Neutral copper-phosphido-borane complexes: synthesis, characterization, and use as precatalysts in C(sp)-P bond formation

Copper-phosphido-borane complexes were synthesized and isolated for the first time. Their structures were experimentally and computationally investigated. They were shown to display catalytic activity in C(sp)-P bond formation.     

Lanthanide-based hexanuclear complexes usable as molecular precursors for new hybrid materials

Lanthanide containing octahedral hexanuclear complexes with general chemical formula [Ln₆O(OH)₈(NO₃)₆(H₂O)_x].2NO₃.yH₂O where Ln = Ce–Lu (except Pm) or Y, x = 0, 6, 12, 14 or 16 and y = 0, 2, 4 or 5 constitute a great family of polymorphic compounds. The synthesis and the crystal structures of all these compounds are overviewed. The hydration/dehydration processes that allow the structural transitions from one compound to another are described. The crystal structure of compounds with general chemical formula [Ln₆O(OH)₈(NO₃)₆(H₂O)₆].2NO₃ where Ln = Ce–Lu (except Pm) or Y is described. It has been solved on the basis of a powder XRD diagram. The use of such hexanuclear complexes as molecular precursors for new materials is also discussed.     

Development of a radiochemical separation for selenium with the aim of measuring its isotope 79 in low and intermediate nuclear wastes by ICP-MS

Selenium (Se) 79 is a beta emitter produced from ²³⁵U fission thus occurring as one of the fission products found in nuclear reactors. Due to its long half life (about 10⁵ years), ⁷⁹Se is one of the radionuclides of interest for the performance of assessment studies of waste storage or disposal. Thus, the National Radioactive Waste Management Agency (Andra, France) requests its monitoring in wastes packages before their disposal in specific sites.Measurement of ⁷⁹Se is difficult owing to its trace level concentration and its low activity in nuclear wastes. A radiochemical procedure has to be carried out in order to separate selenium from the matrix and to concentrate it before the measurement with a mass spectrometric or a nuclear technique. The beginning of the development is presented in this paper. The optimised protocol firstly developed in view of an ICP-MS measurement, includes five steps based on microwave digestion, evaporation and separations on ion exchange resins. It was tested first on synthetic solutions and was optimised in order to be applicable to a large number of sample types. The recoveries of the whole procedure were evaluated using natural ⁸²Se or the gamma emitter ⁷⁵Se as a radioactive spiker. Then, the protocol was applied to two solid samples spiked with natural selenium, a glass microfiber filter and an ion exchange resin, and two liquid samples spiked with ⁷⁵Se, a synthetic solution and an effluent. The yields obtained for both samples ranged from 70 up to 80%.     

Determination of carbon monoxide with palladium chloride

The gas sample transfer apparatus, as shown in Fig. 1, was designed to facilitate the analysis of several carbon monoxide samples during a normal work day. With this apparatus, gas samples were accurately transferred from the Tedlar bags leak free through the system to the reaction flask. Changing from one gas concentration to another was greatly simplified by using the quick-connect fittings; gas volumes were readily calculated from direct readings taken from the differential pressure gauge.Tests were conducted to assess the performance of the PdCl₂ method and the sampling system with various gas mixtures. The proposed PdCl₂ method utilizing this particular sampling scheme is capable of successfully measuring carbon monoxide in nitrogen cylinder gas at levels of 0.1, 0.05, and 0.01% with expected average recoveries of 96, 96, and 93%, respectively.     

Modelling the Epidemiological Consequences of HIV Infection and AIDS: A Contribution from Operational Research

As part of the worldwide effort involved in modelling the spread of AIDS, nothing has been heard from the operational research community. Yet OR, with its client-oriented modelling philosophy, patently has a role to play in assisting clinicians, epidemiologists and health planners to obtain greater understanding of this staggering threat to public health. Using the methods of system dynamics, a model of the spread of AIDS in the UK homosexual population has been developed. It is implemented on a personal computer, and possesses the capability to capture complex virological and behavioural features of the epidemic whilst portraying the consequences in an easily digested graphical form. Some examples of these results are presented. The model's structure and parameters are also fully described, together with sensitivity tests on the crucial incubation time distribution.     

Engineering an obligate domain-swapped dimer of cyanovirin-N with enhanced anti-HIV activity

The anti-HIV cyanobacterial protein cyanovirin-N can undergo domain swapping to form an intertwined dimer. The dimeric form is stable at low pH and millimolar concentrations. By deleting an amino acid from the hinge linker about which domain swapping occurs, we have constructed an obligate domain-swapped dimer of cyanovirin-N that represents a new tetravalent carbohydrate binding protein that is stable over a large range of pH values. This obligate dimer displays enhanced anti-HIV activity relative to the wild-type cyanovirin-N monomer with an observed 3.5-fold decrease in IC(50) (9nM for the dimer vs 32 nM for the monomer) for inhibition of HIV-1 envelope-mediated cell fusion and, when expressed in Escherichia coli, can be rapidly obtained in >98% purity in a single chromatographic step.     

Qualitative and Quantitative Studies of Neodymium Doped Silica Gel-Glasses

The effect of neodymium ions on establishment of the sol-gel matrix and of the sol-gel matrix on the metal ion chemistry has been investigated by spectroscopic techniques including FTIR, FTNIR and UV/Vis. Materials were prepared by the pre-doping method and contained metal ions added as the chloride in either the aqueous or alcoholic phase at initial solution concentrations between 10–7 M and 1 M. The presence of the metal ions affects the rate of drying of the gel-silicas and can reduce the time required for weight stabilization by ca. 50%. Irrespective of the solvent system used for metal ion incorporation, the metal ions are found in 9-fold coordination sites in the as-prepared gels, in 8-fold coordination sites in gels where solvent has been removed by evacuation and for gels heated to 800°C, a highly symmetric environment (concentrations of 0.01 M and below) or 8-fold solid-state coordination where there is a degree of covalent character between the liganding atoms and the metal ions. Metal ion concentration and absorbance are related for gel-silicas prepared with dopants at initial concentrations of 0.01–1 M, although the absorbances measured depend on the solvent used for incorporation of the metal ion. Experimental data suggest that the matrix and metal ions interact at various stages in the formation of the doped gel-silica, however, we do not presently know if these interactions are transitory or permanent in nature.     

A New Mechanism of Thermochromism for Silica Sol-gel Materials

Summary.  In this contribution we provide evidence for thermochromic colour changes unique to silica based materials formed at low temperature by the sol-gel process. The mechanism requires the matrix and the dopant to interact in order to produce the observed colour changes. The materials formed have potential application as temperature sensitive light filters, visual temperature indicators, self-diagnostic labels for electronic devices and infrared recording media. Corresponding author. E-mail: carole.perry@ntu.ac.uk Received June 28, 2002; accepted July 30, 2002     

Synthesis of a fluoroalkene peptidomimetic precursor of N-acetyl-l-glutamyl-l-alanine

Fluoroolefin peptidomimetics of the dipeptide N-acetyl-Glu-Ala were synthesized via an Evans asymmetric aldol reaction and an Overman rearrangement. This dipeptide is the N-terminal of an octapeptide implicated in the signal transduction via PKCε.