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Coordination Number (coordination + number)
Selected AbstractsSteric Modulation of Coordination Number and Reactivity in the Synthesis of Lanthanoid(III) FormamidinatesCHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2007Marcus Abstract Reactions of a range of the readily prepared and sterically tunable N,N,-bis(aryl)formamidines with lanthanoid metals and bis(pentafluorophenyl)mercury (Hg(C6F5)2) in THF have given an extensive series of tris(formamidinato)lanthanoid(III) complexes, [Ln(Form)3(thf)n], namely [La(o -TolForm)3(thf)2], [Er(o -TolForm)3(thf)], [La(XylForm)3(thf)], [Sm(XylForm)3], [Ln(MesForm)3] (Ln=La, Nd, Sm and Yb), [Ln(EtForm)3] (Ln=La, Nd, Sm, Ho and Yb), and [Ln(o -PhPhForm)3] (Ln=La, Nd, Sm and Er). [For an explanation of the N,N, - bis(aryl)formamidinate abbreviations used see Scheme,1.] Analogous attempts to prepare [Yb(o -TolForm)3] by this method invariably yielded [{Yb(o -TolForm)2(,-OH)(thf)}2], but [Yb(o -TolForm)3] was isolated from a metathesis synthesis. X-ray crystal structures show exclusively N,N, - chelation of the Form ligands and a gradation in coordination number with Ln3+ size and with Form ligand bulk. The largest ligands, MesForm, EtForm and o -PhPhForm give solely homoleptic complexes, the first two being six-coordinate, the last having an ,1 -,-ArLn interaction. Reaction of lanthanoid elements and Hg(C6F5)2 with the still bulkier DippFormH in THF resulted in CF activation and formation of [Ln(DippForm)2F(thf)] (Ln=La, Ce, Nd, Sm and Tm) complexes, and o -HC6F4O(CH2)4DippForm in which the formamidine is functionalised by a ring-opened THF that has trapped tetrafluorobenzyne. Analogous reactions between Ln metals, Hg(o -HC6F4)2 and DippFormH yielded [Ln(DippForm)2F(thf)] (Ln=La, Sm and Nd) and 3,4,5-F3C6H2O(CH2)4DippForm. X-ray crystal structures of the heteroleptic fluorides show six-coordinate monomers with two chelating DippForm ligands and cisoid fluoride and THF ligands in a trigonal prismatic array. The organometallic species [Ln(DippForm)2(CCPh)(thf)] (Ln=Nd or Sm) are obtained from reaction of Nd metal, bis(phenylethynyl)mercury (Hg(CCPh)2) and DippFormH, and the oxidation of [Sm(DippForm)2(thf)2] with Hg(CCPh)2, respectively. The monomeric, six-coordinate, cisoid [Ln(DippForm)2(CCPh)(thf)] complexes have trigonal prismatic geometries and rare (for Ln) terminal CCPh groups with contrasting LnCC angles (Ln=Nd, 170.9(4)°; Ln=Sm, 142.9(7)°). Their formation lends support to the view that [Ln(DippForm)2F(thf)] complexes arise from oxidative formation and CF activation of [Ln(DippForm)2(C6F5)] intermediates. [source] Ligand Association/Dissociation Paths and Ill-Defined Coordination NumbersCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2010Antonio Ruiz-Martínez Abstract The continuous shape measures approach provides a means for handling the common situation in which a metal atom presents an ill-defined coordination number. Those cases are characterized by the presence of secondary interactions to Lewis bases at distances significantly longer than those expected for a chemical bond. Systematic ways of analyzing ligand association/dissociation pathways that describe such structures and their application to a variety of specific cases are presented. The concepts and methodology presented here apply to molecules and extended solids as well and provide, when needed, a more flexible and precise stereochemical description of the metal coordination sphere than that of an integer coordination number and the associated polyhedral shape. El mètode de les mesures contínues de forma ens permet tractar la situació trobada freqüentment, en què un àtom metàl,lic presenta un nombre de coordinació no ben definit, caracteritzat per la presència d,interaccions secundàries amb bases de Lewis a distàncies significativament més llargues que les que correspondrien a un enllaç químic. Aquí es presenten formes sistemàtiques d,analitzar els camins d,associació/dissociació de lligands que descriuen aquestes estructures, així com llur aplicació a diverses famílies de compostos. Els conceptes i la metodologia presentats en aquest article s'apliquen tant a molècules com a sòlids infinits i ens proporcionen una descripció estereoquímica més flexible i alhora precisa de les esferes de coordinació que no pas un nombre de coordinació enter i la forma polièdrica associada. [source] Ytterbium Coordination Polymer with Four Different Coordination Numbers: The First Structural Characterization of Lanthanide Phthalate ComplexCHINESE JOURNAL OF CHEMISTRY, Issue 9 2002Yong-Hong Wan Abstract The novel ytterbium coordination polymer is a two-dimensional framework in which the central metal ions have four different coordination numbers and form four kinds of coordination polyhedra. The four kinds of coordination polyhedra connect into infinite chains by sharing oxygen atoms. [source] Optimization of an Iron Intercalated Montmorillonite Preparation for the Removal of Arsenic at Low Concentrations,ENGINEERING IN LIFE SCIENCES (ELECTRONIC), Issue 1 2007D. Masih Abstract A series of iron intercalated montmorillonites (Fe-Monts) were prepared using (i) ion exchange of native sodium and calcium ions with iron ions, (ii) base hydrolysis of inserted iron ions in montmorillonite suspension, and (iii) insertion of pre-hydrolyzed iron colloid in montmorillonite. The materials were characterized by X-ray diffraction and gas adsorption-desorption techniques. The basal d(001)-spacing and BET specific surface area increased after the intercalation of iron species in montmorillonite. Local iron structure studied by X-ray absorption fine structure (XAFS) spectroscopy showed an unsaturation of the Fe···Fe coordination number (N 2.5) of the intercalated iron species as compared to the bulk iron oxyhydroxides (N 6). The Fe-Monts were employed for arsenic removal from aqueous solutions at low concentration (0.2,16 mg/L). Among the Fe-Monts, the one prepared by the hydrolysis of inserted iron ions, was the best in performance. The saturation adsorption amount of the optimized iron-montmorillonite was 4 and 28 times higher for the removal of arsenite and arsenate, respectively, as compared to bulk iron oxyhydroxide (goethite). Compared with bulk iron oxyhydroxide, the Fe-Monts were superior for arsenate uptake and comparable for arsenite. In addition, arsenite adsorbed on the Fe-Monts was found to be oxidized to arsenate based on XAFS spectroscopy. [source] Molecular modeling of metal complexation by a fluoroquinolone antibioticENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2008Ludmilla Aristilde Abstract An understanding of the factors controlling the chemodynamics of fluoroquinolone antibiotics in different environmental matrices is a necessary prerequisite to the assessment of their potential impact on nontarget organisms in soils and receiving waters. Of particular interest are the complexes formed between fluoroquinolones and metal cations, which are believed to be important in the mechanism of sequestration of the antibiotic by minerals and natural organic matter. The structures of these complexes have not been fully resolved by conventional spectroscopy; therefore, molecular simulations may provide useful complementary insights. We present results from apparently the first molecular dynamics simulations of a widely used fluoroquinolone antibiotic, ciprofloxacin (Cipro), in aqueous complexes with five metal cations typically found in soils and surface waters: Ca2+, Mg2+, Fe2+, Na+, and K+. The interatomic potential functions employed in the simulations were validated by comparison with available structural data for solid-phase Cipro-hexahydrate and for the metal cations in aqueous solution. Although no comprehensive structural data on the aqueous complexes appear to be available, properties of the metal complexes predicted by our simulations agree with available data for solid-phase metal,Cipro complexes. Our results indicate that the ionic potential of the metal cation controls the stability of the complex formed and that the hydration number of the metal cation in aqueous solution determines its coordination number with O atoms in the metal,Cipro complex. In respect to environmental chemodynamics, our results imply that Cipro will form two configurations of bidendate chelates with metal centers on exposed surfaces of mineral oxides, water-bridged surface complexes with exchangeable cations in clay mineral interlayers, and cation-bridged complexes with functional groups in natural organic matter. [source] Linear Coinage Metal Complexes Stabilized by a Group 13 Metalloid LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 28 2010Ganesan Prabusankar Abstract The synthesis and structural characterization of the linear, cationic complex [{(DDP)Ga}2Cu][OTf]·2C6H5F (2, Tf = O2SCF3) stabilized by the monodentate, gallium-based ligand Ga(DDP) (DDP = [HC{(CMe)N(2,6- iPr2C6H3)}2],), as well as NMR spectroscopic evidence for the formation of the corresponding silver compound [{(DDP)Ga}2Ag][Al(hfip)4] (3, hfip = [OC(H)(CF3)2]), are reported. The remarkable steric properties of this gallium-based ligand permit the stabilization of 2, which exhibits an unusual linear geometry and a coordination number of two. [source] A Dynamically Entangled Coordination Polymer: Synthesis, Structure, Luminescence, Single-Crystal-to-Single-Crystal Reversible Guest Inclusion and Structural TransformationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2010Arshad Aijaz Abstract A ZnII coordination polymer {[Zn2(cpa)2(bpy)]·3H2O}n (1) (cpa2, = 4-(methoxycarbonyl)benzoate and bpy = 4,4,-bipyridine) has been synthesized under solvothermal condition and structurally characterized. This coordination polymer has nanotubular threefold entangled (2D,3D) structure with embedded water molecules; the water molecules can be partially exchanged in reversible single-crystal-to-single-crystal (SC-SC) fashion by different solvent molecules like methanol, ethanol and acetone giving rise to {[Zn2(cpa)2(bpy)]·(0.5MeOH)·(2.5H2O)}n (2), {[Zn2(cpa)2(bpy)]·(0.5EtOH)·(0.5H2O)}n (3) and {[Zn2(cpa)2(bpy)]·(0.5Me2CO)·(H2O)}n (4). Inclusion of EtOH or MeOH leaves the size of the voids in the framework unaltered. Inclusion of acetone, however, is accompanied by shrinking of the voids in the framework. Heating of 1 at 100 °C under vacuum for 4 h affords the de-solvated compound, {Zn2(cpa)2(bpy)}n (1,). Single-crystal X-ray structure of 1, shows sliding of the individual nanotubular components expanding the overall framework. Thus, the coordination polymer exhibits dynamic motion of the molecular components in SC-SC fashion. All compounds were further characterized via IR spectroscopy, PXRD, elemental and TGA analysis. When 1 is placed in benzene at 100 °C for 2 days, compound {[Zn2(cpa)2(bpy)]·(2.5H2O)}n (5) is formed in a SC-SC fashion where coordination number of ZnII ion increases from four to five. Compound 1 also exhibits reversible guest-dependent photoluminescence properties. [source] Treating [K4(H2O)0.5][SnTe4] with PPh4+ or AsPh4+: Formation of a Highly Air-Stable Tellurostannate Salt, a Tellurolate with One-Coordinate Te Atoms, and a Polytelluride Salt with ,1[Te4(H2O)2]2, Chains,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 28 2007Eugen Ruzin Abstract In reactions of potassium orthotellurostannates with EPh4Cl (E = P, As), different single-crystalline products are obtained. An air-stable dimeric tellurostannate forms by partial replacement of strongly interacting K+ cations by weakly interacting or noninteracting [PPh4]+ ions. For E = As, an equally air-stable tellurophenolate, in which Te has the rare coordination number 1, results upon As,C bond cleavage at a tetraphenylarsonium cation. Oxidation of the air-sensitive [Sn2Te6]4, anion and Te2, by O2 (air) affords a tetratelluride salt with strands of H2O-bridged [Te4]2, anions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Synthesis and Luminescence Properties of New Dinuclear Complexes of Lanthanide(III) IonsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2004Violetta Patroniak Abstract As a result of coordination of the ligand L, containing two tridentate binding units, to samarium(III), europium(III), terbium(III), and dysprosium(III) ions, new dinuclear architectures containing two ions with coordination number 9 were formed. The structures of the complexes have been assigned on the basis of their solution spectroscopic and microanalytical data, and confirmed by X-ray crystallography in the case of the europium(III) complex 2. The structural analysis of the dinuclear complex 2 showed the presence of two europium centers [Eu(1) and Eu(2)] and four ligands L. Each europium ion is coordinated by 9 donor atoms with typical Eu,N and Eu,O distances. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Extended X-ray Absorption Fine Structure Study of Copper(I) and Copper(II) Complexes in Atom Transfer Radical PolymerizationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2003Tomislav Pintauer Abstract Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy has been used to investigate structural features of CuIBr and CuIIBr2 complexes with dNbpy, PMDETA, Me6TREN, tNtpy, and Me4CYCLAM in various solvents {dNbpy = 4,4,-bis(5-nonyl)-2,2,-bipyridine, PMDETA = N,N,N,,N,,,N,, -pentamethyldiethylenetriamine, Me6TREN = tris[2-(dimethylamino)ethyl]amine, tNtpy = 4,4,,4,,-tris(5-nonyl)-2,2,:6,,2,,-terpyridine, Me4CYCLAM = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane}. The structures of the CuI and CuII complexes were found to depend on the solvent polarity and the number of nitrogen atoms in the ligand. Generally, in non-polar media and with monomers typically used in ATRP, CuI complexes preferred a tetracoordinate geometry, and were either ionic as observed in [CuI(dNbpy)2]+[CuIBr2], (CuI,NAV = 2.00 Å, CuI,BrAV = 2.25 Å) and [CuI(Me4CYCLAM)]+[CuIBr2], (CuI,NAV = 2.06 Å, CuI,BrAV = 2.23 Å), or neutral as in [CuI(PMDETA)Br] (CuI,NAV = 2.12 Å, CuI,BrAV = 2.33 Å), and [CuI(tNtpy)Br] (CuI,NAV = 2.03 Å, CuI,BrAV = 2.29 Å). The EXAFS analysis of CuIIBr2 complexes indicated a preference for a coordination number of five, such as in [CuII(dNbpy)2Br]+[Br], (CuII,NAV = 2.03 Å, CuII,BrAV = 2.43 Å), [CuII(PMDETA)Br2] (CuII,NAV = 2.03 Å, CuII,Br1,AV = 2.44 Å, CuII,Br2,AV = 2.64 Å) and [CuII(Me6TREN)Br]+[Br], (CuII,NAV = 2.09 Å, CuII -BrAV = 2.39 Å), with the exception of the neutral tetracoordinate [CuII(dNbpy)Br2] (CuII,NAV = 2.02 Å, CuII,BrAV = 2.36 Å), which has been observed in non-polar media. Additionally, polar media were found to favor bromide dissociation in [CuII(Me6TREN)Br]+[Br], and [CuII(PMDETA)Br2], as indicated by a decrease in the Br and Cu coordination numbers at the Cu- and Br- K -edges, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Outer sphere mutagenesis of Lactobacillus plantarum manganese catalase disrupts the cluster coreFEBS JOURNAL, Issue 6 2003Mechanistic implications X-ray crystallography of the nonheme manganese catalase from Lactobacillus plantarum (LPC) [Barynin, V.V., Whittaker, M.M., Antonyuk, S.V., Lamzin, V.S., Harrison, P.M., Artymiuk, P.J. & Whittaker, J.W. (2001) Structure9, 725,738] has revealed the structure of the dimanganese redox cluster together with its protein environment. The oxidized [Mn(III)Mn(III)] cluster is bridged by two solvent molecules (oxo and hydroxo, respectively) together with a µ1,3 bridging glutamate carboxylate and is embedded in a web of hydrogen bonds involving an outer sphere tyrosine residue (Tyr42). A novel homologous expression system has been developed for production of active recombinant LPC and Tyr42 has been replaced by phenylalanine using site-directed mutagenesis. Spectroscopic and structural studies indicate that disruption of the hydrogen-bonded web significantly perturbs the active site in Y42F LPC, breaking one of the solvent bridges and generating an ,open' form of the dimanganese cluster. Two of the metal ligands adopt alternate conformations in the crystal structure, both conformers having a broken solvent bridge in the dimanganese core. The oxidized Y42F LPC exhibits strong optical absorption characteristic of high spin Mn(III) in low symmetry and lower coordination number. MCD and EPR measurements provide complementary information defining a ferromagnetically coupled electronic ground state for a cluster containing a single solvent bridge, in contrast to the diamagnetic ground state found for the native cluster containing a pair of solvent bridges. Y42F LPC has less than 5% of the catalase activity and much higher Km for H2O2 (,1.4 m) at neutral pH than WT LPC, although the activity is slightly restored at high pH where the cluster is converted to a diamagnetic form. These studies provide new insight into the contribution of the outer sphere tyrosine to the stability of the dimanganese cluster and the role of the solvent bridges in catalysis by dimanganese catalases. [source] Effects of Zinc(II) on the Luminescence of Europium(III) in Complexes Containing , -Diketone and Schiff BasesHELVETICA CHIMICA ACTA, Issue 11 2009Yuko Hasegawa Abstract The UV, excitation, and luminescence spectra of tris(pivaloyltrifluoroacetonato)europium(III) ([Eu(pta)3]; Hpta=1,1,1-trifluoro-5,5-dimethylhexane-2,4-dione=HA) were measured in the presence of bis(salicylidene)trimethylenediamine (H2saltn), bis[5-(tert- butyl)salicylidene]trimethylenediamine (H2(tBu)saltn), or bis(salicylidene)cyclohexane-1,2-diyldiamine (H2salchn), and the corresponding ZnII complexes [ZnB] (B=Schiff base). The excitation and luminescence spectra of the solution containing [Eu(pta)3] and [Zn(salchn)] exhibited much stronger intensities than those of solutions containing the other [ZnB] complexes. The introduction of a tBu group into the Schiff base was not effective in sensitizing the luminescence of [Eu(pta)3]. The luminescence spectrum of [ZnB] showed a band around 450,nm. The intensity decreased in the presence of [Eu(pta)3], reflecting complexation between [Eu(pta)3] and [ZnB]. On the basis of the change in intensity against the concentration of [ZnB], stability constants were determined for [Eu(pta)3Zn(saltn)], [Eu(pta)3Zn{(tBu)saltn}], and [Eu(pta)3Zn(salchn)] as 4.13, 4.9 and 5.56, respectively (log,, where =[[Eu(pta)3ZnB]]([[Eu(pta)3]][[ZnB]]),1). The quantum yields of these binuclear complexes were determined as 0.15, 0.11, and 0.035, although [Eu(pta)3Zn(salchn)] revealed the strongest luminescence at 613,nm. The results of X-ray diffraction analysis for [Eu(pta)3Zn(saltn)] showed that ZnII had a coordination number of five and was bridged with EuIII by three donor O-atoms, i.e., two from the salicylidene moieties and one from the ketonato group pta. [source] Copper-Binding Motifs: Structural and Theoretical AspectsHELVETICA CHIMICA ACTA, Issue 5 2003Amy Kaufman, Katz In this paper, we report the results of a study involving the coordination geometries of CuI, CuII, and CuIII crystal structures in the Cambridge Structural Database, and on Cu binding sites in proteins taken from the Protein Data Bank. The motifs used to bind two bridged Cu ions are also described. In addition, we report the results of ab initio molecular-orbital calculations performed on a variety of model CuI/CuII complexes (CuI/CuII,XnYm (X, Y=NH3, SH2); n+m=4; n=0,4) to provide data on the structural and energetic changes that occur in isolated complexes when the oxidation state of the Cu ion is changed from II to I while the coordination number is conserved. The use of such simple ligands in these calculations eliminates constraints on the geometric changes that may be imposed by more-complicated ligands. [source] Sandwich compounds with central hypercoordinate carbon, nitrogen, and oxygen: A quantum-chemical studyHETEROATOM CHEMISTRY, Issue 6 2006Ruslan M. Minyaev Ab initio (MP2(fu)/6-311+G**) and DFT (B3LYP/6-311+G**) calculations predict stable structures of sandwich compounds with the central carbon, nitrogen, and oxygen atoms surrounding lithium counterions between three-membered (BH)3 rings. The lithium counterions play a crucial role in the stabilization of these systems. The topological Bader analysis performed for these sandwich systems demonstrates that the central atom is linked to the ligands by nine-bond paths and, consequently, should be regarded as a nine-coordinated atom. Such a coordination number of carbon is the highest for all known main group organoelement compounds. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:464,474, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20268 [source] Structures and stability of lithium monosilicide clusters SiLin (n = 4,16): What is the maximum number, magic number, and core number for lithium coordination to silicon?JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2008Ning He Abstract In the coordination, hypervalent and cluster chemistry, three important characteristic properties are the maximum coordination number, magic number, and core coordination number. Yet, few studies have considered these three numbers at the same time for an MLn cluster with n larger than 8. In this article, we systematically studied the three properties of SiLin (n = 4,16) clusters at the B3LYP/6-31G(d), B3LYP/6-311++G(2d), and CCSD(T)/6-311++G(3df)//B3LYP/6-311++G(2d) (for energy only) levels. Various isomeric forms with different symmetries were calculated. For each SiLin (n = 4,9), silicon cohesive energy (cE) from SiLin , Si + Lin reaction, vertical ionization potential (vIP), and vertical electron affinity (vEA) were obtained for the lowest-energy isomer. We found that the maximum Li-coordination number of Si is 9, which is the largest number among the known MLin clusters. All cE, vIP, and vEA values predicted that 6 is the magic Li-coordination number of Si. For small SiLin (n , 6) clusters, Li atoms favor direct coordination to Si, whereas for larger SiLin (n , 7) clusters, there is a core cluster that is surrounded by excessive Li atoms. The core Li-coordination number is 6 for SiLin (n = 7,8), 7 for SiLin (n = 9,10), 8 for SiLin (n = 11,15) and 9 for SiLin (n , 16). Through the calculations, we verified the relationship between the structure and stability of SiLin with the maximum coordination number, magic number, and core coordination number. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Raman spectra of apatites: La10,x Si6,y (Al,Fe)y O26±,JOURNAL OF RAMAN SPECTROSCOPY, Issue 1 2007G. Lucazeau Abstract Raman spectra of eight polycrystalline apatites of the general formula La10,xSi6,yM,yO26 ± , with M, = Al or Fe were obtained at 300 K. Raman spectra of La10Si4Fe2O26 and La9.83Si4.5Al1.5O26 were investigated in the range 80,1000 K and 80,623 K, respectively. Tentative assignments of bands to stretching and bending modes of SiO4 tetrahedra and to M'O vibrations are proposed. Except for the two new bands, which appear around 700 cm,1 when Al is replaced by Fe, only some band broadenings and relative intensity changes are observed as a function of the rate of O5 or La vacancies. Most of the bands soften and broaden continuously when raising the temperature. This is an indication that the Al- and Fe-substituted apatites do not undergo any structural change up to 1000 K. Above 1000 K, the broad and weak shoulder observed at 850 cm,1 for La10Si4Fe2O26 is replaced by a strong band at 868 cm,1, suggesting that SiO4 tetrahedra undergo a structural modification. All compounds show the same residual band broadening at 80 K. This suggests that there is a small rate of static disorder preferentially related to the solubility of Al and Fe in the Si sublattice rather than to other defects. Moreover, the observation of FeO modes indicates that the dynamics of the solid solution obeys the so-called two-mode behavior. The occurrence of FeO stretching vibrations 150 cm,1 lower than for those of SiO suggests that the coordination number of iron could be larger than 4, particularly for the Fe4+ species. Copyright © 2006 John Wiley & Sons, Ltd. [source] On the neglecting of higher-order cumulants in EXAFS data analysisJOURNAL OF SYNCHROTRON RADIATION, Issue 6 2009A. Sanson The cumulant expansion is one of the most powerful and useful methods for EXAFS data analysis, in which the higher-order cumulants allow to consider deviations from a simple Gaussian distribution. In this work, analytical expressions have been derived to show the effects of neglecting higher-order cumulants in EXAFS analysis by the ratio method. The errors in the best-fitting procedure owing to the omission of the higher-order cumulants, as well as of the coordination number, can be determined. [source] Modeling and Simulation of Elementary Processes in Ideal SinteringJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2006Fumihiro Wakai Three-dimensional numerical simulation of sintering was performed to illustrate the interplay between shapes and forces in particle scale. Elementary rules for topological transition of triple lines were derived from the analysis of micromechanical models of sintering: bond formation, pore channel closure, and pore disappearance. The particle motion was driven by the force acting on a particle, i.e., a vector sum of sintering forces. The non-equilibrium sintering stress was defined to analyze the pore shrinkage. A particle changed its own shape through interaction with neighbor particles; then, the coordination number affected particle motion, pore shrinkage, coarsening, and grain growth. [source] First-Principles Calculations of Anion Vacancies in Oxides and NitridesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Isao Tanaka The formation energy, structural relaxation, and defect-induced states of neutral anion vacancies of five oxides (i.e., MgO, Al2O3, ZnO, In2O3, and SnO2) and four nitrides (i.e., AlN, Si3N4, Ge3N4, and InN) are systematically discussed, based on first-principles plane-wave pseudopotential calculations. Two types of polymorphs for each compound are compared. The number of atoms included in the supercells ranged from 54 to 96. When a localized vacancy-induced state appears within the band gap, as in a typical ionic crystal, the formation energy can be well scaled by the band gap of the perfect crystal. On the other hand, when an empty and localized vacancy-induced state is located above the highest occupied band or no localized state is formed, the formation energy has a tendency to be smaller. In compounds such as ZnO and SnO2, the formation energy is dependent largely on the crystal structure. This result can be explained by the transition of the vacancy-induced state from occupied to unoccupied, which is caused by the change in atomic arrangement, as represented by the cation coordination number. [source] Solid-state NMR characterization of 69Ga and 71Ga in crystalline solidsMAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2006Jason T. Ash Abstract Gallium model systems containing four- and six-coordinate gallium sites have been investigated using solid-state NMR. Measurement of the isotropic chemical shift and electric field gradient (EFG) have been performed at 9.4 T on ,-Ga2O3, ,-Ga2O3, LiGaO2, NaGaO2, KGaO2, Ga2(SO4)3, and LaGaO3 using a variety of techniques on both NMR active nuclei (69Ga and 71Ga) including static, high speed magic-angle spinning (MAS), satellite transition (ST) spectroscopy, and rotor-assisted population transfer (RAPT). The chemical shift is found to correlate well with the coordination number, with four-coordinate gallium having values of approximately 50 ppm and six-coordinate gallium having values near 225 ppm (referenced to 1 M gallium nitrate solution). The magnitude of the EFG is found to be correlated to the distortion of the gallium polyhedra, with the strained systems having EFGs of 3 × 1021 Vm,2 or more, while the less strained systems have values of 1.5 × 1021 Vm,2 or less. A plot of chemical shift versus EFG suggests that solid-state NMR of gallium oxyanions can be more discriminating than liquid state NMR chemical shifts alone. Copyright © 2006 John Wiley & Sons, Ltd. [source] Iron oxidation state in impact glass from the K/T boundary at Beloc, Haiti, by high-resolution XANES spectroscopyMETEORITICS & PLANETARY SCIENCE, Issue 11 2005Gabriele Giuli The samples have been analyzed by Fe K-edge high-resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre-edge peak of our high-resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four- and five-coordinated Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. There is no evidence for six-fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash-form tektites, followed by progressive oxidation. [source] Reentrant phenomenon in the exactly solvable mixed spin-1/2 and spin-1 Ising,Heisenberg model on diamond-like decorated planar latticesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2010Lucia, anová Abstract Ground-state and finite-temperature behaviour of the mixed spin-1/2 and spin-1 Ising,Heisenberg model on decorated planar lattices consisting of inter-connected diamonds is investigated by means of the generalised decoration,iteration mapping transformation. The obtained exact results clearly point out that this model has a rather complex ground state composed of two unusual quantum phases, which is valid regardless of the lattice topology as well as the spatial dimensionality of the investigated system. It is shown that the diamond-like decorated planar lattices with a sufficiently high coordination number may exhibit a striking critical behaviour including reentrant phase transitions with two or three consecutive critical points. [source] Study of Mn2+ luminescence in Zn(PO3)2 glassesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007M. C. Flores J. Abstract The optical properties of Mn2+ doped Zn(PO3)2 glass are discussed. Depending on the manganese concentration in the glass samples, the Mn2+ emission band appears in the wavelength range from 515 to 620 nm. This behavior depends on the possible coordination number, a result confirmed by different lifetimes and also different structure of the absorption spectra. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Surface and interface investigations using radioactive atomsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004K. Potzger Abstract The magnetic behavior of isolated nonmagnetic probe atoms (Cd) at ferromagnetic Ni surfaces and interfaces have been studied using perturbed angular correlation (PAC) spectroscopy. Atomic resolution is achieved making it possible to distinguish between probe atoms at various surface sites, i.e., in/at terraces, atomic steps, kinks. These different atomic surface sites have differing Ni coordination numbers, (i.e., the number of Ni nearest neighbours). With decreasing coordination number, a continuous nonlinear dependent correlation was found with increasing magnetic hyperfine fields (Bhf). Furthermore, experimental evidence is presented, that the induced s-moments on the Cd isolated impurity is independent of the details of the surface symmetry but essentially determined only by the coordination number. Finally, it has been observed that monolayers of Pd on Ni form large unit cells, which offer selected sites to the Cd impurity. We discuss this finding as basic research towards future applications with self-organizing patterned structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Quasiperiodic plane tilings based on stepped surfacesACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2008A. V. Shutov Static and dynamic characteristics of layerwise growth in two-dimensional quasiperiodic Ito,Ohtsuki tilings are studied. These tilings are the projections of three-dimensional stepped surfaces. It is proved that these tilings have hexagonal self-similar growth with bounded radius of neighborhood. A formula is given for the averaged coordination number. Deviations of coordination numbers from its average are quasiperiodic. Ito,Ohtsuki tiling can be decomposed into one-dimensional sector layers. These sector layers are one-dimensional quasiperiodic tilings with properties like Ito,Ohtsuki tilings. [source] Production and isolation of ligated metal(IV)-oxo ions by tandem mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 8 2010Ivan F. Taylor High valent metal(IV)-oxo species, [M(O)(MeIm)n(OAc)]+ (M,=,Mn,Ni, MeIm,=,1-methylimidazole, n,=,1,2), which are relevant to biology and oxidative catalysis, were produced and isolated in gas-phase reactions of the metal(II) precursor ions [M(MeIm)n(OAc)]+ (M,=,Mn,Zn, n,=,1,3) with ozone. The precursor ions [M(MeIm)(OAc)]+ and [M(MeIm)2(OAc)]+ were generated via collision-induced dissociation of the corresponding [M(MeIm)3(OAc)]+ ion. The dependence of ozone reactivity on metal and coordination number is discussed. Copyright © 2010 John Wiley & Sons, Ltd. [source] (NH4)2WTe2O8 at 5.09,GPa: a single-crystal study using synchrotron radiationACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2010A. Grzechnik The polar crystal structure of diammonium [octaoxidoditellurato(IV)]tungstate, (NH4)2WTe2O8, was studied at high pressures using single-crystal X-ray diffraction in a diamond-anvil cell at the HASYLAB synchrotron (DESY, Hamburg, Germany). No phase transition was observed up to 7.16,GPa. However, a full structure determination at 5.09,GPa shows that the coordination number of one of the two non-equivalent Te atoms has decreased from four to three. [source] The new ternary phases of La3(Zn0.874Mg0.126)11 and Ce3(Zn0.863Mg0.137)11ACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2010Volodymyr Pavlyuk The new ternary intermetallic title compounds, namely trilanthanum undeca(zinc/magnesium), La3(Zn0.874Mg0.126)11, (I), and tricerium undeca(zinc/magnesium), Ce3(Zn0.863Mg0.137)11, (II), are isostructural and crystallize in the orthorhombic La3Al11 structure type. These three phases belong to the same structural family, the representative members of which may be derived from the tetragonal BaAl4 structure type by a combination of internal deformation and multiple substitution. Compared to the structure of La3Al11, in (I), a significant decrease of 11.9% in the unit-cell b axis and an increase in the other two directions, of 3.6% along a and 5.2% along c, are observed. Such an atypical deformation is caused by the closer packing of atoms in the unit cell due to atom shifts that reflect strengthening of metallic-type bonding. This structural change is also manifested in a significant difference in the coordination around the smaller atoms at the 8l Wyckoff position (site symmetry m). The Al atom in La3Al11 is in a tricapped trigonal prismatic environment (coordination number 9), while the Zn atoms in (I) and (II) are situated in a tetragonal antiprism with two added atoms (coordination number 10). [source] Strontium tetrafluoridoborate and barium tetrafluoridoborateACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2007Tina Buni In Sr(BF4)2, which is isomorphous with the previously published Ca(BF4)2, the metal atom possesses a coordination number of 8 with a square-antiprismatic environment. Each tetrafluoridoborate anion is bonded to four metal centers. In the barium derivative, the metal center, with symmetry 2/m, is surrounded by 14 F atoms. The B atom and two of the three independent F atoms occupy special positions with symmetry m. Each anion is connected to five Ba atoms. This structure differs significantly from an earlier published structure of Ba(BF4)2 [published as Ba2(BF4)4; Lin, Cheng, Chen & Huang (1998). Jiegon Huaxue, 17, 245]. The radial distribution functions for the present Ba(BF4)2 and earlier Ba2(BF4)4 structures differ significantly. [source] A capped trigonal prismatic cadmium complex with tetra- and tridentate ligands: bis(triethanolamine)-,3N,O,O,;,4N,O,O,,O,,-cadmium(II) squarate monohydrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2004brahim Uçar In the crystal structure of the title compound, [Cd(C6H15NO3)2](C4O4)·H2O, a supramolecular structure is observed. The asymmetric unit consists of one unit of the cationic Cd complex, one water molecule and two half-squarate anions, each sitting on a crystallographic inversion center. The different coordinations of the two triethanolamine (TEA) ligands results in an unusual example of coordination number seven for the CdII ion. Both TEA ligands coordinate to the CdII ion, forming a distorted monocapped trigonal prismatic geometry with approximate C2v symmetry. One of the TEA ligands acts as an N,O,O,-tridentate ligand, whereas the other behaves as an N,O,O,,O,,-tetradentate donor. The anions and cations are linked to one another by hydrogen bonds between hydroxy H atoms of the TEA ligands and squarate O atoms. The crystal structure is stabilized by O,H,O hydrogen bonds between the unligated water molecule and a squarate O atom, together with a weak ,,ring interaction between the ethylene group of a TEA ligand and a squarate anion. [source] | |||||||||||||||||||||||||||||||||||||