Using polarization effects to alter chemical reactivity: A simple host which enhances amine nucleophilicity

The rational design of a bis(phosphine oxide) host which is capable of binding a benzylic amine is presented. The ability of this host to increase the rate of addition of 4-fluorobenzylamine to N-phenylmaleimide is rationalized in terms of the enhancement of the nucleophilicity of the benzylic amine.     

Effects of cell volume fraction changes on apparent diffusion in human cells

Diffusion-weighted imaging was used to study the relationship between apparent diffusion coefficient (ADC) and cell volume fraction in cell suspensions and packed arrays. Cell volume fraction was varied by changing extracellular fluid osmolarity (for human glial cells) and by changing cell density (for human glial and red blood cells). In packed arrays of glial cells, ADC increased 10% when cells shrank and decreased 13% when cells swelled. ADC decreased 34% as cell density increased from 0 to 72%. In erythrocyte suspensions, ADC decreased 90% as the cell density increased from 0 to 89%. These results agree with theoretical predictions.     

Thermal Decomposition and Crystallization of Aqueous Sol-Gel Derived Zirconium Acetate Gels: Effects of the Additive Anions

Zirconia powders were prepared by forming gels by desiccation of aqueous precursor solutions of zirconium acetate containing nitric or sulfuric acid at pH 2.4 and 1.4 and pyrolyzing the gels to temperatures up to 825°C. The structure development in the gels and solid pyrolysis products was investigated. The crystalline zirconia structures produced monoclinic (m), metastable cubic (c) and tetragonal (t) polymorphs. The structure transition temperatures were strongly dependent on the pH, the anions and the stoichiometry of the zirconium complex in the precursor solution. The monoclinic polymorph fraction in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing sulfuric acid at pH 2.4 to 750°C approaches zero while this ratio in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing nitric acid at pH 1.4 to 825°C is 0.7.     

Sharp concentration of the chromatic number on random graphsG n,p

The distribution of the chromatic number on random graphsG _{n, p} is quite sharply concentrated. For fixedp it concentrates almost surely in √n ω(n) consecutive integers where ω(n) approaches infinity arbitrarily slowly. If the average degreepn is less thann ^1/6, it concentrates almost surely in five consecutive integers. Large deviation estimates for martingales are used in the proof.     

Muonium in bulk fused quartz: Test case for RAHD relaxation theory

Muonium (μ ⁺ e ⁻) in bulk fused quartz is a unique system in that theμ ⁺ spin polarization (in the muonium state) relaxes almost entirely via random anisotropic hyperfine distortions (RAHD). As such, this system provides an excellent test case for a new RAHD spin relaxation theory. This theory is quantitatively compared to static zero field data and the functional characteristics in both the high field and dynamic limits are considered as well.     

Introduction of bis-discotic and bis-calamitic mesogenic addends to C 60

The synthesis and characterization of four C 60 Bingel cyclopropanation adducts incorporating bis-biphenylene (three adducts) and bis-triphenylene (one adduct) moieties are described. The thermal analysis (POM and DSC) of these materials reveals that they are not liquid crystalline. However, two of the precursor bis-biphenylene malonate esters possess monotropic mesophases. Furthermore, each of the corresponding C 60 adducts is miscible in the melts of the precursor malonate ester, and at low dopings, retains the liquid crystalline monotropic mesophases of the precursor.     

Stress in elastic plates reinforced by fibres lying in concentric circles

We Consider fibre-reinforced elastic plates with the reinforcement continuously distributed in concentric circles ; such a material is locally transversely isotropic, with the circumferential direction as the preferred direction. For an annulus bounded by concentric circles, the exact solution of the traction boundary value problem is obtained. When the extension modulus in the fibre direction is large compared to other extension and shear moduli, the material is strongly anisotropic. For this case a simpler approximate solution is obtained by treating the material as inextensible in the fibre direction. It is shown that the exact solution reduces to the inextensible solution in the appropriate limit. The inextensible theory predicts the occurrence of stress concentration layers in which the direct stress is infinite ; for materials with small but finite extensibility these layers correspond to thin regions of high stress and high stress gradient. A boundary layer theory is developed for these regions. For a typical carbon fibre-resin composite, the combined boundary layer and inextensible solutions give an excellent approximation to the exact solution. The theory is applied to the problem of an isotropic plate, under uniform stress at infinity, containing a circular hole which is strengthened by the addition of an annulus of fibre-reinforced material.