Observation of multiple peaking and a possible enhanced diffusion effect in channelling angular distributions

Multiple peaking has been observed in a channelling angular distribution obtained with N ions back-scattered from a Br-implanted Fe single crystal. An effect associated with the same distribution is seen which is attributed to the enhanced diffusion of N in Fe.     

Prediction of Comparative Catalytic Activity in the Series of Single Crystalline Surfaces in a Water-Gas Shift Reaction

A method is proposed for predicting comparative catalytic activity. This method includes the following stages: (1) formulation of a general reaction mechanism that is applicable to the whole series of catalysts, (2) calculation of the Arrhenius activation energies and the preexponential factors, and (3) kinetic simulations for the preset conditions. The method is illustrated by the model water-gas shift reaction (WGSR) CO + H₂O = CO₂ + H₂ and a series of catalytic single crystalline surfaces Cu(111), Ag(111), Au(111), Ni(111), Pd(111), Pt(111), and Fe(110). The mechanism is formulated using the computer program MECHEM. The activation energies are calculated using the UBI-QEP method. The reaction kinetics is simulated for a plug-flow reactor. The following series of the catalytic activity is obtained: Cu(111) > Ni(111) > Fe(111) > Pt(111), Pd(111) > Ag(111) > Au(111).__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 543–549.Original Russian Text Copyright © 2005 by Zeigarnik, Callaghan, Datta, Fishtik, Shustorovich.     

Diffusion, relaxation, and chemical exchange in casein gels: a nuclear magnetic resonance study

Water in protein/water mixtures can be described in terms of bound water and free water, by exchange between these two states, and by its exchange with appropriate sites on the protein. 1H-NMR diffusion and relaxation measurements provide insights into the mobility of these states. T2 relaxation-time dispersions (i.e., T2 relaxation times at different echo pulse spacings) reveal additional information about exchange. We present a comprehensive set of diffusion and T2 dispersion measurements on casein gels for which the protein/water ratio ranges from 0.25 to 0.5. The combination of these methods, taken in conjunction with concentration dependence, allows a good estimate of the parameters required to fit the data with Luz/Meiboom and Carver/Richards models for relaxation and chemical exchange. We compare the exchange (a) between water and protein and (b) between free water and bound water. Further, we attempt to distinguish chemical site exchange and diffusion/susceptibility exchange.     

Structural chemistry of arachno-nonaboranes

Single-crystal conventional-tube and synchrotron X-ray diffraction studies of the anions in [NMe(4)][arachno-B(9)H(12)-4,8-Br(2)] 1 and K[arachno-B(9)H(14)] 2, and also of the series of adducts [arachno-B(9)H(13)-4-L], where L is P(CCH)(3) (3), NHEt(2) (4), NC(5)H(5) (5), or NH(2)CH(2)Ph (6), are reported. Structural studies of 1-6, determined at low temperatures, located all atoms, including bridging and endo-terminal hydrogen atoms. The basic boron-hydride clusters of these, and of all the other known species with the arachno nine-vertex i-nonanborane geometry reported in the literature, are isostructural and feature three bridging and two endo-terminal hydrogen atoms on the open face. This arrangement is different from that previously reported for Cs[arachno-B(9)H(14)] 7 and for [arachno-B(9)H(13)-4-(NCMe)] 9. However, a new X-ray diffraction data set and refinement experimentally confirm the [3 x mu-H, 2 x endo] arrangement for 9 also. The experimental results for 1-6 support recently reported calculations for [B(9)H(14)](-), which predict both the structures and the (11)B NMR chemical shifts. These conclusions are also supported by calculations for 3, 4, and 9 and also for the [arachno-B(9)H(13)-4-(NCS)](-) anion in [NMe(4)][B(9)H(13)(NCS)] 8.     

The influence of polymer size and shape on self-diffusion of polysaccharides and solvents

Self-diffusion data obtained using pulsed-field-gradient nuclear magnetic resonance spectroscopy are presented for the poly-d-glucose molecules, dextran, amylopectin and glycogen in water and dimethyl sulphoxide. A model is developed for the influence of macroparticle shape on the concentration-dependence of polymer self-diffusion. The concentration-dependence of solvent and polymer self-diffusion indicates that amylopectin is planar with an axial ratio in excess of 6. In contrast, the dextran and glycogen data are consistent with random coil and spherical particle models, respectively. Both amylopectin and glycogen exhibit significantly different dimensions in the two solvent used.     

Field dependence of the vortex core size in a multiband superconductor

The magnetic field dependence of the vortex core size in the multiband superconductor NbSe2 has been determined from muon spin rotation measurements. The spatially extended nature of the quasiparticle core states associated with the smaller gap leads to a rapid field-induced shrinkage of the core size at low fields, while the more tightly bound nature of the states associated with the larger gap leads to a field-independent core size for fields greater than 4 kOe. A simple model is proposed for the density of delocalized core states that establishes a direct relationship between the field-induced reduction of the vortex core size and the corresponding enhancement of the electronic thermal conductivity. We show that this model accurately describes both NbSe2 and the single-band superconductor V3Si.     

Time-dependent NMR-velocimetry of a colloidal glass

The time-dependent velocity profiles in a Couette geometry of a soft glass made of colloidal star polymers suspended in squalene is presented. A shear-banded structure, hinted at by conventional rheological methods, is unequivocally confirmed with the local shear-rate of the low shear-rate branch below the measurable threshold. The evolution of the extent of the ‘zero-shear-band’ band is tracked and found to follow a Gompertz-like growth. The effects of altering shear rate, waiting time and temperature on the Gompertz fit parameters are investigated and show, among other relations, that the growth rate of the zero-shear band is proportional to the inverse of the applied shear rate, the time taken to reach unit strain.