Home About us Contact
|
Home About us Contact | ||
|
|
||
Octahedral
Terms modified by Octahedral Selected AbstractsSix-coordinate Co2+ with imidazole, NH3, and H2O ligands: Approaching spin crossoverINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2007Ann M. Schmiedekamp Abstract Octahedral, six-coordinate Co2+ can exist in two spin states: S = 3/2 and S = 1/2. The difference in energy between high spin (S = 3/2) and low spin (S = 1/2) is dependent on both the ligand mix and coordination stereochemistry. B3LYP calculations on combinations of neutral imidazole, NH3, and H2O ligands show that low-spin isomers are stabilized by axial H2O ligands and in structures that also include trans pairs of equatorial NH3 and protonated imidazole ligands, spin crossover structures are predicted from spin state energy differences. Occupied Co d orbitals from the DFT calculations provide a means of estimating effective ligand strength for homoleptic and mixed ligand combinations. These calculations suggest that in a labile biological system, a spin crossover environment can be created. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Iron-Containing Poly(propylene imine) Dendromesogens with Photoactive PropertiesMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 7 2010Natalia E. Domracheva Abstract Here, we report the first results of investigation the local structure and photoactive properties of iron-containing dendromesogens based on decyloxybenzoate substituted poly(propylene imine) dendrimers of the first to fifth generations. Iron ions existing in a high-spin state are coordinated in dendrimer ligands by two kinds of iron-complexing sites with an octahedral and a tetrahedral symmetry. Octahedral (high-symmetry) centers are located at the border of the dendrimeric core, while the tetrahedral centers with strong rhombic distortion of iron environment are distributed throughout all branching of the dendrimeric core. It has been found that all iron-containing dendromesogens exhibit light-harvesting and fluorescence properties. [source] Stabilization of High-Valent FeIVS6 -Cores by Dithiocarbamate(1,) and 1,2-Dithiolate(2,) Ligands in Octahedral [FeIV(Et2dtc)3,n(mnt)n](n,1), Complexes (n=0, 1, 2, 3): A Spectroscopic and Density Functional Theory Computational Study,CHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2010Carsten Milsmann Dr. Abstract A detailed spectroscopic and quantum chemical analysis is presented to elucidate the electronic structures of the octahedral complexes [Fe(Et2dtc)3,n(mnt)n]n, (1,4, n=3, 2, 1, 0) and their one-electron oxidized analogues [Fe(Et2dtc)3,n(mnt)n](n,1), (1ox,4ox); (mnt)2, represents maleonitriledithiolate(2,) and (Et2dtc)1, is the diethyldithiocarbamato(1,) ligand. By using X-ray crystallography, Mössbauer spectroscopy, and Fe and S K-edge X-ray absorption spectroscopy (XAS) it is convincingly shown that, in contrast to our previous studies on [Fe(cyclam)(mnt)]1+ (cyclam=1,4,8,11-tetraazacyclotetradecane), the oxidation of 1,4 is metal-centered yielding the genuine FeIV complexes 1ox,4ox. For the latter complexes, a spin ground state of S=1 has been established by magnetic susceptibility measurements, which indicates a low-spin d4 configuration. DFT calculations at the B3LYP level support this electronic structure and exclude the presence of a ligand , radical coordinated to an intermediate-spin ferric ion. Mössbauer parameters and XAS spectra have been calculated to calibrate our computational results against the experiment. Finally, a simple ligand-field approach is presented to correlate the structural features obtained from X-ray crystallography (100,K) with the spectroscopic data. [source] Synthesis and Characterisation of ,3 -Octahedral [Ni36Pd8(CO)48]6, and [Ni35Pt9(CO)48]6, Clusters Displaying Unexpected Surface Segregation of Pt Atoms and Molecular and/or Crystal Substitutional Ni/Pd and Ni/Pt DisorderCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2004Cristina Femoni Dr. Abstract The synthesis and structure, as well as the chemical and electrochemical characterisation of two new ,3 -octahedral bimetallic clusters with the general [Ni44,xMx(CO)48]6, (M = Pd, x = 8; M = Pt, x = 9) formula is reported. The [Ni35Pt9(CO)48]6, cluster was obtained in reasonable yields (56,% based on Pt) by reaction of [Ni6(CO)12]2, with 1.1 equivalents of PtII complexes, in ethyl acetate or THF as the solvent. The [Ni36Pd8(CO)48]6, cluster was obtained from the related reaction with PdII salts in THF, and was isolated only in low yields (5,10,% based on Pd), mainly because of insufficient differential solubility of its salts. The unit cell of the [NBu4]6[Ni35Pt9(CO)48] salt contains a substitutionally Ni,Pt disordered [Ni24(Ni14,xPtx)Pt6(CO)48]6, (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. The unit cell of the corresponding [Ni36Pd8(CO)48]6, salt contains two independent [Ni30(Ni8,xPdx)Pd6(CO)48]6, (x = 2) hexaanions. The two display similar substitutional Ni,Pd disorder, which probably arises only from crystal disorder. The structure of [Ni36Pd8(CO)48]6, establishes the first similarity between the chemistry of Ni-Pd and Ni-Pt carbonyl clusters. A comparison of the chemical and electrochemical properties of [Ni35Pt9(CO)48]6, with those of the related [Ni38Pt6(CO)48]6, cluster shows that surface colouring of the latter with Pt atoms decreases redox as well as protonation propensity of the cluster. In contrast, substitution of all internal Pt and two surface Ni with Pd atoms preserves the protonation behaviour and is only detrimental with respect to its redox aptitude. A qualitative rationalisation of the different surface-site selectivity of Pt and Pd, based on distinctive interplays of MM and MCO bond energies, is suggested. [source] Reductive Activation of tripod Metal Compounds: Identification of Intermediates and Preparative Application,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2008Jürgen Mautz Abstract [tripodCoCl2] {tripod = CH3C(CH2PPh2)3} when treated with KC8 in THF solution under an argon atmosphere produces a reactive species ["tripodCo0"] (A) which undergoes oxidative additions with stannanes, [tripodCo(H)2(SnBu3)] (4), formed, for example, by addition of Bu3SnH. Silanes, R3SiH, undergo the same type of reaction producing [tripodCo(H)2(SiR3)] (R = Et: 5a; R = Ph: 5b). The solid-state structures of all the compounds [tripodCo(H)2(ER3)] (E = Si, R = Ph; E = Sn, R = Ph, Bu) are rather similar. While they contain six-coordinate cobalt with the formal oxidation state of cobalt being +III the coordination geometry is not octahedral: the heteroelement E deviates from the position which it would have in octahedral coordination by around 40° while the other five ligands, three phosphorus and two hydrogen, have the expected interligand angles of around 90° and 180°, respectively. The deviation of the heteroelement E is such that it approaches the metal bonded hydrogen atoms leading to short H···E contacts of only about 190 pm (E = Si) and 230 pm (E = Sn), respectively. The generation of a reactive species ["tripodCo0"] (A) was transferred to the synthesis of a reactive tripodnickel(0) species by treating a THF solution of [(DME)NiBr2] with KC8 in the presence of tripod. This species reacts with two electron donor ligands L to produce the pseudo tetrahedral compounds [tripodNi(L)] {L = PPh3 (6), AsPh3 (7), cHexNC (8), tBuNC (9), C2H4 (10)}. The identity of the reactive nickel(0) species as unequivocally deduced from NMR experiments is [tripod4Ni3] (12). All compounds were characterised by the usual analytic techniques including X-ray analysis where applicable.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Structural Diversity in Organotin Compounds Derived from Bulky Monoaryl Phosphates: Dimeric, Tetrameric, and Polymeric Tin Phosphate ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2008Ramaswamy Murugavel Abstract Monoaryl phosphates with a bulky aryl substituent have been used to synthesize new organotin clusters and polymers. The equimolar reaction between 2,6-diisopropylphenylphosphate (dipp-H2) and Me2SnCl2 in ethanol at 25 °C leads to the formation of [Me2Sn(,3 -dipp)]n (1), while the reaction of 2,6-dimethylphenylphosphate (dmpp-H2) with Me2SnCl2 in either a 1:1 or 2:1 molar ratio proceeds to produce exclusively [Me2Sn(,-dmpp-H)2]n·nH2O (2). Compounds 1 and 2 are 1D polymers with different architectures. In compound 1, the tin atom is five-coordinate (trigonal bipyramidal). Each dipp ligand bridges three different tin atoms to form an infinite ladder-chain structure. In 2, each six-coordinate (octahedral) tin atom is surrounded by four phosphate oxygen atoms originating from four different bridging dmpp-H ligands, thus forming a spirocyclic coordination polymeric chain. The use of nBu2SnO as the diorganotin source in its reaction with dipp-H2 leads to the isolation of dimeric [nBu2Sn(,-dipp-H)(dipp-H)]2 (4), which contains a central Sn2O4P2 unit. There are two chemically different half molecules of 4 in the asymmetric part of the unit cell and hence it actually exists as a 1:1 mixture of [nBu2Sn(,-dipp-H)(dipp-H)]2 and [nBu2Sn(,-dipp)(dipp-H2)]2 in the solid state. The reaction of the monoorgano tin precursor nBuSn(O)(OH)·xH2O with dipp-H2 takes place in acetone at room temperature to yield the tetrameric cluster 5, which has different structures in the solution and in the solid state. 31P NMR spectroscopy clearly suggests that 5 has the formula [nBu4Sn4(,-O)2(,-dipp-H)8] in solution. The single-crystal X-ray diffraction studies in the solid state, however, reveal that compound 5 exists as [nBu4Sn4(,-OH)2(,-dipp-H)6(,-dipp)2]. The use of compounds 1,4 as possible precursors for the preparation of ceramic tin phosphate materials has been investigated. The thermolysis of 1 at 500 °C leads to the formation of quantitative amounts of Sn2P2O7, while the thermolysis of 2, 3, and 4 under similar conditions results in the formation of SnP2O7. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Synthesis and Characterization of MoOI2(PMe3)3 and Use of MoOX2(PMe3)3 (X = Cl, I) in Controlled Radical PolymerizationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2006José A. Mata Abstract Complex MoOCl2(PMe3)3 smoothly reacts with NaI in acetone to produce MoOI2(PMe3)3 in good yields. The geometry of the compound is mer - cis octahedral, that is, identical to that of the dichloride precursor, as shown by NMR spectroscopy and by an X-ray crystallographic study. Electrochemical investigations of MoOX2(PMe3)3 show irreversible oxidation waves at Ep,a = +0.18 and +0.39 V for X = Cl and I, respectively. A study of the halide exchange between MoOCl2(PMe3)3 and NaI, or between MoOI2(PMe3)3 and Bu4NCl, shows two equilibrated isomers for the mixed halide intermediate MoOICl(PMe3)3. The diiodide complex rapidly exchanges the iodo ligands with chloride upon dissolution in chloroform at room temperature, and with bromide from (1-bromoethyl)benzene (BEB) under more forcing conditions. The equilibrium favors the softer halide (I) on C and the harder one (Cl or Br) on MoIV. Both oxido compounds catalyze the atom transfer radical polymerization (ATRP) of styrene in combination with the BEB initiator, yielding polymers with quite narrow molecular weight distributions (down to 1.11). The apparent polymerization rate constant is approximately doubled in the presence of 1 equiv. of the Al(OiPr)3 cocatalyst. On the other hand, the system is not capable of efficiently controlling the radical chain growth for methyl acrylate polymerization. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] A Study of the Electronic Spin-State Crossover in {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 16 2004Daniel L. Reger Abstract The synthesis, structural, magnetic, and Mössbauer spectroscopic properties of {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2 are reported. The single-crystal X-ray structure results indicate that at 150 K {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2 has a structure which is very similar to that observed at 220 K for the trigonally distorted octahedral, high-spin {Fe[HC(3,5-Me2pz)3]2}(BF4)2 complex. Both the magnetic and Mössbauer spectroscopic results indicate that {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2 is high spin between 160 and 296 K. Upon cooling, {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2 exhibits a complete electronic spin-state crossover from the high-spin to the low-spin state at approximately 110 K and remains completely low spin down to 4.2 K; upon subsequent warming from 4.2 K, the transition from the low-spin to the high-spin state occurs at 148 to 150 K. {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2 exhibits a rather large thermal hysteresis of 38 K in its spin-state crossover. Thus, {Fe[HC(3,4,5-Me3pz)3]2}(BF4)2 behaves differently from both {Fe[HC(3,5-Me2pz)3]2}(BF4)2, which is known to show a unique spin-state crossover of one-half of its iron(II) ions associated with a phase transition, and Fe[HB(3,4,5-Me3pz)3]2, which is known to remain high-spin even upon cooling to 1.7 K. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Synthesis and Structure of the First Non-Metallocene TiIII Fluoride Complex LTiF2·2Me3SnCl Supported by a ,-Diketiminato LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2003Grigori B. Nikiforov Abstract The TiIII dichloride complex LTiCl2 (1) and the first non-metallocene TiIII fluoride complex LTiF2·2Me3SnCl (2) supported by the ,-diketiminato ligand 2-{[2-(diethylamino)ethyl]amino}-4-{[2-(diethylamino)ethyl]imino}pent-2-ene have been synthesized. Elemental analysis, mass spectrometry and X-ray structural analysis show that 1 is monomeric, neutral, and free of solvent and lithium salt. The complex adopts a pseudo-octahedral geometry with the two chlorine atoms arranged in trans position to each other. Compound 1 is soluble in common organic solvents and thermally surprisingly robust. Complex 2 was prepared using Me3SnF as a fluorinating agent. X-ray structural analysis revealed that complex 2 consists of the LTiF2 unit and two molecules of Me3SnCl coordinated through a fluorine bridge to the titanium center. The fluorine atoms in 2 are located in trans positions to each other and the geometry around the titanium atom is distorted octahedral. Elemental analysis and mass spectrometry proved that 2 releases the coordinated Me3SnCl under vacuum or during sublimation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] 1D and 3D Ionic Liquid,Aluminum Hydroxide Hybrids Prepared via an Ionothermal Process,ADVANCED FUNCTIONAL MATERIALS, Issue 14 2007S. Park Abstract Room-temperature ionic liquids (RTILs) are used as hierarchically multifunctional components by employing them not only as templates and co-solvents for fabricating nanostructured materials but also proton conductors for electrochemical devices. RTIL/aluminum hydroxide (RTIL,Al) hybrids containing various nanometer-sized shapes, including 1D nanorods with hexagonal tips, straight and curved nanofibers, nanofibers embedded in a porous network, and 3D octahedral-, polyhedral-, and angular spherical shapes are synthesized via a one-pot ionothermal process. The structures or shapes of the RTIL,Al hybrids are related to the anionic moieties, alkyl chain length of the RTILs, and the humidity during fabrication. In particular, the introduction of water molecules into the interface led to 3D isotropic growth of the hybrids by influencing intermolecular interactions between the RTILs and the building blocks. The shapes of the nanohybrids fabricated from RTILs containing short alkyl chains were dependent on the types of anions and on the level of humidity. These results indicate that the cooperative interactions between RTILs and aluminum hydroxides induces emerging shape-controlled hybrids. The shape-controlled nanohybrids show enhanced electrochemical properties compared to those of a conventional hybrid prepared by mixing RTILs and aluminum hydroxides, exhibiting tenfold or higher proton conductivity under anhydrous condition and thermal stability as a result of the continuous proton conduction channel and the one-pot-assembled nanoconfinement. This method is expected to be a useful technique for controlling the diverse shapes of nanometer-sized crystalline inorganic materials for a variety of applications, such as fuel cells, solar cells, rechargeable batteries, and biosensors. [source] A comparative in situ Rietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxidesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2008Rune E. Johnsen Rietveld refinement based on in situ X-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107,s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl,NO3 LDH and spinel/zincite for a CoZnAl,NO3 LDH. Correlating these data with those from the TGA,MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. The a axis of the spinel formed by decomposition of the CoAl,NO3 LDH increases almost linearly from approximately 598 to 1163,K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl,NO3 shows a more complex temperature dependency. Between approximately 698 and 1073,K, the a axis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163,K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas. [source] Chemistry and Morphology of Hydrogarnets Formed in Cement-Based CASH Hydroceramics Cured at 200° to 350°CJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2009Konstantinos Kyritsis We have studied the chemistry and the morphology of hydrogarnet crystals produced in cement-based hydroceramic materials at elevated temperatures (200°,350°C) with silica and alumina additions. Such materials lie within the hydrothermal CaO,Al2O3,SiO2,H2O (CASH) system. Hydrogarnet Ca3Al2(SiO4)3,y(OH)4y is the dominant aluminum bearing phase formed and its composition is influenced mainly by the curing temperature and to a lesser degree by the addition of silica. The composition parameter y was estimated by Rietveld refinement of X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) shows that the hydrogarnets incorporate minor elements such as Fe, Mg, and S. EPMA data confirmed the hydrogarnet composition estimated from XRD. Both octahedral and icositetrahedral forms are observed. The icositetrahedral form is associated with higher minor element content. [source] Microwave Plasma Synthesis of Nanostructured ,-Al2O3 PowdersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2003L. Fu Nanostructured Al2O3 powders have been synthesized by combustion of aluminum powder in a microwave oxygen plasma, and characterized by X-ray diffraction and electron microscopy. The main phase is ,-Al2O3, with a small amount of ,-Al2O3. The particles are truncated octahedral in shape, with mean particle sizes of 21,24 nm. The effect of reaction chamber pressure on the phase composition and the particle size was studied. The ,-alumina content increases and the mean particle size decreases with decreasing pressure. No ,-Al2O3 appears in the final particles. Electron microscopy studies find that a particle may contain more than one phase. [source] Iron-Containing Poly(propylene imine) Dendromesogens with Photoactive PropertiesMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 7 2010Natalia E. Domracheva Abstract Here, we report the first results of investigation the local structure and photoactive properties of iron-containing dendromesogens based on decyloxybenzoate substituted poly(propylene imine) dendrimers of the first to fifth generations. Iron ions existing in a high-spin state are coordinated in dendrimer ligands by two kinds of iron-complexing sites with an octahedral and a tetrahedral symmetry. Octahedral (high-symmetry) centers are located at the border of the dendrimeric core, while the tetrahedral centers with strong rhombic distortion of iron environment are distributed throughout all branching of the dendrimeric core. It has been found that all iron-containing dendromesogens exhibit light-harvesting and fluorescence properties. [source] The oxygen vacancy in Ga2O3: a double resonance investigation,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005H. J. Kümmerer Abstract When produced under reducing conditions, ,-Ga2O3 is transformed into an n -type semiconductor with delocalized conduction electrons that exhibit a very strong electron spin resonance (ESR) and a strong hyperfine coupling to the gallium nuclei of the host lattice. We apply the Overhauser-shift technique to investigate single crystals of this compound. With extension to the high magnetic field of a W-band spectrometer, we were able to resolve all spectral lines that were recorded and to assign them to their corresponding electronic and nuclear states. This separate analysis was the basis to access additional sample characteristics: the hyperfine coupling that is actually averaged out in the ESR signal, as well as the nuclear relaxation rates could be analyzed. Systematic measurements by varying the microwave power revealed the Overhauser shift in thermal equilibrium. The signal could be tracked to very small microwave saturation parameters, at which the deviation from the usual linear relation between power and shift becomes evident and the shift clearly approaches a constant value. This value in equilibrium was determined directly from a fit to a sequence of measurements, whereas standard X-band experiments only provided indirect conclusions. The probability densities of the electrons at the nuclei in the two nonequivalent crystallographic positions,the lattice sites with octahedral and tetrahedral coordination,could also be determined directly. The enhanced resolution revealed an otherwise hidden substructure in the nuclear resonance signals. On the basis of a microscopic model, this structure could be used to probe the environment of the oxygen vacancy more precisely and to determine the extension of the electronic wave function of the donor electrons. Copyright © 2005 John Wiley & Sons, Ltd. [source] Transmission electron microscopy and theoretical analysis of AuCu nanoparticles: Atomic distribution and dynamic behaviorMICROSCOPY RESEARCH AND TECHNIQUE, Issue 7 2006J.A. Ascencio Abstract Though the application of bimetallic nanoparticles is becoming increasingly important, the local atomistic structure of such alloyed particles, which is critical for tailoring their properties, is not yet very clearly understood. In this work, we present detailed study on the atomistic structure of Au,Cu nanoparticles so as to determine their most stable configurations and the conditions for obtaining clusters of different structural variants. The dynamic behavior of these nanoparticles upon local heating is investigated. AuCu nanoparticles are characterized by high resolution transmission electron microscopy (HRTEM) and energy filtering elemental composition mapping (EFECM), which allowed us to study the internal structure and the elemental distribution in the particles. Quantum mechanical approaches and classic molecular dynamics methods are applied to model the structure and to determine the lowest energy configurations, the corresponding electronic structures, and understand structural transition of clusters upon heating, supported by experimental evidences. Our theoretical results demonstrate only the core/shell bimetallic structure have negative heat of formation, both for decahedra and octahedral, and energetically favoring core/shell structure is with Au covering the core of Cu, whose reverse core/shell structure is not stable and may transform back at a certain temperature. Experimental evidences corroborate these structures and their structural changes upon heating, demonstrating the possibility to manipulate the structure of such bimetallic nanoparticles using extra stimulating energy, which is in accordance with the calculated coherence energy proportions between the different configurations. Microsc. Res. Tech., 2006. © 2006 Wiley-Liss, Inc. [source] Theoretical study of electron paramagnetic resonance spectra and local structure of trigonal [Cr(H2O)6]3+ complex in GASH:Cr3+ and AlCl3,·,6H2O:Cr3+ systems at different temperaturesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2010Yan-Fang Li Abstract The electron paramagnetic resonance (EPR) parameters and local structure of the octahedral [Cr(H2O)6]3+ complex for Cr3+ doped in GASH and AlCl3,·,6H2O crystals with trigonal symmetry have been studied on the basis of the 120,×,120 complete energy matrices, respectively. By simulating the EPR and optical spectra, the local structure distortion parameters are determined in the temperature range of 4.2,297,K. The calculated results show that the local structure of [Cr(H2O)6]3+ octahedron exhibits an elongated distortion and the value of distortion depends on the temperature. Furthermore, the interrelation between the g -factors and orbit-reduction factor k has been studied, and the influence of the local structure on the EPR parameters has also been discussed. [source] Temperature-induced phase transition in simulated amorphous Al2O3PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2006Vo Van Hoang Abstract Our previous simulation studies indicated the existence of pressure-induced phase transition in liquid and amorphous Al2O3. In the current paper, we present the structural transformation of simulated amorphous Al2O3 from an octahedral to a tetrahedral network structure by heating from the high-density amorphous (hda) model at constant pressure P = 0 GPa. The structure of the models was analyzed through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Furthermore, significant differences in structural characteristics of low-density amorphous (lda) models obtained by heating from the hda one and by cooling from the low-density melt have been found and are presented. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spin-polarized charge transport through ionic clusters of magnetic oxidesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2006G. A. Gehring Abstract We analyze the spin-controlled charge transfer through a heterostructure consisting of one octahedral and one tetrahedral iron,oxygen ionic clusters, which are site-coupled, sharing an oxygen ion. A number of charge carriers can be manipulated by valence-uncompensated doping. The electron-energy structure of the clusters and that of the heterostructure are found on the basis of the Anderson model. Current,voltage (I,V ) characteristics, derived from the Landauer-like formula, turn out to be highly sensitive to the position of the Fermi level. We also calculated the magnetoresistance for the heterostructure with different orientations of the magnetic field. The result confirmed the empirical data for Ca:YIG, which indicate strong anisotropy of the magnetoresistance. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Catalytic activities of polymer-supported metal complexes in oxidation of phenol and epoxidation of cyclohexenePOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 3 2008K. C. Gupta Abstract The metal complexes of N, N,-bis (o -hydroxy acetophenone) propylene diamine (HPPn) Schiff base were supported on cross-linked polystyrene beads. The complexation of iron(III), copper(II), and zinc(II) ions on polymer-anchored HPPn Schiff base was 83.4, 85.7, and 84.5,wt%, respectively, whereas the complexation of these metal ions on unsupported HPPn Schiff base was 82.3, 84.5, and 83.9,wt%. The iron(III) complexes of HPPn Schiff base were octahedral in geometry, whereas copper(II) and zinc(II) ions complexes were square planar and tetrahedral. Complexation of metal ions increased the thermal stability of HPPn Schiff base. Catalytic activity of metal complexes was tested by studying the oxidation of phenol and epoxidation of cyclohexene in the presence of hydrogen peroxide. The polymer-supported HPPn Schiff base complexes of iron(III) ions showed 73.0,wt% conversion of phenol and 90.6,wt% conversion of cyclohexene at a molar ratio of 1:1:1 of substrate to catalyst and hydrogen peroxide, but unsupported complexes of iron(III) ions showed 63.8,wt% conversion for phenol and 83.2,wt% conversion for cyclohexene. The product selectivity for catechol (CTL) and epoxy cyclohexane (ECH) was 93.1 and 98.3,wt%, respectively with supported HPPn Schiff base complexes of iron(III) ions but was lower with HPPn Schiff base complexes of copper(II) and zinc(II) ions. Activation energy for the epoxidation of cyclohexene and phenol conversion with unsupported HPPn Schiff base complexes of iron(III) ions was 16.6,kJ,mol,1 and 21.2,kJ,mol,1, respectively, but was lower with supported complexes of iron(III) ions. Copyright © 2007 John Wiley & Sons, Ltd. [source] Crystal packing of a bacteriophage MS2 coat protein mutant corresponds to octahedral particlesPROTEIN SCIENCE, Issue 10 2008Pavel Plevka Abstract A covalent dimer of the bacteriophage MS2 coat protein was created by performing genetic fusion of two copies of the gene while removing the stop codon of the first gene. The dimer was crystallized in the cubic F432 space group. The organization of the asymmetric unit together with the F432 symmetry results in an arrangement of subunits that corresponds to T = 3 octahedral particles. The octahedral particles are probably artifacts created by the particular crystal packing. When it is not crystallized in the F cubic crystal form, the coat protein dimer appears to assemble into T = 3 icosahedral particles indistinguishable from the wild-type particles. To form an octahedral particle with closed surface, the dimer subunits interact at sharper angles than in the icosahedral arrangement. The fold of the covalent dimer is almost identical to the wild-type dimer with differences located in loops and in the covalent linker region. The main differences in the subunit packing between the octahedral and icosahedral arrangements are located close to the fourfold and fivefold symmetry axes where different sets of loops mediate the contacts. The volume of the wild-type virions is 7 times bigger than that of the octahedral particles. [source] On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2006The crystal structure and order,disorder transition of Cr2N were investigated utilizing transmission electron microscopy (TEM). Based on the analyses of selected-area diffraction (SAD) patterns, the crystal structure of the ordered Cr2N superstructure was confirmed to be trigonal (), characterized by three sets of superlattice reflections (001), (0) and (1). During electron irradiation, the superlattice reflections gradually disappeared in the regular sequence (001), (0) and (1), indicating that the order,disorder phase transition of Cr2N occurred. The convergent-beam electron diffraction (CBED) observation revealed that the space group of disordered Cr2N is P63/mmc, which corresponds to an h.c.p. (hexagonal close packed) sublattice of metal atoms with a random distribution of N atoms in six octahedral interstices. The redistribution model of N atoms through the order,disorder transition is discussed based on the characteristics and disappearing sequence of superlattice reflections. [source] The Zintl,Klemm concept applied to cations in oxides.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2003The structures of 94 ternary aluminates are reinterpreted on the basis of the Zintl,Klemm concept and Pearson's generalized octet rule. In aluminates of highly electropositive metals such as alkali, alkaline-earth and rare-earth metals, the Al atoms form three-dimensional skeleta which can be interpreted as if the Al atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity. The O atoms are then located close to both the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. When more electronegative elements, such as W or Si, are present in the compound, the electron transfer towards the Al atoms does not take place. In this case, aluminium behaves as a base, transferring its electrons to the more electronegative atoms and the coordination sphere of aluminium becomes octahedral. In some compounds the Al atoms clearly show amphoteric character so that some Al atoms act as donors (bases) and hence are octahedrally coordinated, whereas others behave as acceptors (acids), adopting a tetrahedral coordination. From this it is concluded that the coordination sphere of aluminium is not a function of the ionic radius of the Al3+ cations, but it depends on the nature of the other cations accompanying them in the structure. The networks formed by these aluminates are, in many instances, similar to those of the binary oxides of the main-group elements. For this reason, a systematic survey of these oxides is also reported. Compounds such as stuffed cristobalites and trydimites and also perovskites are examples of this new interpretation. Perovskites are then reinterpreted as a stuffed pseudo-TeO3 structure. Other families of compounds such as silicates and phosphates are susceptible to a similar interpretation. This study provides additional examples of how cations recognize themselves in spite of being embedded in an oxygen matrix. [source] Structure-forming components in crystals of ternary and quaternary 3d -metal complex fluoridesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2003E. V. Peresypkina Crystallochemical analysis and classification were performed for 139 ternary and quaternary complex fluorides with the general formula M1nM2mM3F6, belonging to 33 structure types. Using coordination sequences and the uniformity criterion the structure-forming ionic sublattices or their combinations were found, which are responsible for the formation of stable periodic frameworks. Analysis of structure-forming motifs allows the interpretation of the crystal structures of complex fluorides as close packings of F ions with M1, M2 and M3 cations, partially occupying tetrahedral and octahedral voids, or as the packings of [M3F6] complex ions with M1 and M2 countercations in the voids. Cationic sublattices are noted to play an essential role, while forming crystal structures of complex fluorides. Relationships between the composition of structure-forming sublattices, the composition of compounds, and the size and charge of ions belonging to the sublattices were analysed under normal conditions, with thermal and high-pressure polymorphic transitions. Rules were formulated to predict the crystal structures of complex fluorides with a given chemical composition. [source] Polymorphs of DABCO monohydrate as structural analogues of NaClACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2010Barbara Wicher A new crystalline form of 1,4-diazabicyclo[2.2.2]octane (DABCO) monohydrate, C6H12N2·H2O, crystallizing in the space group P31, has been identified during screening for cocrystals. There are three DABCO and three water molecules in the asymmetric unit, with two DABCO molecules exhibiting disorder over two positions related by rotation around the N...N axis. As in the monoclinic C2/c (Z, = 2) polymorph, the molecular components are connected via O,H...N hydrogen bonds into a polymeric structure that consists of linear O,H...N(CH2CH2)3N...H,O segments, which are approximately mutually perpendicular. The two polymorphic forms of DABCO monohydrate can be considered as structural analogues of NaCl, with the nearly globular DABCO molecules showing distorted cubic closest packing and all octahedral interstices occupied by water molecules. [source] Poly[[tetraaqua(,7 -pyridine-2,3,5,6-tetracarboxylato)dicadmium(II)] monohydrate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2010Sitang Yan The title compound, {[Cd2(C9HNO8)(H2O)4]·H2O}n, consists of two crystallographically independent CdII cations, one tetrabasic pyridine-2,3,5,6-tetracarboxylate (pdtc) anion, four coordinated water molecules and one solvent water molecule. The CdII cations have distorted square-antiprismatic (one pyridine N, six carboxylate O and one water O atom) and octahedral (three carboxylate O and three water O atoms) coordination environments. Each pdtc ligand employs its pyridine and carboxylate groups to chelate and bridge seven CdII cations. The square-antiprismatic coordinated CdII cations are linked by pdtc ligands into a lamellar framework structure, while the octahedral coordinated CdII cations are bridged by the ,2 -carboxylate O atoms and the pdtc ligands into a chain network that further joins neighbouring lamellae into a three-dimensional porous network. The cavities are filled with solvent water molecules that are linked to the host through complex hydrogen bonding. [source] 12-Membered borophosphate rings in KNi5[P6B6O23(OH)13]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2009Olga V. Yakubovich The title compound, potassium pentanickel hexaborophosphate tridecahydroxide, was synthesized under hydrothermal conditions from the NiCl2,K3PO4,B2O3,K2CO3,H2O system. The crystal structure was determined using single-crystal X-ray diffraction at 100,K. The KNi5[P6B6O23(OH)13] phase is cubic. For the three crystallographically distinct Ni centers, two occupy sites with 3 symmetry, while the third Ni and the K atom are located on sites. The structure is built from alternating borate and phosphate tetrahedra forming 12-membered puckered rings with K+ ions at the centers. These rings are arranged as in cubic dense sphere packing. A novel feature of the new crystal structure is the presence of linear trimers of face-sharing [NiO6] octahedra occupying the octahedral interstices of this sphere packing, and of single [NiO6] octahedra in the tetrahedral interstices. All oxygen corners of the Ni octahedra are linked to phosphate or borate tetrahedra of the 12-membered rings to form a mixed anionic framework. [source] The coordination polymers poly[,-4,4,-bipyridyl-di-,-formato-copper(II)] and catena -poly[[[diaqua(1-benzofuran-2,3-dicarboxylato)copper(II)]-,-1,2-di-4-pyridylethane] dihydrate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009Rajesh Koner The title compounds, [Cu(CHO2)2(C10H8N2)]n, (I), and {[Cu(C10H4O5)(C12H12N2)(H2O)2]·2H2O}n, (II), are composed of one-dimensional linear coordination polymers involving copper(II) ions and bidentate bipyridyl species. In (I), the polymeric chains are located on twofold rotation axes at (x, x, 0) and are arranged in layered zones centered at z = 0, , ½ and parallel to the ab plane of the tetragonal crystal. Weak coordination of the formate anions of one layer to the copper centers of neighboring layers imparts a three-dimensional connectivity to this structure. In (II), the polymeric chains propagate parallel to the a axis of the crystal. Noncoordinated water molecules link the chains through O,H...O hydrogen bonding in directions perpendicular to c, imparting to the entire structure three-dimensional connectivity. The metal ions adopt distorted octahedral and square-based pyramidal environments in (I) and (II), respectively. This study indicates that, under the given conditions, extended coordination involves CuII centers associating with the bipyridyl ligands rather than with the competing benzofurandicarboxylate entities. [source] A three-dimensional hybrid framework based on novel [Co4Mo4] bimetallic oxide clusters with 3,5-bis(3-pyridyl)-1,2,4-triazole ligandsACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2009Quan-Guo Zhai In the title organic,inorganic hybrid complex, poly[[[,-3,5-bis(3-pyridyl)-1,2,4-triazole]tri-,3 -oxido-tetra-,2 -oxido-oxidodicobalt(II)dimolybdenum(VI)] monohydrate], {[Co2Mo2O8(C12H9N5)]·H2O}n, the asymmetric unit is composed of two CoII centers, two [MoVIO4] tetrahedral units, one neutral 3,5-bis(3-pyridyl)-1,2,4-triazole (BPT) ligand and one solvent water molecule. The cobalt centers both exhibit octahedral [CoO5N] coordination environments. Four CoII and four MoVI centers are linked by ,2 -oxide and/or ,3 -oxide bridges to give an unprecedented bimetallic octanuclear [Co4Mo4O22N4] cluster, which can be regarded as the first example of a metal-substituted octamolybdate and exhibits a structure different from those of the eight octamolybdate isomers reported to date. The bimetallic oxide clusters are linked to each other through corner-sharing to give two-dimensional inorganic layers, which are further bridged by trans -BPT ligands to generate a three-dimensional organic,inorganic hybrid architecture with six-connected distorted ,-Po topology. [source] Manganese(II) and cobalt(II) complexes of 1,4-bis(diphenylphosphinoyl)butaneACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2009Anthony M. J. Lees The title complexes, catena -poly[[[diaquadiethanolmanganese(II)]-,-1,4-bis(diphenylphosphinoyl)butane-,2O:O,] dinitrate 1,4-bis(diphenylphosphinoyl)butane solvate], {[Mn(C2H6O)2(C28H28O2P2)(H2O)2](NO3)2·C28H28O2P2}n, (I), and catena -poly[[[diaquadiethanolcobalt(II)]-,-1,4-bis(diphenylphosphinoyl)butane-,2O:O,] dinitrate 1,4-bis(diphenylphosphinoyl)butane solvate], {[Co(C2H6O)2(C28H28O2P2)(H2O)2](NO3)2·C28H28O2P2}n, (II), are isostructural and centrosymmetric, with the MII ions at centres of inversion. The coordination geometry is octahedral, with each metal ion coordinated by two trans ethanol molecules, two trans water molecules and two bridging 1,4-bis(diphenylphosphinoyl)butane ligands which link the coordination centres to form one-dimensional polymeric chains. Parallel chains are linked by hydrogen bonds to uncoordinated 1,4-bis(diphenylphosphinoyl)butane molecules, which are bisected by a centre of inversion. Further hydrogen bonds, weak C,H...O interactions to nitrate anions, and weak C,H..., interactions serve to stabilize the structure. This study reports a development of the coordination chemistry of bis(diphenylphosphinoyl)alkanes, with the first reported structures of complexes of the first-row transition metals with 1,4-bis(diphenylphosphinoyl)butane. [source] | ||||||||||||||||