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PubMed2005

Structure determination of homoleptic AuI, AgI, and CuI aryl/alkylethynyl coordination polymers by X-ray powder diffraction.

This article describes the structure determination of five homoleptic d(10) metal-aryl/alkylacetylides [RC triple bond CM] (M=Cu, R=tBu 1, nPr 2, Ph 3; R=Ph, M=Ag 4; Au 5) by using X-ray single-crystal and powder diffraction. Complex 1.C6H6 reveals an unusual Cu20 catenane cluster structure that has various types of tBuC triple bond C-->Cu coordination modes. By using this single-crystal structure as a starting model for subsequent Rietveld refinement of X-ray powder diffraction data, the structure of the powder synthesized from CuI and tBuC triple bond CH was found to have the same structure as 1. Complex 2 has an extended sheet structure consisting of discrete zig-zag Cu4 subunits connected through bridging nPrC triple bond C groups. Complex 3 forms an infinite chain structure with extended Cu-Cu ladders (Cu-Cu=2.49(4)-2.83(2) A). The silver(I) congener 4 is iso-structural to 3 (average Ag-Ag distance 3.11 A), whereas the gold(I) analogue 5 forms a Au...Au honeycomb network with PhC triple bond C pillars (Au-Au=2.98(1)-3.26(1) A). Solid-state properties including photoluminescence, nu(C triple bond C) stretching frequencies and thermal stability of these polymeric systems are discussed in the context of the determined structures.

Acetylene↗

PubMed2004

Shadowing and absorption corrections of high-pressure powder diffraction data: toward accurate electron-density determinations.

The shadowing of the primary beam and of reflections from a powder crystal sample enclosed in a diamond-anvil high-pressure cell (DAC) is described for unrestricted data-collection procedures. The corrections account for the shadowing of the data measured with point- or area-detector diffractometers and can be applied for any containers of environment devices enclosing a powdered sample. The general procedure for correcting the powder diffraction data can be applied for analysing and optimizing diffractometric procedures of data collection and provides facilities for collecting high-pressure powder diffraction data with the DAC positioned optimally and rotated to counteract the preferred-orientation and particle-size effects, and to increase the resolution of data. The quadrature effects of numerical integration for the accuracy of computed corrections have been analysed.

Journal Article↗

PubMed2004

Binding of N-acetylglucosamine oligosaccharides to hen egg-white lysozyme: a powder diffraction study.

The binding of N-acetylglucosamine oligosaccharides (NAGn, n = 2-6) to hen egg-white lysozyme (HEWL; EC 3.2.1.17) was investigated by X-ray powder diffraction at room temperature. Each NAGn examined was found to bind to lysozyme in rapid-precipitation preparations in 1.0 M NaCl pH 6.0 buffer. The location of each NAGn was easily found from difference Fourier maps generated from structure factors extracted during preliminary Rietveld refinements. Full NAGn-protein structures were subjected to combined Rietveld and stereochemical restraint refinements (Rwp = 2.28-2.59%; Rp = 1.81-2.04%; RF2 = 3.91-5.80%) and revealed binding modes for NAGn that depended on the length of the NAG oligosaccharide. The NAG2 ligand was found in the BC sites in the cleft of HEWL, NAG3 was found to bind in both the ABC and BCD sites in the ratio 35:65 and NAG4 and NAG5 bound to the ABCD and ABCDE sites, respectively, while NAG6 only bound to sites ABCDE, leaving the F site empty with the remaining saccharide ring located in a solvent region adjacent to the A site. All protein powder diffraction patterns in this study consisted of extremely sharp Bragg peaks consistent with approximately 1 microm crystallites that were devoid of line-broadening defects. Details of the stereochemical restraints used in these refinements and their impact on structural validation are also discussed.

Acetylglucosamine↗

PubMed2005

Application notes on the use of softer X-rays for anomalous powder diffraction.

An in-vacuum diffractometer has been modified to collect powder diffraction data from a sample of promazine hydrochloride. Strong anomalous contrast has been observed at, or close to, the sulphur edge. Based on the data collected, a bespoke diffractometer has been designed that, together with minor changes to the geometry of the experiment, will enable the routine collection of powder data at absorption edges below 5.5 keV.

Equipment Design↗

PubMed2001

Structure determination of two intercalated compounds VOPO4.(CH2)4O and VOPO4.OH-(CH2)2-O-(CH2)2-OH; synchrotron powder diffraction and molecular modelling.

The crystal structures of two intercalated compounds have been determined using a combination of synchrotron powder diffraction and molecular mechanics simulations: (1) vanadyl phosphate intercalated with tetrahydrofuran, VOPO(4).(CH(2))(4)O, and (2) vanadyl phosphate intercalated with diethylene glycol, VOPO(4).HO(CH(2))(2)O(CH(2))(2)OH. Both intercalates preserve the tetragonal space group P4/n, as found in the host structure VOPO(4).2H(2)O. (1): a = 6.208, c = 8.930 A, Z = 2, D(x) = 2.51 g cm(-3); (2): a = 6.223, c = 11.417 A, Z = 2, D(x) = 2.66 g cm(-3). Both intercalates exhibit the same type of orientational disorder in the arrangement of guest molecules, as observed in the same host compound intercalated with water. These two intercalates also exhibit, rather surprisingly, perfect ordering in layer stacking without the displacement disorder, characteristic of many intercalated layered structures. Thanks to this regularity in the arrangement of guests and layers, synchrotron powder diffraction could be used in the present structure determination. The present results also enabled the analysis of the effect of geometrical parameters characterizing the mutual host-guest complementarity and the effect of host-guest and guest-guest interaction on the crystal packing of intercalates.

Journal Article↗

PubMed2000

Structure determination from conventional powder diffraction data: application to hydrates, hydrochloride salts, and metastable polymorphs.

Recent advances in crystallographic computing have made it possible to solve by powder diffraction methods structures that have not been possible to solve by single-crystal methods. Although there is vast improvement in the quality of data obtained from high-intensity synchrotron radiation, we found that surprisingly reliable results can be obtained from conventional laboratory sources. In this article we examine the application of Monte Carlo/simulated annealing methods for the determination of structures ranging in complexity from 9 to 15 degrees of freedom. We re-determine the structures of papaverine hydrochloride and erythromycin A dihydrate by the powder diffraction method and compare the structures to those determined by single-crystal diffraction methods. The structure of a metastable polymorphic form of acetohexamide, form B, is solved and examined spectroscopically. Its structure has not previously been solved by single-crystal techniques because of the small size of its crystals.

Acetohexamide↗

PubMed2000

Structural characterization of three crystalline modifications of telmisartan by single crystal and high-resolution X-ray powder diffraction.

Three crystalline modifications (A, B, and C) of 4'-[[2-n-propyl-4-methyl-6-(1-methyl-benzimidazol-2-yl)benzi midazol-1-yl]methyl]biphenyl-2-carboxylic acid (INN name, telmisartan) have been detected and their crystal structures have been determined by single-crystal X-ray diffraction (pseudopolymorph C) and the method of simulated annealing from high-resolution X-ray powder diffraction data (polymorphs A and B). The compound is of interest because of its use as an angiotensin II receptor antagonist. Polymorph A crystallizes in space group P2(I)/c, Z = 4, with unit cell parameters a = 18.7798(3), b = 18.1043(2), and c = 8.00578(7) A, beta = 97.066(1) degrees, and V = 2701.31 A(3). Polymorph B crystallizes in space group P2(I)/a, Z = 4, with unit cell parameters a = 16.0646(5), b = 13.0909(3), and c = 13.3231(3) A, beta = 99.402(1) degrees, and V = 2764.2(1) A(3). The solvated form C crystallizes in space group C2/c, Z = 8, with unit cell parameters a = 30.990(5), b = 13.130(3), and c = 16.381(3) A, beta = 95.02(2) degrees, and V = 6639(2) A(3). For the structure solutions of polymorphs A and B, 13 degrees of freedom (3 translational, 3 orientational, 7 torsion angles) were determined in approximately 2 h of computer time, demonstrating that the crystal packing and the molecular conformation of medium-sized (MW approximately 500) pharmaceutical compounds can now be solved quickly and routinely from high-resolution X-ray powder diffraction data.

Angiotensin-Converting Enzyme Inhibitors↗

PubMed2005

X-ray powder diffraction structure determination of gamma-butyrolactone at 180 K: phase-problem solution from the lattice energy minimization with two independent molecules.

The crystal structure of the solid phase of the dipolar aprotic solvent gamma-butyrolactone (BL1), C(4)H(6)O(2), has been solved using the atom-atom potential method and Rietveld-refined against powder diffraction data collected at T = 180 K with a curved position-sensitive detector (INEL CPS120) using Debye-Scherrer diffraction geometry with monochromatic X-rays. It was first deduced from the X-ray experiment that the lattice parameters are a = 10.1282 (4), b = 10.2303 (5), c = 8.3133 (4) A, beta = 93.291 (2) degrees and that the space group is P2(1)/a, with Z = 8 and two independent molecules in the asymmetric unit. The structure was then solved by global energy minimization of the crystal-lattice atom-atom potentials. The subsequent GSAS-based Rietveld refinement converged to the final crystal-structure model indicator R(F(2)) = 0.0684, profile factors R(p) = 0.0517 and R(wp) = 0.0694, and a reduced chi(2) = 1.671. After further cycles of heating and cooling, a powder diffraction pattern markedly different from the first pattern was obtained, again at T = 180 K, which we tentatively assign to a second polymorph (BL2). All the observed diffraction peaks are well indexed by a triclinic unit cell essentially featuring a doubling of the a axis. An excellent Le Bail fit is obtained, for which R(p) = 0.0312 and R(wp) = 0.0511.

4-Butyrolactone↗

PubMed2005

Analysis of the acid-base reaction between solid indomethacin and sodium bicarbonate using infrared spectroscopy, X-ray powder diffraction, and solid-state nuclear magnetic resonance spectroscopy.

Indomethacin was used as a model compound to investigate acid-base reactions of solid materials, a common type of drug-excipient interaction. In a typical experiment, 500 mg of pure alpha-form indomethacin were mixed with 500 mg of sodium bicarbonate. The mixture was kept at 40 degrees C and at several relative humidities. The reaction was monitored by IR spectroscopy, X-ray powder diffraction, and solid-state NMR. At 40 degrees C and 80% RH, the reaction is nearly complete after 300 h. As observed by IR spectroscopy, the characteristic peaks of alpha-indomethacin disappear during the course of the reaction with the appearance of the characteristic peaks of the salt product, sodium indomethacin trihydrate. Solid-state NMR spectra and X-ray powder diffraction patterns of the reaction mixtures confirm the transformation of the mixtures to sodium indomethacin trihydrate; the reduced peak intensities in the diffraction patterns of the product relative to the initial mixtures indicate the formation of a microcrystalline product. A change in the reaction rate of sodium bicarbonate with alpha-indomethacin is observed when the mixtures are stored at different relative humidities. At 40 degrees C and 66% RH, the reaction of sodium bicarbonate with alpha-indomethacin is about 86% complete after 500 h. No detectable reaction was observed for sodium bicarbonate with the alpha form of indomethacin at 40 degrees C and 11% RH after 15 months. The combination of these solid-state characterization techniques is demonstrated to be essential to detect and monitor acid-base reactions in solid materials, which are impossible to monitor using solution-chemistry methods. The reaction kinetics at 66% RH fits the Jander equation very well, which is consistent with a diffusion-controlled mechanism.

Calibration↗

PubMed2005

Single crystal and powder diffraction characterization of three polymorphic forms of Acitretin.

Acitretin [all-trans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethyl-2,4,6,8-nonatetraenoic acid or 3-methoxy-2-methyl-17-nor-1,2,3,4-tetradehydroretinoic acid], a widely marketed oral synthetic retinoid, introduced for clinical use as effective therapy against psoriasis, was found to crystallize in three polymorphic modifications (hereafter, I, II, and III), the crystal structures of which have been determined by single-crystal diffractometry (form I) or X-ray powder diffraction methods (form II and III) from conventional laboratory data only. In these latter cases, real space techniques (simulated annealing and whole-profile pattern matching) have been employed. Polymorph I crystallizes in space group P2(1), Z = 8, with unit cell parameters a = 7.894(1), b = 58.454(6), c = 8.161(1) angstroms, beta = 102.04(1) degrees, and V = 3682.9(8) angstroms3. Polymorph II crystallizes in space group P2(1)/n, Z = 4, with unit cell parameters a = 13.999(2), b = 10.714(1), c = 12.465(2) angstroms, beta = 98.76(5) degrees, and V = 1847.9(3) angstroms3. Polymorph III crystallizes in space group P2(1)/c, Z = 4, with unit cell parameters a = 3.0751(4), b = 4.0487(4), c = 14.956(2) angstroms, beta = 100.41(7) degrees, and V = 1831.3(4) angstroms3. Polymorph I, found to be identical with that deposited in the European Pharmacopeia, shows four crystallographically independent Acitretin molecules, arranged in pairs through conventional hydrogen-bonded carboxylic dimers; also in form II, carboxylic dimers are observed, located on crystallographic inversion centres, while in form III, a catameric arrangement of the carboxylic residues, winding up about the rather short monoclinic axis, generates one-dimensional chains of hydrogen-bonded Acitretin molecules. Thermal analysis showed that form I can be quantitatively transformed into form II by moderate heating near 200 degrees C, under vacuum. These results show that ab initio structural studies from conventional laboratory X-ray powder diffraction (XRPD) data are fully providing the opportunity to investigate the structural aspects of moderately complex substances also in the absence of single crystals, disclosing the crystal chemistry of a few polymorphs of pharmaceutically relevant species.

Acitretin↗

PubMed2005

X-ray powder diffraction patterns for certain beta-lactam, tetracycline and macrolide antibiotic drugs.

X-ray powder diffraction (XRD) data for eight beta-lactam viz., ampicillin sodium, ampicillin trihydrate, penicillin G procaine, benzathine penicillin, benzyl penicillin sodium, cefalexin, cefotaxime sodium and ceftriaxone sodium; three tetracyclines viz., doxycycline hydrochloride, oxytetracycline dihydrate and tetracycline hydrochloride; and two macrolide viz., azithromycin and erythromycin estolate antibiotic drugs were obtained using a powder diffractometer. The drugs were scanned from Bragg angles (2theta) of 10 degrees to 70 degrees. The obtained data were tabulated in terms of the lattice spacing (A) and relative line intensities (I/I(I)). This new information may be useful for identifying these drugs from confiscated materials, which has been frequently encountered in forensic laboratories.

Anti-Bacterial Agents↗

PubMed2004

Crystal structure determination of thymoquinone by high-resolution X-ray powder diffraction.

The crystal structure of 2-isopropyl-5-methyl-1,4-benzoquinone (thymoquinone) and its thermal behavior--as necessary physical and chemical properties--were determined in order to enhance the current understanding of thymoquinone chemical action by using high resolution x-ray powder diffraction, Fourier transform infrared spectroscopy (FTIR), and 3 thermo-analytical techniques thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The findings obtained with high-resolution x-ray powder diffraction and molecular location methods based on a simulated annealing algorithm after Rietveld refinement showed that the triclinic unit cell was a = 6.73728(8) A, b = 6.91560(8) A, c = 10.4988(2) A, alpha = 88.864(2) degrees, beta = 82.449(1) degrees, gamma = 77.0299(9) degrees; cell volume = 472.52(1) A3, Z = 2, and space group P1. In addition, FTIR spectrum revealed absorption bands corresponding to the carbonyl and C-H stretching of aliphatic and vinylic groups characteristically observed in such p-benzoquinones. Also, a chemical decomposition process starting at 65 degrees C and ending at 213 degrees C was noted when TGA was used. DSC allowed for the determination of onset at 43.55 degrees C and a melting enthalpy value of DeltaH(m) = 110.6 J/g. The low value obtained for the fusion point displayed a van der Waals pattern for molecular binding, and the thermograms performed evidence that thymoquinone can only be found in crystalline triclinic form, as determined by DRX methods.

Benzoquinones↗

PubMed2004

Combinatorial screening employing nylon loops and micro-X-ray powder diffraction.

A combinatorial screening method is described that employs nylon loops and micro-X-ray powder diffraction. The nylon loops are used to position the sample in a three-circle X-ray diffractometer in such a way as to allow for Gandolfi-like scans. These scans maximize mechanical tumbling of the sample, which in turn results in higher quality data. The diffractometer utilizes a pinhole source and two-dimensional area detection for rapid data collection. The method can be readily automated by employing single-crystal screening procedures. The combinatorial analysis of the three known polymorphs of D-mannitol is discussed and a simple procedure to produce all three polymorphs from the same starting solutions is presented.

Combinatorial Chemistry Techniques↗

PubMed2000

Rietveld analysis of electron powder diffraction data from nanocrystalline anatase, TiO2

The structure of nanocrystalline anatase (TiO2) was successfully refined from electron powder diffraction data using the Rietveld technique. A polycrystalline sample (average crystal size about 70 A) was characterised by selected area electron diffraction in a conventional transmission electron microscope operated at 300 kV. Radially integrated intensities were extracted from digitised photographic films and used in the course of structure refinements by a standard program for Rietveld analysis. The structure was refined in space group I4(1)/amd (#141) with lattice parameters a = 3.7710(9) A and c = 9.430(2) A. The reliability factors of the refinement are Rwp = 5.2% and R(B) = 2.6%. The close agreement of the refined structural parameters with previous results obtained from neutron diffraction on coarse-grained powders proves the applicability of the method for characterising nanocrystalline powders. The present study shows that Rietveld analysis on electron powder data is a good compliment to the existing methods for accurate structural investigations on nanocrystalline materials and thin films.

Journal Article↗

PubMed2000

Metastable beta-phase of benzophenone: independent structure determinations via X-ray powder diffraction and single crystal studies

Benzophenone was the first organic molecular material to be identified as polymorphic. It is well known that benzophenone crystallizes in a stable orthorhombic alpha-form (m.p. 321 K) with space group P2(1)2(1)2(1) and a = 10.28, b = 12.12, c = 7.99 A, [Girdwood (1998). Ph.D. thesis. Strathclyde University, Glasgow, Scotland]. Here we report two separate structure determinations of the metastable beta-form (m.p. 297-299 K). Crystalline material of the metastable polymorph was obtained from a melt supercooled to approximately 243 K. The structure was determined from X-ray powder diffraction data by employing a novel, computational systematic search procedure to identify trial packing arrangements for subsequent refinement. Unit-cell and space-group information, determined from indexing the powder diffraction data, was used to define the search space. The structure was also determined from single-crystal diffraction data at room temperature and at 223 K. The metastable phase is monoclinic with space group C2/c and a = 16.22, b = 8.15, c = 16.33 A, beta = 112.91 degrees (at 223 K). The structures derived from the individual techniques are qualitatively the same. They are compared both with each other and with the stable polymorph and other benzophenone derivatives.

Journal Article↗

PubMed2004

Beyond classical applications of powder diffraction.

This article provides a short tutorial review of how laboratory powder diffraction methods can be used to establish a variety of "non-traditional" pieces of information about solid state materials-information beyond simple phase identification or structure determination at a specific temperature. The examples show how unique insights into synthetic pathways, reaction mechanisms, reaction kinetics, polymorphism, phase transitions and physical properties can be obtained for a variety of different types of materials.

Journal Article↗

PubMed2004

How to determine structures when single crystals cannot be grown: opportunities for structure determination of molecular materials using powder diffraction data.

Many crystalline solids cannot be prepared as single crystals of sufficient size and/or quality for structure determination to be carried out using single crystal X-ray diffraction techniques. In such cases, when only polycrystalline powders of a material are available, it is necessary instead to tackle structure determination using powder X-ray diffraction. This article highlights recent developments in the opportunities for determining crystal structures directly from powder diffraction data, focusing on the case of molecular solids and giving particular attention to the most challenging stage of the structure determination process, namely the structure solution stage. In particular, the direct-space strategy for structure solution is highlighted, as this approach has opened up new opportunities for the structure determination of molecular solids. The article gives an overview of the current state-of-the-art in structure determination of molecular solids from powder diffraction data. Relevant fundamental aspects of the techniques in this field are described, and examples are given to highlight the application of these techniques to determine crystal structures of molecular materials.

Journal Article↗

PubMed2003

Characterization of monoclinic crystals in tablets by pattern-fitting procedure using X-ray powder diffraction data.

The purpose of this study is to characterize the monoclinic crystals in tablets by using X-ray powder diffraction data and to evaluate the deformation feature of crystals during compression. The monoclinic crystals of acetaminophen and benzoic acid were used as the samples. The observed X-ray diffraction intensities were fitted to the analytic expression, and the fitting parameters, such as the lattice parameters, the peak-width parameters, the preferred orientation parameter and peak asymmetric parameter were optimized by a non-linear least-squares procedure. The Gauss and March distribution functions were used to correct the preferred orientation of crystallites in the tablet. The March function performed better in correcting the modification of diffraction intensity by preferred orientation of crystallites, suggesting that the crystallites in the tablets had fiber texture with axial orientation. Although a broadening of diffraction peaks was observed in acetaminophen tablets with an increase of compression pressure, little broadening was observed in the benzoic tablets. These results suggest that "acetaminophen is a material consolidating by fragmentation of crystalline particles and benzoic acid is a material consolidating by plastic deformation then occurred rearrangement of molecules during compression". A pattern-fitting procedure is the superior method for characterizing the crystalline drugs of monoclinic crystals in the tablets, as well as orthorhombic isoniazid and mannitol crystals reported in the previous paper.

Acetaminophen↗