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PubMed2006

Self-assembly and characterization of layered double hydroxide/DNA hybrids.

The purpose of this study was to control the fabrication of new labile supramolecular assemblies by formulating associations of DNA molecules with inorganic layered double hydroxides (LDHs). The results show that LDH/DNA hybrids synthesized by a coprecipitation route involving the in situ formation of LDHs around DNA molecules acting as templates were characterized by a lamellar organization, with DNA molecules sandwiched between hydroxide layers, exhibiting a regular spacing of 1.96 nm. Our results indicate that labile complexes resulting from the association of nucleic acids and inorganic materials can be obtained not only by anion exchange but also by a direct self-assembly route.

DNA↗
PubMed2006

Novel expansion/shrinkage modulation of 2D layered MOF triggered by clathrate formation with CO(2) molecules.

Crystal-to-crystal transformation from a 3D interpenetrated-type MOF {[Cu(BF(4))(2)(bpy)(H(2)O)(2)] (bpy)} (1) to a 2D square-grid-type [Cu(BF(4))(2)(bpy)(2)] (2) (bpy = 4,4'-bipyridine) was observed. It was derived from dehydration and confirmed by in situ FT-IR, TG, and elemental analysis. Moreover, we elucidate the novel expansion/shrinkage dynamic modulation of 2 triggered by clathrate formation with gas molecules.

Adsorption↗
PubMed2002

Neotame anhydrate polymorphs. II: Quantitation and relative physical stability.

PURPOSE: To study the relative thermodynamic and kinetic stabilities of neotame anhydrate polymorphs A, D, F, and G, and to develop a quantitative method for analyzing polymorphic mixtures of A and G by powder X-ray diffractometry (PXRD). METHODS: Based on the melting points, heats of fusion, and densities of the four polymorphs, thermodynamic rules were applied to study their thermodynamic relationships. The phase transition temperature of Forms A and G was estimated from their heats of solution and intrinsic dissolution rates (J) in 2-propanol. Using PXRD, a method for the quantitative analysis of polymorphic mixtures of Forms A and G was developed. Binary polymorphic mixtures of Forms A, D, F, or G were stored under zero relative humidity at 23 or 70 degrees C, and their compositions were monitored by PXRD. RESULTS: The endothermic enthalpy of solution of A, D, F, and G follows the rank order: G (29.71 +/- 0.82 kJ/mol, n = 4) > A (28.48 +/- 0.51 kJ/mol, n = 4) > D (20.43 +/- 0.45 kJ/mol, n = 4) > F (18.77 +/- 0.31 kJ/mol, n = 4). The van't Hoff plots of ln(J) against 1/T for A and G show good linearity between 25 degrees C and 32 degrees C. At 23 degrees C polymorphic mixtures remain unchanged for 4 months. However, at 70 degrees C the phase transition is fast and the relative stability of the four polymorphs follows the rank order: G > D > F and G > A. CONCLUSIONS: PXRD provides a reliable and accurate technique for the quantitative analysis of polymorphic mixtures of Forms A and G. Among the four polymorphs, A-G and A-D are enantiotropic pairs, whereas D-F, D-G, F-G are monotropic pairs. The phase transition temperature between A and G lies within the range 35-70 degrees C.

Anhydrides↗
PubMed2002

Investigation of solid-state reactions using variable temperature X-ray powder diffractometry. II. Aminophylline monohydrate.

PURPOSE: The object of this investigation was to demonstrate the utility of X-ray powder diffractometry (XRD) to study the kinetics of a complex pharmaceutical solid-state reaction wherein the reactant, product and intermediate phases were all simultaneously quantified. METHODS: Aminophylline monohydrate (I) decomposed to anhydrous theophylline (III) either directly or through an intermediate (anhydrous aminophylline, II). The reaction kinetics were studied isothermally at several temperatures ranging from 65 to 100 degrees C. By measuring the intensities of the XRD peaks unique to I, II and III, it was possible to simultaneously quantify the 3 phases during the entire reaction. RESULTS: Assuming that all the reaction steps follow first-order kinetics. the three equations describing the concentrations of I, II and III as a function of time, were derived. By fitting the experimental data to these equations, it was possible to obtain the rate constants for the three reaction steps. The rate constants were obtained at different temperatures and were used to draw Arrhenius type plots from which the activation energies were determined. At lower temperatures (< 80 degrees C). the concentration of the intermediate phase, i.e., II, was low throughout the reaction while at higher temperatures (> 90 degrees C), there was rapid formation and accumulation of II during the early stages of the reaction. These differences could be attributed to the fact that k1 (I --> II) had a more pronounced temperature dependence than k2 (I --> III) and k3 (II --> II). The XRD results were confirmed with isothermal thermogravimetry. CONCLUSIONS: Variable temperature XRD is a powerful tool to probe reaction kinetics in crystalline pharmaceuticals since it permits simultaneous quantification of multiple solid phases.

Aminophylline↗
PubMed2006

Room-temperature miscibility gap in LixFePO4.

The rechargeable lithium-ion cell is an advanced energy-storage system. However, high cost, safety hazards, and chemical instability prohibit its use in large-scale applications. An alternative cathode material, LiFePO(4), solves these problems, but has a kinetic problem involving strong electron/hole localization. One reason for this is believed to be the limited carrier density in the fixed monovalent Fe(3+)PO(4)/LiFe(2+)PO(4) two-phase electrode reaction in LixFePO4. Here, we provide experimental evidence that LixFePO4, at room temperature, can be described as a mixture of the Fe(3+)/Fe(2+) mixed-valent intermediate LialphaFePO4 and Li1-betaFePO4 phases. Using powder neutron diffraction, the site occupancy numbers for lithium in each phase were refined to be alpha=0.05 and 1-beta=0.89. The corresponding solid solution ranges outside the miscibility gap (0<x<alpha,1-beta<x<1) were detected by the anomaly in the configurational entropy, and also by the deviation of the open-circuit voltage from the constant equilibrium potential. These findings encourage further improvement of this important class of compounds at ambient temperatures.

Journal Article↗
PubMed2004

Li(+) ion conductivity in rock salt-structured nickel-doped Li(3)NbO(4).

Two mechanisms of doping Li(3)NbO(4), which has an ordered, rock salt superstructure, have been established. In the "stoichiometric mechanism", the overall cation-to-anion ratio is maintained at 1:1 by means of the substitution 3Li(+) + Nb(5+) --> 4Ni(2+). In the "vacancy mechanism", Li(+) ion vacancies are created by means of the substitution 2Li(+) --> Ni(2+). Solid solution ranges have been determined for both mechanisms and a partial phase diagram constructed for the stoichiometric join. On the vacancy join, the substitution mechanism has been confirmed by powder neutron diffraction; associated with lithium vacancy creation, a dramatic increase in Li(+) ion conductivity occurs with increasing Ni content, reaching a value of 5 x 10(-4) Omega(-1) cm(-1) at 300 degrees C for composition x= 0.1 in the formula Li(3-2x)Ni(x)NbO(4). This is the first example of high Li(+) ion conductivity in complex oxides with rock salt-related structures.

Journal Article↗
PubMed2004

The pyrochlore family -- a potential panacea for the frustrated perovskite chemist.

Many known complex oxides of general formula A(2)B(2)X(7) adopt the pyrochlore structure, a key structure-type that has been shown to demonstrate a vast range of useful physical properties. Areas currently of much interest with respect to pyrochlores, include metal-insulator transitions, magnetic frustration/spin ices, magnetoresistance, superconductivity, ferroelectrics, O/F ionic conductivity, mixed conductivity, pigments and catalysis. We present some recent results on three types of pyrochlore materials that show unusual magnetic, optical and electronic behaviours associated with subtle structural and compositional changes. High-resolution powder neutron diffraction studies of the superconducting Cd(2)Re(2)O(7) and the ferroelectric Cd(2)Nb(2)O(7) have been undertaken on material cooled below room temperature. Both Cd(2)Re(2)O(7) and Cd(2)Nb(2)O(7) exhibit small structure distortions, in each case involving a distortion from a cubic unit cell, on cooling below approximately 180 K and possible models that can be used to describe the low-temperature structures and associated atomic displacements are developed and described in this article. A range of materials of the general formula Ca(1-x)Ln(x)TaO(2-x)N(1+x), x= 0.5 and x= 1, Ln = La-Yb have been synthesised and shown to adopt pyrochlore and/or perovskite structures. The absorption spectra of these materials are discussed in terms of their structures and compositions.

Journal Article↗
PubMed2004

Rubidium doped zeolite rho: structure and microwave conductivity of a metallic zeolite.

Large crystallites of high purity zeolite rho were synthesized by controlled monitoring of the aging and heating period of the mother gel. The microwave conductivity of Rb-rho doped with up to 20 Rb atoms per unit cell was measured over the temperature range 15-300 K, and the structures of three of the samples were examined through Rietveld analysis of powder neutron diffraction data. At low concentrations of rubidium dopant the observed microwave responses were dominated by polarization effects. In the sample Rb(17)/Rb-rho a strongly temperature-dependent electronic contribution to the conductivity was observed above approximately =150 K. In Rb(20)/Rb-rho, conductivities in the range 1.5-2.3 Sm(-1) were observed between 15 and 300 K. This residual conductivity at 15 K, unprecedented in a zeolite, indicates that the sample is indeed metallic; however, the values of conductivity measured are low in comparison to conventional metals and comparable to those of doped semiconductors. The evolution of the conducting behaviour is discussed in relation both to observed structural and to possible electronic changes occurring within the samples on metal doping.

Journal Article↗
PubMed2005

Thiol-functionalized mesostructured silica vesicles.

The direct supramolecular assembly of organofunctional mesostructures with a vesicular hierarchical morphology is reported for the first time for (SiO2)1-x(LSiO1.5)x compositions, where L is a mercaptopropyl group and x = 0.10-0.30.

Microscopy, Electron, Scanning Transmission↗
PubMed2005

One teflon-like channelled nanoporous polymer with a chiral and new uninodal 4-connected net: sorption and catalytic properties.

Zn(C17H8F6O4) is the first example of a fluoro-lined nanotube organo-inorganic 3D polymeric chiral structure, which possesses two different types of isolated channels, one of them being laid out with a double spiral of CF3-groups from the ligand molecule; the structure is a new uninodal 4-connected net that only exists when bent ligands connect the centres, and the compound exhibits selective sorption and catalytic chiral recognition properties.

Adsorption↗
PubMed2005

Selective polymorph transformation via solvent-drop grinding.

A method of inducing specific polymorph transformations is exemplified with two single-component systems, whereby a given crystal form undergoes conversion when subjected to solid state grinding in the presence of a minor quantity of a certain solvent.

Combinatorial Chemistry Techniques↗
PubMed2005

Conformational polymorphism of methacrylamide.

The industrially important compound methacrylamide crystallizes as concomitant conformational polymorphs; the monoclinic form contains only the s-cis conformer, while the orthorhombic form contains only the s-trans conformer.

Acrylamides↗
PubMed2006

Crystal structure and thermochemical reactivity of an unusual copper complex that contains copper in four different coordination geometries.

According to X-ray single-crystal structure analysis, {[Cu(II)(en)2]2[Cu(I)2(CN)6]}[Cu(II)(en)2]2[Cu(I)(CN)3]2.2H2O contains copper in four different coordination environments: trigonal planar, square planar, square pyramidal and tetrahedral. The different coordination geometries of copper were investigated by quantum chemical calculations of model compounds, thus allowing to tentatively assign the different CN-bands in the IR spectrum. The thermolysis led to sub-microm sized rod-like copper(II) oxide particles.

Copper↗
PubMed2006

Raman spectroscopic study of hydrogen ordered ice XIII and of its reversible phase transition to disordered ice V.

Raman spectra of recovered ordered H(2)O (D(2)O) ice XIII doped with 0.01 M HCl (DCl) recorded in vacuo at 80 K are reported in the range 3600-200 cm(-1). The bands are assigned to the various types of modes on the basis of isotope ratios. On thermal cycling between 80 and 120 K, the reversible phase transition to disordered ice V is observed. The remarkable effect of HCl (DCl) on orientational ordering in ice V and its phase transition to ordered ice XIII, first reported in a powder neutron diffraction study of DCl doped D(2)O ice V (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758), is demonstrated by Raman spectroscopy and discussed. The dopants KOH and HF have only a minor effect on hydrogen ordering in ice V, as shown by the Raman spectra.

Hydrogen↗
PubMed2003

Ba2LnSbO6 and Sr2LnSbO6 (Ln = Dy, Ho, Gd) double perovskites: lanthanides in the geometrically frustrating fcc lattice.

Magnetic ground states in solids often arise as a result of a delicate balance between competing factors. One currently active area of research in magnetic materials involves compounds in which long-range magnetic ordering at low temperatures is frustrated by the geometry of the crystalline lattice, a situation known as geometrical magnetic frustration. The number of systems known to display the effects of such frustration is growing, but those that are sufficiently simple from theoretical, chemical, and physical perspectives to allow for detailed understanding remain very few. A search for model compounds in this family has led us to the double perovskites Ba2LnSbO6 and Sr2LnSbO6 (Ln = Dy, Ho, and Gd) reported here. Ba2DySbO6,Ba2HoSbO6,Sr2DySbO6, and Sr2HoSbO6 are structurally characterized by powder neutron diffraction at ambient temperature. The trivalent lanthanides and pentavalent antimony are found to be fully ordered in the double-perovskite arrangement of alternating octahedra sharing corner oxygens. In such a structure, the lanthanide sublattice displays a classical fcc arrangement, an edge-shared network of tetrahedra known to result in geometric magnetic frustration. No magnetic ordering is observed in any of these compounds down to temperatures of 2 K, and in the case of the Dy-based compounds in particular, frustration of the magnetic ordering is clearly present. Lanthanide-based double perovskites are proposed to be excellent model systems for the detailed study of geometric magnetic frustration.

Journal Article↗