Solvophobic acceleration of Diels-Alder reactions in supercritical carbon dioxide

The rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthracene in supercritical carbon dioxide (scCO(2)) was determined by following the disappearance of 9-hydroxymethylanthracene with in situ UV/vis absorption spectroscopy. The reaction conditions were 45-75 degrees C and 90-190 bar, which correspond to fluid densities (based on pure carbon dioxide) ranging between approximately 340 and 730 kg m(-3). The measured reaction rate at low scCO(2) fluid densities was nearly 25x faster than that reported in acetonitrile at the same temperature (45 degrees C). An inverse relationship between reaction rate and fluid density/pressure was observed at all temperatures in scCO(2). The apparent activation volumes were large and positive (350 cm(3) mol(-1)) and only a weak function of reduced temperature. A solvophobic mechanism analogous to those observed in conventional solvents is postulated to describe (a) the rate acceleration observed for this reaction in scCO(2) relative to that in acetonitrile, (b) the observed relationship between reaction rate and pressure/temperature/density, and (c) the large, positive activation volumes. Solubility measurements in scCO(2), rate measurements in conventional solvents, and an empirical correlation are used to support this theory. Our results advance the general understanding of reactivity in supercritical fluids and provide a rationale for selecting reactions which can be accelerated when conducted in scCO(2).     

Reactions of Charged Substrates. 7. The Methoxymethyl Carbenium Ion Problem. 2. A Semiempirical Study of the Kinetic and Thermodynamic Stabilities of Linear and Cyclic Oxo- and Thiocarbenium Ions Generated from Aldehyde Hydrates, Hemiacetals, Acetals, and Methyl Ribosides and Glucosides

Factors affecting the cleavage of the carbon-oxygen bond in linear and cyclic aldehyde hydrates, heimacetals, acetals, and methyl ribosides and glucosides have been investigated using semiempirical calculations (AM1 and PM3). (For some systems, low- and high-level ab initio energies are available for comparison with the semiempirical results. With one exception, the results obtained by the two methods show excellent agreement in relative energies and trends in reactivity.) The effects on reactivity and stability caused by substituting a sulfur for the alpha oxygen in the oxocarbenium ion were also studied. In general, systems that can have an antiperiplanar alignment of lone pairs on the leaving group and potential oxocarbenium ion oxygens undergo spontanteous cleavage. An examination of various conformers of the leaving group relative to the potential oxocarbenium oxygen shows, however, that lone pair repulsion and steric factors for MeOH as the leaving group are more important than the antiperiplanar effect for bond cleavage. All compounds in which the alpha-oxygen in the potential carbenium ion is replaced by sulfur undergo spontaneous cleavage regardless of the leaving group or structure of the compound. Energy profiles, DeltaH(), and DeltaH(R) values show that linear and cyclic thiocarbenium ions are much more stable than the corresponding oxocarbenium ions. Comparison of results for methyl ribosides and glucosides with results for corresponding pyridinium substrates suggests that both should hydrolyze through an A-1 mechanism. General-acid catalysis with hydronium as the acid was studied. With solution results, the computations suggest that substrates with either a good leaving group or stable oxocarbenium ion react with rate-limiting proton transfer from the acid to the leaving group but that substrates with both a good leaving group and stable carbenium ion react with concerted proton transfer and bond cleavage.     

Solid targets for 99mTc production on medical cyclotrons

Recent disruptions in the molybdenum-technetium generator supply chain prompted a review of non-reactor based production methods for both ⁹⁹Mo and ^99mTc. Small medical cyclotrons (E _p ~ 16–24 MeV) are capable of producing Curie quantities of ^99mTc from isotopically enriched ¹⁰⁰Mo using the ¹⁰⁰Mo(p,2n)^99mTc reaction. Unlike most other metallic target materials for routine production of medical radioisotopes, molybdenum cannot be deposited by reductive electroplating from aqueous salt solutions. To overcome this issue, we developed a new process for solid molybdenum targets based on the electrophoretic deposition of fine ¹⁰⁰Mo powder onto a tantalum plate, followed by high temperature sintering. The targets obtained were mechanically robust and thermally stable when irradiated with protons at high power density.     

Spectral Interferences in Light Element Analysis

 Interferences (overlaps) occurring when lines of other elements affect either peak or background measurements can cause errors in quantitative WD analysis, but may be minimised by suitable choices of analysis conditions such as spectrometer crystal, background offsets, and pulse-height analyser settings. Computer spectrum-simulation is much more effective than reference to wavelength tables for investigating interferences. The ‘Virtual WDS’ simulation program developed by the present authors, hitherto applied only to ‘ordinary’ elements (Z ≥ 11), has been extended to light elements for which evaporated multilayers are used in place of true crystals. ‘Virtual WDS’ utilises experimentally recorded light-element K spectra and L and M spectra of heavier elements in the same wavelength range. It is impractical to record all high-order peaks, so computed line profiles are used, with widths and intensities interpolated from a limited set of observations. The relative positions of first and higher order peaks are affected significantly by the refractive index of the multilayer, requiring modification of the Bragg equation. Suppression of high orders by pulse-height analysis is less effective than for ‘normal’ wavelengths, owing to the breadth of the pulse-height distribution for low X-ray energies. Simulation using a Gaussian expression aids optimisation of threshold and window-width settings.     

A small angle neutron scattering and electrochemical impedance spectroscopy study of the nanostructure of the iron chalcogenide glass ion-selective electrode

This paper presents a preliminary structural and interfacial study of the iron chalcogenide glass [i.e., Fe_x(Ge₂₈Sb₁₂Se₆₀)_100−x] ion-selective electrode (ISE) using small angle neutron scattering (SANS) and electrochemical impedance spectroscopy (EIS). SANS detected variations in the neutron scattering as a function of iron content in the chalcogenide glass. Furthermore, a change in the chalcogenide glass structure was observed at elevated iron dopant levels. Conversely, EIS was used to show that the iron chalcogenide membrane comprises various time constants, and the interfacial charge transfer reaction depends on the membrane iron content. Equivalent circuit modeling revealed that the charge transfer resistance decreases at elevated iron levels, and this may be related to the presence of iron defects in the glass. It is proposed that the iron chalcogenide membrane comprises an iron nanostructural network embedded in the amorphous matrix, and this directly influences the electrical conductivity and concomitant electrochemical reactivity of the glass.     

Oxygen harvesting from carbon dioxide: simultaneous epoxidation and CO formation

Due to increasing concentrations in the atmosphere, carbon dioxide has, in recent times, been targeted for utilisation (Carbon Capture Utilisation and Storage, CCUS). In particular, the production of CO from CO₂ has been an area of intense interest, particularly since the CO can be utilized in Fischer–Tropsch synthesis. Herein we report that CO₂ can also be used as a source of atomic oxygen that is efficiently harvested and used as a waste-free terminal oxidant for the oxidation of alkenes to epoxides. Simultaneously, the process yields CO. Utilization of the atomic oxygen does not only generate a valuable product, but also prevents the recombination of O and CO, thus increasing the yield of CO for possible application in the synthesis of higher-order hydrocarbons.

Selective formation of atomic oxygen to form epoxides in a waste free process is reported. Simultaneously generating carbon monoxide from carbon dioxide for further use.     

Holographic projector using one lens

It is shown that the lens count in a Fourier holographic projector can be reduced by encoding the equivalent lens power in sets of Fresnel holograms. By using appropriately calculated Fresnel holograms in a reflective configuration to effectively share a lens between the beam-expansion and demagnification stages of a holographic projector, a reduction in lens count from four to one is demonstrated.