three amine salts readily eluted from both Ethyl-pyridine-bonded silica and Amino-bonded silica phases without the need of additive although the peak shapes were less than desirable. The addition of sulfonate salt to the mobile phase again sharpened the peaks. In the presence of a mixture of methanol and CO₂, we suggest that these stationary phases are positively charged. We describe here also for the first time the employment of a strong silica-based anion exchange (SAX) column for supercritical fluid separation of cationic species. Two elution mechanisms were proposed for the three amine salts depending upon whether the ionic additive was incorporated into the mobile phase. Separation as the ion-pair was proposed with an ionic additive in the mobile phase; whereas separation of the intact amine salt from the positively charged basic stationary phases was suggested to be operational without the ionic additive.     

Ring-opening of unsymmetrical 1,2-dioxines using cobalt(II) salen complexes

The regioselectivity of the metal-catalyzed ring opening of unsymmetrical 1,2-dioxines to cis-gamma-hydroxyenones was investigated using two different Co(II) salen complexes. Regioselectivity was determined by direct examination of the enone ratios and by derivitization with a stabilized phosphorus ylide. The steric influence of the substituents on the 1,2-dioxine was the primary influence on regioselectivity. Temperature played little role; however, solvent and selection of Co(II) complex could be used to mildly influence the outcome of the rearrangement for selected substrates. The origins of the selectivity for the reaction are discussed.     

Synthesis of conjugated polyrotaxanes

A series of conjugated polyrotaxane insulated molecular wires are synthesised by aqueous Suzuki polymerisation, using hydrophobic binding to promote threading of the cyclodextrin units. These polyrotaxanes have conjugated polymer cores based on poly(para-phenylene), polyfluorene, and poly(diphenylene-vinylene), threaded through 0.9-1.6 cyclodextrins per repeat unit. Bulky naphthalene-3,6-disulfonate endgroups prevent the macrocycles from slipping off the conjugated polymer chains. Dialysis experiments show that the cyclodextrins become unthreaded only if smaller stoppers are used. MALDI TOF mass spectra detect oligomers with up to ten threaded cyclodextrins, and reveal the presence of some defects that result for oxidative homo-coupling of boronic acids. Weight-average molecular weights were determined by analytical ultracentrifugation, demonstrating that step-growth polymerisation is efficient enough to achieve degrees of polymerisation up to approximately 20 repeat units (84 para-phenylenes). The fluorescence spectra of these polyrotaxanes indicate that the presence of the threaded cyclodextrin macrocycles reduces the flexibility of the conjugated polymer pi-systems. Both the solution and the solid-state photoluminescence quantum yields are enhanced upon threading of the conjugated polyaromatic cores through alpha- or beta-cyclodextrins, and the emission spectra of the polyrotaxanes are blue-shifted compared to the corresponding unthreaded polymers. The greater weight of the 0-0 transition in the emission spectra, as well as the smaller Stokes shift, indicate that the polyrotaxanes are more rigid than the unthreaded polymers.