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The crystal structure of a previously unknown triclinic polymorph of adipamide has been solved from laboratory X-ray powder diffraction data using a new direct space global optimisation method based on differential evolution.
The objective of this study was to demonstrate the applicability of parallel beam X-ray powder diffraction (XRPD) and a new method for whole pattern fitting to the quantification of the residual amount of amorphous content in a pharmaceutical solid using lactose as a model system. Lactose monohydrate, prepared by slurry conversion of anhydrous lactose, was mixed with different amounts of amorphous lactose produced by lyophilization. X-ray powder diffractograms of each mixture were recorded and analyzed by whole pattern fitting using Percentage Crystallinity Determination Software from Kratos Analytical Inc. The polycapillary X-ray optic, which provides a parallel beam of X-radiation, has advantages over Bragg-Brentano Optics with respect to sample height artifacts. Significant shifts in peak position with changes in sample height of lactose monohydrate were observed using Bragg-Brentano Optics while no change was detected for the polycapillary X-ray optic. A technique to normalize all diffractograms to have the same total integrated intensity was necessary to eliminate tube fluctuation effects. After normalization, the amorphous content of lactose in the range of 1-10% was reproducibly predicted (small standard deviation between samplings) using whole pattern fitting. The limit of detection was calculated to be 0.37% amorphous content. The results indicated that parallel beam XRPD and whole pattern fitting can provide accurate analysis of relatively small amounts of amorphous content in pharmaceuticals compared to typical XRPD analysis.
The present study reports the re-examination of the crystal structure of mannan I by the X-ray powder diffraction study combined with the miniature crystal model simulation. A primary objective of this study was to investigate an adequate chain staggering position along the fiber axis. Among the several crystal structure models proposed for mannan I, that derived from the electron diffraction study of the single crystals was adopted as a starting model. The X-ray crystallographic residuals were calculated for the single crystal structure at different chain staggering positions, while the ab projection was kept invariant. In a similar fashion, the miniature crystal models consisting of seven mannotetraoses were constructed, each having different chain staggering values and the three-dimensional structures of all the models were subsequently optimized by using the MM3(92) program without introducing any constraint. Both the X-ray residual and lattice energy plots with respect to the chain staggering position showed the common minima around the -0.25 x c chain staggering. The minimum models were subjected to a search for preferred orientations of O2 and O6 hydroxyl groups. It was found that the hydroxyl groups present inside the minicrystal tended to rotate into particular orientations during the structure optimizations to form the O5-H-O2 and O3-H-O6 intermolecular hydrogen bonds between the adjacent oligomers in an alternative direction.
Synthetic hydrocerussite [trilead dihydroxide dicarbonate, Pb(3)(CO(3))(2)(OH)(2)] can be easily obtained, as a white powder, by the action of carbon dioxide and water on either lead or litharge at pH 4-5. This compound is also found in lead corrosion technological products as a fine-grained phase. Ab initio crystal structure determination was carried out on X-ray powder diffraction data. The heavy-atom method and the Patterson function helped determine the crystallographic model and the atom locations. The Rietveld fitting procedure was used for the final refinement. The atomic arrangement is closely related to the structures of other lead hydroxide carbonates. The hydrocerussite structure can be viewed as a sequence of two types of layers stacked along [001]. Layer A is composed of Pb and CO(3), and layer B is composed of Pb and OH. The stacking sequence is ...BAABAA...
The crystal structure of the defect perovskite Ce1/3NbO3, cerium niobium oxide, has been re-examined by neutron powder and electron diffraction. The results of a powder neutron Rietveld refinement indicate that the structure is monoclinic: space group P2/m with Z = 4, a = 5.5267 (3), b = 7.8824 (2), c = 5.5245 (3) A, beta = 90.294 (1)degrees, V= 240.67 (2) A3 at 298 K with chi2 = 2.570. Previous reports have described the Ce1/3NbO3 structure in a smaller (V/2) orthorhombic cell based solely upon X-ray powder diffraction data. The presence of weak reflections in the electron diffraction pattern provides conclusive evidence for a monoclinic superstructure of the orthorhombic cell. While these superlattice reflections are barely detectable with X-ray radiation, they are clearly visible in the neutron diffraction experiments. The superlattice reflections are shown to arise from a tilting of the NbO6 octahedra which results in the reduction of symmetry from orthorhombic to monoclinic. It is also found that the Ce3+ and Nb5+ cations are displaced from the centres of their respective polyhedra to accommodate the bond-valence requirements of the crystal structure. It is likely that distortions of this type are present in other Ln1/3NbO3 and Ln1/3TaO3 defect perovskites.
The structures of three substituted arenesulfonamides have been solved from laboratory X-ray powder diffraction data, using a new direct-space structure solution method based on a differential evolution algorithm, and refined by the Rietveld method. In 2-toluenesulfonamide, C(7)H(9)NO(2)S (I) (tetragonal I4(1)/a, Z = 16), the molecules are linked by N-H...O=S hydrogen bonds into a three-dimensional framework. In 3-nitrobenzenesulfonamide, C(6)H(6)N(2)O(4)S (II) (monoclinic P2(1), Z = 2), N-H...O=S hydrogen bonds produce molecular ladders, which are linked into sheets by C-H...O=S hydrogen bonds: the nitro group does not participate in the hydrogen bonding. Molecules of 4-nitrobenzenesulfonamide, C(6)H(6)N(2)O(4)S (III) (monoclinic P2(1)/n, Z = 4), are linked into sheets by four types of hydrogen bond, N-H...O=S, N-H...O(nitro), C-H...O=S and C-H...O(nitro), and the sheets are weakly linked by aromatic pi...pi stacking interactions.
The purpose of this paper and a test case study is to assess a method of ab initio structure solution from powder diffraction data using f' difference techniques. A theoretical foundation for the approach used is first provided. Then, with a test case (nickel sulfate hexahydrate), it is shown that both the position of the anomalous scatterer (Ni) can be determined and the structure can be developed in full. Specifically, synchrotron-radiation data were collected at two wavelengths close to the K edge for Ni and three wavelengths remote from the Ni absorption edge, at 1.3, 1.8 and 2.16 A. These five wavelengths then allowed various combinations to be tried to establish which wavelength pairs gave the optimum signal in the Patterson maps using dispersive amplitude differences. The initial phases derived from the metal-atom position then allowed the structure to be fully developed by difference Fourier cycling. The relevance of these developments to structure-solution possibilities for proteins via powder dispersive difference data is then outlined.
The structure of lead tartrate, Pb(2+).C(4)H(4)O(6)(2-), has been solved from X-ray powder diffraction data. The cation exhibits ninefold coordination and the tartrate groups are linked through Pb.O contacts to form a three-dimensional network.
The crystal structure of a new hybrid product comprised of two rigid building blocks, namely dirhodium(II) tetraacetate, [Rh(2)(O(2)CCH(3))(4)] (1), and 2,6-diselenaspiro[3.3]heptane, Se(2)C(5)H(8) (2), has been solved ab initio using laboratory source X-ray powder diffraction (XRPD) data. The rigid body refinement approach has been applied to assist in finding an adequate model and to reduce the number of the refined parameters. Complex [Rh(2)(O(2)CCH(3))(4).mu(2)-Se(2)C(5)H(8)-Se,Se'] (3) conforms to the triclinic unit cell with lattice parameters of a = 8.1357(4), b = 8.7736(4), and c = 15.2183(8) A, alpha = 77.417(3), beta = 88.837(3), and gamma = 69.276(4) degrees, V = 989.66(8) A(3), and Z = 2. The centrosymmetric P space group was selected for calculations. The final values of the reduced wR(p), R(p), and chi(2) were calculated at 0.0579, 0.0433, and 5.95, respectively. The structure of 3 is a one-dimensional zigzag polymer built on axial Rh...Se interactions at 2.632(6) A. The 2,6-diselenaspiro[3.3]heptane ligand acts as a bidentate linker bridging dirhodium units via both selenium atoms. The geometrical parameters of individual groups for rigid body refinement have been obtained from X-ray powder data for dirhodium(II) tetraacetate (1) and from single-crystal X-ray diffraction for diselenium molecule 2. The crystal structures of 1 and 2 are reported here for the first time. For 1 indexing based on XRPD data has resulted in the triclinic unit cell P with lattice parameters of a = 8.3392(7), b = 5.2216(5), and c = 7.5264(6) A, alpha = 95.547(10), beta = 78.101(6), and gamma = 104.714(13) degrees, V = 309.51(5) A(3), and Z = 1. The final values were wR(p) = 0.0452, R(p) = 0.0340, and chi(2) = 1.99. The 1D polymeric motif built on axial Rh.O interactions of the centrosymmetric dirhodium units has been confirmed for the solid-state structure of 1. Compound 2,6-diselenaspiro[3.3]heptane (2) conforms to the monoclinic space group P2(1)/c with the unit cell parameters of a = 5.9123(4), b = 19.6400(13), and c = 5.8877(4) A, beta = 108.5500(10) degrees, V = 648.15(8) A(3), and Z = 4.
Solid-state NMR is used to dramatically improve the efficiency and reliability of molecular crystal structure determination from X-ray powder diffraction data.
We describe a model-independent approach for the extraction of detailed lattice dynamics information from neutron powder diffraction data, based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement. Phonon dispersion curves for MgO extracted in this way are shown to reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. By means of molecular dynamics simulations, we quantify the extent to which the diffraction data are sensitive to lattice dynamics in this system.
The binding of N-acetylglucosamine (NAG) to chicken egg lysozyme (E.C. 3.2.1.17) was investigated by high-resolution X-ray powder diffraction at room temperature. NAG was found to bind to lysozyme in a rapid precipitation preparation with 0.05 M NaCl buffer pH 6.0, but not 0.05 M NaCl buffer pH 5.0. Binding was indicated by significant and readily apparent changes in the diffraction pattern from that of the apo protein precipitated from the same solvent. The location of NAG bound to lysozyme was easily found from a difference Fourier map generated from structure factors extracted during a preliminary combined Rietveld and stereochemical restraint refinement. Full protein and protein-NAG structures were refined with these techniques (R(wp) = 2.22-2.49%, R(p) = 1.79-1.95%, R(F)(2) = 4.95-6.35%) and revealed a binding mode for NAG which differed from that found in an earlier single-crystal study and probably represents a precursor trapped by rapid precipitation.
The complex [Fe(teec)6](BF4)2 (teec = chloroethyltetrazole) shows a two-step complete spin-crossover transition in the temperature range 300-90 K. Time-resolved synchrotron powder diffraction experiments have been carried out in this temperature range, and crystal structure models have been obtained from the powder patterns by using the parallel tempering technique. Of these models, the low-spin state structure at 90 K has been refined completely with Rietveld refinement. Its structural characteristics are discussed in relation to the high-spin state model and other spin-crossover compounds. The complex shows a remarkable anisotropic unit-cell parameter contraction that is dependent on the applied cooling rate. In addition, the possible important implications for the interpretation of spin-crossover behavior in terms of structural changes are discussed.
The structure of thallium selenate, Tl2SeO4, in a paraelastic phase (above 661 K) has been analysed by Rietveld analysis of the X-ray powder diffraction pattern. Atomic parameters based on the isomorphic K2SO4 crystal in the paraelastic phase were used as the starting model. The structure was determined in the hexagonal space group P6(3)/mmc, with a = 6.2916 (2) A and c = 8.1964 (2) A. From the Rietveld refinement it was found that two orientations are possible for the SeO4 tetrahedra, in which one of their apices points randomly up and down with respect to [001]. One Tl atom lies at the origin with 3m symmetry, the other Tl and one of the O atoms occupy sites with 3m symmetry, the Se atom is at a site with 6m2 symmetry and the remaining O atom is at a site with m symmetry. Furthermore, it was also found that the Tl atoms display anomalously large positional disorder along [001] in the paraelastic phase.
The stable anhydrous form of alpha-lactose has been obtained by the dehydration of alpha-lactose monohydrate in methanol. An X-ray powder diffraction pattern was recorded at room temperature with a laboratory diffractometer equipped with an INEL curved sensitive detector CPS120. The starting structural model of this form was found by a Monte-Carlo simulated annealing method. The structure was obtained through Rietveld refinements and the minimization of crystalline energy for the localization of the H atoms of the hydroxyl groups. Soft restraints were applied to bond lengths and angles. Networks of O-H...O hydrogen bonds account for the crystalline cohesion. A comparison is made between the hydrogen-bond networks of this form and those of the monohydrate and hygroscopic anhydrous forms of alpha-lactose.
The crystal structure of silicalite (SiO2) with a substantial amount of structural hydroxyl groups [Si(1-x)O(2-4x)(OH)4x, with 0.08<x<0.10] has been refined from neutron powder diffraction data measured using the HRPD instrument at the ISIS pulsed neutron source. Powder data were collected on the as-synthesized orthorhombic sample at 298 K, and on the deuterated monoclinic sample at 100 K. Preferential location of Si-atom vacancies was found on four out of 12 independent T sites in the orthorhombic silicalite [Si(6), Si(7), Si(10) and Si(11)], although the H atoms of the substituting hydroxyl groups could not be located because of the low statistical site occupancy on multiple sites. No significant population of D atoms or of Si vacancies was found in the tetrahedral sites of the monoclinic sample. The detected long-range order of adjacent Si atoms in defective orthorhombic [MFI] structures is compatible with a mechanism of Si vacancy clustering and with the model of hydroxyl nests assumed in the literature on the basis of IR spectroscopic evidence. Crystal data: orthorhombic, Pnma, Z = 8, a = 20.0511 (1), b = 19.8757 (1), c = 13.36823 (9) A, V = 5327.62 (5) A3, Dx = 1.798 g cm(-3), Mr = 721.01; monoclinic, P2(1)/n, Z = 4, a = 19.8352 (2), b = 20.0903 (2), c = 13.3588 (1) A, beta = 90.892 (1), V = 5322.78 (6) A3, Dx = 1.799 g cm(-3), Mr = 1442.02.
The crystal structure of glipizide, used as a major treatment of type-2 diabetes, has been determined ab initio using variable-temperature laboratory X-ray powder diffraction combined with a direct-space Monte Carlo/simulated annealing methodology. The strengths of the intermolecular interactions (van der Waals, pi-pi stacking, hydrogen bonding and steric interlock) were quantitatively estimated using the thermal expansion data, which were collected in the same set of experiments as those used to determine the structure.
The crystal symmetries of lead hafnate titanate (Pb(HfxTi1-x)O3, PHT) powders with 0.10<or=x<or=0.50 were investigated by high-resolution neutron powder diffraction. Samples with x<or=0.40 were tetragonal (space group P4 mm), while the sample with x=0.50 contained both monoclinic Cm and rhombohedral (modeled using the R3c space group) phases. The role of the B cations (Hf and Ti) and the oxygen octahedra network, in addition to the displacement of Pb ions from their ideal sites, in promoting the phase transformation between the P4 mm and Cm phases was considered. Two types of structural disorder were identified. Diffuse scattering between Bragg reflection peaks was assigned to Pb ion displacement. A second type of structural disorder, revealed by the weak intensities of observed pseudo-cubic 00l reflections with l even and as 00l reflection peak widths significantly broader than the l00 reflection peaks, was observed. This behavior was attributed to disorder in the arrangement of the O-B-O rows parallel to the c axis. For small values of x, this shift was predominantly along the c axis, whereas shifts perpendicular to the c axis increased with increasing x. These features were modeled using an hkl-dependent line-broadening model. The origin of the hkl-dependent line broadening was assigned to the microstrain accompanying a spatial-composition variation. Structural models were tested by computing valence sums and spontaneous polarization values.