X-ray Diffraction Methods (x-ray + diffraction_methods)

Distribution by Scientific Domains
Distribution within Chemistry

Selected Abstracts

Developing the Kharasch Reaction in Aqueous Media: Dinuclear Group 8 and 9 Catalysts Containing the Bridging Cage Ligand Tris(1,2-dimethylhydrazino)diphosphane,

Alba E. Díaz-Álvarez
Abstract The dinuclear complexes [{RuCl2(,6 - p -cymene)}2(,-THDP)] (4) and [{MCl(,4 -cod)}2(,-THDP)] [M = Rh (5), Ir (6)], containing the bridging cage-type ligand tris(1,2-dimethylhydrazino)diphosphane (THDP), have been synthesized in high yields (89,95,%) by treatment of dimers [{RuCl(,-Cl)(,6 - p -cymene)}2] (1) and [{M(,-Cl)(,4 -cod)}2] [M = Rh (2), Ir (3)] with one equivalent of THDP. The structure of the (,6 -arene)ruthenium(II) derivative 4 has been unequivocally confirmed by means of X-ray diffraction methods. All these complexes have been found to be active catalysts for the atom-transfer radical addition of bromotrichloromethane to olefins (Kharasch reaction) in heterogeneous aqueous media under mild conditions (room temp.).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Multifunctional Microporous MOFs Exhibiting Gas/Hydrocarbon Adsorption Selectivity, Separation Capability and Three-Dimensional Magnetic Ordering,

Kunhao Li
Abstract Microporous materials [M3(HCOO)6],·,DMF (M,=,Mn, Co, Ni) were synthesized solvothermally and structurally characterized by single crystal and powder X-ray diffraction methods. The metal network exhibits diamondoid connectivity and the overall framework gives rise to zigzag channels along the b axis where guest dimethylformamide molecules reside. The effective pore size of these channels is ,5,6,Å. The materials feature high thermal stability and permanent porosity with relatively small pore diameters which are attributed to the extensive strong dative bonding between the metal centers and formate molecules. The title compounds exhibit interesting multi-fold gas adsorption and magnetic properties. The adsorption study of a series of alcohols, aromatics, and linear hydrocarbons revealed strong control of the adsorbent channel structures on their adsorption capacity and selectivity. The study also indicated possibility of using these materials for separation of close boiling chemicals (e.g., ethylbenzene and p-xylene) via pressure swing adsorption (PSA) process and molecules with different diffusion parameters via kinetic-based process. Three-dimensional spontaneous magnetic ordering was found in all three network structures investigated and at ground states they behave somewhat like soft magnets. [source]

Solution NMR and X-Ray Structural Studies on Phthalocyaninatoiron Complexes

Ignacio Fernández
Abstract The addition of primary amines as solubilizing reagents for phthalocyaninatoiron complexes is shown to afford six-coordinate bis(amine)phthalocyaninato complexes, i.e., [Fe(amine)2(pc)] 2 (amine,=,decan-1-amine) and 3 (amine,=,benzylamine), with the two new N-donors occupying the trans -axial positions. The new complexes were characterized by extensive NMR measurements in THF solution. For complex 3 with the benzylamine ligand, the solid-state structure was determined by X-ray diffraction methods. Complex 2 is sufficiently labile in THF solution to exchange one amine ligand against CO (gas) affording an equilibrium mixture containing [Fe(amine)(CO)(pc)] 4. [source]

Structural determination of the stable and meta-stable forms of atomoxetine HCl using single crystal and powder X-ray diffraction methods

Gregory A. Stephenson
Abstract StratteraÔ is the first FDA-approved nonstimulant medication for the treatment of Attention Deficit Hyperactivity Disorder (ADHD) in children, adolescents, and adults. Two polymorphic forms and an amorphous form of the active pharmaceutical ingredient, atomoxetine HCl, were discovered during drug development. The thermodynamically stable polymorphic form was selected for the commercial product. The stable form readily grows as crystals suitable for single crystal diffraction. The meta-stable crystal form is isolated by rapid crystallization, providing crystals that are too small for routine single crystal methods; consequently its structure was determined by X-ray powder diffraction. © 2006 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95: 1677,1683, 2006 [source]

The change in characteristics of microcrystalline cellulose during wet granulation using a high-shear mixer

Tatsuya Suzuki
The objective of this study was to investigate the mechanism of hard granule formation and to demonstrate the applicability of X-ray diffraction methods for studying the polymeric pharmaceutical excipients. Using a high-shear mixer, microcrystalline cellulose (MCC) was granulated with water as the granulating liquid. The hardness of the MCC granules increased with granulation time and the amount of water added. The specific surface area measured by the N2 adsorption method was reduced during the process. Crystallite size of cellulose, calculated by Scherrer's equation adapted for wide angle X-ray diffraction method, decreased with granulation time and with increasing amounts of water added. Debye plots for X-ray small scattering patterns suggested that the average magnitude of the continuous solid region in MCC granules became significantly greater, whereas the specific surface area of the MCC granules, calculated from Debye plots, became smaller in comparison with that of intact MCC. These findings suggested that the long-chain structures in MCC were disrupted, resulting in smaller units with shorter chain lengths due to the strong shear force of the impeller. These smaller units then form a network within the granules. Thus, MCC granules are strengthened with longer granulation time and greater amounts of water, resulting in a more intricate network. The change in MCC chain length and physical structure can be experimentally detected using the small-angle X-ray scattering and wide-angle powder X-ray diffraction methods. [source]

Structural and vibrational study of 2-(2,- furyl)-4,5-1H -dihydroimidazole

Juan Zinczuk
Abstract In this study 2-(2,-furyl)-4,5-1H -dihydroimidazole (1) was prepared and then characterized by infrared, Raman, and multidimensional nuclear magnetic resonance (NMR) spectroscopies. The crystal and molecular structures of 1 were determined by X-ray diffraction methods. The density functional theory (DFT) and second-order Møller,Plesset theory (MP2) with Pople's basis set show that there are two conformers for the title molecule that have been theoretically determined in the gas phase, and that only one of them, conformer I, is present in the solid phase. NMR spectra observed for 1 were successfully compared with the calculated chemical shifts at the B3LYP/6-311++G** level theorized for this conformer. The harmonic vibrational frequencies for the optimized geometry of the latter conformer were calculated at the B3LYP/6-311++G** level in the approximation of the isolated molecule. For a complete assignment of the IR and Raman spectra in the solid phase of 1, DFT calculations were combined with Pulay´s scaled quantum mechanics force field (SQMFF) methodology to fit the theoretical frequency values to the experimental ones. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Growth and characterization of ZnMgTe/ZnTe layered structures grown by molecular beam epitaxy

S. Imada
Abstract ZnMgTe/ZnTe layered structures were grown on ZnTe substrates by molecular beam epitaxy, and the crystal structures were characterized using X-ray diffraction methods. This structure would be the waveguide for various optoelectronic devices. Therefore, the crystal quality of this layered structure would be very crucial for the realization of high performance devices. ZnMgTe is lattice mismatched to ZnTe, and the increase of the ZnMgTe layer thickness or Mg mole fraction ratio would result in the crystal quality deterioration of the layered structure. The critical layer thickness (CLT) was theoretically derived, and various structures with various ZnMgTe layer thickness and Mg mole fraction were grown. The lattice mismatch strain relief and crystal quality of those samples were investigated by means of X-ray reciprocal space mapping (RSM) and cross sectional transmission electron microscopy (TEM). The dislocation formation and the lattice mismatch relaxation were confirmed for various samples and it was revealed that the calculated CLT values could be used as an appropriate guideline to design the dislocation free and high performance device structures (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effect of thermal aging on the crystal structural characteristics of poly(tetra fluoro ethylene)

Anjana Jain
The residual effects of cumulative thermal aging on the crystal structural characteristics of the fluoro carbon poly(tetra fluoro ethylene) (PTFE) have been studied by X-ray diffraction methods. The initial hexagonal arrangement of the PTFE chains in a 157 helical conformation is left unaffected by the exposures to temperatures (T), up to and beyond its melting point, Tm. The unit cell registers a residual anisotropic volume expansion. The anisotropy arises from the enhanced enlargement of the basal plane dimension a compared with the axial dimension c. Conformational changes contributing to the observed increase in the chain length have been examined. Enhancement of residual crystallinity of samples aged at T's < Tm suggests that the selective thermal aging could be used as an effective tool to improve the initial crystallinity of commercially available PTFE. The activation energy for 50% enhancement in initial crystallinity has been estimated as 53.9 kJ mol,1. Aging at 400°C, a temperature above Tm, is accompanied by markedly different features viz., deterioration in crystallinity and other structural characteristics. The overall behavior of thermally aged PTFE bears a marked similarity to many polyamides. POLYM. ENG. SCI., 47:1724,1729, 2007. © 2007 Society of Plastics Engineers [source]

Polymorphic behavior of nylon 6/saponite and nylon 6/montmorillonite nanocomposites

Tzong-Ming Wu
X-ray diffraction methods and DSC thermal analysis have been used to investigate the structural change of nylon 6/clay nanocomposites. Nylon 6/clay has prepared by the intercalation of ,-caprolactam and then exfoliaton of the layered saponite or montmorillonite by subsequent polymerization. Both X-ray diffraction data and DSC results indicate the presence of polymorphism in nylon 6 and in nylon 6/clay nanocomposites. This polymorphic behavior is dependent on the cooling rate of nylon 6/clay nanocomposites from melt and the content of saponite or montmorillonite in nylon 6/clay nanocomposites. The quenching from the melt induces the crystallization into the , crystalline form. The addition of clay increases the crystallization rate of the , crystalline form at lower saponite content and promotes the heterophase nucleation of , crystalline form at higher saponite or montmorillonite content. The effect of thermal treatment on the crystalline structure of nylon 6/clay nanocomposites in the range between Tg and Tm is also discussed. [source]

Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298,<,T,<,100,K)

Günther J. Redhammer
The alkali-metal clinopyroxene NaTi3+Si2O6, one of the rare compounds with trivalent titanium, was synthesized at high temperature/high pressure and subsequently investigated by single-crystal X-ray diffraction methods between 298 and 100,K. One main difference between the high- and the low-temperature form is the sudden appearance of two different Ti3+,Ti3+ interatomic distances within the infinite chain of the TiO6 octahedra just below 197 K. This change can be seen as direct evidence for the formation of Ti,Ti singlet pairs in the low-temperature phase. Mean Ti,O bond lengths smoothly decrease with decreasing temperature and the phase transition is associated with a slight jump in the Ti,O bond length. The break in symmetry, however, causes distinct variations, especially with respect to the two Ti,Oapex bond lengths, but also with respect to the four Ti,O bonds in the equatorial plane of the octahedron. The TiO6 octahedron appears to be stretched in the chain direction with a slightly larger elongation in the P low-temperature phase compared with the C2/c high-temperature phase. Polyhedral distortion parameters such as bond-length distortion and octahedral angle variance suggest the TiO6 octahedron in P to be closer to the geometry of an ideal octahedron than in C2/c. Mean Na,O bond lengths decrease with decreasing temperature and the variations in individual Na,O bond lengths are the result of variations in the geometry of the octahedral site. The tetrahedral site acts as a rigid unit, which does not show pronounced changes upon cooling and through the phase transitions. There are neither large changes in bond lengths and angles nor in polyhedral distortion parameters, for the tetrahedral site, when they are plotted. In contrast with the C2/c,P21/c phase transition, found especially in LiMSi2O6 clinopyroxenes, no very large variations are found for the tetrahedral bridging angle. Thus, it is concluded that the main factor inducing the phase transition and controlling the structural variations is the M1 octahedral site. [source]

Unambiguous determination of H-atom positions: comparing results from neutron and high-resolution X-ray crystallography

Anna S. Gardberg
The locations of H atoms in biological structures can be difficult to determine using X-ray diffraction methods. Neutron diffraction offers a relatively greater scattering magnitude from H and D atoms. Here, 1.65,Å resolution neutron diffraction studies of fully perdeuterated and selectively CH3 -protonated perdeuterated crystals of Pyrococcus furiosus rubredoxin (D-rubredoxin and HD-rubredoxin, respectively) at room temperature (RT) are described, as well as 1.1,Å resolution X-ray diffraction studies of the same protein at both RT and 100,K. The two techniques are quantitatively compared in terms of their power to directly provide atomic positions for D atoms and analyze the role played by atomic thermal motion by computing the , level at the D-atom coordinate in simulated-annealing composite D-OMIT maps. It is shown that 1.65,Å resolution RT neutron data for perdeuterated rubredoxin are ,8 times more likely overall to provide high-confidence positions for D atoms than 1.1,Å resolution X-ray data at 100,K or RT. At or above the 1.0, level, the joint X-ray/neutron (XN) structures define 342/378 (90%) and 291/365 (80%) of the D-atom positions for D-rubredoxin and HD-rubredoxin, respectively. The X-ray-only 1.1,Å resolution 100,K structures determine only 19/388 (5%) and 8/388 (2%) of the D-atom positions above the 1.0, level for D-rubredoxin and HD-rubredoxin, respectively. Furthermore, the improved model obtained from joint XN refinement yielded improved electron-density maps, permitting the location of more D atoms than electron-density maps from models refined against X-ray data only. [source]

The influence of an internal electric field upon protein crystallization using the gel-acupuncture method

N. Mirkin
In this work, the influence of an internal electric field upon the crystallization of lysozyme and thaumatin is explored using a modified design of the gel-acupuncture setup. From a crystallographic point of view, the orientation of crystals that grow preferentially over different types of electrodes inside capillary tubes is also evaluated. Finally, the crystal quality and the three-dimensional structure of these proteins grown with and without the electric field influence are analyzed by means of X-ray diffraction methods. [source]

Homoleptic Organoderivatives of High-Valent Nickel(III)

Abstract Homoleptic perhalophenyl derivatives of divalent nickel complexes with the general formula [NBu4]2[NiII(C6X5)4] [X=F (1), Cl (2)] have been prepared by low-temperature treatment of the halo-complex precursor [NBu4]2[NiBr4] with the corresponding organolithium reagent LiC6X5. Compounds 1 and 2 are electrochemically related by reversible one-electron exchange processes with the corresponding organometallate(III) compounds [NBu4][NiIII(C6X5)4] [X=F (3), Cl (4)]. The potentials of the [NiIII(C6X5)4],/[NiII(C6X5)4]2, couples are +0.07 and ,0.11,V for X=F or Cl, respectively. Compounds 3 and 4 have also been prepared and isolated in good yield by chemical oxidation of 1 or 2 with bromine or the amminium salt [N(C6H4Br-4)3][SbCl6]. The [NiIII(C6X5)4], species have SP -4 structures in the salts 3 and 4, as established by single-crystal X-ray diffraction methods. The [NiII(C6F5)4]2, ion in the parent compound 1 has also been found to exhibit a rather similar SP -4 structure. According to their SP -4 geometry, the NiIII compounds (d7) behave as S=1/2 systems both at microscopic (EPR) and macroscopic levels (ac and dc magnetization measurements). The spin Hamiltonian parameters obtained from the analysis of the magnetic behavior of 3 and 4 within the framework of ligand field theory show that the unpaired electron is centered mainly on the metal atom, with >97,% estimated d contribution. Thermal decomposition of 3 and 4 proceeds with formation of the corresponding C6X5C6X5 coupling compounds. [source]