PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Powder Diffraction”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMed2003

Development of a multipopulation parallel genetic algorithm for structure solution from powder diffraction data.

Previously, the genetic algorithm (GA) approach for direct-space crystal structure solution from powder diffraction data has been applied successfully in the structure determination of a range of organic molecular materials. In this article, we present a further development of our approach, namely a multipopulation parallel GA (PGA), which is shown to give rise to increased speed, efficiency, and reliability of structure solution calculations, as well as providing new opportunities for further optimizing our GA methodology. The multipopulation PGA is based on the independent evolution of different subpopulations, with occasional interaction (e.g., transfer of structures) allowed to occur between the different subpopulations. Different strategies for carrying out this interpopulation communication are considered in this article, and comparisons are made to the conventional single-population GA. The increased power offered by the PGA approach creates the opportunity for structure determination of molecular crystals of increasing complexity.

Journal Article↗
PubMed2005

Polymorphism of ceramide 6: a vibrational spectroscopic and X-ray powder diffraction investigation of the diastereomers of N-(alpha-hydroxyoctadecanoyl)-phytosphingosine.

A preparative chromatographic method was developed for the quantitative isolation of the diastereomers of synthetic N-(alpha-hydroxyoctadecanoyl)-phytosphingosine (DL-CER6). The L- and the D-compound were studied each by means X-ray powder diffraction, FT-Raman and FT-IR spectroscopy. The diastereomers exhibit different thermotropic polymorphism. Three lamellar crystalline and a lamellar liquid crystalline phase were found for L-CER6. The natural occurring D-CER6 forms an Lalpha phase with a larger repeating distance than the L-CER6. The two lamellar crystalline phases of the D-compound have a significant larger dimension than those of the L-compound. The addition of water lowers the phase transition temperatures but does not induce structural changes such as incorporation into the lamellar sheets.

Calorimetry↗
PubMed2001

A probabilistic approach to space-group determination from powder diffraction data.

An algorithm for the determination of the space-group symmetry of a crystal from powder diffraction data, based upon probability theory, is described. Specifically, the relative probabilities of different extinction symbols are assessed within a particular crystal system. In general, only a small number of extinction symbols are relatively highly probable and a single extinction symbol is often significantly more probable than any other. Several examples are presented to illustrate this approach.

Journal Article↗
PubMed2003

A new method for the detection of low levels of free fibres of chrysotile in contaminated soils by X-ray powder diffraction.

A new method for the determination of free fibres of chrysotile in contaminated soils is described. The detection limit of 0.5 wt per thousand is reached by an enrichment process of the asbestos fraction of the sample using a standard laboratory elutriator for sedimentation analysis. The analysis of the enriched fraction is performed by X-ray powder diffraction using a conventional instrument. The procedure can be successfully applied to several soils of different nature throughout thermal treatment and removal of possible interferences due to some matrix components. This method is straightforward, routinized and has been especially developed to fulfil the request of public and private institutions for an appropriate quantitative determination of chrysotile free fibres in contaminated soils.

Asbestos, Serpentine↗
PubMed2003

Neutron powder diffraction studies of silicon-substituted hydroxyapatite.

The incorporation of a small amount of silicon in the hydroxyapatite (HAp) lattice is known to improve the bioactivity of the material. The effect of silicon substitution was studied by comparing samples of pure and 0.4wt% silicon-substituted HAp prepared by an aqueous precipitation method. High-resolution neutron powder diffraction and Rietveld refinement methods were used to investigate the effect of silicon substitution on the crystal structure parameters of the HAp lattice from room temperature down to 20K. Small structural changes in the lattice constants, interatomic distances, site occupancies and distortion of the phosphate tetrahedron were found. Modifications of the Fourier transform infrared spectra as well as appearance of new modes were observed in the silicon-substituted sample. Heat treatment and silicon substitution also affected the morphology and crystallinity of this bioapatite.

Biocompatible Materials↗
PubMed2004

Bulk material vs. single crystal: powder diffraction to the rescue.

The crystal structure of bulk microcrystalline material obtained by interaction of two rigid building blocks, namely dirhodium(ii) tetra(trifluoroacetate), [Rh2(O2CCF3)4], and bis(4'-pyridyl)diphenylsilane, (C6H5)2Si(C5H4N)2, has been solved ab initio using X-ray powder diffraction data. The title product of the 1 [ratio] 1 composition, [Rh2(O2CCF3)4.(mu2-(C6H5)2Si(C5H4N)2)], is a one-dimensional zigzag polymer built on axial Rh...N interactions averaged at 2.16 A. Its structural characterization complements the previously reported product of the 2:1 composition obtained from the same reaction, namely {[Rh2(O2CCF3)4]2.(mu4-(C6H5)2Si(C5H4N)2)}. The latter has a 2D layered network revealed by the single crystal diffraction study. A combination of powder and single crystal X-ray techniques is shown to be methodologically important and complementary for understanding of product assembling in the system.

Journal Article↗
PubMed2004

Synchrotron X-ray powder diffraction and computational investigation of purely siliceous zeolite Y under pressure.

High-pressure synchrotron X-ray powder diffraction measurements of a sample of purely siliceous zeolite Y (faujasite) were carried out up to 8.0 GPa at room temperature using a diamond anvil cell. Measurements using silicone oil as the pressure-transmitting medium show compression of the zeolite followed by a loss of long-range ordering at 2.2 GPa. The experimentally determined bulk modulus, 38(2) GPa, is, within experimental error, identical to that of quartz. When using a methanol:ethanol:water mixture (16:3:1) as the pressure-transmitting medium, two distinct compressibility regions are observed with a dramatic change in the compression mechanism at 4 GPa. Rietveld refinement analysis of the powder patterns provides a detailed description of the underlying chemistry, with sequential pore filling the main response up to 4 GPa and framework distortions at higher pressures.

Journal Article↗
PubMed2003

Polymorphism of ceramide 3. Part 2: a vibrational spectroscopic and X-ray powder diffraction investigation of N-octadecanoyl phytosphingosine and the analogous specifically deuterated d(35) derivative.

In order to characterize the arrangements of the hydrocarbon chains of ceramide 3, the thermotropic phase behaviour of the ceramides N-octadecanoylphytosphingosine (CER3) and its chain deuterated derivative N-(d(35)-octadecanoyl)phytosphingosine (d(35)CER3) was studied by means of X-ray powder diffraction, FT-IR and Raman spectroscopy. CER3 and d(35)CER3 exhibit an identical thermotropic polymorphism involving three different crystalline phases. The selective deuteration of the fatty acid chain enables to distinguish the sphingoid part from the fatty acid part by means of FT-IR and Raman spectroscopy. It could be shown that both hydrocarbon chains are arranged in different subcells. Temperature dependent Raman measurements elucidate simultaneously the changes in the trans/gauche ratios and the packing of both the hydrocarbon chains of the fatty acid and of the sphingoid part. The phase behaviour of CER3 and d(35)CER3, both dry and hydrated, was investigated.

Deuterium↗
PubMed2005

An unprecedented process involving normal and redox transmetallation reactions between Hg and Pt affording the unexpected K[Pt2[CH2C(O)Me]6(mu-Cl)3] complex: the key role of X-ray powder diffraction in unravelling its nature and structure.

[Hg{CH2C(O)Me}2] reacts with K[PtCl3(CH2=CH2)](2 : 1 molar ratio) to give K[Pt2{CH2C(O)Me}6(mu-Cl)3] (1); the intermediate [Pt{CH2C(O)Me}Cl2(CH2=CH2)]- has been detected in solution and isolated as a Me4N+ salt; the process occurs through successive normal and redox transmetallation reactions and an ab initio X-ray powder diffraction study of has proven to be essential to establish its nature.

Crystallization↗
PubMed2001

Quantitative analysis of crystalline pharmaceuticals in powders and tablets by a pattern-fitting procedure using X-ray powder diffraction data.

A pattern-fitting procedure for quantitative analysis of crystalline pharmaceuticals in solid dosage forms using X-ray powder diffraction data is described. This method is based on a procedure for pattern-fitting in crystal structure refinement, and observed X-ray scattering intensities were fitted to analytical expressions including some fitting parameters, i.e. scale factor, peak positions, peak widths and degree of preferred orientation of the crystallites. All fitting parameters were optimized by the non-linear least-squares procedure. Then the weight fraction of each component was determined from the optimized scale factors. In the present study, well-crystallized binary systems, zinc oxide-zinc sulfide (ZnO-ZnS) and salicylic acid-benzoic acid (SA-BA), were used as the samples. In analysis of the ZnO-ZnS system, the weight fraction of ZnO or ZnS could be determined quantitatively in the range of 5-95% in the case of both powders and tablets. In analysis of the SA-BA systems, the weight fraction of SA or BA could be determined quantitatively in the range of 20-80% in the case of both powders and tablets. Quantitative analysis applying this pattern-fitting procedure showed better reproducibility than other X-ray methods based on the linear or integral intensities of particular diffraction peaks. Analysis using this pattern-fitting procedure also has the advantage that the preferred orientation of the crystallites in solid dosage forms can be also determined in the course of quantitative analysis.

Benzoic Acid↗
PubMed2001

Disordered crystal structure of pentamethylcyclopentadienylsodium as seen by high-resolution X-ray powder diffraction.

The crystal structure of pentamethylcyclopentadienylsodium, [NaC10H15] (NaCp*), has been determined from high-resolution X-ray powder diffraction. The compound crystallizes in space group Cmcm with lattice parameters a = 4.61030 (3), b = 16.4621 (3), c = 14.6751 (2) A, V = 1113.77 (4) A(3) (Z = 4). NaCp* forms polymeric multidecker chains along the a axis. The Rietveld refinement (R(p) = 0.050 and R(F) = 0.163) shows that the Cp* moieties occupy, with disorder, two different orientations rotated away from the eclipsed conformation by +/-13.8 degrees.

Journal Article↗
PubMed1998

Structural studies of endohedral metallofullerenes by synchrotron radiation powder diffraction.

The endohedral natures of the metallofullerenes Y@C(82) and Sc(2)@C(8)4 are described based on synchrotron radiation powder diffraction experiments. For structural analysis, a combination of the maximum-entropy method (MEM) and Rietveld refinement was employed to analyse the complicated powder pattern. The obtained MEM charge densities show a clear distinction of the endohedral natures of the mono- and dimetallofullerenes.

Journal Article↗
PubMedDate not supplied

X-ray powder diffraction data for selected drugs: furosemide, hydrochlorothiazide, naproxen, naproxen sodium, propranolol hydrochloride, and the halopheniramine maleates.

X-ray powder diffraction data for 9 commonly used drug substances are reported. The data for furosemide, hydrochlorothiazide, propranolol hydrochloride, dexchlorpheniramine maleate, and brompheniramine maleate have been indexed by reference to published crystal structure data. The racemic modifications of propranolol hydrochloride and brompheniramine maleate are shown to exist as racemic compounds rather than racemic mixtures.

Chlorpheniramine↗
PubMed2001

Quantifying amorphous content of lactose using parallel beam X-ray powder diffraction and whole pattern fitting.

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.

Lactose↗
PubMed1997

An evaluation of crystal structure of mannan I by X-ray powder diffraction and molecular mechanics studies.

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.

Cell Wall↗
PubMed2002

Synthetic hydrocerussite, 2PbCO(3) x Pb(OH)(2), by X-ray powder diffraction.

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...

Journal Article↗