N.M.R. solid echoes in systems of dipolar-coupled inequivalent spins-1 or dipolar-coupled inequivalent pairs of spins-1/2

Proton solid-echo transverse relaxation functions for many thermotropic and lyotropic mesophases, mapped by measurement of the echo amplitude S_yx (t′ = τ) as a function of τ using a P_y (90°)-τ-P_x (90°)-t′ sequence, yield gaussian behaviour of the form exp [-½M ^E ₂τ²] for decays up to 20–30 per cent of the value at τ = 0. M ^E ₂, the second moment for the dipolar interactions between the spin-½ pairs, is related to the van Vleck second moment M ^VV ₂ through a factor f. Whilst experiments suggested a value of 0·70–0·72 for f = M ^E ₂/M ^VV ₂, simple models that ignored the non-equivalence of the dipolar-coupled spin-pairs had predicted f = 0·65. In this paper we derive an exact analytic expression for the spin response of a model of two dipolar-coupled inequivalent spins-1 to the pulse sequence P_y (90°)-τ-P _α(β)-t′, and show that the present model, with the quenching of the spin-flip terms of the dipolar hamiltonian, resolves the afore-mentioned discrepancy. We also reconcile the differences between the experimental and the earlier predicted values of f for deuteron N.M.R. spin echoes in perdeuterated solids.     

Dianionic Mononuclear Cyclo‐P4 Complexes of Zero‐Valent Molybdenum: Coordination of the Cyclo‐P4 Dianion in the Absence of Intramolecular Charge Transfer

Relative to other cyclic poly‐phosphorus species (that is, cyclo‐P_n), the planar cyclo‐P₄ group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η⁴‐P₄ complexes are presented that can be viewed as the simple coordination of the [cyclo‐P₄]^2? dianion to a neutral metal fragment. Treatment of the neutral, molybdenum cyclo‐P₄ complexes Mo(η⁴‐P₄)I₂(CO)(CNAr^Dipp2)₂ and Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)₂ with KC₈ produces the dianionic, three‐legged piano stool complexes, [Mo(η⁴‐P₄)(CO)(CNAr^Dipp2)₂]^2? and [Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)]^2?, respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η⁶‐benzene complex (η⁶‐C₆H₆)Mo(CO)₃ regarding the metal‐center valence state and electronic population of the planar‐cyclic ligand π system.     

Investigation of Boron Distribution in Solids Demonstrated in Nickel and in Silica Glass

By means solid state track autoradiography has been demonstrated that boron is enriched on grain boundaries of nickel and on texture boundaries of silica glass. This result has been proved on nickel by SIMA and on silica glass by luminescence autoradiography.     

Sintering and crystallization of titanium-containing lead borate glasses

The sintering and crystallization of glass powders with chemical composition 47 PbO—38 B₂O₃—15 TiO₂ (mole %) was investigated under non-isothermal conditions by shrinkage measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD), dielectric measurement and positron annihilation (Doppler broadening). The correlation between changes of macroscopic properties and positron annihilation results recommends this method for the detection of structural changes in glasses.     

Oxidative-Insertion Reactivity Across a Geometrically Constrained Metal→Borane Interaction

While interest in cooperative reactivity of transition metals and Lewis acids is receiving significant attention, the scope of known reactions that directly exploit the polarized reverse-dative σ-bond of metal-borane complexes (i.e., M → BR₃) remains limited. Described herein is that the platinum (boryl)iminomethane (BIM) complex [Pt(κ²-N,B-^Cy2BIM)(CNAr^Dipp2)] can effect the oxidative insertion of a range of unsaturated organic substrates, including azides, isocyantes, and nitriles, as well as CO₂ and elemental sulfur (S₈). In addition, alkyl migration processes available to the BIM framework allow for post-insertion reaction sequences resulting in product release from the metal center.     

An advanced decision support system for European disaster management: the feature of the skills taxonomy

Central European Journal of Operations Research - Mankind has faced a huge increase in severe natural and man-made disasters worldwide in the last few years. Emergency responders on a strategic,...     

A simple precision phase calibrator

A circuit is described which produces two square wave outputs with precise phase differences between them. The range of phase differences which may be selected is from 0° to 360° in increments of 22·5°. However, any increment given by 180°/n, where n is an integer, can be achieved by a suitable choise of circuit components. The device is intended as a means of checking the calibration of phase meters or even for actually calibrating such meters.     

Flow Properties of Selected Pharmaceutical Powders from a Vibrating Spatula

Studies have been performed to characterize the flow of certain powders of pharmaceutical interest from a vibrating spatula. Lactose, cromolyn sodium, sodium chloride and charcoal powders were prepared by milling or sieving. Jet milling yielded micronized powder (1–10 μm), which was recovered from the product jar or metal cyclone. Particles in the larger size ranges were prepared by sieving. Photomicrograph images of the products were obtained by scanning electron microscopy. All of the powders were subjected to flow studies from vibrating beds to a top loading pan balance. Cumulative mass versus time, flow profiles were plotted. Bulk flow properties, as reflected in mass flow rates (M_i), were assessed. Irregular flow patterns were obtained for lactose, cromolyn sodium and charcoal, whereas sodium chloride was free flowing. The lengths of the lines depicting cumulative mass versus time, for samples of each material, were estimated using the inswing structured walk technique. Stride lengths (γ) of 1–100 mm were employed. Fractal dimensions were obtained from graphs of the logarithm of the cumulative mass versus time line length plotted against the logarithm of the stride length. Thus, flow irregularities were indicated by fractal dimensions (δ). In all of the powders studied, mass flow rates increased as the particle size increased. Sodium chloride did not exhibit fractal behavior (δ ≤ 1.007), as there were no flow irregularities. Lactose, cromolyn sodium and charcoal did not appear to exhibit fractal behavior (δ ≤ 1.010) at large stride lengths (γ = 15–100 mm). At small stride lengths (γ = 1–10 mm) these materials did exhibit fractal behavior (δ equals; 1.029–1.059) indicative of flow irregularity. This method may be used to numerically quantify uniformity of flow from a vibrating spatula, and would be useful in a number of industrial processes.