Br K-edge Research Articles

X-Ray Absorption Spectroscopic Study of New High-TcSuperconducting Intercalation Compound of (HgX2)0,5Bi2Sr2CaCu2Oy(X = Br, I)

We have systematically performed X-ray absorption spectroscopic (XAS) studies on the new mercuric halide intercalated superconductors, (HgX 2 ) 0.5 Bi 2 Sr 2 CaCu 2 O y with X = Br and I, in order to investigate the electronic and crystal structures of these compounds. According to the I L 1- and Br K-edge X-ray absorption near edge structure (XANES) spectroscopic analyses, it is found that a small amount of electron is transferred from the intercalant HgX 2 to the host lattice, resulting in the negatively charged state of (HgX 2 ) δ- . The Cu K-edge extended X-ray absorption fine structure (EXAFS) analysis reveals that the bond distance or (Cu-O axial ) is slightly shortened upon intercalation, reflecting the oxidation of CuO 2 layer, which is well consistent with the I L 1- and Br K-edge XANES results. On the other hand, the present Hg L III -edge EXAFS results indicate that the metal is coordinated with two bromide or iodide ligands with bond length of 2.46 A for the HgBr 2 -intercalate and 2.65 A for the HgI 2 -one, respectively. This is the first example of linear molecular salt of HgX 2 stabilized in the solid lattice.

Multiple-Edge EXAFS Study of Solid and Liquid CuBr

X-ray absorption measurements of solid and liquid CuBr have been carried out around the Cu and Br K-edge at 295, 400, 600, 670, 793 K. EXAFS data of both edges have been analyzed simultaneously, for the first time on a binary compound, by using the ab-initio multiple-scattering GNXAS method. Multiple-edge EXAFS refinement allowed us to obtain an accurate determination of the first-neighbor distribution as a function of temperature. Mean bond distance, variance and skewness of the distribution have been measured and their statistical errors evaluated. A slight contraction of the most probable Cu-Br bond-length is observed at high temperatures approaching the superionic and the liquid phases (T ≥ 670 K). The asymmetry of the first-neighbor distribution is found to increase at high-temperature. Evidence for nearly-covalent bonding is found in liquid CuBr.

Scanning and Energy Dispersive EXAFS Studies of Nickel Alkene Oligomerisation Catalysts

In sirs EDE and scanning EXAFS studies of homogeneous alkene oligomerisation catalysts are reported. EDE spectra were acquired either on Station 9.3 at the SRS Daresbury Laboratory using a bent Si(220) crystal monochromator (Ni and Br K-edges) or on ID24 at the ESRF using a Si(111) monochromator in a Bragg geometry (Ni K-edge). The catalysts studied were derived from mixtures of Ni(acac) 2 (acac = 2,4-pentanedionato) and AlEt 2 (OEt) or NiX 2 (PEt 3 ) 2 (X = Cl, Br) and AlEt 3 . For Ni(acac) 2 AlEt 2 (OEt), Ni K-edge EDE is consistent with a mean Structure similar in nature to trimeric Ni(acac) 2 but with partial substitution of acac ligands by alkene and alkyl groups. At 25°C, further breakdown of the trimer occurs to give alkylated monomeric nickel species. For NiCl 2 (PEt 3 ) 2 /AlEt 3 , low temperature Ni Kedge EDE has been used to monitor alkylation of the Ni centre, and for ambient temperature mixtures of NiBr 2 (PEt 3 ) 2 /AlEt 3 , Br K-edge EDE data shows complete loss of Br from the metal centre occurring within minutes of adding AlEt 3 . Instrumental difficulties (detector and beamline set-up) at the current time prevent acquisition of analysable EXAFS in a timescale faster than 0.1.10 s for these types of systems.

Intracrystalline Structure of Molecular Mercury Halide Intercalated in High-Tc Superconducting Lattice of Bi2Sr2CaCu2Oy

X-ray absorption spectroscopic studies have been systematically carried out for the new high-Tc superconducting intercalation compounds, (HgX2)0.5Bi2Sr2CaCu2Oy (X = Br and I), with a hybridized crystal lattice consisting of superconducting layer and insulating one. From the Hg LIII-edge EXAFS analysis, it is found for the first time that the intercalated mercuric halide is stabilized as a linear molecule with the bond distance (Hg−X) of 2.46 Å for the HgBr2 intercalate and 2.65 Å for the HgI2 one, respectively. These are cross-confirmed not only by micro-Raman studies but also by one-dimensional electron density mapping based on (00l) X-ray reflections, which is well consistent with the EXAFS fitting results. The present I LI- and Br K-edge XANES results indicate a partial electron transfer from the host lattice of Bi2Sr2CaCu2Oy to the intercalant HgX2 layer with the partial oxidations of Bi2O2 and CuO2 layers. And the latter was also observed clearly from the Bi LIII-edge XANES and Cu K-edge EXAFS analyses, which are in good agreement with the I LI- and Br K-edge XANES results. It is, therefore, concluded that the charge transfer between host and guest is mainly responsible for the Tc evolution upon intercalation, and the Tc of this compound is surely related to the variation of hole concentration rather than electronic coupling along the c-axis.

Quantitative reaction kinetics in the liquid state by in situ XAFS

By quantitative evaluation of in situ recorded Br K-edge XAFS spectra we obtained kinetic parameters for the halogen exchange in 2-bromopropane with sodium iodide (‘Finkelstein reaction’). The results compare well with the classical data by Ingold and co-workers. Thus, XAFS is a suitable method to quantitatively monitor reactions in the liquid state.

Temperature Dependence of Extended X-Ray Absorption Fine Structure ofCuBr2-: Dynamical Effect of the Scattering Amplitude

We observed a noticeable dynamical effect of the scattering amplitude in Br K-edge extended X-ray-absorption fine structure (EXAFS) spectra of linear triatomic CuBr 2 - . The conventional static treatment of the scattering amplitude was found to give appropriate thermal damping for the first-nearest neighbor (NN) Br–Cu shell, while it showed fatal discrepancy for the second-NN Br–Br ′ shell which contains strong multiple-scattering paths due to the collinear arrangement. We have thus employed the dynamical treatment of the scattering amplitude for the second-NN shell with the assumption of the harmonic oscillators. The thermal damping of the second-NN shell was estimated appropriately by taking the bending motions into account. The present study gives the first clear example for the dynamical effect of the scattering amplitude and indicates that great care should be taken in the analysis of higher-NN shells containing multiple-scattering paths.

Short-range order in solid and liquid KBr probed by EXAFS

Br K-edge EXAFS spectra of solid and liquid KBr have been performed and analysed using advanced techniques for data analysis (GNXAS). Structural results on solid KBr at room temperature and near melting are compared with molecular dynamics (MD) simulations and diffraction data. The first-neighbour distribution is found to be strongly asymmetric even at room temperature, as also shown by MD simulations. This confirms the existence of important anharmonic effects in solid KBr. MD simulations are in agreement with EXAFS data at room temperature. For solid KBr near melting, anharmonic effects become very important. In comparison with MD calculations, the rise of the first-neighbour peak is found to be steeper, while the foot of the first-neighbour distribution is found to be slightly shifted toward longer distances. Experimental short-range partial pair distribution functions and are derived in liquid KBr for the first time. The first peak of the distribution is accurately determined and found to be in good agreement with MD simulations, confirming a slight contraction of the most probable Br - K distance in the liquid phase. The first peak of the distribution, measured with less accuracy, is found to be sharper and slightly shifted to longer distances, lowering the overlap with the first-neighbour K shell.

EXAFS of bromide ion in solvents and at air/solution interface

Solvation structures of Br − ion dissolved in 23 solvents have been studied at the Br K-edge using extended X-ray absorption fine structure (EXAFS). The results are summarized as follows. 1) Coordination bond lengths r are 0.31–0.32 nm for BrO in protic solvents and 0.35–0.36 nm for BrC in aprotic solvents. 2) The r values for water and formamide are larger than those for other protic solvents, possibly due to the strong solvent-solvent interactions of these two solvents. 3) Coordination numbers N widely differ, ranging between 3–6, from solvent to solvent. The value of N for alcohol linearly correlates with the Mayer-Gutmann's acceptor number A N. 4) In aprotic solvents containing two methyl groups per molecule, Br − is found to contact with the two methyl groups, thus the number of primary solvent molecules must be equal to half the N value. 5) EXAFS amplitude is found to correlate with A N of the solvent. To study the surface structure, a new EXAFS technique, tocal-reflection total-conversion-electron-yield EXAFS, has been developed and applied. The new technique utilizes polarized X-rays from synchrotron radiation, providing not only information on the solvation structures of ions at the aqueous solution surface but also on the preferential orientation of coordination bond at the solution surface.

Local symmetry influence on the formation of X-ray absorption near edge structure in KBr and RbCl

X-ray absorption near edge structure (XANES) of the Br and Rb K-edge of high-pressure (B2) phase of KBr and RbCl has been analyzed. We have used a full multiple-scattering method to investigate the conduction band structure of KBr and RbCl. Theoretical K-edge absorption spectra obtained over a large energy interval were used to analyze experimental data. Like in the case of low-pressure (B1) phase, XANES is found to be determined by the multiple scattering of the photoelectron in a large size cluster (30 or more atoms). For the energy interval up to 50 eV above the main edge, the single-scattering approximation seems to be less satisfactory. Comparative analysis of XANES of two different crystal phases made it possible to conclude on the role of local symmetry around absorbing site and characteristics of surrounding atoms in the formation of XANES and conduction band of compounds investigated.

Probing the conformational properties of guest molecules in solid inclusion compounds via EXAFS spectroscopy: bromine K-edge EXAFS studies of the bromocyclohexane/thiourea and trans-1-bromo-2-chlorocyclohexane/thiourea inclusion compounds

Thiourea inclusion compounds comprise a thiourea host structure containing one-dimensional (1D) tunnels within which guest molecules are accommodated. Bromine K-edge EXAFS spectroscopy has been used to investigate local structrural properties of bromocyclohexane and trans-1-bromo-2-chlorocyclohexane guest molecules included within the thiourea tunnel structure.For the trans-1-bromo-2-chlorocyclohexane/thiourea inclusion compound, the intramolecular Br⋯Cl distance (ca. 4.50 A) determined from Br K-edge EXAFS spectra recorded between 295 and 80 K indicates that the trans-1-bromo-2-chlorocyclohexane guest molecules are in the diaxial conformation, and the intramolecular Br⋯C(3,5) distance (ca. 3.27 A) further confirms that the bromine substituent is in an axial position with respect to the cyclohexane ring. Furthermore, an intermolecular Br⋯Br distance is observed for this inclusion compound at 80 K, suggesting that there is head-to-head ordering of the guest molecules within the thiourea tunnel structure at this temperature.For the bromocyclohexane/thiourea inclusion compound, the intramolecular Br⋯C(3,5) distance (ca. 3.27 A) indicates that the major conformation of the bromocyclohexane guest molecules is the axial conformation. An intermolecular Br⋯Br distance is evident from the EXAFS data (at both 295 and 80 K), suggesting that there is head-to-head ordering of the bromocyclohexane guest molecules in this inclusion compound.

Solvation structures for bromide ion in various solvents by extended X-ray absorption fine structure

The Br K-edge X-ray absorption spectra have been obtained for bromide ion dissolved in various protic and aprotic solvents. The Br ⋯ O distances in protic solvents which are fixed by the hydrogen-bond interaction and Br ⋯ C in aprotic solvents by the ion–dipole interaction were determined to be 3.1–3.2 and 3.5–3.6 A, respectively. The co-ordination numbers of Br– were found to be widely different, 3–6, from solvent to solvent. In protic solvents the solvation number correlates with the Mayer–Gutmann acceptor number, AN. Aprotic solvents can be divided into two groups in terms of the solvation structures. The solvation numbers in dimethylformamide, dimethylacetamide, dimethyl sulfoxide, acetone and ethyl methyl ketone are larger than those in other aprotic solvents possibly because of their bonding to Br–through two methyl groups. The amplitude of the extended X-ray absorption fine structure (EXAFS) oscillation evaluated from the peak intensity in the Fourier-transform spectrum correlates with AN throughout the series of solvents studied. This correlation is attributed to the similarity between the mechanism determining the amplitude of EXAFS and that of AN.

Structural investigation of gaseous, liquid, and solid Br2 by x-ray absorption.

Br K-edge x-ray-absorption measurements have been performed on gaseous (300 K), liquid (294 and 278 K), and solid (200 and 80 K) ${\mathrm{Br}}_{2}$. The experimental data have been analyzed by using a comparison with calculated ``ab initio'' structural signals. The molecular Br-Br average distance, determined with the accuracy of 0.003 \AA{}, shows an expansion of about 0.02 \AA{} going from the gas phase to the low-temperature condensed phases. The bond-distance variance is found to be 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ A${\mathrm{\r{}}}^{2}$ in the gas and liquid phases. The intermolecular contribution in the condensed phases, which is found to affect considerably the near-edge region of the spectrum k3.5 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, has been isolated by means of a subtraction procedure. Significant differences between the liquid samples at the two temperatures are observed and the potential of these measurements to provide information on orientational correlations between neighboring molecules is addressed.

Multielectron Excitation Structures in X-Ray Absorption Fine Structure Spectra of Bromide Ion in Tetra-n-Propylammonium Salt and Solutions

Abstract Strong multielectron excitation structures (MEES) in the extended X-ray absorption fine structure (EXAFS) region have been observed in Br K-edge absorption spectra of tetra-n-propylammonium bromide and bromide ion in aqueous and acetonitrile solutions. The MEES were extracted from the background of EXAFS spectra. The positions of steps due to MEES are well predicted by the Z+1 approximation.

Multiple scattering x-ray absorption analysis of simple brominated hydrocarbon molecules

A multiple-scattering analysis of Br K-edge extended X-ray absorption spectra of some brominated hydrocarbon molecules is presented. A new method for calculating multiple-scattering contributions and to perform data analysis has been used. The brominated hydrocarbons solubilized in different molecular assemblies and macromolecular systems may be used as probe molecules. An accurate analysis of the intramolecular contribution to the absorption spectra allows the possibility of identifying the intermolecular contacts related to the systems in which these molecule are solubilized to be evaluated

EXAFS Studies of Zinc Halide Glasses–Mixed Halide Systems

Structure of mixed halide glasses of ZnCl2-ZnBr2, ZnCl2-ZnI2 and ZnX2-KX' (X, X'=C1, Br, I) systems was investigated by means of Zn K-edge, Br K-edge and I L3-edge EXAFS spectroscopy. The network structure of the ZnCl2-ZnBr2 and ZnCl2-ZnI2 glasses is composed of corner-sharing tetrahedral units, ZnClnX4-n (X=Br, I). The bond distances between zinc and halide ions are similar to those of the corresponding end members though they are slightly varied by the substitution of halide ion. The addition of K+ ions introduces non-bridging halide ions and decreases the bond distance.

Temperature-Dependent XAFS Study on AgBr1-xIx Solid Solution

Local structures of rock salt type AgBr1-xIx solid solutions with x varying from 0.07 to 0.4, substances interesting from both viewpoints of crystal structure and excellent properties as a photographic material, have been investigated by EXAFS analysis of three sets of spectra, i.e., I K-edge, Br K-edge and Ag K edge, at various temperature from 10K to room temperature. The Br-Ag and I-Ag interatomic distances in the solid solution were different from each other and deviated from the average Ag-(Br,I), or virtual crystal, distance determined from X-ray diffraction at any temperature. The iodide content x dependence of I-Ag and Br-Ag interatomic distances changed with temperature. At low temperature the I-Ag and Br-Ag distances increased with increasing the iodide content, whereas at room temperature they did not exhibit significant change with composition. This suggested that the bonding nature of the AgBr1-xIx solid solution is rather ionic at low temperature, while it is rather covalent at room temperature.

Application of X-ray absorption spectroscopy to the structural characterisation of monodispersed benzotriazole coatings on partly oxidised copper thin films

X-Ray absorption spectroscopy using synchrotron radiation has been used to probe the structure of Copper-benzotriazole complexes and that of the benzotriazole involved in surface films following adsorption onto partly oxidised Cu thin films. Data recorded at the Cu-K edge and, using 4- and 5-bromo-substituted benzotriazole, at the Br-K edge are consistent with the CuI atoms coordinated to three or four benzotriazole molecules through the N(2) position on the triazole ring. Examination of the thin films using glancing-angle geometry suggests the adsorbed film to be only a few monolayers in thickness with the benzotriazole molecules coordinated to CuI ions in the oxidised Cu surface forming a hydrogen-bonded surface complex similar, in part, to that proposed by Fang et al. (Surf. Sci., 1986, 176, 476). At the interface the surface reaction appears to involve a topotactic arrangement with the (100) plane of Cu2O where the in-plane Cu–Cu interatomic distances closely match that expected for the surface complex. The overall limitations of the glancing-angle technique to the characterisation of this kind of interfacial system are discussed.

Splice Method in the EXAFS Analysis Applied to Cuprous Halide

The splice method in the EXAFS analysis, which was recently proposed for the direct determination of a radial distribution function (RDF), was applied to the systems with large disorder. The 4th-order cumulant expansion method cannot reproduce a strongly asymmetric RDF, while the splice method successfully gives the original RDF. Temperature dependence of the 1st nearest neighbor Br-Cu RDF in CuBr was investigated by analyzing the Br K-edge EXAFS with the splice method.

EXAFS spectroscopic studies of the bromine environment in the crystalline inclusion compounds formed between urea and α,ω-dibromoalkanes

EXAFS spectroscopy of the bromine K-edge has been used to investigate local structural properties of urea inclusion compounds containing α,ω-dibromoalkane ‘guest’ molecules [Br(CH2)nBr; n= 6–11]. In these inclusion compounds, the α,ω-dibromoalkane molecules are included within a urea ‘host’ substructure containing linear, parallel tunnels. For the inclusion compounds containing Br(CH2)nBr ‘guest’ molecules with n= 7–11, the host substructure is that of the conventional urea inclusion compounds, whereas the inclusion compound containing Br(CH2)6Br as the guest has a different host substructure.The principal aim of these studies was to determine the Br⋯Br distance between adjacent guest molecules in the urea tunnel. For the inclusion compounds containing Br(CH2)nBr molecules with n= 7–11, however, an accurate determination of the Br⋯Br distance has not been possible from data collected at room temperature and at 77 K (this was also found for data collected at 9 K for 1,10-dibromodecane/urea), probably because of dynamic disorder at high temperature and static positional disorder at low temperature. On the other hand, backscattering from the first three intramolecular carbon neighbours on the Br(CH2)nBr molecule is detected in the Br EXAFS, and structural parameters for these neighbours have been determined. In these inclusion compounds, the periodicities of the host and guest substructures along the tunnel axis are incommensurate, and therefore each guest molecule within a given tunnel is in a different environment with respect to the host substructure. As a consequence, no well defined features in the Br EXAFS spectra arising from backscattering by atoms in the host substructure are expected or observed.In the 1,6-dibromohexane/urea inclusion compound, the guest substructure is commensurate with the host substructure. The Br K-edge EXAFS results for 1,6-dibromohexane/urea are consistent with known structural features of this inclusion compound.

Strand breaks in plasmid DNA, natural and brominated, by low-energy X-rays.

Single- and double-strand breaks were measured in Col E1 plasmid DNA, natural and partially brominated, irradiated with monoenergetic X-rays (from a synchrotron radiation source) on both sides of the K-absorption edge for Br. The fraction of the undamaged supercoiled form decreased exponentially with the photon dose; its yield in the brominated DNA did not exhibit any energy-dependence. This result is consistent with the calculated relative contributions of photoelectric interactions with individual component atoms which show that an electron flux originating in light atoms outweighs that from bromine. However, X-rays of energy above the Br K-edge appear slightly more efficient in producing double-strand breaks. This result, as well as those reported in the literature on the Auger enhancement of different biological endpoints in cells containing brominated DNA, seem to suggest that the positive charge and submolecular effects associated with the photo-absorption in bromine play some role in damaging processes, besides the initial distribution of deposited electron energy.