First-shell Coordination Research Articles

The Structure of the Homogeneous Oxidation Catalyst, Mn(II)(Br-1)x, in Supercritical Water: An X-ray Absorption Fine-Structure Study

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure about Mn(II) and Br(-1) ions that exist as contact ion pairs in supercritical water. This work was performed to clarify why solutions of MnBr2 in supercritical water are known to effectively catalyze the aerobic oxidative synthesis of terephthalic acid from p-xylene as well as a number of other methylaromatic compounds. The Mn and Br K-edge spectra were collected at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne National Laboratory. The first-shell coordination structure about the Mn(II) ion changes from octahedral at ambient conditions to tetrahedral at supercritical conditions. Under supercritical conditions, the measured bond distances of Mn-OH2 and Mn-Br are 2.14 and 2.46 A, respectively. Direct contact ion pairs form with about 2 Br(-1) ions present in the first coordination shell of the Mn(II) ion. The structure of dissolved MnBr2, below 1.0 m, changes from essentially [Mn(II)(H2O)6]+2 to [Mn(II)(H2O)2(Br(-1))2] in supercritical water (scH2O). When an excess of Br(-1) ion is added, the bromide coordination number increases and the number of water molecules decreases. The results show that the initial MnBr2 catalyst in scH2O is tetrahedral with two Mn-Br contact ion pairs. The presence of the acetate anion deactivates the catalyst by formation of insoluble MnO.

A XANES and EXAFS Study of Hydration and Ion Pairing in Ambient Aqueous MnBr2 Solutions

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure of Mn(II) in aqueous MnBr2 solutions at ambient conditions from very dilute to the near saturation limit. The Mn K-edge EXAFS spectra for 0.05 and 0.2 m solutions showed that there was no Br(−I) in the first shell, and that the Mn(II) was fully hydrated with six water molecules in an octahedral arrangement. In contrast, for 6 m solution, the coordination number of water was reduced to about 5, and an average of about one bromine atom was present in the first shell as a contact ion pair. The 1s → 4p transition at 6545.5 eV confirmed the observation of Mn–Br contact ion pairs at high concentrations and the 1s → 3d transition at 6539.5 eV showed that the first shell coordination symmetry remained octahedral even in the presence of Mn–Br ion pairs.

A theoretical study of trivalent lanthanide ion microsolvation in water clusters from first principles

Although metal ion–water clusters M m+ (H 2O) n have been widely studied for many singly charged metal ions, thermodynamic and structural studies of di- or trivalent metal ion–water clusters remain relatively rare. We have investigated the structural and thermodynamic properties of Ln 3+(H 2O) n clusters (Ln 3+ = Nd 3+, Eu 3+, Er 3+ and Yb 3+) by means of Monte Carlo simulations using newly-developed, polarizable model potentials parameterized on the basis of ab initio calculations for small clusters. We report total cluster enthalpies and stepwise cluster binding enthalpies predicted by our simulations. Our results also indicate that Ln 3+ ions exhibit a well-defined interior solvation shell structure. At small cluster sizes ( n = 6–12), the first-shell coordination numbers are close to 6 or 7, whereas convergence towards bulk-like coordination numbers seems to be achieved at cluster size n ≥ 24. In contrast, convergence of the thermodynamic properties towards bulk values only occurs at much larger cluster sizes, n ≥ 64.

Hydration effect on structural parameter of chromium loaded in silica

This study used CrO3-loaded silica as an example to illustrate how the hydration of the samples could affect the structural parameters of an environmental sample. Our results showed that as the moisture content in samples increased, the amplitude of the EXAFS increased, while the oscillation frequency of the EXAFS was virtually unchanged. The hydration of Cr(VI) enhanced the first-shell coordination number (Cr–O); the Debye–Waller factor was also changed. The first-shell coordination number was 4.55 for CrO3-loaded silica slurry and 2.67 for the dry CrO3-loaded silica. The Debye–Waller factor (σ2) was 0.0036 for the slurry and 0.0014 for the dry one. In addition, hydration caused a considerable change in the pre-edge height and the white band of the XANES spectra of Cr(VI).

Nature of metal binding sites in Cu(II) complexes with histidine and related N-coordinating ligands, as studied by EXAFS.

Knowledge of the complexes formed by N-coordinating ligands and Cu(II) ions is of relevance in understanding the interactions of this ion with biomolecules. Within this framework, we investigated Cu(II) complexation with mono- and polydentate ligands, such as ammonia, ethylenediamine (en), and phthalocyanine (Pc). The obtained Cu-N coordination distances were 2.02 A for [Cu(NH(3))(4)](2+), 2.01 A for [Cu(en)(2)](2+), and 1.95 A for CuPc. The shorter bond distance found for CuPc is attributed to the macrocyclic effect. In addition to the structure of the first shell, information on higher coordination shells of the chelate ligands could be extracted by EXAFS, thus allowing discrimination among the different coordination modes. This was possible due to the geometry of the complexes, where the absorbing Cu atoms are coplanar with the four N atoms forming the first coordination shell of the complex. For this reason multiple scattering contributions become relevant, thus allowing determination of higher shells. This knowledge has been used to gain information about the structure of the 1:2 complexes formed by Cu(II) ions with the amino acids histidine and glycine, both showing a high affinity for Cu(II) ions. The in-solution structure of these complexes, particularly that with histidine, is not clear yet, probably due to the various possible coordination modes. In this case the square-planar arrangements glycine-histamine and histamine-histamine as well as tetrahedral coordination modes have been considered. The obtained first-shell Cu-N coordination distance for this complex is 1.99 A. The results of the higher shells EXAFS analysis point to the fact that the predominant coordination mode is the so-called histamine-histamine one in which both histidine molecules coordinate Cu(II) cations through N atoms from the amino group and from the imidazole ring.

Nearly uniform MgO-supported pentaosmium cluster catalysts.

[Os5C(CO)14]2- was synthesized on the surface of MgO by reductive carbonylation of adsorbed Os3(CO)12 at 548 K and 1 bar. The supported species were characterized by infrared (IR), 13C NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR and EXAFS data are consistent with the presence of [Os5C(CO)14]2-, formed in a yield of about 65%, along with smaller osmium carbonyl clusters. As the supported clusters were decarbonylated in flowing He or H2, they were characterized by IR and EXAFS spectroscopies, which indicate that the decarbonylation was complete after each treatment at 573 K. The EXAFS data characterizing the sample treated in He determine an Os-Os first-shell coordination number of 3.4, matching that of [Os5C(CO)14]2- and indicating that the Os5C frame was retained after decarbonylation in He. Treatment of MgO-supported [Os5C(CO)14]2- in H2 at 573 K resulted in the formation of aggregated osmium clusters larger than Os5C. The catalytic activity of Os5C for toluene hydrogenation was found to be an order of magnitude less than that of the aggregated osmium clusters, which are metallic in character.

EXAFS investigation of local structure ofEr3+andYb3+in low-silica calcium aluminate glasses

Erbium and ytterbium environments in low-silica calcium aluminate glasses, with nominal composition 58 CaO, $27.1\ensuremath{-}x$ ${\mathrm{Al}}_{2}{\mathrm{O}}_{3},$ 6.9 MgO, 8 ${\mathrm{SiO}}_{2},$ x ${\mathrm{Er}}_{2}{\mathrm{O}}_{3},$ or ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3},$ $0.2<~x<~1.5(\mathrm{mol}%),$ were investigated using x-ray-absorption fine-structure spectroscopy (EXAFS) on the Er and Yb ${L}_{\mathrm{III}}$ edge. The average Er-O bond separation was found to vary only slightly between 2.24 and 2.21 \AA{} and the Yb-O bond between 2.20 and 2.18 \AA{}. The first-shell coordination number decreased as the rare-earth oxide replaced ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ For ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$-doped samples this decrease was about 28%, from 6.5 to 4.7 atoms, whereas for ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$-doped ones it was about 14%, from 6.4 to 5.5 atoms. The decrease in the coordination number is attributed to the difficulty of rare-earth atoms to coordinate a larger number of the nonbridging oxygens that appear as the ${\mathrm{C}\mathrm{a}\mathrm{O}/\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}$ ratio decreases. The Debye-Waller factor $2{\ensuremath{\sigma}}^{2}$ varied from 0.026 to 0.012 ${\mathrm{\AA{}}}^{2}$.

The effects of solute-solvent electrostatic interactions on solvatochromic shifts in supercritical CO2.

Solvent clustering around attractive solutes is an important feature of supercritical solvation. We examine here the effects of the local density enhancement on solvatochromic shifts in electronic absorption and emission spectra in supercritical CO2. We use molecular dynamics (MD) simulation to study the spectral line shifts for model diatomic solutes that become more polar upon electronic excitation. The electronic transition is modeled as either a change from a quadrupolar to a dipolar solute charge distribution or as an increase in the magnitude of the solute dipole. Our main focus is on the density dependence of the line shifts at 320 K, which corresponds to about 1.05 times the solvent critical temperature, Tc, but results for higher temperatures are also obtained in order to determine the behavior of the line shifts in the absence of local density enhancement. We find that the extent of local density enhancement at 1.05Tc is strongly correlated with solute-solvent electrostatic attraction and that the density dependence of the emission line shifts resembles the behavior of the effective local densities, rho(eff), obtained from the first-shell coordination numbers. The differences that are seen are shown to be due to solute-solvent orientational correlations which provide an additional source of enhancement for electrostatic solvation energies and spectral line shifts.

In situ X-ray absorption spectroscopy study of lithium insertion in a new disordered manganese oxi-iodide

Mn K-edge X-ray absorption measurements were carried out on an amorphous manganese oxi-iodide (Li 0.60Na 0.16MnO 2.33I ≈0.05) as cathode material in plastic lithium batteries. X-ray absorption spectroscopy experiments were performed in situ for every lithium intercalation increment Δ x≈0.10 per formula unit along a discharge with 0≤ x≤0.67. The X-ray absorption near edge structure (XANES) spectra demonstrate a smooth decrease of Mn valence with in-situ reduction, confirmed by a monotonous increase in Mn–O distance from extended X-ray absorption fine structure (EXAFS). The first-shell octahedral coordination of manganese is hardly affected by lithium insertion, while the evolution of the Mn–Mn coordinence shows a trend towards the formation of MnO n polyhedra chains on deep discharge. The material discharged to an average manganese valence 3.25+ showed negligible static Jahn–Teller effect. Together with the inherent flexibility of disordered structures towards intercalation and bond length changes, this is probably a major cause of the stability of this oxi-iodide on cycling in lithium batteries.

Mechanisms of arsenic uptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite, and pyrite: an X-ray absorption spectroscopy study.

The mechanisms whereby As(III) and As(V) in aqueous solution (pH 5.5-6.5) interact with the surfaces of goethite, lepidocrocite, mackinawite, and pyrite have been investigated using As K-edge EXAFS and XANES spectroscopy. Arsenic species retain original oxidation states and occupy similar environments on the oxyhydroxide substrates, with first-shell coordination to four oxygens at 1.78 A for As(III) and 1.69 A for As(V). In agreement with other workers, we find that inner sphere complexes form, apparently involving bidentate (bridging) arsenate or arsenite. Interaction of As(III) and As(V) with the sulfide surfaces shows primary coordination to four oxygens (As-O: 1.69-1.76 A) with further sulfur (approximately 3.1 A) and iron (3.4-3.5 A) shells suggesting outer sphere complexation. Arsenic species were also coprecipitated with mackinawite (pH 4.0), and these samples were further studied following oxidation. At high As(III) or As(V) concentrations, arsenate or arsenite species form, probably as sorption complexes, along with poorly crystalline arsenic sulfide (the only product at low As(V) concentrations). All oxidized samples show primary coordination to four oxygens at 1.7 A, indicating As(V); these arsenates may show both outer sphere complexation with residual mackinawite and inner sphere complexation with new oxyhydroxides. These experiments help to clarify our understanding of As mobility in near-surface environments.

Synthesis and Structure of Tetrairidium Clusters on TiO2 Powder: Characterization by Infrared and Extended X-ray Absorption Fine Structure Spectroscopies

Site-isolated [Ir4(CO)12] on the surface of TiO2 powder (calcined at 200 or 400 °C) was prepared by deposition of [Ir4(CO)12] from n-hexane solution and, alternatively, by reductive carbonylation of TiO2-supported [Ir(CO)2(acac)] in the presence of CO at 1 atm and 100 °C. The preparation of the supported clusters and their subsequent decarbonylation by treatment in He or H2 were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The first-shell Ir−Ir coordination number representing the supported iridium carbonyl clusters was found to be 3.0 ± 0.3, with an Ir−Ir bond distance of 2.68 ± 0.03 A, consistent with X-ray diffraction data characterizing crystalline [Ir4(CO)12]. Decarbonylation in He at 300 °C gave TiO2-supported clusters retaining the tetrahedral Ir4 frame of the precursor [Ir4(CO)12], with an Ir−Ir first-shell coordination number of 3.0 ± 0.3. In contrast, decarbonylation of TiO2-supported [Ir4(CO)12] in H2 at 300 °C led to (nonuniform) aggregate...

A neutron diffraction study of yttrium- and lanthanum-aluminate glasses

Neutron diffraction data for three aluminate glasses have been collected for a range of scattering vector from 0.3 to 25 A ̊ −1 . A single phase glass, representing the high-density glass polymorph of yttrium-aluminate (AY25) composition has a structure resembling that of levitated YAG liquid. This glass has a first-neighbor Al–O distance of 1.82 Å and a first-shell coordination number of 4.16±0.21. The Y–O distance is represented by a single Gaussian peak centered at 2.28 Å which yields a first-neighbor coordination number of 6.64±0.33. These values indicate an increase in volume on melting of YAG solid solution. An equivalent lanthanum-aluminate glass (La25), also produced from the stable high-temperature aluminate liquid, has an Al–O distance of 1.79 Å and a coordination number of 4.55±0.23. The La–O distance is increased due to an increase in ionic radius for La(III). The magnitude of this La–O peak is lower than expected and there is evidence for a second La–O distance at 2.79 Å. The T( r) for the lanthanum-aluminate glass shows stronger features than the yttrium-aluminate glass at radial distances greater than 3 Å; these suggest a higher degree of order in the lanthanum-aluminate liquid. A two-phase yttrium-aluminate (AY20) glass, which represents a partly complete transition between a high-density liquid and a lower density polymorph, has an Al–O distance of 1.81 Å and a mean Al–O coordination number of 4.38±0.22. There is an increase in the Y–O radial distance on transition and also additional mid-range ordering. This mid-structure is similar to that seen in the La25 glass. The data show that rare earth aluminate liquids are dominated by a tetrahedral aluminate framework and that the liquid–liquid (transition) in the yttrium-aluminate liquids results from differences in packing among AlO 4 and Y–O polyhedra and not from a change in the mean Al–O coordination number.

X-ray absorption spectroscopy and imaging of heterogeneous hydrothermal mixtures using a diamond microreactor cell

Hydrothermal synthesis is an important route to novel materials. Hydrothermal chemistry is also an important aspect of geochemistry and a variety of waste remediation technologies. There is a significant lack of information about the speciation of inorganic compounds under hydrothermal conditions. For these reasons we describe a high-temperature, high-pressure cell that allows one to acquire both x-ray absorption fine structure (XAFS) spectra and x-ray transmission and absorption images of heterogeneous hydrothermal mixtures. We demonstrate the utility of the method by measuring the Cu(I) speciation in a solution containing both solid and dissolved Cu phases at temperatures up to 325 °C. X-ray imaging of the various hydrothermal phases allows micro-XAFS to be collected from different phases within the heterogeneous mixture. The complete structural characterization of a soluble bichloro-cuprous species was determined. In situ XAFS measurements were used to define the oxidation state and the first-shell coordination structure. The Cu–Cl distance was determined to be 2.12 Å for the CuCl2− species and the complete loss of tightly bound waters of hydration in the first shell was observed. The microreactor cell described here can be used to test thermodynamic models of solubility and redox chemistry of a variety of different hydrothermal mixtures.

Synthesis and Characterization of Site-Isolated Hexarhodium Clusters on Titania Powder

[Rh6(CO)16] was prepared on the surface of TiO2 (calcined at 200 or 400 °C) by deposition from n-hexane solution and by a surface-mediated synthesis from TiO2-supported [Rh(CO)2(acac)] in the presence of CO at 1 atm and 100 °C. The cluster preparation and subsequent decarbonylation by treatment in He or H2 were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. Deposition from solution gave aggregated [Rh6(CO)16] on TiO2; removal of the carbonyl ligands led to destruction of the Rh6 frame and sintering to give rhodium aggregates. In contrast, the reductive carbonylation of TiO2-supported [Rh(CO)2(acac)] gave site-isolated TiO2-supported [Rh6(CO)16] in high yield, paralleling the chemistry of rhodium carbonyls in neutral solutions and on neutral surfaces. Removal of the carbonyl ligands from the site-isolated clusters by treatment in H2 at 300 °C led to rhodium aggregates, but decarbonylation in He at 300 °C gave site-isolated Rh6 clusters on the TiO2. The first-shell Rh−Rh coordination number of these clusters was 4.4 ± 0.4 with a bond distance of 2.64 ± 0.03 Å. Thus, the clusters formed by decarbonylation of site-isolated TiO2-supported [Rh6(CO)16] are represented as octahedral Rh6 (which has a Rh−Rh first-shell coordination number of 4). EXAFS spectroscopy indicates that the decarbonylated Rh6 clusters on TiO2 calcined at 200 °C have a small amount of carbon bonded to them, but no such ligands were indicated in the spectra of the Rh6 clusters on TiO2 calcined at 400 °C.

Partially decarbonylated tetrairidium clusters on MgO: structural characterization and catalysis of toluene hydrogenation

Supported [HIr 4(CO) 11] −, prepared by adsorption of [Ir 4(CO) 12] from n-pentane solution onto MgO powder that had been partially dehydroxylated in vacuum at 400°C, was identified by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The clusters were decarbonylated to various degrees by treatment in He or H 2 and characterized by infrared and EXAFS spectroscopies. When the decarbonylation was carried out in H 2, the iridium aggregated; infrared data show that the CO ligands reacted with H 2, resulting in the formation of water and hydrocarbons. When the decarbonylation was carried out in He, the tetrahedral cluster frame remained unchanged; the fully decarbonylated clusters, modeled as Ir 4 tetrahedra, had an Ir–Ir first-shell coordination number of 3.0±0.1 at a distance of 2.67±0.01 Å. The partially decarbonylated tetrairidium clusters chemisorbed hydrogen. Temperature-programmed desorption (TPD) data show that the strength of bonding of hydrogen to the clusters increased with increasing cluster decarbonylation. The completely decarbonylated clusters were characterized by low chemisorption capacities (H/Ir and CO/Ir values, e.g. 0.27 and 0.39, respectively). Chemisorption and EXAFS data show that the amount of hydrogen or CO chemisorbed on Ir 4 increases with increasing size of the iridium clusters and aggregates, consistent with a support effect that is maximized for the smallest clusters, Ir 4. Data representing the catalytic hydrogenation of toluene show that the rate depends on the amount of hydrogen on the clusters — and not just on the degree of decarbonylation — consistent with the suggestion that iridium atoms free of CO ligands are not sufficient for catalysis to proceed. Toluene adsorbed on the support is implicated in the catalysis.

X-Ray Absorption Spectroscopy of Strontium(II) Coordination: I. Static and Thermal Disorder in Crystalline, Hydrated, and Precipitated Solids and in Aqueous Solution

Detailed analyses of crystalline, hydrated, and precipitated strontium compounds and an aqueous strontium solution by synchrotron extended X-ray absorption fine structure (EXAFS) were used to quantify local thermal and static disorder and to characterize strontium coordination in a variety of oxygen-ligated bonding environments. Analysis of anharmonic vibrational disorder (i.e., significant contribution from a third cumulant term (C3) in the EXAFS phase-shift function) in compounds with low and high static disorder around strontium showed that first-shell anharmonic contributions were generally not significant above experimental error in the EXAFS fits (R±0.02 Å with and without C3). The only case in which a significant apparent decrease in Sr–O distance was observed with increasing temperature, and for which a third cumulant term was significant, was for dilute strontium in aqueous solution. Empirical parameterization of Debye–Waller factor (σ2) for strontium compounds as a function of backscatterer atomic number (Z), interatomic Sr–Z distance, and temperature of spectral data collection showed systematic increases in σ2 as a function of increasing temperature and Sr–Z bond length. At values of σ2 greater than ≈0.025 Å2 (for N<12 and RSr-Z>3 Å), backscattering was generally not significant above noise levels in spectra of compounds of known crystal structure. Comparison of the EXAFS spectra of freshly precipitated SrCO3 (spectra collected wet) to that of dry, powdered strontianite (SrCO3(s)) indicated no significant differences in the local atomic structure around strontium. Analysis of partially hydrated strontium in natural Ca-zeolite (heulandite) showed that strontium is substituted only in the calcium (Ca2) site. Backscattering from aluminum and silicon atoms in the zeolite framework were apparent in the EXAFS spectra at low and room temperature at distances from central strontium of <4.2 Å. Comparison of strontium structural coordination determined in this and previous studies suggests that previous EXAFS determinations of hydrated strontium may have underestimated first-shell interatomic distances and coordination numbers because minor contributions to the EXAFS phase-shift and amplitude functions were not accounted for, either theoretically or empirically.

High-energy x-ray diffraction study of pure amorphous silicon

Medium and high-energy x-ray diffraction has been used to study the atomic structure of pure amorphous Si prepared by MeV Si implantation into crystalline silicon. Both as-implanted and annealed samples were studied. The inelastically scattered x rays were removed by fitting the energy spectrum for the scattered x rays. The atomic scattering factor of silicon, previously known reliably up to 20 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$, has been extended to 55 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$. The radial distribution function of amorphous Si, before and after annealing, has been determined through an unbiased Fourier transformation of the normalized scattering data. Gaussian fits to the first neighbor peak in these functions shows that scattering data out to at least 40 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ is required to reliably determine the radial distribution function. The first-shell coordination number increases from 3.79 to 3.88 upon thermal annealing at 600 \ifmmode^\circ\else\textdegree\fi{}C, whereas that of crystalline Si determined from similar measurements on a Si powder analyzed using the same technique is 4.0. Amorphous Si is therefore under coordinated relative to crystalline Si. Noise in the distribution function, caused by statistical variations in the scattering data at high-momentum transfer, has been reduced without affecting the experimental resolution through filtering of the interference function after subtracting the contribution of the first-neighbor peak. The difference induced by thermal annealing in the remainder of the radial distribution functions, thus revealed, is much smaller than previously believed.

Microstructure of N-Picolylpolyurethane Transition Metal Complexes

Spectroscopic methods are used to investigate coordination structure of N-picolylpolyurethane transition metal complexes (PUPYM, M = Co2+ and Ni2+). Geometrical arrangement of ligands in first-shell coordination sphere of metal ions is postulated to be tetrahedral CoL2Cl2 and octahedral NiL2Cl2Z2, where L is the picolyl group and Z is a hydrate. From extended X-ray absorption fine structure (EXAFS) analysis, bond lengths for metal−chlorine and metal−ligand of PUPYM are similar to those of small molecular weight transition metal complexes. A two-phase model of PUPYM, which best describes the experimental data of DMTA and SAXS, is proposed. One microphase is the hard domain of self-segregated hard segments brought about by metal−ligand interaction, and the other phase is the matrix of soft segments. Transition metal ion−ligand moieties and their interactions dominate the macroscopic thermal behavior of PUPYM. The ligand field stabilization energy difference (ΔLFSE) between metal d-electrons in complexes with two picolyl ligands in the coordination sphere of metal ions and complexes maintaining one picolyl ligand as coordination pendent group is calculated on the basis of observed coordination structure, and it represents the energy supplied to split coordination cross-links. ΔLFSE of polyurethane nickel(II) complex is larger than that of the cobalt(II) complex. Since the mobility of hard segments is in inverse proportion to the strength of coordination cross-links, a higher α-transition temperature of PUPYNi2+ with respect to PUPYCo2+ is found as expected.

Characterisation of Ion-Implantation-Induced Disorder in GaAs by EXAFS

The Ga and As local atomic environments in ion-implanted GaAs have been characterised with extended x-ray absorption fine structure spectroscopy. The accumulation of disorder prior to the onset of amorphisation has been investigated. For implantation at -188 °C, a heterogenous amorphisation process was identified and an increase in bondlength and Debye-Waller factor and a decrease in first-shell coordination number were observed. Analysis of the EXAFS data using the cumulant method revealed a monotonic increase in asymmetry in the interatomic distance distribution as a function of dose. Amorphised samples implanted to ion doses two orders of magnitude beyond the amorphisation threshold have also been studied and the structural parameters of amorphous GaAs were shown to be independent of implant conditions.

Transition metal complexes ofN-picolyl polyurethane

N-Picolyl polyurethanes (PUPY) were synthesized by nucleophilic substitution. The blends of these polyurethanes with various of transition metal chlorides [cobalt(II), nickel(II), and copper(II)] were studied by spectroscopic and thermal analysis. Ultraviolet-visible and infrared spectroscopic evidence indicates that a tetrahedral cobalt(II) complex with two pendent picolyl groups in the first-shell coordination sphere of Co2+ is formed in a series of blends with different molar ratio (from 10/1 to 2/1) of picolyl groups to cobalt(II) ions. According to the result of Small-Angle X-ray Scattering (SAXS), Differential Scanning Calorimetry (DSC), and Dynamic Mechanical Thermal Analysis (DMTA), coordination interaction between ligands in hard segments and metal ions provides a driving force for phase separation. The coordination strength of pyridine with Ni2+ is stronger than Co2+ and Cu2+. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1539–1546, 1998