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Building Units (building + unit)
Kinds of Building Units Selected AbstractsDirect Synthesis of Highly Stable Mesoporous Molecular Sieves Containing Zeolite Building Units,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2005A. Sakthivel Abstract A novel, one-step synthesis of a highly stable mesoporous molecular sieve (MMS-H), which has a structure analogous to MCM-48 but which contains zeolite building units, is reported. A variety of experimental techniques,X-ray diffraction (XRD), N2 adsorption/desorption, transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, hyperpolarized 129Xe,NMR, and solid-state 27Al and 31P,magic-angle spinning (MAS) NMR spectroscopies,have been used to characterize the framework structure, porosity, and acidity of this novel mesoporous/microporous composite material, which is also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. [source] The Synthesis of an Extra-Large-Pore Zeolite with Double Three-Ring Building Units and a Low Framework Density,ANGEWANDTE CHEMIE, Issue 29 2010Jiuxing Jiang Weit geöffnet: Mithilfe von Hochdurchsatztechniken wurde ein Zeolith mit 18×12×12-Ring-Porensystem erhalten (siehe Bild). Das Material hat die niedrigste Gerüstdichte aller vollständig verbundenen Zeolithe auf Siliciumoxidbasis; außerdem handelt es sich um das erste Gerüst mit doppelten Dreierringen als Strukturmotiv. [source] ChemInform Abstract: Cu(II)-Azide Polymers of Cu3 and Cu6 Building Units: Synthesis, Structures and Magnetic Exchange Mechanism.CHEMINFORM, Issue 20 2010Sandip Mukherjee Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: A New Organic,Inorganic Molybdenum Arsenate Complex Based on [(CuO6) (As3O3)2Mo6O18] 4- Cluster as Secondary Building Units.CHEMINFORM, Issue 23 2009Fangyong Su Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Magic Silica Clusters as Nanoscale Building Units for Super-(tris)tetrahedral Materials.CHEMINFORM, Issue 23 2006Jacek C. Wojdel Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] BaSm5[Si9Al3N20]O , A Nitridoaluminosilicate Oxide with a New Structure Type Composed of "Star-Shaped" [N[4]((Si,Al)N3)4] Units as Secondary Building Units.CHEMINFORM, Issue 20 2006Alexandra Lieb Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Hybrid 2D and 3D Frameworks Based on ,-Keggin Polyoxometallates: Experiment and SimulationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2005Anne Dolbecq Abstract The ,-Keggin polyoxomolybdate {,-PMoV8MoVI4O40,x(OH)xM4} is a versatile building unit, with M being either a ZnII or a LaIII capping ion located at the vertices of a slightly distorted tetrahedron. The charge of the Keggin unit depends on the number of protonated oxo bridging ligands, which has been shown to vary from 0 to 5. The Keggin entity can thus be either an anion (M = Zn, x = 0) or a cation (M = La, x = 3,5). The Zn derivative has been generated in situ by hydrothermal synthesis and forms a 2D material built from the connection of the cations by 4,4'-bipyridine ligands linked to the capping ZnII ions. The reaction of the chloride salt of the La derivative with di-, tri- and tetrasubstituted benzenecarboxylate ligands has allowed us to isolate 2D and 3D materials. The 3D materials seem to be the first examples of hybrid open frameworks based on Keggin building blocks. The 3D framework built from the connection of ,-Keggin units by trimesate ions exhibits tunnels filled only by water molecules, which can be partly removed and reintroduced at room temperature. Besides these experimental results, simulation has allowed us to generate two virtual hybrid structures derived from those of known silicates by replacing the Si ions by hypothetical ,-Keggin cations and the O -bridging ligands by terephthalate ions, thus showing that 3D frameworks with large pores can be envisioned in the chemistry of hybrid organic,inorganic materials based on ,-Keggin units and motivating further experimental investigations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Novel Phosphovanadate Layered Structure Assembled from a Tetrametallic Cubane-Like VV ClusterEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2004Fa-Nian Shi Abstract A novel phosphovanadate layered structure intercalated by 4,4,-bipyridinium cations, (C10H10N2)[(VO2)4(PO4)2], was synthesised under hydrothermal conditions and its crystal structure determined using single-crystal X-ray diffraction. The anionic [(VO2)4(PO4)2]n2n, perforated layers are strongly hydrogen bonded to the interlayer 4,4,-bipyridinium cations and are assembled by an unprecedented secondary tetrametallic VV building unit constructed from a distorted cubane-like {V4O4} cluster. The compound was further characterised by IR, Raman, 1H, 1H- 13C, 31P and 51V MAS NMR spectroscopy, and by thermal and elemental analysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] A novel threefold-interpenetrating primitive cubic network based on a dinuclear Zn2 nodeACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2009Yun-Peng Diao In the mixed-ligand metal,organic polymeric compound poly[[,2 -1,4-bis(imidazol-1-yl)benzene](,2 -terephthalato)dizinc(II)], [Zn2(C8H4O4)2(C12H10N4)]n or [Zn2(bdc)2(bib)]n [H2bdc is terephthalic acid and bib is 1,4-bis(imidazol-1-yl)benzene], the asymmetric unit contains one ZnII ion, with two half bdc anions and one half bib molecule lying around inversion centers. The ZnII ion is in a slightly distorted tetrahedral environment, coordinated by three carboxylate O atoms from three different bdc anions and by one bib N atom. The crystal structure is constructed from the secondary building unit (SBU) [Zn2(CO2)2N2O2], in which the two metal centers are held together by two bdc linkers with bis(syn,syn -bridging bidentate) bonding modes. The SBU is connected by bdc bridges to form a two-dimensional grid-like (4,4)-layer, which is further pillared by the bib ligand. Topologically, the dinuclear SBU can be considered to be a six-connected node, and the extended structure exhibits an elongated primitive approximately cubic framework. The three-dimensional framework possesses a large cavity with dimensions of approximately 10 × 13 × 17,Å in cross-section. The potential porosity is filled with mutual interpenetration of two identical equivalent frameworks, generating a novel threefold interpenetrating network with an ,-polonium topology [Abrahams, Hoskins, Robson & Slizys (2002). CrystEngComm, 4, 478,482]. [source] Formation of 1,D and 3,D Coordination Polymers in the Solid State Induced by Mechanochemical and Annealing Treatments: Bis(3-cyano-pentane-2,4-dionato) Metal ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2008Jun Yoshida Abstract Bis(3-cyano-pentane-2,4-dionato) (CNacac) metal complex, [M(CNacac)2], which acts as both a metal-ion-like and a ligand-like building unit, forms supramolecular structures by self-assembly. Co-grinding of the metal acetates of MnII, CoII, NiII, CuII and ZnII with CNacacH formed a CNacac complex in all cases: mononuclear complex was formed in the cases of MnII, CuII and ZnII, whereas polymeric ones were formed in the cases of FeII, CoII and NiII. Subsequent annealing converted the mononuclear complexes of MnII, CuII and ZnII to their corresponding polymers as a result of dehydration of the mononuclear complexes. The resultant MnII, FeII, CoII, NiII and ZnII polymeric complexes had a common 3,D structure with high thermal stability. In the case of CuII, a 1,D polymer was obtained. The MnII, CuII and ZnII polymeric complexes returned to their original mononuclear complexes on exposure to water vapour but they reverted to the polymeric complexes by re-annealing. Co-grinding of metal chlorides with CNacacH and annealing of the mononuclear CNacac complexes prepared from solution reactions were also examined for comparison. [Mn(CNacac)2(H2O)2], [M(CNacac)2(H2O)] (M=CuII and ZnII) and [M(CNacac)2], (M=MnII, FeII and ZnII) are new compounds, which clearly indicated the power of the combined mechanochemical/annealing method for the synthesis of varied metal coordination complexes. [source] Inorganic,Organic Hybrid Structures: Open-Framework Iron Phosphite,Oxalates of Varying DimensionalityCHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2007Sukhendu Mandal Abstract Inorganic,organic hybrid structures belonging to the family of iron phosphite,oxalates have been prepared by employing hydrothermal methods. Their structures, determined by single-crystal X-ray diffraction, show a hierarchy within the family. While compounds I and II are low dimensional, III,VI have three-dimensional structures. Compound I has edge-shared ladders of iron phosphite with oxalate units hanging from the iron centers. Compound II has a layer structure with a honeycomb-like arrangement. The three-dimensional hybrid structures have the oxalate units connected in both in-plane and out-of-plane modes. A newly identified secondary building unit (SBU-7) and the oxalate units satisfying the valence and coordination requirements in the structure of V are novel and noteworthy structural features. Magnetic studies show that the dominant interactions between the iron centers are antiferromagnetic. Similar to other known hybrid structures, the phosphite,oxalate structures appear to show wide compositional and structural diversity. [source] A Zeolite-Like Zinc Phosphonocarboxylate Framework and Its Transformation into Two- and Three-Dimensional StructuresCHEMISTRY - AN ASIAN JOURNAL, Issue 12 2007Zhenxia Chen Abstract Three zinc phosphonocarboxylates, Zn2(pbc)2,Hdma,H3O,2H2O (1), Zn(pbc),Hdma (2), and Zn4.5(pbc)3(OH)(H2O)0.5,Hdma (3) (H3pbc=4-phosphonobenzoic acid, dma=dimethylamine) were synthesized by the mixed solvothermal reaction of Zn(Ac)2,2H2O and 4-phosphonobenzoic acid in N,N -dimethylformamide (DMF) and water. The zigzag and ladderlike chains completely constructed by triply fused 4-membered rings (denoted SBU-1) are linked by the organic moieties to form the 3D zeolite-like structure 1 and the layered structure 2, respectively. As for structure 3, a new second building unit (SBU-2) formed by the inset of the [Zn3O12] trimer into the 4-membered ring as well as SBU-1 is observed. The connections between the two types of SBUs lead to a "zinc phosphate" layer, which is linked by the organic groups to generate a 3D pillar-layered structure. Both solution-mediated and solid-state transformations of 1 to 2 and 3 were observed. A possible mechanism for the transformation is proposed. Gas sorption studies show that 1 has accessible pores for methanol and water and exhibits size selectivity for alcohols. [source] Syntheses, Structures and Luminescence Properties of Ln-Coordination Polymers Based on Flexible Thiodiacetic Acid LigandCHINESE JOURNAL OF CHEMISTRY, Issue 8 2009Xiangyang HOU Abstract Two novel lanthanide complexes with the formulas [Ln2(tda)3(H2O)2]n [Ln=Sm (1), Dy (2), H2tda=thiodiacetic acid] have been prepared by using the corresponding lanthanide salt and H2tda under hydrothermal conditions. Single crystal X-ray diffraction studies show that the two compounds are isostructural and present a 2D network based on the 1D metallic chain, in which an edge-sharing dinuclear polyhedral [Ln2O16] was found to be the building unit of the 1D chains. Interestingly, in the network, the tda ligands exhibit two different coordination modes, including the mode La: bis(syn,syn -bridging bidentate, chelating-bridging tridentate), and mode Lb: bis(chelating- bridging tridentate, syn, anti- bridging bidentate). Furthermore, through the tda ligand in modes La and Lb, the 1D metallic chains are cross-linked to generate a 2D network structure with (3,4,5,6)-connected (47·68)(44·66)(45·6)- (46)(43) topology. Photoluminescence measurements indicate that the dysprosium complex is yellow emitters, and the samarium complex shows salmon pink light emission in the solid state at room temperature. [source] Porous Porphyrin Nanoarchitectures on SurfacesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2010Rie Makiura Abstract Controlling the size and the growth direction of porous hybrid objects , metal,organic frameworks (MOFs) or porous coordination polymers (PCPs) , at the nanoscale is a critical issue for enabling their use in a number of potential applications that have arisen from the current remarkable activity in studying such porous materials. This microreview describes the recent progress in the design, growth, and characterization of multidimensional nanoarchitectures by employing porphyrin-based components. The versatility of the sequential bottom-up fabrication process, which uses multitopic molecular building units assembled by appropriately chosen linkers, is suitable to be extended to the formation of a rich variety of nanostructures endowed with pores on surfaces. [source] A Chiral Metal,Organic Framework Based on Heptanuclear Zinc CoresEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2009Hai-Yang Liu Abstract Two zinc(II) complexes, [Zn9(L1)6(H2O)2]·13.5H2O (1) and [Zn9(L2)6(H2O)3]·C2H5OH·8.5H2O (2), were synthesized. Both 1 and 2 are constructed from molecular building units (MBUs) that contain heptanuclear zinc clusters as cores. By introducing an additional chiral site into the original ligand, we achieved the transformation of the MOF from a nonchiral to a chiral structure, which provides a new strategy for designing chiral compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Rarely Decorated Rutile Frameworks Built from Triangular Organic Spacers and Distorted Octahedral Co3 Building BlocksEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2007Feng Luo Abstract Under mild temperatures, the self-assembly of CoCl2 and H3BTC in a solution of dmso generates a guest-free metal,organic polymer, namely Co3(BTC)2(,1 -dmso)2(,2 -dmso)2(1: dmso = dimethylsulfoxide, H3BTC = 1,3,5-benzenetricarboxylic acid). In 1, the CoII ions show the six-coordinate octahedral geometry completed by the BTC3, ions and the dmso oxygen atoms; remarkably, the coordinated dmso ligands not only act as terminal ligands to complete the octahedral geometry of the CoII ions, but also play an important role in bridging CoII ions together to give the Co3 secondary building units (SBUs). From a topological viewpoint, this novel polymer is classified to be the decorated (3,6)-connected rutile net with the (4.62)2(42.610.83) topology, where BTC3, ligands and Co3 SBUs are viewed to be the 3- and 6-connected nodes, respectively. In addition, the magnetic properties of 1 are explored by using a linear trinuclear cobalt mode, thusleading to g = 2.41, J = ,34.92 cm,1, TIP = 340,×,10,6 cm3,mol,1,, = ,6 K (a , parameter was included to take into account intertrinuclear interactions).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Structural Relationships in High-Nuclearity Heterobimetallic Bismuth-Oxo ClustersEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2005Michael Mehring Abstract The novel heterobimetallic sodium-bismuth-oxo clusters [Bi2Na4O(OSiMe3)8] (1), [Bi10Na5O7(OH)6(OSiMe3)15]·1.5C7H8 (2·1.5C7H8), [Bi15Na3O18(OSiMe3)12]·C7H8 (3·C7H8) and [Bi14Na8O18(OSiMe3)14(THF)4]·C6H6 (4·C6H6) were prepared starting from BiCl3 and NaOSiMe3. Compound 1 crystallises in the trigonal space group Rc with the lattice constants a = 12.8844(3) Å and c = 54.6565(3) Å, compound 2·1.5C7H8 crystallises in the triclinic space group P with the lattice constants a = 15.0377(2) Å, b = 16.0373(2) Å, c = 27.8967(5) Å, , = 87.1321(6)°, , = 86.6530(7)° and , = 63.6617(6)°, compound 3·C7H8 crystallises in the monoclinic space group C2/c with the lattice constants a = 54.311(11), b = 19.846(4), c = 22.885(5) Å and , = 112.32(3)°, and compound 4·C6H6 crystallises in the trigonal space group R with the lattice constants a = 15.9786(4) Å and c = 46.8329(17) Å. The formation of M,O,M bonds results from both partial hydrolysis followed by condensation as well as from elimination of Me3SiOSiMe3 from M,OSiMe3 groups. The hexanuclear metal-oxo silanolate 1 is more conveniently synthesised by the addition of NaOSiMe3 to a toluene solution of in situ-prepared [Bi(OSiMe3)3]. The metal-oxo(hydroxo) silanolates differ significantly in composition, but show similar building units. Thermal decomposition of the metal-oxo silanolates in the solid state gave heterogeneous decomposition products containing bismuth silicates. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Direct Synthesis of Highly Stable Mesoporous Molecular Sieves Containing Zeolite Building Units,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2005A. Sakthivel Abstract A novel, one-step synthesis of a highly stable mesoporous molecular sieve (MMS-H), which has a structure analogous to MCM-48 but which contains zeolite building units, is reported. A variety of experimental techniques,X-ray diffraction (XRD), N2 adsorption/desorption, transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, hyperpolarized 129Xe,NMR, and solid-state 27Al and 31P,magic-angle spinning (MAS) NMR spectroscopies,have been used to characterize the framework structure, porosity, and acidity of this novel mesoporous/microporous composite material, which is also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. [source] Toward direct determination of conformations of protein building units from multidimensional NMR experiments VI.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2005Chemical shift analysis of his to gain 3D structure, protonation state information Abstract NMR,chemical shift structure correlations were investigated by using GIAO-RB3LYP/6-311++G(2d,2p) formalism. Geometries and chemical shifts (CSI values) of 103 different conformers of N,-formyl-L-histidinamide were determined including both neutral and charged protonation forms. Correlations between amino acid torsional angle values and chemical shifts were investigated for the first time for an aromatic and polar amino acid residue whose side chain may carry different charges. Linear correlation coefficients of a significant level were determined between chemical shifts and dihedral angles for CSI[1H,]/,, CSI[13C,]/,, and CSI[13C,]/,. Protonation of the imidazole ring induces the upfield shift of CSI[13C,] for positively charged histidines and an opposite effect for the negative residue. We investigated the correspondence of theoretical and experimental 13C,, 13C,, and 1H, chemical shifts and the nine basic conformational building units characteristic for proteins. These three chemical shift values allow the identification of conformational building units at 80% accuracy. These results enable the prediction of additional regular secondary structural elements (e.g., polyProlineII, inverse ,-turns) and loops beyond the assignment of chemical shifts to ,-helices and ,-pleated sheets. Moreover, the location of the His residue can be further specified in a ,-sheet. It is possible to determine whether the appropriate residue is located at the middle or in a first/last ,-strand within a ,-sheet based on calculated CSI values. Thus, the attractive idea of establishing local residue specific backbone folding parameters in peptides and proteins by employing chemical shift information (e.g., 1H, and 13C,) obtained from selected heteronuclear correlation NMR experiments (e.g., 2D-HSQC) is reinforced. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1307,1317, 2005 [source] Toward direct determination of conformations of protein building units from multidimensional NMR experiments.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2003Abstract Knowledge of chemical shift,structure relationships could greatly facilitate the NMR chemical shift assignment and structure refinement processes that occur during peptide/protein structure determination via NMR spectroscopy. To determine whether such correlations exist for polar side chain containing amino acid residues the serine dipeptide model, For- L -Ser-NH2, was studied. Using the GIAO-RHF/6-31+G(d) and GIAO-RHF/TZ2P levels of theory the NMR chemical shifts of all hydrogen (1HN, 1H,, 1H,1, 1H,2), carbon (13C,, 13C,, 13C,) and nitrogen (15N) atoms have been computed for all 44 stable conformers of For- L -Ser-NH2. An attempt was made to establish correlation between chemical shift of each nucleus and the major conformational variables (,0, ,, ,, ,1, ,,1 and ,2). At both levels of theory a linear correlation can be observed between 1H,/,, 13C,/,, and 13C,/,. These results indicate that the backbone and side-chain structures of For- L -Ser-NH2 have a strong influence on its chemical shifts. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1157,1171, 2003 [source] Transition metal complexes of a cyclic pseudo hexapeptide: synthesis, complex formation and catalytic activities,,JOURNAL OF PEPTIDE SCIENCE, Issue 9 2008Huong Ngyen Abstract To contribute to a better understanding of metalloenzymes, we studied ion selectivity, complex formation tendencies and catalytic activities of linear and cyclic pseudopeptides. In this contribution, a linear and cyclic pseudo hexapeptide is described. The complex with transition metal ions and the sequence were designed using the programme COSMOS. Different routes of solid-phase synthesis were performed and compared using anchoring by C -terminus or a His side chain, using preformed pseudodipeptide building units or formation of N -functionalized peptide bond during stepwise assembly. The different strategies were compared regarding cyclization tendency, yield and purity. Side-chain anchoring to solid support favours the cyclization but leads to the formation of difficult to separate dioxopiperazine. Both routes require preformed building units. Complex-formation tendencies and selectivity for certain bivalent transition metal ions were experimentally estimated and compared to ones predicted theoretically. CD measurements indicate conformational changes by complex formation with different metal ions. Catalytic activities on oxidation of catechol and hydrolysis of bis-phosphate esters by some metal complexes of linear and cyclic peptide show only low catalytic activities compared to other model peptides and related metalloenzymes. The preference of the cyclic peptide for complexation of Ni2+ corresponds well to the predictions of COSMOS-NMR force field calculations. Copyright © 2008 European Peptide Society and John Wiley & Sons, Ltd. [source] Nanoscale uniformity of pore architecture in diatomaceous silica: a combined small and wide angle x-ray scattering studyJOURNAL OF PHYCOLOGY, Issue 1 2000Engel G. Vrieling Combined small and wide angle X-ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO2 matrix of diatom frustules. One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra. These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance (d -value ,0.06 nm) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of ,4.0 to ,2.5, with two clear common regions among species: d < 10 nm (slopes ,4, denoted as region I and also called the Porod region), and 10.0 < d < 40.0 nm (slopes ,2.9 to ,3.8, denoted as region II). The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO2 matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl3 to the growth medium. The results indicate that diatoms deposit silica with pores <50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennate or centric, the formation of silica at a nanoscale level is a uniform process. [source] Structure of Disodium Dimolybdate Synthesized Using Thermodynamically Stable Molybdenum (VI) Oxide Clusters as PrecursorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2009Dragana J. Jovanovi The structure of disodium dimolybdate (Na2Mo2O7) synthesized by a low-temperature method in the process of ultrasonic spray pyrolysis using acidified aqueous solutions of thermodynamically stable molybdenum (VI) oxide clusters as a precursor was refined down to an R -factor of 7%. The refinement of the diffraction data showed that Na2Mo2O7 powder synthesized at 300°C belongs to the base-centered orthorhombic type of structure with a space group of Cmca (no. 64). It was found that the basic units of the octahedral MoO6 precursor complexes exist in the structure of Na2Mo2O7. Tetrahedral MoO4 building units that coexist together with octahedral units in the structure of Na2Mo2O7 are most likely developed by the termination of weak octahedral bonds and by the placement of the molybdenum atom in the center of the tetrahedra. [source] Self-Assembly of Large Multimolecular Micelles from Hyperbranched Star CopolymersMACROMOLECULAR RAPID COMMUNICATIONS, Issue 5 2007Haiyan Hong Abstract This work focused on the synthesis and aqueous self-assembly of a series of novel hyperbranched star copolymers with a hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many linear poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) arms. The copolymers can synchronously form unimolecular micelles (around 10 nm) and large multimolecular micelles (around 100 nm) in water at room temperature. TEM measurements have provided direct evidence that the large micelles are a kind of multimicelle aggregates (MMAs) with the basic building units of unimolecular micelles. It is the first demonstration of the self-assembly mechanism for the large multimolecular micelles generated from the solution self-assembly of hyperbranched copolymers. [source] Determination of zinc incorporation in the Zn-substituted gallophosphate ZnULM-5 by multiple wavelength anomalous dispersion techniquesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010M. Helliwell The location of isomorphously substituted zinc over eight crystallographically different gallium sites has been determined in a single-crystal study of the gallophosphate ZnULM-5, GaZn(PO4)14(HPO4)2(OH)2F7, [H3N{CH2}6NH3]4, 6H2O, in an 11 wavelength experiment, using data from Station 9.8, SRS Daresbury. The measurement of datasets around the K edges of both Ga and Zn, as well as two reference datasets away from each absorption edge, was utilized to selectively exploit dispersive differences of each metal atom type in turn, which allowed the major sites of Zn incorporation to be identified as the metal 1 and 3 sites, M1 and M3. The preferential substitution of Zn at these sites probably arises because they are located in double four-ring (D4R) building units which can relax to accommodate the incorporation of hetero atoms. As the crystal is non-centrosymmetric, with space group P21212, it was also possible to use anomalous differences to corroborate the results obtained from the dispersive differences. These results were obtained firstly from difference Fourier maps, calculated using a phase set from the refined structure from data measured at the Zr K edge. Also, refined dispersive and anomalous occupancies, on an absolute scale, could be obtained using the program MLPHARE, allowing estimates for the Zn incorporation of approximately 22 and 18 at. % at the M1 and M3 sites to be obtained. In addition, f, and f,, values for Ga and Zn at each wavelength could be estimated both from MLPHARE results, and by refinement in JANA2006. The fully quantitative determinations of the dispersive and anomalous coefficients for Ga and Zn at each wavelength, as well as metal atom occupancies over the eight metal atom sites made use of the CCP4's MLPHARE program as well as SHELXL and JANA2006. The results by these methods agree closely, and JANA2006 allowed the ready determination of standard uncertainties on the occupancy parameters, which were for M1 and M3, 20.6,(3) and 17.2,(3),at %, respectively. [source] Mullite-type Ga4B2O9: structure and order,disorder phenomenonACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2010Rihong Cong Ga4B2O9, an aluminium-free mullite-type compound, was prepared by a boric-acid flux method and its structure was determined using powder X-ray diffraction techniques, in combination with transmission electron microscopy, solid-state 11B MAS-NMR and IR spectroscopies. GaO6 octahedra share edges in a trans -manner forming one-dimensional chains along the b direction, and the chains are further cross-linked by GaO5, BO3 and BO4 groups into a three-dimensional mullite-type structure. The disorder of the inter-chain groups results in a small unit cell for Ga4B2O9 compared with that for Al4B2O9, an ordered compound with a superstructure. By deconstructing the structure of Ga4B2O9, we were able to identify the fundamental building units and their linking rules which can be used to reconstruct the ordered and disordered structures. For Ga4B2O9, we found that the structure is intrinsically disordered within the ac plane, but ordered along the b axis. The three-dimensional structure can then be constructed by stacking the disordered ac sheets along the b axis (½b) with a ½a shift. The fundamental building units and exclusivity rules identified in this gallium borate mullite may also be useful for understanding other related mullite phases. The structure analysis applying the proposed method is used to recognize the structural features of Al4B2O9 and Al18B4O33. [source] Incommensurate structure of Ca2Al2O5 at high temperatures , structure investigation and Raman spectroscopyACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2008Biljana Lazic A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca2Al2O5 transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca2Al2O5 were synthesized in an end-loaded piston cylinder press at 2.5,GPa and 1273,K. The diffraction pattern observed at 1090,(10),K by in situ single-crystal diffraction experiments can be indexed by an I -centred orthorhombic cell and a modulation wavevector of q = 0.595,(1)c*. A (3,+,1)-dimensional model in superspace group Imma(00,)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO6] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO4] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca2Al2O5 is isotypic to the modulated high-temperature modification of Ca2Fe2O5. A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075,(10),K; on cooling, satellite reflections can be observed down to 975,(10),K. The characterization of Ca2Al2O5 is completed by Raman spectroscopy, including a partial interpretation of the spectra. [source] Structure determination of Mg3(OH)5Cl·4H2O (F5 phase) from laboratory powder diffraction data and its impact on the analysis of problematic magnesia floorsACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2007Kunihisa Sugimoto The crystal structure with the idealized formula Mg3(OH)5Cl·4H2O, the so-called F5 phase according to 5Mg(OH)2·MgCl2·8H2O in the system MgCl2,MgO,H2O, has been solved ab initio from high-quality laboratory powder diffraction data at room temperature. The F5 phase is structurally related to 3Mg(OH)2·MgCl2·8H2O (F3 form). The F5 phase consists of infinite triple chains with one Mg(OH)6 and two Mg(OH)4(OH2)2 octahedra as building units intercalated by chlorides, which are partly substituted by disordered hydroxides in the real structure. The F5 phase is of technological importance as the most important binder phase in Sorel cements. Knowledge of the crystal structure enables the full quantitative phase analysis of magnesia cements for the first time, which turns out to be very helpful in the search for possible causes of broken or bleached magnesia floors. Two real-life examples are given. [source] Two-dimensional metal-organic frameworks containing linear dicarboxylatesACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2006Samuel M. Hawxwell The solvothermal synthesis of four two-dimensional metal-organic frameworks containing linear dicarboxylic acids as ligands for ZnII centres is described. Zn(BDC)(DMF) [(1) where BDC = benzene-1,4-dicarboxylic acid; DMF = N,N -dimethylformamide] adopts a common paddlewheel motif leading to a 44 grid network, whereas Zn3(BDC)3(EtOH)2 (2), Zn3(BDC)3(H2O)2·4DMF (3) and Zn3(BPDC)3(DMF)2·4DMF (4) each form networks with the relatively uncommon 36 topology based upon Zn3(O2CR)6 secondary building units. All contain coordinated solvent molecules, namely DMF [(1) and (4)], ethanol (2) or H2O (3). Comparison of structures (2) and (3) illustrates a clay-like flexibility in interplanar spacing which sheds light on the ability of the Zn3(BDC)3 framework to undergo desolvation and uptake of small solvent and gas molecules. [source] New phosphate langbeinites, K2MTi(PO4)3 (M = Er, Yb or Y), and an alternative description of the langbeinite frameworkACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2002Stefan T. Norberg Three new potassium rare-earth/titanium phosphate structures, K2ErTi(PO4)3 (KErTP), K2YbTi(PO4)3 (KYbTP) and K2YTi(PO4)3 (KYTP), are presented, all of which are characterized by single-crystal X-ray diffraction studies. In addition, a fourth structure, K2CrTi(PO4)3 (KCrTP), has been reinvestigated. All structures are isostructural to the langbeinite-type structure and result from changes made to the growth constituents in high-temperature flux-growth experiments intended to give structurally modified potassium titanyl phosphate (KTP). The two crystallographically independent octahedra sites (site symmetry 3) have a mixed Ti/M (M = Er, Yb, Y or Cr) population, although the rare-earth metals favour one site while chromium favours the other. An alternative approach for the description of the channels and cation cages in langbeinite and related structures is given using [M5X6O39] units. The framework of langbeinite is compared with that of nasicon using these alternative building units. All of the investigated structures crystallize in space group P213 with Z,=,4; a,=,10.1053,(2),Å, R,=,0.023 (KErTP); a,=,10.0939,(8),Å, R,=,0.022 (KYbTP); a,=,10.1318,(6),Å, R,=,0.047 (KYTP); a,=,9.8001,(2),Å, R,=,0.016 (KCrTP). [source] | ||||||||||||||||||||||||||||