57Fe NMR study of a spatially modulated magnetic structure in BiFeO3

The ⁵⁷ Fe spin echo spectra were studied in local magnetic fields of a BiFeO₃ ferroelectric antiferromagnet over the temperature range 77–304 K. The line shape analysis confirmed the presence of a spatially modulated, incommensurate cycloidal spin structure in BiFeO₃ and allowed the actual spin distribution to be reproduced throughout the cycloid length. The distribution was found to be essentially anharmonic. The modulated structure is stable over the whole temperature range studied. The cycloid wave becomes more harmonic with temperature elevation.     

Consensus bond-charge increments fitted to electrostatic potential or field of many compounds: Application to MMFF94 training set

Bond-charge increments (BCIs) are additive parameters used to assign atomic charges for the MMFF force field. BCI parameters are classified parsimoniously according to two atom types and the bond order. We show how BCIs may be fitted rapidly by linear least squares to the calculated ab initio electrostatic potential (ESP) or to the electrostatic field. When applied simultaneously to a set of compounds or conformations, the method yields consensus values of the BCIs. The method can also derive conventional “ESP-fit” atomic charges with improved numerical stability. The method may be generalized to determine atom multipoles, multicenter charge templates, or electronegativities, but not polarizability or hardness. We determine 65 potential-derived (PD) BCI parameters, which are classified as in MMFF, by fitting the 6-31G* ESP or the electrostatic field of the 45 compounds in the original MMFF94 training set. We compare the consensus BCIs with classified BCIs that were fit to each molecule individually and with “unique-bond” BCIs (ESP-derived atom charges). Consensus BCIs give a satisfactory representation for about half of the structures and are robust to the adjustment of the alkyl CH bond increment to the zero value employed in MMFF94. We highlight problems at three levels: Point approximation: the potential near lone pairs on sulfur and to some extent nitrogen cannot be represented just by atom charges. Bond classification: BCIs classified according to MMFF atom types cannot represent all delocalized electronic effects. The problem is especially severe for bonds between atoms of equivalent MMFF type, whose BCI must be taken as zero. Consensus: discrepancies that occur in forming the consensus across the training set indicate the need for a more detailed classification of BCIs. Contradictions are seen (e.g., between acetic acid and acetone and between guanidine and formaldehydeimine). We then test the three sets of PD-BCIs in energy minimizations of hydrogen-bonded dimers. Unique-bond BCIs used with the MMFF buffered 14–7 potential reproduce unscaled quantum chemical dimer interaction energies within 0.9 kcal/mol root mean square (or 0.5, omitting two N-oxides). These energies are on average 0.7 (or 0.5) kcal/mol too weak to reproduce the scaled quantum mechanical (SQM) results that are a benchmark for MMFF parameterization. Consensus BCIs tend to weaken the dimer energy by a further 0.4–0.6 kcal/mol. Thus, consensus PD-BCIs can serve as a starting point for MMFF parameterization, but they require both systematic and individual adjustments. Used with a “harder” AMBER-like Lennard–Jones potential, unique-bond PD-BCIs without systematic adjustment give dimer energies in fairly good agreement with SQM. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1495–1516, 1999     

Kinetics and mechanism of glycolide and ethylenoxalate copolymerization. Characteristics of the copolymers formed and mechanism of the biodegradation

The study of glycolide (GL) and ethylenoxalate (EO) copolymerization with the use of SnCl₂ 2 H₂O as a catalyst was carried out by ¹H NMR spectroscopy. Monomer reactivities are found to be close and equal to 1,1 and 1,11 for GL and 0,71 and 0,85 for EO at 150 and 170 °C, respectively. Kinetic data obtained show that the reaction proceeds without induction period up to entire consumption of every monomer. The detailed identification of ¹H spectra enables to follow the polymer microstructure in the course of copolymerization. Thermal properties of copolymers and their biodegradation abilities have been studied. The biodegradation rate is shown to increase with growing EO proportion in the copolymer. The lactone biodegradation mechanism where the electrophility of carbon and nucleophility of OH group are playing the most important role, is suggested. The correlation between the biodegradation rate and the chemical shift in ¹³C NMR spectra of the carbon atom of the carbonyl group has been established.     

Space‒time inhomogeneity of the electron flow in pyroelectric X-ray sources

The operating conditions of X-Ray sources based on LiNbO₃ crystals are investigated during heating-cooling cycles. It is demonstrated that the radiation intensity is unstable. The radiation is accompanied by electrical breakdowns in the Z plane of the crystals and emission of photon packets not described by the Poisson distribution. Visualization of the electron beam through the grid electrode by a luminescent screen showed that the electron beam is not uniform in the Z-plane of the crystal and greatly changes with temperature. It is found that, under definite conditions, the numerous redistributions of the intense emission zones occurred between different areas at Z-surface of the crystal. Possible reasons for the observed effects are examined. The obtained data are important for creating pyroelectric X-ray and neutron sources presuming the usage of strong electric fields whose strength reaches 10⁵ V/cm.     

Nature of Local Symmetry Violations of Ions in the Magnetic Subsystem of Magnetoplumbite Crystal,According to Raman Scattering Spectroscopy Data

The structure of an M-type lead hexaferrite (magnetoplumbite) crystal is studied via the diffraction of synchrotron radiation. The crystal model contains two iron ions positions with dynamic disorder. The dynamics of the crystal lattice in the temperature range of 80 to 775 K is studied by Raman scattering. Particular attention is given to the discussing the nature of the local disorder of iron ions in the trigonal bipyramidal environment of oxygen ions.     

Spin dynamics in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> low-dimensional helical magnets

Comprehensive NMR investigation of low-frequency spin dynamics of LiCu₂O₂ (LCO) and NaCu₂O₂ (NCO) low-dimensional helical magnets in the paramagnetic state has been carried out for the first time. Temperature dependences of the spin–lattice relaxation rate and anisotropy on various LCO/NCO nuclei have been determined at various orientations of single crystals in an external magnetic field. The spatial asymmetry of spin fluctuations in LCO multiferroic has been discovered. The quantitative analysis of the anisotropy of spin–lattice relaxation in LCO/NCO has allowed estimating the contributions of individual neighboring Cu²⁺ ions to the transferred hyperfine field on Li⁺(Na⁺) ions.     

Features of the Jahn-Teller transition in Ni1 − x Co x Cr2O4 solid solutions

The structure of Ni_{1 ? x} Co _x Cr₂O₄ solid solutions has been investigated by synchrotron X-ray powder diffraction in the temperature range of 90–350 K for compositions x = 0, 0.005, 0.01, 0.015, 0.02, 0.1, 0.5, and 0.8, and their structures have been refined using the Rietveld analysis. The T-x phase diagram for solid solutions in the studied range of temperatures and concentrations has been constructed based on the obtained data. The revealed structural phase transitions have been explained from the viewpoint of the crystal field theory and the Jahn-Teller cooperative effect. Distortions of polyhedra, which are the cause of the structural phase transition, have been evaluated. The applicability of the Landau phenomenological theory and the possibility of using the magnitude of spontaneous strains as the order parameter have been discussed.     

Levitating states of superconducting rings in the field of a fixed ring with constant current

We consider the possibility of designing a plasma trap with a magnetic system formed by super-conducting rings and coils levitating in the field of a fixed coaxial coil carrying constant current. An analytic dependence of the potential energy of such a system with one or two levitating superconducting rings having trapped preset magnetic fluxes on their coordinates in the uniform gravitational field is obtained in the thin ring approximation. Calculations performed in the Mathcad system show that equilibrium states of such a system exist for certain values of parameters. Levitating states of a single superconducting ring and two superconducting rings in the field of the coil with constant current are observed experimentally in positions corresponding to calculated values.     

Ionic Liquid–Liquid Extraction and Supported Liquid Membrane Analysis of Lipophilic Wood Extractives from Dissolving-Grade Pulp

In this study, the extraction of lipophilic wood extractives from dissolving pulp samples using ionic liquid–liquid extraction and a two phase hollow fibre supported liquid membrane was investigated. Ionic liquids are capable of dissolving a range of organic and polymeric compounds and are biodegradable, with a negligible vapour pressure. Pulp samples were dissolved in a suitable amount of molten 1-butyl-3-methylimidazolium chloride to give 5 % cellulose solution. Pure cellulose was regenerated by adding water and filtered off. The ionic liquid-aqueous filtrate was first extracted for lipophilic extractives using liquid–liquid extraction. Then, a two phase hollow fibre supported liquid membrane extraction of lipophilic extractives was performed to extract the derivatized compounds prior to analysis by gas chromatography mass spectrometry. The operational parameters of this sample preparation approach were optimised using sterols and fatty acid methyl esters. The variation of enrichment factors and extraction efficiency with respect to liquid membrane, extraction time, stirring speed and sample pH were observed and used to get the optimal parameters. The approach was used in the analysis of oxygen bleached dissolving pulp samples in which main compounds identified were fatty acids, sterols, fatty alcohols, steroid hydrocarbons and ketones. These compounds were similar to those obtained using molecular solvent extraction method, which indicated the absence of chemical reaction between extractives and ionic liquid used.