Narrow beam emission of slow positrons from negative affinity surfaces

We have measured the angular distribution of the slow positrons (e⁺·) from negative work function (φ⁺) surfaces of Cu and Al bombarded by keV e⁺ in ultrahigh vacuum. In analogy with √-point electrons emitted from negative electron affinity surfaces, the majority (>50%) of the slow e⁺ leave the surface with energy ≈ φ⁺ and velocity within ～20° of the surface normal. This agrees qualitatively with the predictions of a simple 1D model.     

Aerosol direct fluorination: Direct synthesis of perfluorinated glyme ethers

Aerosol direct fluorination of 1,2-dimethoxyethane, bis(2-methoxyethyl)ether, triethyleneglycol dimethyl ether, and tetraethyleneglycol dimethyl ether produced the perfluorinated analogs in 36%, 22%, 30% and 15% isolated yields based on total input of hydrocarbon into the reactor. High effluent concentrations in excess of 80% were obtained in all cases indicating that low yields are primarily due to mechanical losses, not fragmentation or incomplete reaction. The clean reaction and the ability to produce high molecular weight fluorocarbons by direct fluorination suggests the applicability of aerosol fluorination to the production of fluorinated fluids and oils.     

Investigation of cation environment and framework changes in silicotitanate exchange materials using solid-state Na, Si, and Cs MAS NMR

Crystalline silicotitanate (CST), HNa₃Ti₄Si₂O₁₄·4H₂O and the Nb-substituted CST (Nb-CST), HNa₂Ti₃NbSi₂O₁₄·4H₂O, are highly selective Cs⁺ sorbents, which makes them attractive materials for the selective removal of radioactive species from nuclear waste solutions. The structural basis for the improved Cs⁺ selectivity in the niobium analogs was investigated through a series of solid-state magic angle spinning (MAS) NMR experiments. Changes in the local environment of the Na⁺ and Cs⁺ cations in both CST and Nb-CST materials as a function of weight percent cesium exchange were investigated using ²³Na and ¹³³Cs MAS NMR. Framework changes induced by Cs⁺ loading and hydration state were investigated with ²⁹Si MAS NMR. Multiple Cs⁺ environments were observed in the CST and Nb-CST material. The relative population of these different Cs⁺ environments varies with the extent of Cs⁺ loading. Marked changes in the framework Si environment were noted with the initial incorporation of Cs⁺, however with increased Cs⁺ loading the impact to the Si environment becomes less pronounced. The Cs⁺ environment and Si framework structure were influenced by the Nb-substitution and were greatly affected by the amount of water present in the materials. The increased Cs⁺ selectivity of the Nb-CST materials arises from both the chemistry and geometry of the tunnels and pores.     

Role du methyle 4β dans la formation des oxetanones en serie steroïde. Cas du cycle B.

The formation of oxetanones $\text{5}$ from bromoketonee $\text{2}$ requires the presence of a β-methyl group at C-4. The yield of the reaction $\text{2}$→$\text{5}$ is improved by addition of water to the medium.     

Regio- and stereospecific models for the biosynthesis of the indole alkaloids—III : The Aspidosperma-Iboga-secodine relationship

In vitro transformation of the Aspidosperma alkaloid (?) tabersonine (1) to (±)-pseudocatharanthine (7) via (+) allocatharanthine (6) and dehydrosecodine A (3) is described as a model for the biochemical interconversion of Aspidosperma and Iboga alkaloids. Facile conversion of 1, 2 and 7 in xylene solution to the carbazole (9) suggests the intermediacy of dehydrosecodines (as 8) in these reactions. In methanol solution the racemic salt (12) is formed in 50% yield from catharanthine at 175°. Further pyrolysis of the salt yields the carbazole (9). Reduction of the salt (12) with NaBH₄ affords (±) dihydrosecodine (16) identical with the natural alkaloid from Rhazya stricta.     

Multinuclear NMR investigations of the oxygen, water, and hydroxyl environments in sodium hexaniobate

Solid-state (1)H, (17)O MAS NMR, (1)H-(93)Nb TRAPDOR NMR, and (1)H double quantum 2D MAS NMR experiments were used to characterize the oxygen, water, and hydroxyl environments in the monoprotonated hexaniobate material, Na(7)[HNb(6)O(19)].15H(2)O. These solid-state NMR experiments demonstrate that the proton is located on the bridging oxygen of the [Nb(6)O(19)](8-) cluster. The solid-state NMR results also show that the NbOH protons are spatially isolated from similar protons, but undergo proton exchange with the water species located in the crystal lattice. On the basis of double quantum (1)H MAS NMR measurements, it was determined that the water species in the crystal lattice have restricted motional dynamics. Two-dimensional (1)H-(17)O MAS NMR correlation experiments show that these restricted waters are preferentially associated with the bridging oxygen. Solution (17)O NMR experiments show that the hydroxyl proton is also attached to the bridging oxygen for the compound in solution. In addition, solution (17)O NMR kinetic studies for the hexaniobate allowed the measurement of relative oxygen exchange rates between the bridging, terminal, and hydroxyl oxygen and the oxygen of the solvent as a function of pH and temperature. These NMR experiments are some of the first investigations into the proton location, oxygen and proton exchange processes, and water dynamics for a base stable polyoxoniobate material, and they provide insight into the chemistry and reactivity of these materials.     

Effect of a natural contaminant on foam fractionation of bromelain

Foam fractionation is a simple, inexpensive method for separating and purifying proteins. Typically, a dilute bromelain solution with a pH ranging from 2.0 to 7.0 foams very well when bubbles are introduced into a foam fractionation column. It was observed, however, that the dilute enzyme solution only foamed between approximately pH 2.0 and 3.0 when the inner wall of the fractionation column was coated with a natural contaminant (okra residue). We studied the separation ratio and the protein mass recovery to explore the effect of a natural antifoaming agent on the foam fractionation of a dilute bromelain solution. The control variables used in this process were the initial bulk solution pH, which ranged from 2.0 to 7.0, and the superficial air velocity, which varied between 1.7 and 6.2 cm/s.