Comparison of the site occupancies determined by combined Rietveld refinement and density functional theory calculations: example of the ternary Mo-Ni-Re σ phase

The site occupancies of the Mo-Ni-Re σ phase have been studied as a function of the composition in the ternary homogeneity domain by both experimental measurements and calculations. Because of the possible simultaneous occupancy of three elements on the five sites of the crystal structure, the experimental determination of the site occupancies was achieved by using combined Rietveld refinement of X-ray and neutron diffraction data, whereas calculation of the site occupancies was carried out by using the density functional theory results of every ordered (i.e., 3(5) = 243) configuration appearing in the ternary system. A comparison of the experimental and calculation results showed good agreement, which suggests that the topologically close-packed phases, such as the σ phase, could be described by the Bragg-Williams approximation (i.e., ignoring the short-range-order contributions). On the other hand, the atomic distribution on different crystallographic sites of the Mo-Ni-Re σ phase was found to be governed by the atomic sizes. Ni, having the smallest atomic size, showed a preference for low-coordination-number (CN) sites, whereas Mo, being the largest in atomic size, preferred occupying high-CN sites. However, the preference of Re, having intermediate atomic size, varied depending on the composition, and a clear reversal in the preference of Re as a function of the composition was evidenced in both the calculated and experimental site-occupancy results.     

Intermetallic compounds as negative electrodes of Ni/MH batteries

This review is devoted to the main families of thermodynamically stable intermetallic compounds (AB₅-, AB₂- and AB-type alloys) that have been researched in the last thirty years as materials for negative electrodes in nickel–metal hydride batteries. The crystal structure of these compounds and their hydrides is widely described. Their solid–gas hydrogenation properties and, particularly, the related desorption isotherm curves are examined as a useful criterion for the selection of suitable battery materials. The electrochemical performances obtained with these alloys are reported and the given solutions to common problems such as corrosion, passivation, decrepitation and short cycle life are discussed. Only AB₅-based compounds have achieved, up to now, enough development for being widely present on the market, and exhibit improved battery performances in comparison with the polluting Ni/Cd batteries. The high capacity of AB₂-based compounds and the remarkable electrochemical activity of some AB-based alloys make, however, further research on all the reviewed families still valuable. Received: 16 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Ring‐opening polymerization of ε‐caprolactone and L‐lactide from silica nanoparticles surface

Coating of silica nanoparticles by biocompatible and biodegradable polymers of ε‐caprolactone and L ‐lactide was performed in situ by ring‐opening polymerization of the cyclic monomers with aluminum, yttrium, and tin alkoxides as catalysts. Hydroxyl groups were introduced on the silica surface by grafting of a prehydrolyzed 3‐glycidoxypropyl trimethoxysilane to initiate a catalytic polymerization in the presence of metal alkoxides. In this manner, free polymer chains were formed to grafted ones, and the graft density was controlled by the nature of the metal and the alcohol‐to‐metal ratio. The grafting reaction was extensively characterized by spectroscopic techniques and quantified. Nanocomposites containing up to 96% of polymer were obtained by this technique. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1976–1984, 2004     

Multi-targeted directed ligands for Alzheimer’s disease: design of novel lead coumarin conjugates

Alzheimer’s Disease (AD) is a neurodegenerative disease characterized by central nervous system insults with progressive cognitive (memory, attention) and non-cognitive (anxiety, depression) impairments. Pathophysiological events affect predominantly cholinergic neuronal loss and dysfunctions of the dopaminergic system. The aim of the current study was to design multi-targeted directed lead structures based on the coumarin scaffold with inhibitory properties at two key enzymes in disease relevant systems, i.e. acetylcholinesterase (AChE) and monoamine oxidase B (MAO-B). Conventional and microwave synthetic methods were utilized to synthesize coumarin scaffold-based novel morpholino, piperidino, thiophene and erucic acid conjugates. Biological assays indicated that the coumarin–morpholine ether conjugate BPR 10 was the most potent hMAO-B inhibitor. The coumarin–piperidine conjugates BPR 13 and BPR 12 were the most potent inhibitors of eeAChE at 100 μM and 1 μM, respectively. Molecular modelling studies were conducted with Accelrys^® Discovery Studio^® V3.1.1 utilising the published hMAO-B (2V61) and hAChE (4EY7) crystal structures. Compound BPR 10 occupies both the entrance and substrate cavities of the active site of MAO-B. BPR 13 resides in both the peripheral anionic site (PAS) and the catalytic anionic site (CAS) of hAChE. This study demonstrated that the coumarin scaffold serves as a promising pharmacophore for MTDLs design.     

Electricity production from lignocellulosic biomass by direct coupling of a gasifier and a nickel/yttria-stabilized zirconia-based solid oxide fuel cell: influence of the H<Subscript>2</Subscript>S content of the syngas onto performances and aging

The aim of this work is to study the reactivity of a Ni-YSZ-based solid oxide fuel cell (SOFC) fueled with gaseous mixtures having the same composition as the syngas issued from a fixed-bed downdraft and staged gasification pilot. The syngas issued from the gasifier contains some ppm(v) of H₂S, and in order to adapt the purification process, the influence of this compound on the Ni-YSZ-based SOFCs is evaluated at 600 and 850 °C. The influence of H₂S depends on fuel composition, temperature but also of current density. In H₂–N₂ mixtures and only at 600 °C, a significant decrease of cell performances is observed for H₂S?>?4.5 ppm(v). For H₂–CO–CO₂–N₂ mixtures, the influence is more important since a small decrease of performance can be observed for 1 ppm(v) of H₂S even at 850 °C. Nevertheless, at 600 °C, it is possible to avoid damage by limiting the current density. Aging experiments, realized at 750 °C, show that the influence of 1 and 2 ppm(v) of H₂S is more important during the first 20 h and is reversible: at this temperature, after poisoning with 1 ppm(v) of H₂S during 72 h, the cell recovers 91% of its initial power density after 100 h in pure hydrogen, and after subsequent poisoning with 2 ppm(v) of H₂S during 77 h, the cell recovers 94% of its initial power density after 168 h in pure hydrogen.     

The elusive atomic rationale for DNA base pair stability

A systematic analysis of the electrostatic interaction between 27 natural DNA base pairs was carried out, based on ab initio correlated wave functions and the topology of the electron density. Using high rank multipole moments we show that the atomic partitioning of the interaction energy contains many substantial contributions between distant atoms. Profiles of cumulative energy versus internuclear distance show large fluctuations and provide an electrostatic fingerprint of the partitioning of interaction energy in a complex. A quantified comparison between each pair of energy profiles, one for each base pair, makes clear that there is no correlation between the total base pair interaction energy and the shape of the profile. In other words, base pairs with similar interaction energy are not stable for the same reasons in terms of atomic partitioning. In summary, simple rules to rationalize the pattern of energetic stability of naturally occurring base pairs in terms of subsets of atoms are elusive. Our work cautions against inappropriate use of Jorgensen's secondary interaction hypothesis.     

Ring dehydration mechanisms and kinetics of polyamic acid models in solution and in the solid state

We have synthesized and studied the ring dehydration mechanisms and kinetics of polyamic acid models in solution and in the solid state using ¹³C-NMR (solid and liquid), HPLC, FTIR, and x-ray diffraction. Results obtained in solution show the role of temperature, catalysts, and the basicity of the amine in ring dehydration mechanisms and kinetics, as well as conformation and intramolecular bonds in the amic acid bond in the solid state. © 1993 John Wiley & Sons, Inc.