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PubMed2000

Structure and solid-state chemistry of anhydrous and hydrated crystal forms of the trimethoprim-sulfamethoxypyridazine 1:1 molecular complex.

The crystal structure of the equimolar trimethoprim (TMP) and sulfamethoxypyridazine (SMPD) complex in the anhydrous form (TMP. SMPD) and that of the species with 1.5 molecules of water of crystallization (TMP.SMPD.W) are reported in this article. X-ray powder diffraction patterns (both computer generated and experimental) and thermal analytical data from differential scanning calorimetry (DSC) and thermogravimetry useful for the characterization of TMP.SMPD and TMP.SMPD.W are provided. The stability of TMP.SMPD.W, which retains its crystallographic order under 0% relative humidity (RH) conditions at room temperature (22 degrees C) and 20 mmHg, is accounted for in terms of crystal structure and hydrogen bonding. Transformation of TMP.SMPD to the hydrate complex by exposure to approximately 100% RH, suspension in water, and wet granulation, and dehydration of TMP.SMPD.W by thermal treatment and by desiccation with methanol were investigated and tentatively interpreted in terms of crystal properties. Interactions in the physical mixture of TMP and SMPD by grinding, compression, heating, and contact with water were also studied. Water-mediated formation of TMP.SMPD.W by wetting and metastable eutectic melting-mediated formation of TMP.SMPD by heating was demonstrated. Mechanical activation by milling makes the physical mixture prone to solid-state transformation into dimorphic anhydrous cocrystals by supply of thermal energy during a DSC scan.

Crystallization↗
PubMed2000

In situ monitoring of solid-state polymerization reactions in sodium chloroacetate and sodium bromoacetate by 23Na and 13C solid-state NMR spectroscopy

The thermally induced solidstate polymerization reactions in sodium chloroacetate and sodium bromoacetate, leading to poly(hydroxyacetic acid) (polyglycolide) and NaCl and NaBr, respectively, were studied by isothermal in situ solid-state NMR spectroscopy at 120, 130 and 140 degrees C with a time resolution of the order of 5 to 25 min. The nuclei probed were 23Na and 13C, allowing the parent compounds (sodium halogenoacetates) and both reaction products (polymer and alkali halide) to be monitored. For sodium chloroacetate, there is no evidence for the involvement of intermediate phases during the reaction whereas this cannot be excluded for sodium bromoacetate. The crystal structure of sodium bromoacetate was determined directly from powder diffraction data by the Monte Carlo method, and was found to be isostructural with sodium chloroacetate. The topochemical reaction mechanism proposed previously for sodium chloroacetate is thus also applicable for the polymerization reaction in sodium bromoacetate. The mechanistic and kinetic information obtained from our in situ solid-state NMR investigations is compared and contrasted with information obtained from other in situ probes of the polymerization reactions in these materials.

Journal Article↗
PubMed2000

Complexation of zolpidem with 2-hydroxypropyl-beta-, methyl-beta-, and 2-hydroxypropyl-gamma-cyclodextrin: effect on aqueous solubility, dissolution rate, and ataxic activity in rat.

The effect of some chemically modified cyclodextrins [namely, 2-hydroxypropyl-beta-, methyl-beta-, and 2-hydroxypropyl-gamma-cyclodextrin (HP-beta-CD, Me-beta-CD, and HP-gamma-CD, respectively)] on the aqueous solubility and dissolution rate of the hypnotic agent Zolpidem (ZP) was investigated. Solid complexes were prepared by freeze drying and characterized by infrared spectroscopy, X-ray powder diffraction, and differential scanning calorimetry. The solubility and dissolution rate of the drug were significantly improved by complexation with HP-beta-CD or Me-beta-CD. The structure of the inclusion complex ZP-HP-beta-CD in CH(3)COOD/D(2)O was investigated by (1)H and (13)C NMR spectroscopy, including NOE measurements. These measurements revealing a weak interaction between the tolyl moiety of the guest molecule and the HP-beta-CD cavity. The ataxic activity in rat was also investigated and it was found that ZP-HP-beta-CD and ZP-Me-beta-CD complexes showed almost 2-fold longer ataxic induction times than controls.

2-Hydroxypropyl-beta-cyclodextrin↗
PubMed2001

Self-assembly and selective guest binding of three-dimensional open-framework solids from a macrocyclic complex as a trifunctional metal building block.

The nickel(II) hexaazamacrocyclic complex (1) containing pendant pyridine groups has been synthesized by the one-pot template condensation reaction of amine and formaldehyde. From the self-assembly of 1 with deprotonated cis,cis-1,3,5-cyclohexanetricarboxylic acid, H2CTC- and CTC3-, three-dimensional supramolecular open-frameworks of [Ni(C20H32N8)][C6H9(COOH)2(COO)]2 x 4H2O (2) and [Ni(C20H32N8)]3[C6H9(COO)3]2 x 16H2O (3), respectively, have been constructed. The solids 2 and 3 are insoluble in all solvents. X-ray crystal structure of 2 indicates that each nickel(II) macrocyclic complex binds two H2CTC- ions in trans position and two pendant pyridine groups of the macrocyclic complex are involved in hydrogen-bonding interactions with the hydroxy groups of H2CTC- belonging to the neighboring macrocyclic complexes, which provides the beltlike one-dimensional chain composed of rectangular synthons. The one-dimensional chains are linked together through lattice water molecules by the hydrogen-bonding interactions to generate two-dimensional networks, which are again connected to each other by the offset pi-pi stacking interactions between the pendant pyridine rings to give rise to a three-dimensional structure in which channels are present. The X-ray crystal structure of 3 indicates that each nickel(II) macrocyclic unit binds two CTC3- ions in trans position and each CTC3- ion coordinates three nickel(II) macrocyclic complexes to form a two-dimensional layer, in which pendant pyridine rings are involved in the hydrogen bonding and the herringbone pi-pi interaction. Between the layers, the pendant pyridine rings belonging to the neighboring layers participate in the offset pi-pi stacking interactions, which gives rise to a three-dimensional network structure. The network creates channels running parallel to the a, b, and c axes, which are filled with guest water molecules. The X-ray powder diffraction patterns indicate that the frameworks of 2 and 3 are deformed upon removal of water guests but restored upon rebinding of water. The host solids 2 and 3 bind [Cu(NH3)4](ClO4)2 in MeCN with a binding constant (Kf) of 210 M(-1) and 710 M(-1), respectively, while they do not bind [Cu(en)2](ClO4)2 (en = ethylenediamine). The dried solids of 2 and 3 do not interact with benzene and toluene, but they differentiate methanol, ethanol, and phenol in toluene solvent with the Kf values of 42, 14, and 12 M(-1), respectively, for 2, and 13, 8.2, and 8.9 M(-1), respectively, for 3. In terms of binding sites for guest molecules, the solid 3 has greater capacity than the solid 2.

Journal Article↗
PubMed2001

NaZn(H2O)2[BP2O8].H2O: a novel open-framework borophosphate and its reversible dehydration to microporous sodium zincoborophosphate N.

Crystals of NaZn(H2O)2[BP2O8].H2O were grown under mild hydrothermal conditions at 170 degrees C. The crystal structure (solved by X-ray single-crystal methods: hexagonal, P6(1)22 (no. 178), a = 946.2(2), c= 1583.5(1) pm, V= 1227.8(4).10(6) pm3, Z = 6) exhibits a chiral octahedral-tetrahedral framework related to the CZP topology and contains helical ribbons of corner-linked borate and phosphate tetrahedra. Investigation of the thermal behavior up to 180 degrees C shows a (reversible) dehydration process; this leads to the microporous compound Na[ZnBP2O8].H2O, which has the CZP topology. The crystal structure of Na[ZnBP2O8].H2O was determined by X-ray powder diffraction by using a combination of simulated annealing, lattice-energy minimization, and Rietveld refinement procedures (hexagonal, P6(1)22 (no. 178), a = 954.04(2), c = 1477.80(3) pm, V= 164.88(5).10(6) pm3, Z = 6). The essential structural difference caused by the dehydration concerns the coordination of Zn2- changing from octahedral to tetrahedral arrangement.

Journal Article↗
PubMed2001

Structural phase transitions in CaC2.

Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1,070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fm3m, Z = 4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z = 2), a low-temperature modification CaC2 II (C2/c, Z =4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z = 4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment. and the purity of the sample, as a comparison with technical CaC2 confirmed.

Journal Article↗
PubMed2001

Ternary alkali metal transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt).

Ternary transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt) can be synthesised by reaction of the respective alkali metal acetylide A2C2 with palladium or platinum in an inert atmosphere at about 350 degrees C. The crystal structures are characterised by (infinity)1[M(C2)(2/2)2-] chains, which are separated by the alkali metals (P3m1, Z = 1). The refinement of neutron powder diffraction data gave C-C = 1.263(3) A for Na2PdC2 (Na2PtC2: 1.289(4) A), which is distinctively longer than the expected value for a C-C triple bond (1.20 A). On the basis of band-structure calculations this can be attributed to a strong back-bonding from the metal into the anti-bonding orbitals of the C2 unit. This was further confirmed by Raman spectroscopic investigations, which showed that the wavenumbers of the C-C stretching vibrations in Na2PdC2 and Na2PtC2 are about 100 cm(-1) smaller than in acetylene. 13C MAS-NMR spectra demonstrated that the acetylenic C2 units in the title compounds are very different from those in acetylene. Electrical conductivity measurements and band-structure calculations showed that the black title compounds are semiconductors with a small indirect band gap (approximately 0.2 eV).

Journal Article↗
PubMed2001

Tl-Pt(CN)5 in the solid state--A multimethod study of an unusual compound containing inorganic wires.

The crystal and molecular structure of a polycrystalline powder with a metal-metal bond and the composition TlPt(CN)5 has been determined by combining results from X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and vibrational spectroscopic studies. The XRD data gave the tetragonal space group P4/nmm (No. 129), with a= 7.647(3), c=8.049(3) A, Z=2, and well-determined positions of the heavy metal atoms. The Pt-TI bond length in the compound is 2.627(2) A. The platinum atom coordinates four equivalent equatorial cyano ligands, with a fifth axial CN ligand and a thallium atom completing a distorted octahedral coordination geometry. The Tl-Pt(CN)5 entities are linked together in linear -NC-Pt-Tl-NC-Pt-Tl chains through the axial cyano ligand. These linear "wires" are the essential structural features and influence the properties of the compound. A three-dimensional network is formed by the four equatorial cyano ligands of the platinum atom that form bridges to the thallium atoms of neighbouring antiparallel chains. The platinum atom and the five nitrogen atoms from the bridging cyano groups form a distorted octahedron around the thallium atom. EXAFS data were recorded at the Pt and Tl L(III) edges for a more complete description of the local structure around the Pt and Tl atoms. The excessive multiple scattering was evaluated by means of the FEFF program. Raman and infrared absorption spectroscopy reveal strong coupling of the vibrational modes of the TlPt(CN)5 entities, in particular the metal-metal stretching mode, which is split into four Raman and two IR bands. Factor group theory shows that a structural unit larger than the crystallographic unit cell must be used to assign vibrational bands. Intra- and intermolecular force constants have also been calculated. The compound exhibits red luminescence at 700 +/- 3 nm in glycerol and has a corresponding excitation maximum at 240 nm. X-ray photoelectron spectra (XPS) show that the metal atoms have intermediate oxidation states, Pt3.2+ and Tl1.6-, between those in the parent Pt(II) and Tl(III) species and the decomposition products, Pt(IV) and Tl(I). The solid compound TlPt(CN)5 is stable to 520 degrees C. However in presence of water, a two-electron transfer between the metal atoms results in the cleavage of the metal-metal bond at 80 degrees C, forming a Pt(IV) pentacyanohydrate complex and a monovalent thallium ion.

Crystallography, X-Ray↗
PubMed2001

Glass-forming binaphthyl chromophores.

The use of the binaphthyl framework to synthesize glass-forming organic chromophores is described. Suzuki coupling reactions of racemic 6,6'-dibromo-2,2'-dialkoxy-1,1'-binaphthyl with 1,1-diphenyl-2-(4-dihydroxyboronphenyl)-ethene using [Pd(dppf)Cl2] (dppf = 1,1'-bis(diphenylphosphino)ferrocene) as the catalyst provide a set of chromophores with the 4-(2,2'-diphenylvinyl)-1-phenyl group at the 6- and 6'-positions and a range of groups on the oxygen atom. Starting with enantiomerically enriched (R)-6,6'-dibromo-2,2'-dihexyloxy-1,1'-binaphthyl ((R)-2Hex), one can obtain (R)-3Hex. Heck coupling reactions of 6,6'-dibromo-2,2'-dialkoxy-1,1'-binaphthyl compounds with styrene provide chromophores of the type 2,2'-dialkoxy-1,1'-binaphthyl-6,6'-bis(2-phenyl-vinyl). Starting with enantiomerically enriched (R)-2Hex, one obtains (R)-4Hex. Molecules of the type 4 contain two 1-naphthyl-2-phenyl ethylene chromophores with a pseudoorthogonal relationship. Similar procedures can be used to obtain fragments with more extended conjugation length. Thus, the Heck coupling reaction of 2Hex with 4-(4'-tert-butylstyryl)styrene, 1-(4'-tert-butylstyryl)-4-(4'-vinylstyryl)benzene, and 1-(3',5'-dihexyloxystyryl)4-(4'-vinylstyryl)benzene provides 5Hex, 6Hex, and 7Hex, respectively. DSC measurements and powder diffraction experiments indicate that the binaphthol chromophores show a resistance to crystallization. In some cases, considerably different thermal behavior is observed between enantiomerically enriched samples and their racemic counterparts. Increasing the size of the conjugated fragment on the binaphthol core leads to materials with higher glass-transition temperatures and a less pronounced tendency to crystallize. Fluorescence spectroscopy gives evidence of "excimer"-type interactions in the solid state, except for the chromophores with 4-(2,2'-diphenylvinyl)-1-phenyl groups. It is possible to obtain amorphous films of these chromophores directly from solution, and to fabricate light-emitting diodes, in which the electroluminescent layer corresponds to the binaphthyl chromophore.

Journal Article↗
PubMed2001

Trimerization of alkali dicyanamides M[N(CN)2] and formation of tricyanomelaminates M3[C6N9] (M = K, Rb) in the melt: crystal structure determination of three polymorphs of K[N(CN)2], two of Rb[N(CN)2], and one of K3[C6N9] and Rb3[C6N9] from X-ray powder diffractometry.

The alkali dicyanamides M[N(CN)2] (M=K, Rb) were synthesized through ion exchange, and the corresponding tricyanomelaminates M3[C6N9] were obtained by heating the respective dicyanamides. The thermal behavior of the dicyanamides and their reaction to form the tricyanomelaminates were investigated by temperature-dependent X-ray powder diffractometry and thermoanalytical measurements. Potassium dicyanamide K[N(CN)2] was found to undergo four phase transitions: At 136 degrees C the low-temperature modification alpha-K[N(CN)2] transforms to beta-K[N(CN)2], and at 187degrees C the latter transforms to the high-temperature modification gamma-K[N(CN)2], which melts at 232 degrees C. Above 310 degrees C the dicyanamide ions [N(CN)2]- trimerize and the resulting tricyanomelaminate K3[C6N9] solidifies. Two modifications of rubidium dicyanamide have been identified: Even at -25 degrees C, the a form slowly transforms to beta-Rb[N(CN)2] within weeks. Rb[N(CN)2] has a melting point of 190 degrees C. Above 260 degrees C the dicyanamide ions [N(CN)2]- of the rubidium salt trimerize in the melt and the tricyanomelaminate Rb3[C6N9] solidifies. The crystal structures of all phases were determined by powder diffraction methods and were refined by the Rietveld method. alpha-K[N(CN)2] (Pbcm, a = 836.52(1), b = 46.90(1), c =7 21.27(1) pm, Z = 4), gamma-K[N(CN)2] (Pnma, a = 855.40(3), b = 387.80(1), 1252.73(4) pm, Z = 4), and Rb[N(CN)2] (C2/c, a = 1381.56(2), b = 1000.02(1), c = 1443.28(2) pm, 116.8963(6) degrees, Z = 16) represent new structure types. The crystal structure of beta-K[N(CN)2] (P2(1/n), a = -726.92(1), b 1596.34(2), c = 387.037(5) pm, 111.8782(6) degrees, Z = 4) is similar but not isotypic to the structure of alpha Na[N(CN)2]. alpha-Rb[N(CN)2] (Pbcm, a = 856.09(1), b = 661.711(7), c = 765.067(9) pm, Z = 4) is isotypic with alpha-K[N(CN)2]. The alkali dicyanamides contain the bent planar anion [N(CN)2]- of approximate symmetry C2, (average bond lengths: C-N(bridge) 133, C-N(term) 113 pm; average angles N-C-N 170 degrees, C-N-C 120 degrees). K3[C6N9] (P2(1/c), a = 373.82(1), b = 1192.48(5), c = 2500.4(1) pm, beta = 101.406(3) degrees, Z = 4) and Rb,[C6N9] (P2(1/c), a = 389.93(2), b = 1226.06(6), c = 2547.5(1) pm, 98.741(5) degrees, Z=4) are isotypic and they contain the planar cyclic anion [C6N9]3-. Although structurally related, Na3[C6N9] is not isotypic with the tricyanomelaminates M3[C6N9] (M = K, Rb).

Journal Article↗
PubMed2002

Hybrid composites of monodisperse pi-conjugated rodlike organic compounds and semiconductor quantum particles.

Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of pi-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6 (Cd), 8 (Cd) or Pb salts 6 (Pb), 8 (Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6 (H), 8 (H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H(2)S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.

Journal Article↗
PubMed2002

Intercalation of cyclic ethers into vanadyl phosphate.

Two cyclic ethers, tetrahydrofuran (THF) and tetrahydropyran (THP), were intercalated into vanadyl phosphate and characterized by X-ray powder diffraction, thermogravimetry, and IR and Raman spectroscopy. Both compounds contain one molecule of ether per formula unit of VOPO(4) and show high thermal stability in comparison with VOPO(4) intercalates with other organic guest molecules. Both ethers are anchored to the VOPO(4) host layers by their oxygen atoms, which are coordinated to the vanadium atoms of the host. The probable arrangement of the tetrahydropyran molecules in the host interlayer space is derived from molecular simulations by the Cerius(2) 4.5 program.

Journal Article↗
PubMed2002

Synthesis, structure and magnetism of homologous series of polycrystalline cobalt alkane mono- and dicarboxylate soaps.

Carboxylate-bridged chain complexes of Co(II) (the diaquacobalt(II) mono- and ,-dialkanoates) form two homologous series of layered compounds which have been fully characterised both structurally and magnetically. The crystal structures of two selected members, [Co[CH3(CH2)10COO]2(H2O)2] and [Co[CH3(CH2)18COO]2(H2O)2], have been solved by X-ray powder diffraction and selected-area electron diffraction methods, and refined by the Rietveld technique. Crystal data: monoclinic, P 2(1)/a; a=9.688(1), b=7.5495(9), c=37.281(5) A, =96.70(3) primary, Z=4; and monoclinic, P 2(1)/a; a=9.7260(7), b=7.5477(7), c=57.53(1) A, =94.66(4) primary, Z=4, respectively. Their isomorphous structures contain layers of octahedral diaquacobalt(II) ions bonded to two chemically inequivalent alkanoates, one chelating and one bridging two Co atoms about 6.3 A apart, thus confirming the rare anti-anti conformation mode of the -RCOO groups recently proposed for diaquacobalt(II) ,-dodecanedioate. Extensive magnetic characterisation allowed estimation of the feeble antiferromagnetic coupling, which is weaker in the mono- than in the dialkanoate series.

Journal Article↗
PubMed2002

Synthesis and self-assembling properties of polymerizable organogelators.

In this paper we report the development of a family of polymerizable urea derivatives that are gelators for organic solvents. The design involves amino-acid-based urea derivatives containing methacrylate functional groups. Several of the bis-ureas show excellent gelation properties in a variety of organic solvents at very low concentrations (1.5 mM). The self-assembling properties of these compounds were studied by X-ray powder diffraction, optical microscopy, and electronic microscopy. The molecules self-assemble into fibrous aggregates as indicated by scanning electron microscopy. Depending on the nature of the monomeric unit, the structure of the aggregates ranges from small to large fibers and planar sheets. In polar solvents, the molecules exhibit multilamellar packing modes. The organogels can be polymerized by photoirradiation leading to significant increases in stability as indicated by changes in phase-transition temperature. The morphology of the aggregates was seen to maintain a similar structure both before and after irradiation. This method provides a novel type of polymer with designed self-assembling architecture and also affords to a route to stabilized polymer gels.

Journal Article↗
PubMed2002

Constructing terbium co-ordination polymers of 4,4'-bipyridine-N,N'-dioxide by means of diffusion solvent mixtures.

Five different co-ordination polymers of terbium(III) and the bidentate ligand 4,4'-bipyridine-N,N'-dioxide (L), [Tb(L)(CH(3)OH)(NO(3))(3)](infinity) (1), ([Tb(L)(1.5)(NO(3))(3)].CH(2)Cl(2))(infinity) (2), ([Tb(L)(1.5)(NO(3))(3)].CH(3)OH.0.8H(2)O)(infinity) (3), ([Tb(L)(1.5)(NO(3))(3)].0.4C(2)Cl(4).0.8CH(3)OH)(infinity) (4), and [Tb(L)(2)(NO(3))(3)](infinity) (5) have been synthesised by the use of different "diffusion solvent mixtures", and structurally characterised by X-ray crystallography. Compound 1, with a Tb:L stoichiometry of 1:1, adopts a zig-zag chain structure, which forms three-fold interpenetrating diamondoid frameworks through interchain hydrogen bonding between co-ordinated methanol and a nitrate group on an adjacent chain. Polymers 2, 3, and 4 all have a Tb:L stoichiometry of 1:1.5, but adopt different topologies. For 2, a ladder arrangement is found and large channels which accommodate solvent CH(2)Cl(2) molecules are formed by superposition of the ladders. For 3 and 4 4.8(2) net structures are observed. The superposition of the 4.8(2) nets in 3 and 4, by disposing adjacent layers such that every octagon is positioned below a tetragon from the neighbouring layer, allows the formation of two kinds of channel, with that inside the tetragons accommodating methanol molecules. The other kind of channel, between tetragons, accommodates water molecules in the case of 3 and tetrachloroethylene molecules in the case of 4. Compound 5, with a Tb:L stoichiometry of 1:2, has a linear polymeric structure with one bridging and one terminal ligand, and forms (6,3) plane nets by means of intermolecular electrostatic interactions between N-oxide moieties. X-ray powder diffraction studies show that upon desolvation, compound 2 maintains its original ladder framework.

Journal Article↗
PubMed2002

Vine-twining polymerization: amylose twines around polyethers to form amylose-polyether inclusion complexes.

In this paper, we describe a new polymerization manner termed as "vine-twining polymerization" to produce amylose-polymer inclusion complexes. The polymerization was achieved by an enzymatic polymerization of alpha-D-glucose-1-phosphate monomer catalyzed by phosphorylase in the presence of polyTHF as a guest polymer. The structure of the product was determined by X-ray powder diffraction and (1)H NMR measurements to be the inclusion complex. The formation process of the inclusion complexes during the polymerization was also evaluated. Furthermore, the formation of the inclusion complexes by this polymerization method by using polyTHFs with various M(n)s and end groups, as well as other polyethers as the guest polymers, was examined.

Amylose↗
PubMed2002

Incommensurate modulation in the microporous silica SSZ-24.

A detailed investigation of the structure of microporous silica, SSZ-24, is presented. It is shown by X-ray powder diffraction and (29)Si MAS NMR experiments that the structure deviates from the previously proposed AlPO(4)-5-type structure. At room temperature, electron diffraction (ED) patterns exhibit extra diffraction spots, which can be attributed to an incommensurate structural modulation along the c axis. This in turn results in a pleat pattern in real space with two different intervals arranged aperiodically along the c axis, as observed with high-resolution electron microscopy (HREM). The modulated structure may easily turn into a disordered one through excessive electron irradiation or heat-treatment. In order to understand the origin of the modulation, soft phonon-modes of the ideal premodulated structure were analyzed by the use of the rigid-unit-mode model. The distribution of soft modes in reciprocal space might correspond roughly to diffuse streaks that could be observed in the diffraction patterns at higher temperatures. It was found that several phonon branches soften at specific wave vectors, which are incommensurate with respect to the original period and might be responsible for the modulation. We present a simple analytic treatment to deduce the wave vectors and associated displacement eigenvectors for the incommensurate soft-modes.

Journal Article↗
PubMed2001

Pentagonal-dodecahedral la2 charge density in [80-ih ]fullerene: la2 @c80.

A double shell of Platonic solids: A trajectory that connects the six-membered rings of Ih -C80 is followed by the two La atoms of La2 @C80 according to maximum entropy/Rietveld analysis of the synchrotron powder-diffraction data. The result is a pentagonal-dodecahedral charge-density distribution for La2 (brown in the picture) inside the icosahedral C80 cage (green).

X-ray diffraction↗