http://journals.iucr.org/b/journalhomepage.html Acta Crystallographica Section B: Structural Science publishes papers in which structure is the primary focus of the work reported. The central themes of Structural Science are the acquisition of structural knowledge from novel experimental observations or from existing data, the correlation of structural knowledge with physico-chemical and other properties, and the application of this knowledge to solve problems in the structural domain. Structural Science has broad chemical coverage, encompassing metals and alloys, inorganics and minerals, metal-organics and purely organic compounds. en-gb Copyright (c) 2008 International Union of Crystallography International Union of Crystallography International Union of Crystallography http://journals.iucr.org urn:issn:0567-7408 Acta Crystallographica Section B: Structural Science publishes papers in which structure is the primary focus of the work reported. The central themes of Structural Science are the acquisition of structural knowledge from novel experimental observations or from existing data, the correlation of structural knowledge with physico-chemical and other properties, and the application of this knowledge to solve problems in the structural domain. Structural Science has broad chemical coverage, encompassing metals and alloys, inorganics and minerals, metal-organics and purely organic compounds. text/html Open access article in Acta Crystallographica Section B Structural Science text yearly 6 2002-02-01T00:00+00:00 med@iucr.org Acta Crystallographica Section B Structural Science Copyright (c) 2008 International Union of Crystallography urn:issn:0567-7408 http://journals.iucr.org/logos/rss10b.gif http://journals.iucr.org/b/journalhomepage.html Still image http://scripts.iucr.org/cgi-bin/paper?bp5012 Ammonium oxopentafluoroniobate, (NH4)2NbOF5, was synthesized in a single-crystal form and the structures of its different phases were determined by X-ray diffraction at three temperatures: phase (I) at 297 K, phase (II) at 233 K and phase (III) at 198 K. The distorted [NbOF5]2− octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF5]2− octahedron related by a jump rotation around the pseudo-threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). The structure of (III) is completely ordered. The F and O atoms in the structures investigated were identified via the Nb—X (X = O and F) distances. The crystals of all three phases are twinned. http://creativecommons.org/licenses/by/2.0/uk urn:issn:0108-7681 Udovenko, A.A. Laptash, N.M. 2008-10-01 doi:10.1107/S0108768108021289 International Union of Crystallography Structural phase transitions in a crystal of (NH4)2NbOF5 are the consequence of dynamic changes in its structural units as the temperature decreases. Using X-ray diffraction, it is possible to identify O and F atoms in the disordered structure of (NH4)2NbOF5 as a result of its dynamic nature. en AMMONIUM OXOPENTAFLUORONIOBATE; DISTORTED OCTAHEDRA; DYNAMIC ORIENTATIONAL DISORDER; PHASE TRANSITIONS; TWINNING; VIBRATIONAL SPECTRA Ammonium oxopentafluoroniobate, (NH4)2NbOF5, was synthesized in a single-crystal form and the structures of its different phases were determined by X-ray diffraction at three temperatures: phase (I) at 297 K, phase (II) at 233 K and phase (III) at 198 K. The distorted [NbOF5]2− octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF5]2− octahedron related by a jump rotation around the pseudo-threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). The structure of (III) is completely ordered. The F and O atoms in the structures investigated were identified via the Nb—X (X = O and F) distances. The crystals of all three phases are twinned. text/html Orientational disorder and phase transitions in crystals of (NH4)2NbOF5 text 5 64 533 2008-10-01 Acta Crystallographica Section B: Structural Science research papers 527 http://scripts.iucr.org/cgi-bin/paper?bs9048 A corrigendum to the paper by Meents et al. (2008), Acta Cryst. B64, 42–49 to correct the nomenclature for CL-20. Copyright (c) 2008 International Union of Crystallography urn:issn:0108-7681 Meents, A. Dittrich, B. Johnas, S.K.J. Thome, V. Weckert, E.F. 2008-08-01 doi:10.1107/S0108768108017497 International Union of Crystallography A corrigendum to the paper by Meents et al. (2008), Acta Cryst. B64, 42–49. en ENERGETIC MATERIALS; TOPOLOGICAL ANALYSIS; CHARGE DENSITY A corrigendum to the paper by Meents et al. (2008), Acta Cryst. B64, 42–49 to correct the nomenclature for CL-20. text/html Charge-density studies of energetic materials: CL-20 and FOX-7. Corrigendum text 4 64 2008-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2008 International Union of Crystallography addenda and errata 519 519 http://scripts.iucr.org/cgi-bin/paper?bk5056 Monte Carlo computer simulation has been used to interpret and model observed single-crystal diffuse X-ray scattering data for pentachloronitrobenzene, C6Cl5NO2. Each site in the crystal contains a molecule in one of six different basic orientations with equal probability. However, no short-range order amongst these different orientations has been detected. The strong, detailed and very distinctive diffraction patterns can be accounted for almost entirely on the assumption of random occupancy of each molecular site, but with very large local relaxation displacements that tend to increase the neighbouring distances for contacts involving NO2⋯NO2 and NO2⋯Cl with a corresponding reduction for those involving Cl⋯Cl. The results show that the mean NO2⋯NO2 distance is increased by ∼ 0.6 Å, compared with that given by the average structure determination. Copyright (c) 2007 International Union of Crystallography urn:issn:0108-7681 Thomas, L.H. Welberry, T.R. Goossens, D.J. Heerdegen, A.P. Gutmann, M.J. Teat, S.J. Lee, P.L. Wilson, C.C. Cole, J.M. 2007-08-01 doi:10.1107/S0108768107024305 International Union of Crystallography Monte Carlo computer simulation has been used to interpret and model observed single-crystal diffuse X-ray scattering data for pentachloronitrobenzene, C6Cl5NO2. The strong, detailed and very distinctive diffraction patterns can be accounted for almost entirely on the assumption of the random occupancy of each molecular site, but with very large local relaxation displacements that tend to increase the neighbouring distances for contacts involving NO2⋯NO2 and NO2⋯Cl with a corresponding reduction for those involving Cl⋯Cl. en DISORDER; DIFFUSE SCATTERING STUDY; MONTE CARLO COMPUTER SIMULATION; RELAXATION DISPLACEMENTS Monte Carlo computer simulation has been used to interpret and model observed single-crystal diffuse X-ray scattering data for pentachloronitrobenzene, C6Cl5NO2. Each site in the crystal contains a molecule in one of six different basic orientations with equal probability. However, no short-range order amongst these different orientations has been detected. The strong, detailed and very distinctive diffraction patterns can be accounted for almost entirely on the assumption of random occupancy of each molecular site, but with very large local relaxation displacements that tend to increase the neighbouring distances for contacts involving NO2⋯NO2 and NO2⋯Cl with a corresponding reduction for those involving Cl⋯Cl. The results show that the mean NO2⋯NO2 distance is increased by ∼ 0.6 Å, compared with that given by the average structure determination. text/html Disorder in pentachloronitrobenzene, C6Cl5NO2: a diffuse scattering study text 4 63 2007-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2007 International Union of Crystallography research papers 663 673 http://scripts.iucr.org/cgi-bin/paper?bk5053 The generalization of the Zintl–Klemm concept provides a universal formulation of a crystal structure in terms of universal building skeletons formed by Klemm's pseudoatoms: atoms that behave structurally according to their formal total electron charge. An important difference in this novel view is that charge is considered to be transferred, in the strict Zintl's sense, from the donor cations to the building skeleton as a whole, not specifically to a given atom or ion. Although application is restricted to (IV)–(IV) compounds (group 14 structures), the principle seems to be universal and can be applied to understand, to relate and to predict the structure of complex compounds on the basis of more simple structures, e.g. a given AB skeleton provides the building block for A2B, AB2, ABXm etc. compounds of a very different nature. The application of such a principle only requires information on the constituent atoms and on the existing phases of the p-block elements (observed under ambient and high-pressure and/or high-temperature conditions). The ideas introduced here demonstrate, for the first time, that a generalization of the Zintl–Klemm concept is possible and that such a generalization helps to establish a univocal link between chemical composition (in terms of pseudoatoms) and the crystalline structures observed experimentally. Copyright (c) 2007 International Union of Crystallography urn:issn:0108-7681 Vegas, A. García-Baonza, V. 2007-06-01 doi:10.1107/S0108768107019167 International Union of Crystallography A generalization of the Zintl–Klemm concept (ZKC) explains the complex structures of alloys as well as the cation arrays of oxides. This generalization helps to establish a univocal link between chemical composition (in terms of Klemm's pseudoatoms) and the structures observed experimentally. en PHASE TRANSITIONS; ZINTL-KLEMM CONCEPT; CATION ARRAYS The generalization of the Zintl–Klemm concept provides a universal formulation of a crystal structure in terms of universal building skeletons formed by Klemm's pseudoatoms: atoms that behave structurally according to their formal total electron charge. An important difference in this novel view is that charge is considered to be transferred, in the strict Zintl's sense, from the donor cations to the building skeleton as a whole, not specifically to a given atom or ion. Although application is restricted to (IV)–(IV) compounds (group 14 structures), the principle seems to be universal and can be applied to understand, to relate and to predict the structure of complex compounds on the basis of more simple structures, e.g. a given AB skeleton provides the building block for A2B, AB2, ABXm etc. compounds of a very different nature. The application of such a principle only requires information on the constituent atoms and on the existing phases of the p-block elements (observed under ambient and high-pressure and/or high-temperature conditions). The ideas introduced here demonstrate, for the first time, that a generalization of the Zintl–Klemm concept is possible and that such a generalization helps to establish a univocal link between chemical composition (in terms of pseudoatoms) and the crystalline structures observed experimentally. text/html Pseudoatoms and preferred skeletons in crystals text 3 63 2007-06-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2007 International Union of Crystallography feature articles 339 345 http://scripts.iucr.org/cgi-bin/paper?og5020 The crystal structure of the compositionally homogeneous ceria–zirconia solid solution CeZrO4 is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296–1831 K. The CeZrO4 exhibits a tetragonal structure with the space group P42/nmc at temperatures from 296 to 1542 K (Z = 1), and a cubic fluorite-type form with the space group Fm\overline 3 m at 1831 K (Z = 2). The isotropic atomic displacement parameters of Ce and Zr atoms B(Ce,Zr) and O atoms B(O) are found to increase with temperature, with B(O) being larger than B(Ce,Zr), suggesting the higher diffusivity of oxygen ions. The ratio of the c axial length to the a length of the pseudo-fluorite lattice (c/aF axial ratio) for the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching unity between 1542 and 1831 K. The displacement of O atoms along the c axis in the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.0 Å between 1542 and 1831 K. These results indicate that the cubic-to-tetragonal phase transition between 1542 and 1831 K is accompanied by oxygen displacement along the c axis and the increase of the c/aF axial ratio from unity. Copyright (c) 2007 International Union of Crystallography urn:issn:0108-7681 Wakita, T. Yashima, M. 2007-06-01 doi:10.1107/S0108768107007720 International Union of Crystallography The crystal structure of the ceria–zirconia solid solution CeZrO4 is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296–1831 K. The cubic-to-tetragonal phase transition between 1542 and 1831 K is accompanied by the displacement of oxygen atoms and elongation of the c axis. en PHASE TRANSITION; NEUTRON DIFFRACTION; HIGH-TEMPERATURE MATERIALS The crystal structure of the compositionally homogeneous ceria–zirconia solid solution CeZrO4 is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296–1831 K. The CeZrO4 exhibits a tetragonal structure with the space group P42/nmc at temperatures from 296 to 1542 K (Z = 1), and a cubic fluorite-type form with the space group Fm\overline 3 m at 1831 K (Z = 2). The isotropic atomic displacement parameters of Ce and Zr atoms B(Ce,Zr) and O atoms B(O) are found to increase with temperature, with B(O) being larger than B(Ce,Zr), suggesting the higher diffusivity of oxygen ions. The ratio of the c axial length to the a length of the pseudo-fluorite lattice (c/aF axial ratio) for the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching unity between 1542 and 1831 K. The displacement of O atoms along the c axis in the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.0 Å between 1542 and 1831 K. These results indicate that the cubic-to-tetragonal phase transition between 1542 and 1831 K is accompanied by oxygen displacement along the c axis and the increase of the c/aF axial ratio from unity. text/html In situ observation of the tetragonal–cubic phase transition in the CeZrO4 solid solution – a high-temperature neutron diffraction study text 3 63 2007-06-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2007 International Union of Crystallography research papers 384 389 http://scripts.iucr.org/cgi-bin/paper?ws5049 1-(2-Chloronicotinoyl)-2-(2-nitrophenyl)hydrazine, C12H9Cl­N4O3, crystallizes in three polymorphic forms, two monoclinic forms in space groups Cc (Ia) and P21 (Ib), and an orthorhombic form in space group Pbcn (Ic). In the Cc polymorph (Ia) the molecules are linked into sheets by combinations of one N—H⋯O and two C—H⋯O hydrogen bonds, while in the P21 polymorph (Ib) the molecules are linked into sheets by combinations of three hydrogen bonds, one each of N—H⋯O, C—H⋯N and C—H⋯O types. In the orthorhombic polymorph (Ic) the molecules are linked into a complex three-dimensional framework structure by a combination of one N—H⋯O, one N—H⋯N and three C—H⋯O hydrogen bonds, and an aromatic π⋯π stacking interaction. In the isomeric compound 1-(2-chloronicotinoyl)-2-(3-nitrophenyl)hydrazine (II) the molecules are again linked into a three-dimensional framework, this time by a combination of three hydrogen bonds, one each of N—H⋯O, N—H⋯N and C—H⋯O types, weakly augmented by a π⋯π stacking interaction. The molecules of 1-(2-chloronicotinoyl)-2-(4-nitrophenyl)hydrazine (III) are linked into sheets by a combination of three hydrogen bonds, one each of N—H⋯O, N—H⋯N and C—H⋯O types. Copyright (c) 2007 International Union of Crystallography urn:issn:0108-7681 Wardell, S.M.S.V. Souza, M.V.N. de Wardell, J.L. Low, J.N. Glidewell, C. 2007-02-01 doi:10.1107/S0108768106041358 International Union of Crystallography In isomeric 1-(2-chloronicotinoyl)-2-(nitrophenyl)hydrazines, the molecules may be linked into supramolecular structures in two or three dimensions by various combinations of N—H⋯O, N—H⋯N, C—H⋯N and C—H⋯O hydrogen bonds, sometimes augmented by π⋯π stacking interactions. en GEOMETRICAL ISOMERS; HYDROGEN BONDING; PHENYLHYDRAZINE; POLYMORPHISM; SUPRAMOLECULAR STRUCTURES 1-(2-Chloronicotinoyl)-2-(2-nitrophenyl)hydrazine, C12H9Cl­N4O3, crystallizes in three polymorphic forms, two monoclinic forms in space groups Cc (Ia) and P21 (Ib), and an orthorhombic form in space group Pbcn (Ic). In the Cc polymorph (Ia) the molecules are linked into sheets by combinations of one N—H⋯O and two C—H⋯O hydrogen bonds, while in the P21 polymorph (Ib) the molecules are linked into sheets by combinations of three hydrogen bonds, one each of N—H⋯O, C—H⋯N and C—H⋯O types. In the orthorhombic polymorph (Ic) the molecules are linked into a complex three-dimensional framework structure by a combination of one N—H⋯O, one N—H⋯N and three C—H⋯O hydrogen bonds, and an aromatic π⋯π stacking interaction. In the isomeric compound 1-(2-chloronicotinoyl)-2-(3-nitrophenyl)hydrazine (II) the molecules are again linked into a three-dimensional framework, this time by a combination of three hydrogen bonds, one each of N—H⋯O, N—H⋯N and C—H⋯O types, weakly augmented by a π⋯π stacking interaction. The molecules of 1-(2-chloronicotinoyl)-2-(4-nitrophenyl)hydrazine (III) are linked into sheets by a combination of three hydrogen bonds, one each of N—H⋯O, N—H⋯N and C—H⋯O types. text/html Three isomeric 1-(2-chloronicotinoyl)-2-(nitro­phenyl)hydrazines, including three polymorphs of 1-(2-chloronicotinoyl)-2-(2-nitrophenyl)hydrazine: hydrogen-bonded supramolecular structures in two and three dimensions text 1 63 2007-02-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2007 International Union of Crystallography research papers 101 110 http://scripts.iucr.org/cgi-bin/paper?gp5013 The structures of five polyiodide salts, [Co([9]aneS3)2]I11 (1), [Ni([9]aneS3)2]I6 (2), [Ni([9]aneS3)2]I10 (3), [Pd([12]aneS4)]I6 (4) and [Pd([14]aneS4)]I10·MeCN (5), containing the template cations [Co([9]aneS3)2]3+, [Ni([9]aneS3)2]2+, [Pd([12]aneS4)]2+ and [Pd([14]aneS4)]2+ ([9]aneS3 = 1,4,7-trithiacyclononane, [12]aneS4 = 1,4,7,10-tetrathiacyclododecane, [14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane) exhibit a range of polyiodide and polyanionic framework structures. In (1) the charge on the CoIII cation is balanced by three I_3^- anions, which along with a neutral di-iodine molecule form I_{11}^{3-} rings in an extended structure comprising undulating chains of alternating I_{11}^{3-} rings and complex cations. In (2) the complex cation is linked to two tri-iodide anions by S⋯I interactions into well separated sheets of cations and anions, while in (3), I_5^- anions are linked by I⋯I interactions into helices which are cross-linked by S⋯I contacts to form sheets. Rather longer I⋯I contacts in (4) assemble I_3^- ions into 2 × 2 rods, which are linked into a three-dimensional network by S⋯I contacts. In (5) the N atom of the acetonitrile solvent molecule forms an array of four weak C—H⋯N hydrogen bonds to the macrocycle. The extended structure comprises corrugated zigzag chains of polyiodide rings formed by linked I_5^- units; the complex cations are attached to the polyiodide network by S⋯I contacts, which link the chains to form layers. Copyright (c) 2007 International Union of Crystallography urn:issn:0108-7681 Blake, A.J. Li, W.-S. Lippolis, V. Parsons, S. Schröder, M. 2007-02-01 doi:10.1107/S0108768106041668 International Union of Crystallography Complex cations of transition metals with thioether macrocycles act as templates for the assembly of a wide range of polyiodide anions and their polyanionic networks. en TEMPLATE EFFECT; POLYIODIDES; FRAMEWORK; SELF-ASSEMBLY; HELICITY The structures of five polyiodide salts, [Co([9]aneS3)2]I11 (1), [Ni([9]aneS3)2]I6 (2), [Ni([9]aneS3)2]I10 (3), [Pd([12]aneS4)]I6 (4) and [Pd([14]aneS4)]I10·MeCN (5), containing the template cations [Co([9]aneS3)2]3+, [Ni([9]aneS3)2]2+, [Pd([12]aneS4)]2+ and [Pd([14]aneS4)]2+ ([9]aneS3 = 1,4,7-trithiacyclononane, [12]aneS4 = 1,4,7,10-tetrathiacyclododecane, [14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane) exhibit a range of polyiodide and polyanionic framework structures. In (1) the charge on the CoIII cation is balanced by three I_3^- anions, which along with a neutral di-iodine molecule form I_{11}^{3-} rings in an extended structure comprising undulating chains of alternating I_{11}^{3-} rings and complex cations. In (2) the complex cation is linked to two tri-iodide anions by S⋯I interactions into well separated sheets of cations and anions, while in (3), I_5^- anions are linked by I⋯I interactions into helices which are cross-linked by S⋯I contacts to form sheets. Rather longer I⋯I contacts in (4) assemble I_3^- ions into 2 × 2 rods, which are linked into a three-dimensional network by S⋯I contacts. In (5) the N atom of the acetonitrile solvent molecule forms an array of four weak C—H⋯N hydrogen bonds to the macrocycle. The extended structure comprises corrugated zigzag chains of polyiodide rings formed by linked I_5^- units; the complex cations are attached to the polyiodide network by S⋯I contacts, which link the chains to form layers. text/html Extended structures of polyiodide salts of transition metal macrocyclic complexes text 1 63 2007-02-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2007 International Union of Crystallography research papers 81 92 http://scripts.iucr.org/cgi-bin/paper?ws5045 High-resolution neutron powder diffraction has been used in order to characterize the order–disorder transition in monoclinic cyclo-octasulphur. Rapid data collection and the novel use of geometrically constrained refinements has enabled a direct and precise determination of the order parameter, based on molecular site occupancies, to be made. The transition is critical and continuous; with a transition temperature, Tc = 198.4 (3) K, and a critical exponent, β = 0.28 (3), which is indicative of three-dimensional ordering. Difficulties encountered as a consequence of the low thermal conductivity of the sample are discussed. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 David, W.I.F. Ibberson, R.M. Cox, S.F.J. Wood, P.T. 2006-12-01 doi:10.1107/S0108768106039309 International Union of Crystallography The order–disorder phase transition in monoclinic β-sulfur has been characterized using high-resolution neutron powder diffraction. The transition is critical and continuous, with a transition temperature, Tc = 198.4 (3) K, and a critical exponent, β = 0.28 (3), which is indicative of three-dimensional ordering. en NEUTRON POWDER DIFFRACTION; LOW-TEMPERATURE CRYSTALLOGRAPHY; CRITICAL PHENOMENA; PHASE TRANSITION High-resolution neutron powder diffraction has been used in order to characterize the order–disorder transition in monoclinic cyclo-octasulphur. Rapid data collection and the novel use of geometrically constrained refinements has enabled a direct and precise determination of the order parameter, based on molecular site occupancies, to be made. The transition is critical and continuous; with a transition temperature, Tc = 198.4 (3) K, and a critical exponent, β = 0.28 (3), which is indicative of three-dimensional ordering. Difficulties encountered as a consequence of the low thermal conductivity of the sample are discussed. text/html Order–disorder transition in monoclinic sulfur: a precise structural study by high-resolution neutron powder diffraction text 6 62 2006-12-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 953 959 http://scripts.iucr.org/cgi-bin/paper?ws5047 The structure of strontium barium niobate crystals SrxBa1 − xNb2O6 is comprehensively studied in the whole range of the tetragonal tungsten bronze phase (x = 0.32–0.82) using both powder and single-crystal X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show an explicit composition dependence which can be consistently explained using simple model calculations. The temperature dependence of the unit-cell parameters exhibits a remarkable anisotropy in a broad temperature region below the phase transition temperature. This proves that the electrostrictive contribution to the thermal expansion plays an important role in strontium barium niobate. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Podlozhenov, S. Graetsch, H.A. Schneider, J. Ulex, M. Wöhlecke, M. Betzler, K. 2006-12-01 doi:10.1107/S0108768106038869 International Union of Crystallography The structure of strontium barium niobate crystals is studied in the whole range of the tetragonal tungsten bronze phase using X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show pronounced composition dependencies which are explained by model calculations. en TUNGSTEN BRONZE; SINGLE CRYSTAL GROWTH; NIOBATES; FERROELECTRIC MATERIALS The structure of strontium barium niobate crystals SrxBa1 − xNb2O6 is comprehensively studied in the whole range of the tetragonal tungsten bronze phase (x = 0.32–0.82) using both powder and single-crystal X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show an explicit composition dependence which can be consistently explained using simple model calculations. The temperature dependence of the unit-cell parameters exhibits a remarkable anisotropy in a broad temperature region below the phase transition temperature. This proves that the electrostrictive contribution to the thermal expansion plays an important role in strontium barium niobate. text/html Structure of strontium barium niobate SrxBa1 − xNb2O6 (SBN) in the composition range 0.32 ≤ x ≤ 0.82 text 6 62 2006-12-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 960 965 http://scripts.iucr.org/cgi-bin/paper?bm5038 The solvothermal synthesis of four two-dimensional metal-organic frameworks containing linear dicarboxylic acids as ligands for ZnII centres is described. Zn(BDC)(DMF) [(1) where BDC = benzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide] adopts a common paddlewheel motif leading to a 44 grid network, whereas Zn3(BDC)3(EtOH)2 (2), Zn3(BDC)3(H2O)2·4DMF (3) and Zn3(BPDC)3(DMF)2·4DMF (4) each form networks with the relatively uncommon 36 topology based upon Zn3(O2CR)6 secondary building units. All contain coordinated solvent molecules, namely DMF [(1) and (4)], ethanol (2) or H2O (3). Comparison of structures (2) and (3) illustrates a clay-like flexibility in interplanar spacing which sheds light on the ability of the Zn3(BDC)3 framework to undergo desolvation and uptake of small solvent and gas molecules. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Hawxwell, S.M. Adams, H. Brammer, L. 2006-10-01 doi:10.1107/S0108768106033283 International Union of Crystallography Metal-organic framework structures adopting either square grid or triangular grid architectures are reported. A family of the latter type incorporating different solvent molecules sheds light on the flexibility of these layered structures in previously reported solvent loss/uptake. en SOLVOTHERMAL SYNTHESIS; METAL-ORGANIC FRAMEWORKS The solvothermal synthesis of four two-dimensional metal-organic frameworks containing linear dicarboxylic acids as ligands for ZnII centres is described. Zn(BDC)(DMF) [(1) where BDC = benzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide] adopts a common paddlewheel motif leading to a 44 grid network, whereas Zn3(BDC)3(EtOH)2 (2), Zn3(BDC)3(H2O)2·4DMF (3) and Zn3(BPDC)3(DMF)2·4DMF (4) each form networks with the relatively uncommon 36 topology based upon Zn3(O2CR)6 secondary building units. All contain coordinated solvent molecules, namely DMF [(1) and (4)], ethanol (2) or H2O (3). Comparison of structures (2) and (3) illustrates a clay-like flexibility in interplanar spacing which sheds light on the ability of the Zn3(BDC)3 framework to undergo desolvation and uptake of small solvent and gas molecules. text/html Two-dimensional metal-organic frameworks containing linear dicarboxylates text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 808 814 http://scripts.iucr.org/cgi-bin/paper?so5004 The effect of pressure on the crystal structure of salicylaldoxime has been investigated. The ambient-pressure phase (salicylaldoxime-I) consists of pairs of molecules interacting through oximic OH⋯O hydrogen bonds; taken with phenolic OH⋯N intramolecular hydrogen bonds, these dimers form a pseudo-macrocycle bounded by an R_4^4 \left({10} \right) motif. The dimers interact principally via π⋯π stacking contacts. Salicylaldoxime derivatives are used industrially as selective solvent extractants for copper; the selectivity reflects the compatibility of the metal ion with the pseudo-macrocycle cavity size. On increasing the pressure to 5.28 GPa the size of the cavity was found to decrease by an amount comparable to the difference in hole sizes in the structures of the Cu2+ salicylaldoximato complex and its Ni2+ equivalent. On increasing the pressure to 5.93 GPa a new polymorph, salicylaldoxime-II, was obtained in a single-crystal to single-crystal phase transition. PIXEL calculations show that the phase transition is driven in part by relief of intermolecular repulsions in the dimer-forming OH⋯O-bonded ring motif, and the ten-centre hydrogen-bonding ring motif of the phase I structure is replaced in phase II by a six-centre ring formed by oximic OH⋯N hydrogen bonds. The transition also relieves repulsions in the π⋯π stacking contacts. The intramolecular OH⋯N hydrogen bond of phase I is replaced in phase II by a intermolecular phenolic OH⋯O hydrogen bond, but the total interaction energy of the pairs of molecules connected by this new contact is very slightly repulsive because the electrostatic hydrogen-bond energy is cancelled by the repulsion term. The intra- to intermolecular hydrogen-bond conversion simply promotes efficient packing rather than contributing to the overall lattice energy. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Wood, P.A. Forgan, R.S. Henderson, D. Parsons, S. Pidcock, E. Tasker, P.A. Warren, J.E. 2006-12-01 doi:10.1107/S0108768106031752 International Union of Crystallography The crystal structure of salicylaldoxime has been determined at room temperature at pressures from 0.75 to 5.28 GPa. Salicylaldoxime forms a pseudo-macrocycle which contains a cavity which decreases in size with pressure. Above 5.28 GPa the structure transforms to a new polymorph, the structure of which has been determined at 5.93 GPa. The changes in intermolecular interactions during the phase transition are interpreted with the aid of PIXEL calculations. en SALICYLALDOXIME; PRESSURE DEPENDENCE; PHASE TRANSITIONS; HYDROGEN BONDING The effect of pressure on the crystal structure of salicylaldoxime has been investigated. The ambient-pressure phase (salicylaldoxime-I) consists of pairs of molecules interacting through oximic OH⋯O hydrogen bonds; taken with phenolic OH⋯N intramolecular hydrogen bonds, these dimers form a pseudo-macrocycle bounded by an R_4^4 \left({10} \right) motif. The dimers interact principally via π⋯π stacking contacts. Salicylaldoxime derivatives are used industrially as selective solvent extractants for copper; the selectivity reflects the compatibility of the metal ion with the pseudo-macrocycle cavity size. On increasing the pressure to 5.28 GPa the size of the cavity was found to decrease by an amount comparable to the difference in hole sizes in the structures of the Cu2+ salicylaldoximato complex and its Ni2+ equivalent. On increasing the pressure to 5.93 GPa a new polymorph, salicylaldoxime-II, was obtained in a single-crystal to single-crystal phase transition. PIXEL calculations show that the phase transition is driven in part by relief of intermolecular repulsions in the dimer-forming OH⋯O-bonded ring motif, and the ten-centre hydrogen-bonding ring motif of the phase I structure is replaced in phase II by a six-centre ring formed by oximic OH⋯N hydrogen bonds. The transition also relieves repulsions in the π⋯π stacking contacts. The intramolecular OH⋯N hydrogen bond of phase I is replaced in phase II by a intermolecular phenolic OH⋯O hydrogen bond, but the total interaction energy of the pairs of molecules connected by this new contact is very slightly repulsive because the electrostatic hydrogen-bond energy is cancelled by the repulsion term. The intra- to intermolecular hydrogen-bond conversion simply promotes efficient packing rather than contributing to the overall lattice energy. text/html Effect of pressure on the crystal structure of salicylaldoxime-I, and the structure of salicylaldoxime-II at 5.93 GPa text 6 62 2006-12-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 1099 1111 http://scripts.iucr.org/cgi-bin/paper?bm5035 The isomeric N-(iodophenyl)nitrobenzamides, C13H9IN2O3, all form different three-dimensional framework structures. Molecules of N-(2-iodophenyl)-3-nitrobenzamide (II) are linked by a combination of N—H⋯O and C—H⋯O hydrogen bonds and a two-centre iodo⋯carbonyl interaction. The supramolecular structure of N-(2-iodophenyl)-4-nitrobenzamide (III) is built from one N—H⋯O and two C—H⋯O hydrogen bonds, but short I⋯O contacts are absent from the structure. In N-(3-iodophenyl)-2-nitrobenzamide (IV), which crystallizes with Z′ = 2 in space group P21, the structure contains two N—H⋯O hydrogen bonds, four C—H⋯O hydrogen bonds, two two-centre iodo⋯nitro interactions and an aromatic π⋯π stacking interaction. The structure of N-(3-iodophenyl)-3-nitrobenzamide (V) contains one N—H⋯O hydrogen bond and three C—H⋯O hydrogen bonds, together with a two-centre iodo⋯nitro interaction and an aromatic π⋯π stacking interaction, while in N-(3-iodophenyl)-4-nitrobenzamide (VI), the combination of one N—H⋯O hydrogen bond and two C—H⋯O hydrogen bonds is augmented not only by a two-centre iodo⋯nitro interaction and an aromatic π⋯π stacking interaction, but also by a dipolar carbonyl⋯carbonyl interaction. In the supramolecular structure of N-(4-iodophenyl)-4-nitrobenzamide (IX), which crystallizes with Z′ = 2 in space group P\overline 1, there are two N—H⋯O hydrogen bonds, four C—H⋯O hydrogen bonds and two three-centre iodo⋯nitro interactions. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Wardell, J.L. Low, J.N. Skakle, J.M.S. Glidewell, C. 2006-10-01 doi:10.1107/S0108768106029053 International Union of Crystallography Seven isomeric N-(iodophenyl)nitrobenzamides all display different direction-specific intermolecular interactions and all form different three-dimensional framework structures. en N-(IODOPHENYL)NITROBENZAMIDES The isomeric N-(iodophenyl)nitrobenzamides, C13H9IN2O3, all form different three-dimensional framework structures. Molecules of N-(2-iodophenyl)-3-nitrobenzamide (II) are linked by a combination of N—H⋯O and C—H⋯O hydrogen bonds and a two-centre iodo⋯carbonyl interaction. The supramolecular structure of N-(2-iodophenyl)-4-nitrobenzamide (III) is built from one N—H⋯O and two C—H⋯O hydrogen bonds, but short I⋯O contacts are absent from the structure. In N-(3-iodophenyl)-2-nitrobenzamide (IV), which crystallizes with Z′ = 2 in space group P21, the structure contains two N—H⋯O hydrogen bonds, four C—H⋯O hydrogen bonds, two two-centre iodo⋯nitro interactions and an aromatic π⋯π stacking interaction. The structure of N-(3-iodophenyl)-3-nitrobenzamide (V) contains one N—H⋯O hydrogen bond and three C—H⋯O hydrogen bonds, together with a two-centre iodo⋯nitro interaction and an aromatic π⋯π stacking interaction, while in N-(3-iodophenyl)-4-nitrobenzamide (VI), the combination of one N—H⋯O hydrogen bond and two C—H⋯O hydrogen bonds is augmented not only by a two-centre iodo⋯nitro interaction and an aromatic π⋯π stacking interaction, but also by a dipolar carbonyl⋯carbonyl interaction. In the supramolecular structure of N-(4-iodophenyl)-4-nitrobenzamide (IX), which crystallizes with Z′ = 2 in space group P\overline 1, there are two N—H⋯O hydrogen bonds, four C—H⋯O hydrogen bonds and two three-centre iodo⋯nitro interactions. text/html Isomeric N-(iodophenyl)nitrobenzamides form different three-dimensional framework structures text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 931 943 http://scripts.iucr.org/cgi-bin/paper?bs5035 The structural response of three members of the family of polycyclic aromatic hydrocarbons (PAHs) to high-pressure recrystallization from dichloromethane solutions is presented. Recrystallization of naphthalene in the 0.2–0.6 GPa pressure range does not result in the formation of a new polymorph. Furthermore, direct compression of a single crystal to 2.1 GPa does not result in a phase transition. A density decrease of 18.2% over the 0.0–2.1 GPa pressure range is observed and the principal effect of pressure is to `tighten' the herringbone structural motif and decrease the size of void regions. A new polymorph of pyrene, form III, has been crystallized at 0.3 and at 0.5 GPa. Structural investigation of this new polymorph by means of topological analysis and comparison of Hirshfeld surfaces and fingerprint plots shows that intermolecular interactions are substantially different from those found in the ambient-pressure structures, and do not fit a previously established packing model for PAHs. Similar discrepancies are found for the high-pressure polymorph of phenanthrene, which is here re-investigated in greater detail. The structures of these high-pressure polymorphs are dominated by π⋯π stacking with a limited contribution from C—H⋯π (peripheral) interactions. It is perhaps not surprising that high-pressure polymorphs deviate from a model that has been devised for ambient-pressure structures, and this may be a direct consequence of the ability of pressure to modify and combine intermolecular interactions in ways that are not usually found at ambient pressure. This is achieved by modifying the relative orientations of molecules and by encouraging the formation of denser structures in which molecules pack together more efficiently. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Fabbiani, F.P.A. Allan, D.R. Parsons, S. Pulham, C.R. 2006-10-01 doi:10.1107/S0108768106026814 International Union of Crystallography Recrystallization from a dichloromethane solution at 0.5 GPa affords new polymorphs of phenanthrene and pyrene. Structural analysis of the high-pressure polymorphs of pyrene and phenanthrene shows that intermolecular interactions are substantially different from those found in the ambient-pressure structures and do not fit a previously established packing model for polycyclic aromatic hydrocarbons. Recrystallization of naphthalene from a dichloromethane solution in the 0.2–0.6 GPa pressure range does not result in the formation of a new polymorph, and its crystal structure is reported to be stable to compression to 2.1 GPa. en HIGH-PRESSURE BEHAVIOUR; POLYCYCLIC AROMATIC HYDROCARBONS The structural response of three members of the family of polycyclic aromatic hydrocarbons (PAHs) to high-pressure recrystallization from dichloromethane solutions is presented. Recrystallization of naphthalene in the 0.2–0.6 GPa pressure range does not result in the formation of a new polymorph. Furthermore, direct compression of a single crystal to 2.1 GPa does not result in a phase transition. A density decrease of 18.2% over the 0.0–2.1 GPa pressure range is observed and the principal effect of pressure is to `tighten' the herringbone structural motif and decrease the size of void regions. A new polymorph of pyrene, form III, has been crystallized at 0.3 and at 0.5 GPa. Structural investigation of this new polymorph by means of topological analysis and comparison of Hirshfeld surfaces and fingerprint plots shows that intermolecular interactions are substantially different from those found in the ambient-pressure structures, and do not fit a previously established packing model for PAHs. Similar discrepancies are found for the high-pressure polymorph of phenanthrene, which is here re-investigated in greater detail. The structures of these high-pressure polymorphs are dominated by π⋯π stacking with a limited contribution from C—H⋯π (peripheral) interactions. It is perhaps not surprising that high-pressure polymorphs deviate from a model that has been devised for ambient-pressure structures, and this may be a direct consequence of the ability of pressure to modify and combine intermolecular interactions in ways that are not usually found at ambient pressure. This is achieved by modifying the relative orientations of molecules and by encouraging the formation of denser structures in which molecules pack together more efficiently. text/html Exploration of the high-pressure behaviour of polycyclic aromatic hydrocarbons: naphthalene, phenanthrene and pyrene text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 826 842 http://scripts.iucr.org/cgi-bin/paper?bm5034 The structures of both known forms of the polymorphic material ortho-acetamidobenzamide, C9H10N2O2, have been determined by low-temperature neutron single-crystal diffraction. Neutron diffraction allows the full description of the H-atom positions in this molecular material, which is vital in benchmarking related crystal-structure predictions. Significant conformational differences are indicated by a number of the torsion angles involving H atoms when compared with previous X-ray studies. A comprehensive description of the hydrogen-bonding scheme in both polymorphs is given. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Leech, C.K. Barnett, S.A. Shankland, K. Gutmann, M. Wilson, C.C. 2006-10-01 doi:10.1107/S0108768106025821 International Union of Crystallography The structures of both known forms of the polymorphic material ortho-acetamidobenzamide have been determined by low-temperature neutron single-crystal diffraction. en SINGLE-CRYSTAL NEUTRON DIFFRACTION; ACCURATE MOLECULAR STRUCTURES; HYDROGEN BONDING; POLYMORPHISM The structures of both known forms of the polymorphic material ortho-acetamidobenzamide, C9H10N2O2, have been determined by low-temperature neutron single-crystal diffraction. Neutron diffraction allows the full description of the H-atom positions in this molecular material, which is vital in benchmarking related crystal-structure predictions. Significant conformational differences are indicated by a number of the torsion angles involving H atoms when compared with previous X-ray studies. A comprehensive description of the hydrogen-bonding scheme in both polymorphs is given. text/html Accurate molecular structures and hydrogen bonding in two polymorphs of ortho-acetamidobenzamide by single-crystal neutron diffraction text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 926 930 http://scripts.iucr.org/cgi-bin/paper?bs5032 The positions of pairs of independent molecules in Z′ = 2 structures have been surveyed for six of the most populated space groups for that class of structure. These results have been compared with the Z′ = 1 situation to reveal whether there are any fundamental differences in the construction of the asymmetric units in these cases. The results indicate that, broadly speaking, the packing of the molecular pairs in Z′ = 2 structures resembles that of single molecules in structures with Z′ = 1; this similarity may be chiefly attributed to the constraints imposed by the symmetry operators of the space group. However, there are key differences, which are particularly marked in the space groups with higher symmetry, that indicate that the asymmetric units in Z′ = 1 and Z′ = 2 structures are not directly comparable. In those cases where the positions of the pair centroids in Z′ = 2 structures are similar to the positions of molecular centroids for Z′ = 1 structures, it follows that the molecular centroids in Z′ = 2 structures must follow a different distribution. A different pattern is produced if the independent molecules in Z′ = 2 structures behave like the individual molecules in Z′ = 1 structures. These two scenarios combine to form the observed distributions of pair centroid positions. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Collins, A. 2006-10-01 doi:10.1107/S0108768106025195 International Union of Crystallography The positions of molecular pairs in structures with Z′ = 2 are considered for space groups common for that class of structure in the Cambridge Structural Database. en Z' = 2; PACKING PATTERNS The positions of pairs of independent molecules in Z′ = 2 structures have been surveyed for six of the most populated space groups for that class of structure. These results have been compared with the Z′ = 1 situation to reveal whether there are any fundamental differences in the construction of the asymmetric units in these cases. The results indicate that, broadly speaking, the packing of the molecular pairs in Z′ = 2 structures resembles that of single molecules in structures with Z′ = 1; this similarity may be chiefly attributed to the constraints imposed by the symmetry operators of the space group. However, there are key differences, which are particularly marked in the space groups with higher symmetry, that indicate that the asymmetric units in Z′ = 1 and Z′ = 2 structures are not directly comparable. In those cases where the positions of the pair centroids in Z′ = 2 structures are similar to the positions of molecular centroids for Z′ = 1 structures, it follows that the molecular centroids in Z′ = 2 structures must follow a different distribution. A different pattern is produced if the independent molecules in Z′ = 2 structures behave like the individual molecules in Z′ = 1 structures. These two scenarios combine to form the observed distributions of pair centroid positions. text/html Distribution of molecular pairs in Z′ = 2 structures text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 897 911 http://scripts.iucr.org/cgi-bin/paper?bs5034 The geometry of hydrogen bonds in 1,6-anhydro-β-d-glucopyranose (levoglucosan) is accurately determined by refinement of time-of-flight neutron single-crystal diffraction data. Molecules of levoglucosan are held together by a hydrogen-bond array formed by a combination of strong O—H⋯O and supporting weaker C—H⋯O bonds. These are fully and accurately detailed by the neutron diffraction study. The strong hydrogen bonds link molecules in finite chains, with hydroxyl O atoms acting as both donors and acceptors of hydroxyl H atoms. A comparison of molecular and solid-state DFT calculations predicts red shifts of O—H and associated blue shifts of C—H stretching frequencies due to the formation of hydrogen bonds in this system. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Smrčok, Ľ. Sládkovičová, M. Langer, V. Wilson, C.C. Koóš, M. 2006-10-01 doi:10.1107/S010876810602489X International Union of Crystallography The geometry of hydrogen bonds in 1,6-anhydro-β-d-glucopyranose is accurately determined by single-crystal neutron diffraction. DFT calculations predict red (blue) shifts of stretching frequencies of O—H (C—H) in this material due to the formation of hydrogen bonds. en HYDROGEN BONDING; NEUTRON DIFFRACTION; DFT; LEVOGLUCOSAN The geometry of hydrogen bonds in 1,6-anhydro-β-d-glucopyranose (levoglucosan) is accurately determined by refinement of time-of-flight neutron single-crystal diffraction data. Molecules of levoglucosan are held together by a hydrogen-bond array formed by a combination of strong O—H⋯O and supporting weaker C—H⋯O bonds. These are fully and accurately detailed by the neutron diffraction study. The strong hydrogen bonds link molecules in finite chains, with hydroxyl O atoms acting as both donors and acceptors of hydroxyl H atoms. A comparison of molecular and solid-state DFT calculations predicts red shifts of O—H and associated blue shifts of C—H stretching frequencies due to the formation of hydrogen bonds in this system. text/html On hydrogen bonding in 1,6-anhydro-β-d-glucopyranose (levoglucosan): X-ray and neutron diffraction and DFT study text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 912 918 http://scripts.iucr.org/cgi-bin/paper?gp5012 The behaviour of the Flack parameter for centrosymmetric and pseudo-centrosymmetric crystal structures based on crystal structures published as being non-centrosymmetric is presented. It is confirmed for centrosymmetric structures that the value obtained for the Flack parameter is critically dependent on the Friedel coverage of the intensity data, approaching 0.5 for a coverage of 100% and sticking near the starting value for a coverage of 0%. For pseudo-centrosymmetric structures, even those very close to being centrosymmetric, it is found that it is often possible to obtain significant values of the Flack parameter. A theoretical basis for this surprising result is established. It has also been possible to establish an a priori estimate of the standard uncertainty of the Flack parameter based only on the chemical composition of the compound and the wavelength of the radiation. The paper concludes with preliminary presentations of bias in the Flack parameter and of inconsistent chemical and crystallographic data. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Flack, H.D. Bernardinelli, G. Clemente, D.A. Linden, A. Spek, A.L. 2006-10-01 doi:10.1107/S0108768106021884 International Union of Crystallography An analysis of the Flack parameter based on published (pseudo-)centrosymmetric crystal-structure determinations. en FLACK PARAMETER; CENTROSYMMETRIC; NON-CENTROSYMMETRIC; PSEUDO-CENTROSYMMETRIC The behaviour of the Flack parameter for centrosymmetric and pseudo-centrosymmetric crystal structures based on crystal structures published as being non-centrosymmetric is presented. It is confirmed for centrosymmetric structures that the value obtained for the Flack parameter is critically dependent on the Friedel coverage of the intensity data, approaching 0.5 for a coverage of 100% and sticking near the starting value for a coverage of 0%. For pseudo-centrosymmetric structures, even those very close to being centrosymmetric, it is found that it is often possible to obtain significant values of the Flack parameter. A theoretical basis for this surprising result is established. It has also been possible to establish an a priori estimate of the standard uncertainty of the Flack parameter based only on the chemical composition of the compound and the wavelength of the radiation. The paper concludes with preliminary presentations of bias in the Flack parameter and of inconsistent chemical and crystallographic data. text/html Centrosymmetric and pseudo-centrosymmetric structures refined as non-centrosymmetric text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 695 701 http://scripts.iucr.org/cgi-bin/paper?bm5031 The crystal structures of triethyl-1,3,5-triazine-2,4,6-tricarb­oxylate (I), triethyl-1,3,5-benzenetricarboxylate (II) and tris-2-hydroxyethyl isocyanurate (III) have been determined from conventional laboratory X-ray powder diffraction data using the differential evolution structure solution technique. The determination of these structures presented an unexpectedly wide variation in levels of difficulty, with only the determination of (III) being without complication. In the case of (I) structure solution resulted in a Rietveld refinement profile that was not ideal, but was subsequently rationalized by single-crystal diffraction as resulting from disorder. Refinement of structure (II) showed significant variation in side-chain conformation from the initial powder structure solution. Further investigation showed that the structure solution optimization had indeed been successful, and that preferred orientation had a dramatic effect on the structure-solution R-factor search surface. Despite the presence of identical side chains in (I) and (II), only the triazine-based system retains threefold mol­ecular symmetry in the crystal structure. The lack of use of the heterocyclic N atom as a hydrogen-bond acceptor in this structure results in the formation of a similar non-centrosymmetric network to the benzene-based structure, but with overall three-dimensional centrosymmetry. The hydrogen-bonded layer structure of (III) is similar to that of other isocyanurate-based structures of this type. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Chong, S.Y. Seaton, C.C. Kariuki, B.M. Tremayne, M. 2006-10-01 doi:10.1107/S0108768106020921 International Union of Crystallography The crystal structures of triethyl-1,3,5-triazine-2,4,6-tricarboxylate, triethyl-1,3,5-benzenetricarboxylate and tris-2-hydroxyethyl isocyanurate have been determined from laboratory X-ray powder and single-crystal diffraction data, despite the presence of disorder and significant preferred orientation. The materials display contrasting packing behaviour, with only the triazine derivative retaining threefold molecular symmetry in its crystal structure. en MOLECULAR SYMMETRY; CRYSTAL SYMMETRY; X-RAY POWDER DIFFRACTION; SINGLE-CRYSTAL DIFFRACTION; DISORDER; PREFERRED ORIENTATION The crystal structures of triethyl-1,3,5-triazine-2,4,6-tricarb­oxylate (I), triethyl-1,3,5-benzenetricarboxylate (II) and tris-2-hydroxyethyl isocyanurate (III) have been determined from conventional laboratory X-ray powder diffraction data using the differential evolution structure solution technique. The determination of these structures presented an unexpectedly wide variation in levels of difficulty, with only the determination of (III) being without complication. In the case of (I) structure solution resulted in a Rietveld refinement profile that was not ideal, but was subsequently rationalized by single-crystal diffraction as resulting from disorder. Refinement of structure (II) showed significant variation in side-chain conformation from the initial powder structure solution. Further investigation showed that the structure solution optimization had indeed been successful, and that preferred orientation had a dramatic effect on the structure-solution R-factor search surface. Despite the presence of identical side chains in (I) and (II), only the triazine-based system retains threefold mol­ecular symmetry in the crystal structure. The lack of use of the heterocyclic N atom as a hydrogen-bond acceptor in this structure results in the formation of a similar non-centrosymmetric network to the benzene-based structure, but with overall three-dimensional centrosymmetry. The hydrogen-bonded layer structure of (III) is similar to that of other isocyanurate-based structures of this type. text/html Molecular versus crystal symmetry in tri-substituted triazine, benzene and isocyanurate derivatives text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 864 874 http://scripts.iucr.org/cgi-bin/paper?bs5027 The hydrostatic compression of l-serine-d7 has been studied to 8.1 GPa by neutron powder diffraction. Over the course of this pressure range the compound undergoes two phase transitions, the first between 4.6 and 5.2 GPa, yielding l-serine-II, and the second between 7.3 and 8.1 GPa, yielding l-serine-III. All three polymorphs are orthorhombic, P212121, and feature chains of serine molecules connected via head-to-tail ND⋯O hydrogen bonds formed between ammonium and carboxylate groups. The chains are linked into a ribbon by a second set of ND⋯O hydrogen bonds. The hydroxyl moieties are distributed along the outer edges of the ribbon and in phase I they connect the ribbons into a layer by chains of OD⋯OD hydrogen bonds. The layers are connected together by a third set of ND⋯O hydrogen bonds, forming R^3_4(14) rings with substantial voids at their centres. In the transition from phase I to II these voids begin to close up, but at the cost of breaking the OD⋯OD chains. The OD⋯OD hydrogen bonds are replaced by shorter OD⋯O hydrogen bonds to carboxylate groups. At 7.3 GPa the O⋯O distance in the OD⋯O hydrogen bonds measures only 2.516 (17) Å, which is short, and we propose that the phase transition to phase III that occurs between 7.3 and 8.1 GPa relieves the strain that has built up in this region of the structure. The hydroxyl D atom now bifurcates between the OD⋯O contact that had been present in phase II and a new OD⋯O contact formed to a carboxylate in another layer. Hirshfeld surface fingerprint plots show that D⋯D interactions become more numerous, while hydrogen bonds actually begin to lengthen in the transition from phase II to III. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Moggach, S.A. Marshall, W.G. Parsons, S. 2006-10-01 doi:10.1107/S010876810601799X International Union of Crystallography The hydrostatic compression of l-serine-d7 has been studied to 8.1 GPa by neutron powder diffraction. Over the course of this pressure range the compound undergoes two phase transitions, the first between 4.6 and 5.2 GPa, and the second between 7.3 and 8.1 GPa. en NEUTRON POWDER DIFFRACTION; HYDROSTATIC COMPRESSION; HYDROGEN BONDING The hydrostatic compression of l-serine-d7 has been studied to 8.1 GPa by neutron powder diffraction. Over the course of this pressure range the compound undergoes two phase transitions, the first between 4.6 and 5.2 GPa, yielding l-serine-II, and the second between 7.3 and 8.1 GPa, yielding l-serine-III. All three polymorphs are orthorhombic, P212121, and feature chains of serine molecules connected via head-to-tail ND⋯O hydrogen bonds formed between ammonium and carboxylate groups. The chains are linked into a ribbon by a second set of ND⋯O hydrogen bonds. The hydroxyl moieties are distributed along the outer edges of the ribbon and in phase I they connect the ribbons into a layer by chains of OD⋯OD hydrogen bonds. The layers are connected together by a third set of ND⋯O hydrogen bonds, forming R^3_4(14) rings with substantial voids at their centres. In the transition from phase I to II these voids begin to close up, but at the cost of breaking the OD⋯OD chains. The OD⋯OD hydrogen bonds are replaced by shorter OD⋯O hydrogen bonds to carboxylate groups. At 7.3 GPa the O⋯O distance in the OD⋯O hydrogen bonds measures only 2.516 (17) Å, which is short, and we propose that the phase transition to phase III that occurs between 7.3 and 8.1 GPa relieves the strain that has built up in this region of the structure. The hydroxyl D atom now bifurcates between the OD⋯O contact that had been present in phase II and a new OD⋯O contact formed to a carboxylate in another layer. Hirshfeld surface fingerprint plots show that D⋯D interactions become more numerous, while hydrogen bonds actually begin to lengthen in the transition from phase II to III. text/html High-pressure neutron diffraction study of l-serine-I and l-serine-II, and the structure of l-serine-III at 8.1 GPa text 5 62 2006-10-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 815 825 http://scripts.iucr.org/cgi-bin/paper?bs5029 Reassessment of the reported single-crystal X-ray diffraction characterization of polymorphs of furosemide and finasteride shows that, in each case, incomplete data collections have resulted in the mistaken identification of two forms that are, in fact, identical. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Karami, S. Li, Y. Hughes, D.S. Hursthouse, M.B. Russell, A.E. Threlfall, T.L. Claybourn, M. Roberts, R. 2006-08-01 doi:10.1107/S0108768106016211 International Union of Crystallography Reassessment of the reported single-crystal X-ray diffraction characterization of polymorphs of furosemide and finasteride shows that, in each case, incomplete data collections have resulted in the mistaken identification of two forms that are, in fact, identical. en FUROSEMIDE; FINASTERIDE; POLYMORPHS Reassessment of the reported single-crystal X-ray diffraction characterization of polymorphs of furosemide and finasteride shows that, in each case, incomplete data collections have resulted in the mistaken identification of two forms that are, in fact, identical. text/html Further errors in polymorph identification: furosemide and finasteride text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography short communications 689 691 http://scripts.iucr.org/cgi-bin/paper?ws5035 The crystallography and microwave dielectric properties of La(Zn1/2Ti1/2)O3 (LZT) ceramics prepared via the mixed-oxide route were investigated in this study. While samples were largely single phase, small amounts of ZnO impurity were detected in sintered pellets. Observed reflections in electron and neutron diffraction patterns indicate that the symmetry of LZT is P21/n. The B site is ordered on {110} or pseudocubic {111}, but the presence of the pseudocubic 1\over2(111) reflection is in itself insufficient to indicate the existence of such order. Rietveld refinements of the neutron diffraction data yield an excellent fit for such a model. The structure is highly twinned, with variants related through common {211} composition planes and 90° rotations about 〈011〉. The microwave dielectric properties measured were ∊r = 34, Qf = 36 090 and τf = −70 MK−1. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Ubic, R. Hu, Y. Abrahams, I. 2006-08-01 doi:10.1107/S0108768106015527 International Union of Crystallography The structure of the perovskite compound La(Zn1/2Ti1/2)O3 is monoclinic, space group P21/n. The B site is ordered on {110}, but the presence of the pseudocubic 1\over2(111) reflection is insufficient by itself to indicate the existence of such order. en PEROVSKITE; NEUTRON DIFFRACTION; ELECTRON DIFFRACTION; MICROWAVE DIELECTRIC The crystallography and microwave dielectric properties of La(Zn1/2Ti1/2)O3 (LZT) ceramics prepared via the mixed-oxide route were investigated in this study. While samples were largely single phase, small amounts of ZnO impurity were detected in sintered pellets. Observed reflections in electron and neutron diffraction patterns indicate that the symmetry of LZT is P21/n. The B site is ordered on {110} or pseudocubic {111}, but the presence of the pseudocubic 1\over2(111) reflection is in itself insufficient to indicate the existence of such order. Rietveld refinements of the neutron diffraction data yield an excellent fit for such a model. The structure is highly twinned, with variants related through common {211} composition planes and 90° rotations about 〈011〉. The microwave dielectric properties measured were ∊r = 34, Qf = 36 090 and τf = −70 MK−1. text/html Neutron and electron diffraction studies of La(Zn1/2Ti1/2)O3 perovskite text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 521 529 http://scripts.iucr.org/cgi-bin/paper?ws5038 The crystal structure of phase II of tertiary butyl alcohol (2-methyl-2-propanol, C4H10O) has been solved using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. This trigonal P\overline 3 phase, which is stable at both low temperature and high pressure, and the triclinic P\overline 1 phase (phase IV) have very similar enthalpies, the calculations revealing only a 3.859 kJ mol−1 enthalpy difference at ambient pressure, despite the substantial change of the intermolecular bonding motif from helical catemer to hexamer with an increase in pressure or reduction in temperature. The hexamers in the trigonal phase adopt a chair conformation. There are two unique hexamers: at low temperature these are centred at (0, 0, 1\over2) and (2\over3, 1\over3, 0.961 (13)), and at high pressure the centres are (0, 0, 1\over2) and (2\over3, 1\over3, 0.958 (14)). A slight flattening of the hexamers is observed at high pressure and the calculations confirm that phase II becomes more stable relative to phase IV on pressure increase. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 McGregor, P.A. Allan, D.R. Parsons, S. Clark, S.J. 2006-08-01 doi:10.1107/S0108768106015424 International Union of Crystallography The crystal structure of phase II of tertiary butyl alcohol (2-methyl-2-propanol) has been solved using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. The calculations indicate that the trigonal P\bar 3 phase II structure is slightly more stable at elevated pressure than that of the triclinic P\bar 1 phase IV. en HEXAMER FORMATION; 2-METHYL-2-PROPANOL The crystal structure of phase II of tertiary butyl alcohol (2-methyl-2-propanol, C4H10O) has been solved using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. This trigonal P\overline 3 phase, which is stable at both low temperature and high pressure, and the triclinic P\overline 1 phase (phase IV) have very similar enthalpies, the calculations revealing only a 3.859 kJ mol−1 enthalpy difference at ambient pressure, despite the substantial change of the intermolecular bonding motif from helical catemer to hexamer with an increase in pressure or reduction in temperature. The hexamers in the trigonal phase adopt a chair conformation. There are two unique hexamers: at low temperature these are centred at (0, 0, 1\over2) and (2\over3, 1\over3, 0.961 (13)), and at high pressure the centres are (0, 0, 1\over2) and (2\over3, 1\over3, 0.958 (14)). A slight flattening of the hexamers is observed at high pressure and the calculations confirm that phase II becomes more stable relative to phase IV on pressure increase. text/html Hexamer formation in tertiary butyl alcohol (2-methyl-2-propanol, C4H10O) text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 599 605 http://scripts.iucr.org/cgi-bin/paper?bm5033 Structures are reported here for eight further substituted N-aryl-2-chloronicotinamides, 2-ClC5H3NCONHC6H4X-4′. When X = H, compound (I) (C12H9ClN2O), the molecules are linked into sheets by N—H⋯N, C—H⋯π(pyridyl) and C—H⋯π(arene) hydrogen bonds. For X = CH3, compound (II) (C13H11ClN2O, triclinic P\bar 1 with Z′ = 2), the molecules are linked into sheets by N—H⋯O, C—H⋯O and C—H⋯π(arene) hydrogen bonds. Compound (III), where X = F, crystallizes as a monohydrate (C12H8ClFN2O·H2O) and sheets are formed by N—H⋯O, O—H⋯O and O—H⋯N hydrogen bonds and aromatic π⋯π stacking interactions. Crystals of compound (IV), where X = Cl (C12H8Cl2N2O, monoclinic P21 with Z′ = 4) exhibit inversion twinning: the molecules are linked by N—H⋯O hydrogen bonds into four independent chains, linked in pairs by C—H⋯π(arene) hydrogen bonds. When X = Br, compound (V) (C12H8BrClN2O), the molecules are linked into sheets by N—H⋯O and C—H⋯N hydrogen bonds, while in compound (VI), where X = I (C12H8ClIN2O), the molecules are linked into a three-dimensional framework by N—H⋯O and C—H⋯π(arene) hydrogen bonds and an iodo⋯N(pyridyl) interaction. For X = CH3O, compound (VII) (C13H11ClN2O2), the molecules are linked into chains by a single N—H⋯O hydrogen bond. Compound (VIII) (C13H8ClN3O, triclinic P\bar 1 with Z′ = 2), where X = CN, forms a complex three-dimensional framework by N—H⋯N, C—H⋯N and C—H⋯O hydrogen bonds and two independent aromatic π⋯π stacking interactions. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Cuffini, S. Glidewell, C. Low, J.N. de Oliveira, A.G. de Souza, M.V.N. Vasconcelos, T.R.A. Wardell, S.M.S.V. Wardell, J.L. 2006-08-01 doi:10.1107/S0108768106015497 International Union of Crystallography Substituted N-aryl-2-chloronicotinamides, 2-ClC5H3NCONHC6H4X-4′, can have supramolecular structures which are one-, two- or three-dimensional, and are all built from different combinations of direction-specific intermolecular interactions. en SUPRAMOLECULAR STRUCTURES; DIRECTION-SPECIFIC INTERMOLECULAR INTERACTIONS; HYDROGEN BONDS; VITAMIN B Structures are reported here for eight further substituted N-aryl-2-chloronicotinamides, 2-ClC5H3NCONHC6H4X-4′. When X = H, compound (I) (C12H9ClN2O), the molecules are linked into sheets by N—H⋯N, C—H⋯π(pyridyl) and C—H⋯π(arene) hydrogen bonds. For X = CH3, compound (II) (C13H11ClN2O, triclinic P\bar 1 with Z′ = 2), the molecules are linked into sheets by N—H⋯O, C—H⋯O and C—H⋯π(arene) hydrogen bonds. Compound (III), where X = F, crystallizes as a monohydrate (C12H8ClFN2O·H2O) and sheets are formed by N—H⋯O, O—H⋯O and O—H⋯N hydrogen bonds and aromatic π⋯π stacking interactions. Crystals of compound (IV), where X = Cl (C12H8Cl2N2O, monoclinic P21 with Z′ = 4) exhibit inversion twinning: the molecules are linked by N—H⋯O hydrogen bonds into four independent chains, linked in pairs by C—H⋯π(arene) hydrogen bonds. When X = Br, compound (V) (C12H8BrClN2O), the molecules are linked into sheets by N—H⋯O and C—H⋯N hydrogen bonds, while in compound (VI), where X = I (C12H8ClIN2O), the molecules are linked into a three-dimensional framework by N—H⋯O and C—H⋯π(arene) hydrogen bonds and an iodo⋯N(pyridyl) interaction. For X = CH3O, compound (VII) (C13H11ClN2O2), the molecules are linked into chains by a single N—H⋯O hydrogen bond. Compound (VIII) (C13H8ClN3O, triclinic P\bar 1 with Z′ = 2), where X = CN, forms a complex three-dimensional framework by N—H⋯N, C—H⋯N and C—H⋯O hydrogen bonds and two independent aromatic π⋯π stacking interactions. text/html Nine N-aryl-2-chloronicotinamides: supramolecular structures in one, two and three dimensions text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 651 665 http://scripts.iucr.org/cgi-bin/paper?bm5032 The structures of five of the possible six isomers of (E,E)-1,4-bis(nitrophenyl)-2,3-diaza-1,3-butadiene are reported, including two polymorphs of one of the isomers. (E,E)-1,4-Bis(2-nitrophenyl)-2,3-diaza-1,3-butadiene, C14H10N4O4 (I), crystallizes in two polymorphic forms (Ia) and (Ib) in which the molecules lie across centres of inversion in space groups P21/n and P21/c, respectively: the molecules in (Ia) and (Ib) are linked into chains by aromatic π⋯π stacking interactions and C—H⋯π(arene) hydrogen bonds, respectively. Molecules of (E,E)-1-(2-nitrophenyl)-4-(3-nitrophenyl)-2,3-diaza-1,3-butadiene (II) are linked into sheets by two independent C—H⋯O hydrogen bonds. The molecules of (E,E)-1,4-bis(3-nitrophenyl)-2,3-diaza-1,3-butadiene (III) lie across inversion centres in the space group P21/n, and a combination of a C—H⋯O hydrogen bond and a π⋯π stacking interaction links the molecules into sheets. A total of four independent C—H⋯O hydrogen bonds link the molecules of (E,E)-1-(3-nitrophenyl)-4-(4-nitrophenyl)-2,3-diaza-1,3-butadiene (IV) into sheets. In (E,E)-1,4-bis(4-nitrophenyl)-2,3-diaza-1,3-butadiene (V) the molecules, which lie across centres of inversion in the space group P21/n, are linked by just two independent C—H⋯O hydrogen bonds into a three-dimensional framework. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Glidewell, C. Low, J.N. Skakle, J.M.S. Wardell, J.L. 2006-08-01 doi:10.1107/S010876810601439X International Union of Crystallography Five isomeric (E,E)-1,4-bis(nitrophenyl)-2,3-diaza-1,3-butadienes all display different direction-specific intermolecular interactions and all give different supramolecular structures. en SUPRAMOLECULAR STRUCTURES; DIRECTION-SPECIFIC INTERMOLECULAR INTERACTIONS; POLYMORPHISM; HYDROGEN BONDING The structures of five of the possible six isomers of (E,E)-1,4-bis(nitrophenyl)-2,3-diaza-1,3-butadiene are reported, including two polymorphs of one of the isomers. (E,E)-1,4-Bis(2-nitrophenyl)-2,3-diaza-1,3-butadiene, C14H10N4O4 (I), crystallizes in two polymorphic forms (Ia) and (Ib) in which the molecules lie across centres of inversion in space groups P21/n and P21/c, respectively: the molecules in (Ia) and (Ib) are linked into chains by aromatic π⋯π stacking interactions and C—H⋯π(arene) hydrogen bonds, respectively. Molecules of (E,E)-1-(2-nitrophenyl)-4-(3-nitrophenyl)-2,3-diaza-1,3-butadiene (II) are linked into sheets by two independent C—H⋯O hydrogen bonds. The molecules of (E,E)-1,4-bis(3-nitrophenyl)-2,3-diaza-1,3-butadiene (III) lie across inversion centres in the space group P21/n, and a combination of a C—H⋯O hydrogen bond and a π⋯π stacking interaction links the molecules into sheets. A total of four independent C—H⋯O hydrogen bonds link the molecules of (E,E)-1-(3-nitrophenyl)-4-(4-nitrophenyl)-2,3-diaza-1,3-butadiene (IV) into sheets. In (E,E)-1,4-bis(4-nitrophenyl)-2,3-diaza-1,3-butadiene (V) the molecules, which lie across centres of inversion in the space group P21/n, are linked by just two independent C—H⋯O hydrogen bonds into a three-dimensional framework. text/html Isomers and polymorphs of (E,E)-1,4-bis(nitrophenyl)-2,3-diaza-1,3-butadienes text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 666 675 http://scripts.iucr.org/cgi-bin/paper?av5050 The crystal structures of the four-membered heterocycles (S)-(−)-4-oxo-2-azetidinecarboxylic acid (I) and 3-azetidinecarboxylic acid (II) were solved by direct methods using powder synchrotron X-ray diffraction data. The asymmetry of the oxoazetidine and azetidine rings is discussed, along with the hydrogen bonding. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Mora, A.J. Brunelli, M. Fitch, A.N. Wright, J. Báez, M.E. López-Carrasquero, F. 2006-08-01 doi:10.1107/S0108768106013887 International Union of Crystallography The crystal structures of the four-membered heterocycles (S)-(−)-4-oxo-2-azetidinecarboxylic acid and 3-azetidinecarboxylic acid were solved by direct methods using powder synchrotron X-ray diffraction data. en POWDER DIFFRACTION; STRUCTURAL SOLUTION; HETEROCYCLES; ANTIBIOTICS; ASYMMETRY; HYDROGEN BONDING The crystal structures of the four-membered heterocycles (S)-(−)-4-oxo-2-azetidinecarboxylic acid (I) and 3-azetidinecarboxylic acid (II) were solved by direct methods using powder synchrotron X-ray diffraction data. The asymmetry of the oxoazetidine and azetidine rings is discussed, along with the hydrogen bonding. text/html Structures of (S)-(−)-4-oxo-2-azetidinecarboxylic acid and 3-azetidinecarboxylic acid from powder synchrotron diffraction data text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 606 611 http://scripts.iucr.org/cgi-bin/paper?bk5029 The commercially available peptide coupling reagent 1-hydroxy-7-azabenzotriazole has been shown to crystallize in two polymorphic forms. The two polymorphs differ in their hydrogen-bonding motif, with form I having an R_2^2(10) dimer motif and form II having a C(5) chain motif. The previously unreported form II was used as an informal blind test of computational crystal structure prediction for flexible molecules. The crystal structure of form II has been successfully predicted blind from lattice-energy minimization calculations following a series of searches using a large number of rigid conformers. The structure for form II was the third lowest in energy with form I found as the global minimum, with the energy calculated as the sum of the ab initio intramolecular energy penalty for conformational distortion and the intermolecular lattice energy which is calculated from a distributed multipole representation of the charge density. The predicted structure was sufficiently close to the experimental structure that it could be used as a starting model for crystal structure refinement. A subsequent limited polymorph screen failed to yield a third polymorphic form, but demonstrated that alcohol solvents are implicated in the formation of the form I dimer structure. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Nowell, H. Frampton, C.S. Waite, J. Price, S.L. 2006-08-01 doi:10.1107/S0108768106012584 International Union of Crystallography The crystal structure of a novel conformational polymorph of 1-hydroxy-7-azabenzotriazole has been independently predicted in an informal blind test. en CONFORMATIONAL POLYMORPHISM; CRYSTAL STRUCTURE PREDICTION; POLYMORPH SCREENING The commercially available peptide coupling reagent 1-hydroxy-7-azabenzotriazole has been shown to crystallize in two polymorphic forms. The two polymorphs differ in their hydrogen-bonding motif, with form I having an R_2^2(10) dimer motif and form II having a C(5) chain motif. The previously unreported form II was used as an informal blind test of computational crystal structure prediction for flexible molecules. The crystal structure of form II has been successfully predicted blind from lattice-energy minimization calculations following a series of searches using a large number of rigid conformers. The structure for form II was the third lowest in energy with form I found as the global minimum, with the energy calculated as the sum of the ab initio intramolecular energy penalty for conformational distortion and the intermolecular lattice energy which is calculated from a distributed multipole representation of the charge density. The predicted structure was sufficiently close to the experimental structure that it could be used as a starting model for crystal structure refinement. A subsequent limited polymorph screen failed to yield a third polymorphic form, but demonstrated that alcohol solvents are implicated in the formation of the form I dimer structure. text/html Blind crystal structure prediction of a novel second polymorph of 1-hydroxy-7-azabenzotriazole text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 642 650 http://scripts.iucr.org/cgi-bin/paper?bk5025 The structures of two neutral and 17 salt forms of the base (1R, 2S)-(−)-ephedrine are reported. These structures are discussed in the light of the conformers of the ephedrine moiety, the existence of bilayers and the structure determining role of the counterions. Overall, most of the salt structures are essentially derived from the observed packing of the neutral base and are dominated by the amphiphilic nature of the ephedrine molecular structure. In a few cases the size and hydrophobicity of the counterion disrupts this behaviour. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Collier, E.A. Davey, R.J. Black, S.N. Roberts, R.J. 2006-06-01 doi:10.1107/S0108768106012018 International Union of Crystallography The structures of anhydrous ephedrine and ephedrine hemihydrate together with 17 salts of the base are reported. It is evident that the observed packings are in most cases related to those of the neutral molecule, incorporating both bilayer and conformational elements. en PACKING ANALYSIS; COUNTERION; MOLECULAR SALTS; PHARMACEUTICALS The structures of two neutral and 17 salt forms of the base (1R, 2S)-(−)-ephedrine are reported. These structures are discussed in the light of the conformers of the ephedrine moiety, the existence of bilayers and the structure determining role of the counterions. Overall, most of the salt structures are essentially derived from the observed packing of the neutral base and are dominated by the amphiphilic nature of the ephedrine molecular structure. In a few cases the size and hydrophobicity of the counterion disrupts this behaviour. text/html 17 salts of ephedrine: crystal structures and packing analysis text 3 62 2006-06-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 498 505 http://scripts.iucr.org/cgi-bin/paper?so5003 The structures of four selenium analogues of the antithyroid drug 6-n-propyl-2-thiouracil [systematic name: 2,3-dihydro-6-n-propyl-2-thioxopyrimidin-4(1H)-one], namely 6-methyl-2-selenouracil, C5H6N2OSe (1), 6-ethyl-2-selenouracil, C6H8N2OSe (2), 6-n-propyl-2-selenouracil, C7H10N2OSe (3), and 6-isopropyl-2-selenouracil, C7H10N2OSe (4), are described, along with that of the dichloromethane monosolvate of 6-isopropyl-2-selenouracil, C7H10N2OSe·CH2Cl2 (4·CH2Cl2). The extended structure of (1) is a two-dimensional sheet of topology 63 with a brick-wall architecture. The extended structures of (2) and (4) are analogous, being based on a chain of eight-membered R86(32) hydrogen-bonded rings. In (3) and (4·CH2Cl2), R22(8) hydrogen bonding links molecules into chains. 6-n-Propyl-2-selenouracil·I2, C7H10N2OSe·I2 (7), is a charge-transfer complex with a `spoke' structure, the extended structure of which is based on a linear chain formed principally by intermolecular N—H⋯O hydrogen bonds. Re-crystallization of 6-ethyl-2-selenouracil or (7) from acetone gave crystals of the diselenides [N-(6′-ethyl-4′-pyr­imidone)(6-ethyl-2-selenouracil)2(Se—Se)]·2H2O (9·2H2O) or [N-(6′-n-propyl-4′-pyrimidone)(6-n-propyl-2-selenouracil)2(Se—Se)] (10), respectively: these have similar extended chain structures formed via N—H⋯O and C—H⋯O hydrogen bonds, stacked to give two-dimensional sheets. Re-crystallization of (7) from methanol/acetonitrile led via deselenation to the formation of crystals of 6-n-propyl-2-uracil (11), in which six symmetry-related molecules combine to form a six-membered R66(24) hydrogen-bonded ring, with each pair of molecules linked by an R22(8) motif. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Antoniadis, C.D. Blake, A.J. Hadjikakou, S.K. Hadjiliadis, N. Hubberstey, P. Schröder, M. Wilson, C. 2006-08-01 doi:10.1107/S0108768106011426 International Union of Crystallography The structures of selenium analogues of the antithyroid drug 6-n-propyl-2-thiouracil, the charge-transfer complex 6-n-propyl-2-selenouracil·I2, two diselenides and the deselenation product 6-n-propyl-2-uracil all show extended architectures. These are based on hydrogen-bonding and, in the case of 6-n-propyl-2-selenouracil·I2, on Se⋯I interactions. en SELENOURACIL; HYDROGEN BONDING; CHARGE-TRANSFER COMPLEX; ANTITHYROID DRUG The structures of four selenium analogues of the antithyroid drug 6-n-propyl-2-thiouracil [systematic name: 2,3-dihydro-6-n-propyl-2-thioxopyrimidin-4(1H)-one], namely 6-methyl-2-selenouracil, C5H6N2OSe (1), 6-ethyl-2-selenouracil, C6H8N2OSe (2), 6-n-propyl-2-selenouracil, C7H10N2OSe (3), and 6-isopropyl-2-selenouracil, C7H10N2OSe (4), are described, along with that of the dichloromethane monosolvate of 6-isopropyl-2-selenouracil, C7H10N2OSe·CH2Cl2 (4·CH2Cl2). The extended structure of (1) is a two-dimensional sheet of topology 63 with a brick-wall architecture. The extended structures of (2) and (4) are analogous, being based on a chain of eight-membered R86(32) hydrogen-bonded rings. In (3) and (4·CH2Cl2), R22(8) hydrogen bonding links molecules into chains. 6-n-Propyl-2-selenouracil·I2, C7H10N2OSe·I2 (7), is a charge-transfer complex with a `spoke' structure, the extended structure of which is based on a linear chain formed principally by intermolecular N—H⋯O hydrogen bonds. Re-crystallization of 6-ethyl-2-selenouracil or (7) from acetone gave crystals of the diselenides [N-(6′-ethyl-4′-pyr­imidone)(6-ethyl-2-selenouracil)2(Se—Se)]·2H2O (9·2H2O) or [N-(6′-n-propyl-4′-pyrimidone)(6-n-propyl-2-selenouracil)2(Se—Se)] (10), respectively: these have similar extended chain structures formed via N—H⋯O and C—H⋯O hydrogen bonds, stacked to give two-dimensional sheets. Re-crystallization of (7) from methanol/acetonitrile led via deselenation to the formation of crystals of 6-n-propyl-2-uracil (11), in which six symmetry-related molecules combine to form a six-membered R66(24) hydrogen-bonded ring, with each pair of molecules linked by an R22(8) motif. text/html Structural characterization of selenium and selenium-diiodine analogues of the antithyroid drug 6-n-propyl-2-thiouracil and its alkyl derivatives text 4 62 2006-08-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography research papers 580 591 http://scripts.iucr.org/cgi-bin/paper?bk0150 The first (so-called) lambda transition in solids was found in the specific heat measurements for NH4Cl at 242 K by F. Simon in 1922 [Simon (1922). Ann. Phys. 68, 241–280]. Analogous phenomena found in many other solids gave rise to doubts (expressed most clearly by A. R. Ubbelohde some 50 years ago) about the applicability of classical thermodynamics to some phase transitions [Ubbelohde (1956). Brit. J. Appl. Phys. 7, 313–321]. However, Y. Mnyukh's studies of enantiotropic phase transitions in eight organic crystals showed that all proceed by a nucleation-and-growth mechanism [summarized in Mnyukh (2001), Fundamentals of Solid State Phase Transitions, Ferromagnetism and Ferroelectricity. 1st Books]. Nucleation is localized at defects in the parent phase; growth can be epitaxic and oriented if parent and daughter phases have closely similar structures, or random (not oriented) if there are substantial structural differences. This conclusion is supported by a critical review of Mnyukh's eight examples and other results published in the interim. It seems that Ubbelohde's invocation of `hybrid crystals' and `smeared transitions' can mostly be accounted for by lack of equilibrium in the phase-transition studies cited by him. However, the intermediate phase in 4,4′-dichlorobenzophenone appears to have structural resemblances to Ubbelohde's' `hybrid crystal'. Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Herbstein, F.H. 2006-06-01 doi:10.1107/S0108768106008640 International Union of Crystallography An integrated and comparative study of more than 20 enantiotropic solid-state phase transitions shows that these are all first-order and proceed by a `nucleation and growth' mechanism in accordance with the description set out especially by Yuri Mnyukh. en SOLID-STATE PHASE TRANSITIONS; POLYMORPHISM; FIRST ORDER The first (so-called) lambda transition in solids was found in the specific heat measurements for NH4Cl at 242 K by F. Simon in 1922 [Simon (1922). Ann. Phys. 68, 241–280]. Analogous phenomena found in many other solids gave rise to doubts (expressed most clearly by A. R. Ubbelohde some 50 years ago) about the applicability of classical thermodynamics to some phase transitions [Ubbelohde (1956). Brit. J. Appl. Phys. 7, 313–321]. However, Y. Mnyukh's studies of enantiotropic phase transitions in eight organic crystals showed that all proceed by a nucleation-and-growth mechanism [summarized in Mnyukh (2001), Fundamentals of Solid State Phase Transitions, Ferromagnetism and Ferroelectricity. 1st Books]. Nucleation is localized at defects in the parent phase; growth can be epitaxic and oriented if parent and daughter phases have closely similar structures, or random (not oriented) if there are substantial structural differences. This conclusion is supported by a critical review of Mnyukh's eight examples and other results published in the interim. It seems that Ubbelohde's invocation of `hybrid crystals' and `smeared transitions' can mostly be accounted for by lack of equilibrium in the phase-transition studies cited by him. However, the intermediate phase in 4,4′-dichlorobenzophenone appears to have structural resemblances to Ubbelohde's' `hybrid crystal'. text/html On the mechanism of some first-order enantiotropic solid-state phase transitions: from Simon through Ubbelohde to Mnyukh text 3 62 2006-06-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography feature articles 341 383 http://scripts.iucr.org/cgi-bin/paper?pf0025 Copyright (c) 2006 International Union of Crystallography urn:issn:0108-7681 Lehmann, C.W. 2006-04-01 doi:10.1107/S0108768106006604 International Union of Crystallography en BOOK REVIEW text/html Crystalline Molecular Complexes and Compounds. Vol. 1 and 2. By F. H. Herbstein. Pp. xxviii + 1273. Oxford: Oxford University Press, 2005. Price (hardback) GBP 125.00. ISBN 0-19-852660-1. text 2 62 2006-04-01 Acta Crystallographica Section B: Structural Science Copyright (c) 2006 International Union of Crystallography book reviews 338 339 http://scripts.iucr.org/cgi-bin/paper?ry5001