| 1. |
- Abrahamsson, Jonas, 1954, et al.
(author)
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The crystal structure of O-ethyl-S-(11-carboxyundecyl)dithiocarbonate
- 1976
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In: Acta Crystallographica Section B: Structural Science. - 0108-7681. ; 32:10, s. 2745-2749
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Journal article (peer-reviewed)abstract
- O-Ethyl S-(I l-carboxyundecyl)dithiocarbonate (CJ5H~8S203) is triclinic (pT) with a=7·534, b=4·797, c=25·304 A, 0:=90·83, jJ=90·72 and y=79·71°. The bond distances and angles agree very well with those reported earlier for the homologue with a shorter carbon chain (C5). The conformations are also very similar in the two compounds. The ethyl end of one molecule just reaches S(2) of a neighbouring one. This results in a packing with only small regions of lateral hydrocarbon chain packing. The chain arrangement cannot be described by any known subcell.
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| 2. |
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| 3. |
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| 4. |
- Bergmann, Justin, et al.
(author)
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Can the results of quantum refinement be improved with a continuum-solvation model?
- 2021
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In: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 77:6, s. 906-918
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Journal article (peer-reviewed)abstract
- Quantum refinement has repeatedly been shown to be a powerful approach to interpret and improve macromolecular crystal structures, allowing for the discrimination between different interpretations of the structure, regarding the protonation states or the nature of bound ligands, for example. In this method, the empirical restraints, used to supplement the crystallographic raw data in standard crystallographic refinement, are replaced by more accurate quantum mechanical (QM) calculations for a small, but interesting, part of the structure. Previous studies have shown that the results of quantum refinement can be improved if the charge of the QM system is reduced by adding neutralizing groups. However, this significantly increases the computation time for the refinement. In this study, we show that a similar improvement can be obtained if the original highly charged QM system is instead immersed in a continuum solvent in the QM calculations. The best results are typically obtained with a high dielectric constant (ε). The continuum solvent improves real-space it Z values, electron-density difference maps and strain energies, and it normally does not affect the discriminatory power of the calculations between different chemical interpretations of the structure. However, for structures with a low charge in the QM system or with a low crystallographic resolution (>2Å), no improvement of the structures is seen.
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| 5. |
- Carlson, Stefan, et al.
(author)
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High-pressure transformations of NbO2F
- 2000
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In: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 56:2, s. 189-196
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Journal article (peer-reviewed)abstract
- The ReO3-type structure NbO2F, niobium dioxyfluoride, has been studied at high pressures using diamond anvil cells and synchrotron X-ray radiation. High-pressure powder diffraction measurements have been performed up to 40.1 GPa. A phase transition from the cubic (Pm3m) ambient pressure structure to a rhombohedral (R3c) structure at 0.47 GPa has been observed. Rietveld refinements at 1.38, 1.96, 3.20, 6.23, 9.00 and 10.5 GPa showed that the transition involves an a-a-a- tilting of the cation-anion coordination octahedra and a change of the anion-anion arrangement to approach hexagonal close packing. Compression and distortion of the Nb(O/F)6 octahedra is also revealed by the Rietveld refinements. At 17-18 GPa, the diffraction pattern disappears and the structure becomes X-ray amorphous.
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| 6. |
- Christensen, Jeppe, et al.
(author)
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Vacancy Ordering Effects in AlB2-type ErGe2-x(0.4 < x < or = 0.5)
- 2008
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In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 64:3, s. 272-280
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Journal article (peer-reviewed)abstract
- In the Er-Ge system, the compostion range ErGe2 to Er2Ge3 has been investigated. Eight samples were produced by arc melting of the elements, and analyzed using X-ray powder diffraction. Nine crystal structures were found to be present in the samples. The structures are described as a homologous series and presented within the superspace formalism using the superspace group X2/m(0)0s, X representing the centring vector (½, ½, 0, ½). In this description the modulation vector q = (a* + c*) is shown to be a direct measure of the Ge content as ErGe2 - ( falls in the range to ½). The large composition range is achieved by extended vacancy ordering in the planar 63 net of Ge with subsequent relaxation.
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| 7. |
- Davies, K., et al.
(author)
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Topological studies of three related metal-organic frameworks of GdIII and 5-nitroisophthalate
- 2012
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In: Acta Crystallographica Section B: Structural Science. - : International Union of Crystallography (IUCr). - 0108-7681 .- 1600-5740. ; 68:5, s. 528-535
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Journal article (peer-reviewed)abstract
- The reaction of 5-nitroisophthalic acid (H(2)NIA) with Gd(NO3)(3)center dot 6H(2)O in DMF afforded three new metal-organic frameworks: [Gd(NIA)(1.5)(DMF)(2)]center dot DMF (I), [Gd-2(NIA)(3)(DMF)(4)]center dot xH(2)O (II) and [Gd-4(NIA)(6)(DMF)(5.5)(H2O)(3)]center dot 4DMF center dot H2O (III). These compounds can be prepared through a variety of methods. Compounds (I) and (II) are more reproducibly formed than compound (III). Network analysis revealed (I) to have a (4(12).6(3))-pcu topology, while (II) displays a (4(2).8(4))(4(2).8(4))-pts topology. Compound (III) was found to present the uncommon 4,5,6T11 topological net, which combines aspects of both the pcu and pts topologies. The short symbol of this net is (4(4).6(2))(4(6).6(4))(2)(4(8).6(6).8).
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| 8. |
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| 9. |
- Dinnebier, R., et al.
(author)
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Crystal structures of the trifluoromethyl sulfonates M(SO3CF3)(2) (M = Mg, Ca, Ba, Zn, Cu) from synchrotron X-ray powder diffraction data
- 2006
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In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 62, s. 467-473
-
Journal article (peer-reviewed)abstract
- The crystal structures of divalent metal salts of trifluoromethyl sulfonic acid ('trifluoromethyl sulfonates') M( SO3CF3)(2) (M = Mg, Ca, Ba, Zn, Cu) were determined from high-resolution X-ray powder diffraction data. Magnesium, calcium and zinc trifluoromethyl sulfonate crystallize in the rhombohedral space group R (3) over bar . Barium trifluoromethyl sulfonate crystallizes in the monoclinic space group I2= a(C2/c) and copper trifluoromethyl sulfonate crystallizes in the triclinic group P (1) over bar. Within the crystal structures the trifluoromethyl sulfonate anions are arranged in double layers with the apolar CF3 groups pointing towards each other. The cations are located next to the SO3 groups. The symmetry relations between the different crystal structures have been analysed.
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| 10. |
- Dopieralski, Przemyslaw D., et al.
(author)
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Proton-transfer dynamics in the (HCO3-)(2) dimer of KHCO3 from Car-Parrinello and path-integrals molecular dynamics calculations
- 2010
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 66, s. 222-228
-
Journal article (peer-reviewed)abstract
- The proton motion in the (HCO3-)(2) dimer of KHCO3 at 298 K has been studied with Car-Parrinello molecular dynamics (CPMD) and path-integrals molecular dynamics (PIMD) simulations. According to earlier neutron diffraction studies at 298 K hydrogen is disordered and occupies two positions with an occupancy ratio of 0.804/0.196. A simulation with only one unit cell is not sufficient to reproduce the disorder of the protons found in the experiments. The CPMD results with four cells, 0.783/0.217, are in close agreement with experiment. The motion of the two protons along the O center dot center dot center dot O bridge is highly correlated inside one dimer, but strongly uncoupled between different dimers. The present results support a mechanism for the disorder which involves proton transfer from donor to acceptor and not orientational disordering of the entire dimer. The question of simultaneous or successive proton transfer in the two hydrogen bonds in the dimer remains unanswered. During the simulation situations with almost simultaneous proton transfer with a time gap of around 1 fs were observed, as well as successive processes where first one proton is transferred and then the second one with a time gap of around 20 fs. The calculated vibrational spectrum is in good agreement with the experimental IR spectrum, but a slightly different assignment of the bands is indicated by the present simulations.
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| 11. |
- Eriksson, Anders, et al.
(author)
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Analysis of the thermal parameters of the water molecule in crystalline hydrates studied by neutron diffraction
- 1983
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; B39, s. 703-711
-
Journal article (peer-reviewed)abstract
- A survey is given of the thermal parameters of 150 water molecules in crystalline hydrates determined by neutron diffraction. The accuracy of the thermal parameters has been examined and rigid-bond tests revealed systematic errors for approximately 25% of the molecules. Considering only the most precise and accurate studies, good agreement is obtained between vibrational amplitudes derived from diffraction and spectroscopy. The influence of the immediate environment on the vibrations of the water molecule has also been investigated. A positive correlation is found between H...O hydrogen-bond distances and librational amplitudes. The coordination geometry around the O atom is shown to influence the vibrational amplitudes of the O atom.
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| 12. |
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| 13. |
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| 14. |
- Hermansson, Kersti, et al.
(author)
-
A deformation electron density study of potassium hydrogen diformate
- 1989
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; B45, s. 252-257
-
Journal article (peer-reviewed)abstract
- KH(HCOO) 2, Mr=130.144, orthorhombic, Pbca, a=17.703(8), b=7.349(4), c=7.302(3) Aring, V=950.1(8) Aring 3, Z=8, Dx=1.820(1) g cm -3, lambda(Mo Kalpha)=0.71069 Aring, mu x(calc.)=0.973 mm -1, F(000)=528, T=120 K, R( F2)=0.033 for 2109 unique reflexions. The deformation electron density in KH(HCOO) 2 has been studied by means of experimental X- N maps obtained at 120 K. The electron distributions in the two chemically similar but crystallographically nonequivalent formate groups are discussed and correlated with bond order. The density distribution in the short intermolecular O-H. . . bond [2.437(1) Aring] is compared with experimental and theoretical maps in related compounds.
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| 15. |
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| 16. |
- Hirsch, T.K., et al.
(author)
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An investigation of H-atom positions in sulfuric acid crystal structures
- 2004
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 60:2, s. 179-183
-
Journal article (peer-reviewed)abstract
- Hydrogen conformations in crystalline H2SO4· 8H2O and H2SO4·6.5H2O have been studied using a system developed by Hirsch [(2003), Z. Anorg. Allg. Chem. 629, 666-672]. New H-atom coordinates, as estimated from DFT calculations, are given for these structures. © 2004 International Union of Crystallography Printed in Great Britain - all rights reserved.
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| 17. |
- Höwing, Jonas, et al.
(author)
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Li3+δVC6O13: a short-range-ordered lithium insertion mechanism
- 2004
-
In: Acta Crystallographica. - 0108-7681. ; B60, s. 382-387
-
Journal article (peer-reviewed)abstract
- The structures of Li3 V6O13 and Li3+V6O13, 0.3, have been determined by single-crystal X-ray diffraction. Both compounds have the space group C2/m, with very similar cell parameters. In Li3V6O13, the Li atoms are found in the Wyckoff positions 4(i) and 2(b) with multiplicities of four and two, respectively. Since Li3V6O13 exhibits no superstructure reflections, it is concluded that Li3V6O13 contains one disordered lithium ion in an otherwise ordered centrosymmetric structure. On inserting more lithium into the structure, the Li3+V6O13 phase is formed with the homogeneity range 0 < < 1. It is concluded that the site for the extra inserted lithium ion is closely coupled to the position of the disordered lithium ion in Li3V6O13. A mechanism for this behaviour and for the further formation of the Li6V6O13 end-phase in the LixV6O13 system is proposed.
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| 18. |
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| 19. |
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| 20. |
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| 21. |
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| 22. |
- Karppinen, Markku
(author)
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Crystal structure, atomic net charges and electric moments in pyroelectric LiNaSO4 at 296 K
- 2015
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 71, s. 334-341
-
Journal article (peer-reviewed)abstract
- The crystal structure and electric charge distribution in LiNaSO4 have been studied at 296 K by X-ray diffraction using a spherical crystal. LiNaSO4 is pyroelectric, nonferroelectric and an optically uniaxial insulator which crystallizes in the space group P31c. Least-squares refinement (MOLLY) was based on 13 026 reflections. The asymmetric unit contains Li+, Na+ and three SO42- ions, where one O and S lie on a threefold axis about which three O atoms are related with a threefold symmetry in each sulfate ion. Two of the O-S-O groups suffer from disorder. The net charges of the atoms in three independent sulfate ions were determined under ionic charge constraints. The S atoms have positive net electric charges and O atoms are negative. The components of the significant electric multipole moments in the principal axis directions are determined from the distribution of net atomic charges in each sulfate ion. Electric moments in the unit cell generate macroscopic electric moments in the crystal which interact with light. This interaction results in two axial vectors of second rank associated with an optical indicatrix. The ratio of the calculated axial vector components in the principal axis directions originating from the asymmetric unit is 1.0061 (1), which compares well with the ratio of 1.006 for the corresponding optical refractive indices of LiNaSO4 for lambda(Na) = 589.29 nm.
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| 23. |
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| 24. |
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| 25. |
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| 26. |
- Krogh Andersen, A.M., et al.
(author)
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Ab Initio Structure Determination and Rietveld Refinement of a High-Temperature Phase of Zirconium Hydrogen Phosphate and a New Polymorph of Zirconium Pyrophosphate from In-Situ Temperature-Resolved Powder Diffraction Data.
- 2000
-
In: Acta Crystallographica Section B. - : Wiley. - 0108-7681. ; 56:4, s. 618-625
-
Journal article (peer-reviewed)abstract
- The collected in situ temperature-resolved synchrotron powder data revealed that the transformation of the recently reported three-dimensional τ-Zr(HPO4)2 to cubic ZrP2O7 goes through two intermediate phases. The first intermediate phase, ρ-Zr(HPO4)2, is formed in a reversible phase transition at 598 K, which involves both rearrangement and disordering of the hydrogen phosphate groups of τ-Zr(HPO4)2. At 688 K condensation of the hydrogen phosphate groups leads to the formation of the second intermediate, a new polymorph of zirconium pyrophosphate (β-ZrP2O7). Heating above 973 K results in the gradual transformation of β-ZrP2O7 to cubic zirconium pyrophosphate (α-ZrP2O7). The crystal structures of the two intermediate phases were solved from the in situ powder diffraction data using direct methods and refined using the Rietveld method. Both phases are orthorhombic, space group Pnnm and Z = 2. The lattice parameters for the two phases are: ρ-Zr(HPO4)2: a = 8.1935 (2), b = 7.7090 (2), c = 5.4080 (1) Å; β-ZrP2O7: a = 8.3127 (5), b = 6.6389 (4), c = 5.3407 (3) Å. The formation mechanism for the new zirconium pyrophosphate polymorph, β-ZrP2O7, is discussed in relation to structurally restricted soft chemistry
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| 27. |
- Krogh Andersen, A.M., et al.
(author)
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High-Pressure Structures of α- and δ-ZrMo2O8
- 2001
-
In: Acta Crystallographica Section B Structural science. - : Wiley. - 0108-7681. ; 57:1, s. 20-26
-
Journal article (peer-reviewed)abstract
- In situ high-pressure synchrotron X-ray powder diffraction studies of trigonal α-ZrMo2O8, zirconium molybdate, have been performed from ambient conditions to 1.9 GPa, over the α–δ phase transition at 1.06–1.11 GPa. The monoclinic structure of δ-ZrMo2O8, stable between 1.1 and 2.5 GPa at 298 K, has been solved by direct methods and refined using the Rietveld method. Significant distortions of the ZrO6 and MoO4 polyhedral elements are observed for δ-ZrMo2O8, as compared to the ambient conditions of the α-phase, while the packing of anions becomes more symmetric at high pressure.
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| 28. |
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| 29. |
- Langer, Vratislav, 1949, et al.
(author)
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Second-degree twinning and dynamic disorder in the crystal structure of deca-dodecasil 3R
- 2005
-
In: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; B61:6, s. 627-634
-
Journal article (peer-reviewed)abstract
- The structure of deca-dodecasil 3R (DD-3R), Si120O240, a very well suited material for the synthesis of inorganic/organic composites structured on a nanometer level, has been investigated in detail. So far, a highly complicated twinning has hampered its structure description at a desirable level of accuracy. This twinning has now been resolved and a new structure determination is presented. Structure refinement in the R-3 space group revealed a large, unusually shaped atomic displacement ellipsoid for oxygen-bridging units (tetrahedra), bridging Si-O bonds shorter than expected and the linear Si-O-Si' bond angle dictated by special positions at a threefold axis. A structure model based on a statistically disordered bridging O atom improved the accuracy of the Si-O bonds of interest, but provided unacceptable O-O contacts. To solve this dilemma, ab initio NVT molecular dynamics calculations were performed to study the possible configurations. Wavelet analysis of the time variations of selected Si-O distances pointed to a synchronous shift of the whole building units (tetrahedra). Low-frequency features of the calculated phonon density of states agree well with the published INS (inelastic neutron scattering) spectra of several silica polymorphs, indicating that the nature of the disorder in DD-3R is dynamic rather than static.
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| 30. |
- Li, Mingrun, et al.
(author)
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A complicated quasicrystal approximant ε16 predicted by the strong-reflections approach
- 2010
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 66:part 1, s. 17-26
-
Journal article (peer-reviewed)abstract
- The structure of a complicated quasicrystal approximant ϵ16 was predicted from a known and related quasicrystal approximant ϵ6 by the strong-reflections approach. Electron-diffraction studies show that in reciprocal space, the positions of the strongest reflections and their intensity distributions are similar for both approximants. By applying the strong-reflections approach, the structure factors of ϵ16 were deduced from those of the known ϵ6 structure. Owing to the different space groups of the two structures, a shift of the phase origin had to be applied in order to obtain the phases of ϵ16. An electron-density map of ϵ16 was calculated by inverse Fourier transformation of the structure factors of the 256 strongest reflections. Similar to that of ϵ6, the predicted structure of ϵ16 contains eight layers in each unit cell, stacked along the b axis. Along the b axis, ϵ16 is built by banana-shaped tiles and pentagonal tiles; this structure is confirmed by high-resolution transmission electron microscopy (HRTEM). The simulated precession electron-diffraction (PED) patterns from the structure model are in good agreement with the experimental ones. ϵ16 with 153 unique atoms in the unit cell is the most complicated approximant structure ever solved or predicted.
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| 31. |
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| 32. |
- Lyxell, D G, et al.
(author)
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Multicomponent polyanions. 53. Structure of tetrakis(trimethylammonium) tetra-mu-oxo-bis(triaquahexadecaoxo(trioxophenylphosphato)hexamolybdate) dihydrate, [NH(CH3)(3)](4)[{(C6H5P)Mo6O21(H2O)(3)}(2)].2H(2)O
- 1998
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 54, s. 424-430
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Journal article (peer-reviewed)abstract
- The title compound crystallized in the monoclinic space group P2(1)/n (No. 14) [a = 11.211 (5), b = 12.862 (3), c = 23.05 (1) Angstrom, beta = 94.37 (3)degrees, V = 3314 (2) Angstrom(3), Z = 2]. The polyanion can be regarded as a dimer of a phenylphosphonatohexamolybdate, (C6H5P)Mo6O23(H2O)(3), Linked by four O atoms. In this monomeric unit the six molybdenum octahedra are grouped into two parts consisting of four and two edge-sharing octahedra, respectively. These two parts are connected by two corner-sharing O atoms to form a bent Mo-6 ring. The phenylphosphonate group coordinates to the Mo-6 ring from the narrow side as a tripodal Ligand. The {(C6H5P)Mo6O21 (H2O)(3)}(2) units form layers parallel to (001) and the structure is stabilized by hydrogen bonds between water and neighboring anions. The monomeric unit has been shown to be a key structure in the process of deducing the aqueous solution structures of the (C6H5P)Mo-6(2-) and (C6H5P)Mo-7(4-) species found in a previous equilibrium study of the H+-MoO42-(C6H5P)O-3(2-) system.
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| 33. |
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| 34. |
- Majerz, Irena, et al.
(author)
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Asymmetric hydrogen bonds in a centrosymmetric environment. III. Quantum mechanical calculations of the potential-energy surfaces for the very short hydrogen bonds in potassium hydrogen dichloromaleate
- 2007
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 63, s. 748-752
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Journal article (peer-reviewed)abstract
- In the crystal structure of potassium hydrogen dichloromaleate there are two short hydrogen bonds of 2.44 angstrom. The 'heavy-atom' structure is centrosymmetric ( space group P 3 (1) over bar) with centers of symmetry in the middle of the O-O bonds, suggesting centered hydrogen bonds. However, earlier unconventional types of refinements of the extensive neutron data taken at 30, 90, 135, 170 and 295 K demonstrated that the H atoms are actually non-centered in the hydrogen bonds, although the environment is centrosymmetric. Traditionally it has been assumed that the hydrogen distribution adopts the same symmetry as the environment. Reviewing these unusual results it was considered of great interest to verify that the non-centered locations of the H atoms are reasonable from an energy point of view. Quantum mechanical calculations have now been carried out for the potential-energy surfaces ( PES) for both the centered and non-centered locations of the H atoms. In all cases the non-centered positions are closer to the energy minima in the PES than the centered positions, and this result confirms that the structure is best described with noncentered H atoms. There is virtually perfect agreement between the quantum-mechanically derived reaction coordinates ( QMRC) and the bond-order reaction coordinates ( BORC) derived using Pauling's bond-order concept together with the principle of conservation of bond order.
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| 35. |
- Majerz, Irena, et al.
(author)
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Comparison of the proton-transfer path in hydrogen bonds from theoretical potential-energy surfaces and the concept of conservation of bond order. II. (N—H…N)+ hydrogen bonds
- 2007
-
In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 63:4, s. 650-662
-
Journal article (peer-reviewed)abstract
- The quantum-mechanically derived reaction coordinates (QMRC) for the proton transfer in (N—H—N)+ hydrogen bonds have been derived from ab initio calculations of potential-energy surfaces. A comparison is made between the QMRC and the corresponding bond-order reaction coordinates (BORC) derived by applying the Pauling bond-order concept together with the principle of conservation of bond order. We find virtually perfect agreement between the QMRC and the BORC for intermolecular (N—H—N)+ hydrogen bonds. In contrast, for intramolecular (N—H—N)+ hydrogen bonds, the donor and acceptor parts of the molecule impose strong constraints on the N—N distance and the QMRC does not follow the BORC relation in the whole range. The X-ray determined hydrogen positions are not located exactly at the theoretically calculated potential-energy minima, but instead at the point where the QMRC and the BORC coincide with each other. On the other hand, the optimized hydrogen positions, with other atoms in the cation fixed as in the crystal structure, are closer to these energy minima. Inclusion of the closest neighbours in the theoretical calculations has a rather small effect on the optimized hydrogen positions. [Part I: Olovsson (2006). Z. Phys. Chem.220, 797–810.]
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| 36. |
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| 37. |
- Norberg, Stefan, 1972, et al.
(author)
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Cation movement and phase transitions in KTP isostructures; X-ray study of sodium-doped KTP at 10.5 K
- 2003
-
In: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; 59:3, s. 353-360
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Journal article (peer-reviewed)abstract
- An accurate structure model of sodium-doped potassium titanyl phosphate, (Na0.114K0.886) K(TiO)(2)(PO4)(2), has been determined at 10.5 K by single-crystal X-ray diffraction. In addition to the low-temperature data, X-ray intensities have been collected at room temperature. When the temperature was decreased from room temperature to 10.5 K, both potassium cations moved 0.033 (2) Angstrom along the c-axis, i.e. in the polar direction within the rigid Ti-O-P network. This alkaline metal ion displacement can be related to the Abrahams-Jamieson-Kurtz T-C criteria for oxygen framework ferroelectrics. Potassium titanyl phosphate (KTP) is a well known material for second harmonic generation (SHG), and the influence of sodium dopant on the TiO6 octahedral geometry and SHG is discussed. The material studied crystallizes in the space group Pna2(1) with Z = 4, a = 12.7919 (5), b = 6.3798 (4), c = 10.5880 (7) Angstrom, V = 864.08 (9) Angstrom(3), T = 10.5 (3) K and R = 0.023.
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| 38. |
- Norberg, Stefan, 1972, et al.
(author)
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Dopant positions in strontium/chromium- and barium-doped KTP, determined with synchrotron X-radiation
- 2001
-
In: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; 56:6, s. 980-987
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Journal article (peer-reviewed)abstract
- Structure factors for strontium/chromium- (Sr/Cr) and barium- (Ba) doped potassium titanyl phosphate (KTiOPO4, KTP) were measured with focused synchrotron X-radiation [0.75000 (9) Angstrom] using a fast avalanche photodiode counter. Space group Pna2(1), Z = 8,a = 12.786 (2), b = 6.3927 (8), c = 10.5585 (9) Angstrom, T = 293 (1) K, R = 0.028 (SrCrKTP); a = 12.851 (6), b = 6.418 (3), c = 10.620 (5) Angstrom, T = 120 (1) K, R = 0.031 (BaKTP). The refinement of the dopant positions showed that Ba2+ is positioned in the larger of the two K cavities of KTP, while the smaller Sr2+ ion is located in both. Split positions are found for the strontium dopant in both cavities and they are located in the positive c direction from the potassium cation. The chromium dopant has two different oxidation states, namely +III and +VI; in both states the dopant is located inside the TiO6 octahedra. The two structures show slightly less distorted TiO6 octahedra than pure KTP.
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| 39. |
- Norberg, Stefan, 1972
(author)
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New phosphate langbeinites, K2MTi(PO4)(3) (M=Er, Yb or Y), and an alternative description of the langbeinite framework
- 2002
-
In: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; 58:5, s. 743-749
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Journal article (peer-reviewed)abstract
- Three new potassium rare-earth/titanium phosphate structures, K2ErTi(PO4)(3) (KErTP), K2YbTi(PO4)(3) (KYbTP) and K2YTi(PO4)(3) (KYTP), are presented, all of which are characterized by single-crystal X-ray diffraction studies. In addition, a fourth structure, K2CrTi(PO4)(3) (KCrTP), has been reinvestigated. All structures are isostructural to the langbeinite-type structure and result from changes made to the growth constituents in high-temperature flux-growth experiments intended to give structurally modified potassium titanyl phosphate (KTP). The two crystallographically independent octahedra sites (site symmetry 3) have a mixed Ti/M (M = Er, Yb, Y or Cr) population, although the rare-earth metals favour one site while chromium favours the other. An alternative approach for the description of the channels and cation cages in langbeinite and related structures is given using [M5X6O39] units. The framework of langbeinite is compared with that of nasicon using these alternative building units. All of the investigated structures crystallize in space group P2(1)3 with Z = 4; a = 10.1053 (2) Angstrom, R = 0.023 (KErTP); a = 10.0939 (8) Angstrom, R = 0.022 (KYbTP); a = 10.1318 (6) Angstrom, R = 0.047 (KYTP); a = 9.8001 (2) Angstrom, R = 0.016 (KCrTP).
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| 40. |
- Norberg, Stefan, 1972, et al.
(author)
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Phase transitions in KTP isostructures: correlation between structure and T-c in germanium-doped RbTiOPO4
- 2003
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In: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; 59:5, s. 588-595
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Journal article (peer-reviewed)abstract
- Crystals of germanium-doped rubidium titanyl phosphate, Rb-2(Ti)(Ge0.121Ti0.879)O-2(PO4)(2) (GeRTP#1) and Rb-2(Ge-0.125-Ti-0.875)(Ge0.225Ti0.775)O-2(PO4)(2) (GeRTP#2), have been structurally characterized from X-ray diffraction data at room temperature. In addition, a third structure, Rb-2(TiO)(2)(PO4)(2) (RTP), has been reinvestigated. The exchange of titanium for germanium results in a less distorted octahedral coordination around the two crystallographically independent titanium sites. Additionally, rubidium split-cation positions have been found in these doped RTP crystals. Dielectric measurements show that the phase-transition temperature, T-c, decreases with increasing germanium concentration, and a direct correlation between the room-temperature split of the rubidium cations and T-c has been discovered. General trends regarding the relationship between the room-temperature structures of KTP-like compounds and their T-c values are discussed.
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| 41. |
- OJAMAE, LARS, et al.
(author)
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STRUCTURAL, VIBRATIONAL AND ELECTRONIC-PROPERTIES OF A CRYSTALLINE HYDRATE FROM AB-INITIO PERIODIC HARTREE-FOCK CALCULATIONS
- 1994
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In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 50, s. 268-279
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Journal article (peer-reviewed)abstract
- The hydrate crystal lithium hydroxide monohydrate LiOH.H2O has been studied by ab initio periodic Hartree-Fock calculations. The influence of the crystalline environment on the local molecular properties (molecular geometry, atomic charges, electron density, molecular vibrations and deuterium quadrupole coupling constants) of the water molecule, the lithium and hydroxide ions has been calculated. A number of crystalline bulk properties are also presented, optimized crystalline structure, lattice energy and electronic band structure. The optimized cell parameters from calculations with a large basis set of triple-zeta quality differ by only 1-3% from the experimental neutron-determined cell, whereas the STO-3g basis set performs poorly (differences of 5-10%). With the triple-zeta basis also the atomic positions and intermolecular distances agree very well with the experiment. The lattice energy differs by approximately 8% from the experimental value, and by at most 3% when a density-functional electron correlation correction is applied. Large electron-density rearrangements occur in the water molecule and in the hydrogen bond and are in qualitative and quantitative agreement with experimental X-ray diffraction results. The quadrupole-coupling constants of the water and hydroxide deuterium atoms are found to be very sensitive to the O-H bond length and are in good agreement with experimental values when the calculation is based on the experimental structure. The anharmonic O-H stretching vibrations in the crystal are presented and found to be very close to results from calculations on molecular clusters. The electronic band and density-of-states spectra are discussed. Model calculations on a hydrogen fluoride chain were used to rationalize the results.
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| 42. |
- Oku, T, et al.
(author)
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Modulated structure of Ag2SnO3 studied by high-resolution electron microscopy
- 2000
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In: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 56:3, s. 363-368
-
Journal article (peer-reviewed)abstract
- The modulated structure of Ag2SnO3, disilver tin trioxide, was investigated by high-resolution electron microscopy and electron diffraction along four different directions. Electron diffraction showed an incommensurate one-dimensional modulated structure with a modulation wavevector of 1/6.4a*. High-resolution images showed a large number of superstructure domains with the size range 10-100 nm and orientations related by hexagonal rotation. The modulation was determined to be displacements along the c axis of the Ag atoms both in octahedral and linear coordination. An approximate structure model with a commensurate sixfold superstructure, with an orthorhombic cell (P2(1)2(1)2(1), a = 2.922, b = 1.267, c = 0.562 nm), is proposed. Calculated images and electron diffraction patterns, based on this model, agree well with experimental observations.
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| 43. |
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| 44. |
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| 45. |
- Ptasiewicz-Bak, H., et al.
(author)
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Charge density in NiCl2.4H2O at 295 and 30 K
- 1999
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In: Acta Crystallographica Section B. - : International Union of Crystallography (IUCr). - 0108-7681 .- 1600-5740. ; 55:6, s. 830-840
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Journal article (peer-reviewed)abstract
- The charge distribution has been determined by multipole refinements against single-crystal X-ray diffraction data. In the refinements a comparison was made between the densities based on H-atom parameters from X-ray and neutron data, respectively. X-ray study: (Mo K ) = 0.71073 Å, F(000) = 408; at 30 K: R(F) = 0.015 for 6686 reflections; at 295 K: R(F) = 0.022 for 4630 reflections. The nickel ion is octahedrally surrounded by four water molecules and two chloride ions, forming a locally neutral Ni(H2O)4Cl2 complex. Two of the water molecules are coordinated to nickel approximately in one of the tetrahedral (`lone-pair') directions; the other two are trigonally coordinated. At 30 K one H atom in one of the trigonally coordinated water molecules is disordered, with equal occupation of two different positions. Owing to the polarizing influence of the nickel ion there are two peaks in the lone-pair plane of the water molecules when these are tetrahedrally coordinated; for those trigonally coordinated there is just one peak. The individual (`partial') charge densities, calculated from the deformation functions of only nickel or the separate water molecules, have also been calculated to study the effects of superposition of the individual densities. In the individual density of nickel an excess is observed in the diagonal directions and a deficiency in the ligand directions. However, owing to the influence of the whole crystalline environment, the maxima around nickel are not found in the planes defined by nickel and the six ligands.
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| 46. |
- Ptasiewicz-Bak, H., et al.
(author)
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Charge Density in Orthorhombic NiSO4.7H2O at Room Temperature and 25 K
- 1997
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In: Acta Crystallographica Section B. - : International Union of Crystallography (IUCr). - 0108-7681 .- 1600-5740. ; 53:3, s. 325-336
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Journal article (peer-reviewed)abstract
- The charge-density distribution in the title compound nickel sulfate heptahydrate has been determined by multipole refinement against single-crystal X-ray intensity data. For the refinement at 25 K hydrogen positions and displacement parameters were fixed to values determined from neutron data. The charge density based on the deformation functions of all atoms in the structure is compared with the individual densities calculated from the deformation functions of only nickel or the separate water molecules. In this way the effects of simple superposition of the individual densities have been studied. The individual deformation density around nickel is in good qualitative agreement with that expected for an approximately octahedral Ni(H2O)62+ complex in a weak ligand field. However, the maximum densities are not found precisely in the planes defined by nickel and the six water ligands, which illustrates that it is necessary to consider the crystal field due to the whole crystalline environment. The individual densities of the water molecules show clear polarization of the lone-pair densities according to the coordination of the water molecules: tetrahedral coordination leads to two resolved lone-pair peaks, whereas planar trigonal coordination leads to just one single peak. Crystal data: NiSO4.7H2O, Mr = 280.87, P212121, Z = 4. At T = 25 K: a = 6.706 (3), b = 11.796 (6), c = 11.949 (6) Å, V = 945 (1) Å3, Dx = 1.977 Mg m-3. At 295 K: a = 6.751 (4), b = 11.746 (7), c = 12.003 (8) Å, V = 952 (1) Å3, Dx = 1.959 Mg m-3. X-ray study: F(000) = 584, (Mo K ) = 0.71069 Å, = 2.255 mm-1. At 25 K: R(F) = 0.014 for 10 185 reflections. At 295 K: R(F) = 0.015 for 5723 reflections. Neutron study at 25 K: = 1.215 Å, = 0.261 mm-1, R(F) = 0.034 for 1381 reflections.
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| 47. |
- Ptasiewicz-Bak, H., et al.
(author)
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Structure, Charge and Spin Density in Na2Ni(CN)4.3H2O at 295 and 30 K
- 1998
-
In: Acta Crystallographica Section B. - : International Union of Crystallography (IUCr). - 0108-7681 .- 1600-5740. ; 54:5, s. 600-612
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Journal article (peer-reviewed)abstract
- The earlier reported structure of the title compound,disodium tetracyanonickelate(II) trihydrate,Na 2 Ni(CN) 4.3H 2O, has been found to be incorrect andhas now been redetermined. The charge distribution hasbeen determined by multipole reÆnements againstsingle-crystal X-ray diffraction data. In the reÆnementbased on 30 K data a comparison was made between theresults obtained using hydrogen positions and displace-ment parameters from X-ray diffraction with those usingthe values determined by neutron diffraction. The spindensity was investigated by polarized neutron diffrac-tion at 1.6 K. Crystal data: at T = 30 K: a = 7.278 (4), b =8.856 (5), c = 15.131 (8) A, = 89.32 (5), = 87.39 (4),= 83.61 (4); at 295 K: a = 7.392 (4), b = 8.895 (4), c =15.115 (8) A, = 89.12 (2), = 87.46 (2), = 84.54 (2).The structure contains practically square planarNi(CN) 2ˇ4 ions, which are stacked on top of each otherin almost linear chains along the a direction. Theseparation between the Ni(CN) 2ˇ4 planes is rather large,with Ni–Ni distances around 3.7 A. The six crystal-lographically independent water molecules are eachcoordinated to two sodium ions, approximately in thetetrahedral (lone-pair) directions, and the polarizinginØuence of these sodium ions also appears to bereØected in the deformation density in the lone-pairplane. The charge density based on the deformationfunctions of all atoms in the structure is compared withthe individual densities calculated from the deformationfunctions of only nickel or the separate water molecules.In this way the effects of simple superposition of theindividual densities have been studied. In the planarNi(CN) 2ˇ4 ion the individual deformation density ofnickel is in qualitative agreement with that expectedfrom crystal-Æeld theory. As the repulsion from theelectrons is much weaker perpendicular to the Ni(CN) 2ˇ4plane than within this plane, the deformation density isconsiderably larger in the perpendicular direction.However, the largest maxima in the individual deforma-tion density around nickel are not found precisely in theplanes deÆned by nickel and the four cyanide ligands orin the perpendicular direction just mentioned, whichillustrates that it is necessary to consider the crystal Æelddue to the whole crystalline environment.
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| 48. |
- Rodrigues, BL, et al.
(author)
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Experimental electron density of urea-phosphoric acid (1/1) at 100 K
- 2001
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In: ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE. - 0108-7681. ; 57, s. 353-358
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Journal article (peer-reviewed)abstract
- The deformation electron density of the urea-phosphoric acid adduct has been studied from 100 K X-ray and neutron diffraction experiments. Data were interpreted according to the Hirshfeld model. The long hydrogen bonds show characteristics of electrostati
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| 49. |
- Smrcok, Lubomir, et al.
(author)
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On hydrogen bonding in 1,6-anhydro-β-D-glucopyranose (levoglucosan): X-ray and neutron diffraction and DFT study
- 2006
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In: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; B62:5, s. 912-918
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Journal article (peer-reviewed)abstract
- 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.
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| 50. |
- Wunschel, Markus, et al.
(author)
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Influence of the molecular structures on the high-pressure and low-temperature phase transitions of plastic crystals
- 2003
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In: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 59:1, s. 60-71
-
Journal article (peer-reviewed)abstract
- The crystal structures of tert-butyl-tris(trimethylsilyl)silane, Si[C(CH3)3]1[Si(CH3)3]3 (Bu1), and di-ferr-butyl-bis(trimethylsilyl)silane, Si[C(CH3) 3]2[Si(CH3)3]2 (Bu2), at room temperature and at 105 K have been determined by X-ray powder diffraction; the high-pressure behavior for pressures between 0 and 5 GPa is reported. The room-temperature structures have cubic Fm3→m symmetry (Z = 4) with a = 13.2645 (2) Å, V= 2333.87 (4) Å3 for Bu1 and a = 12.9673 (1) Å, V = 2180.46 (3) Å3 for Bu2. The molecules are arranged in a cubic close packing (c.c.p.) and exhibit at least 48-fold orientational disorder. Upon cooling both compounds undergo a first-order phase transition at temperatures Tc = 230(5)K (Bu1) and Tc = 250(5)K (Bu2) into monoclinic structures with space group P21/n. The structures at 105 K have a = 17.317 (1), b = 15.598 (1), c = 16.385 (1) Å, γ = 109.477 (4)°, V =4172.7 (8) Å3 and Z = 8 for Bu1 and a = 17.0089(9), b = 15.3159 (8), c = 15.9325 (8) Å, γ = 110.343 (3)°, V= 3891.7 (5) Å3 and Z = 8 for Bu2. The severe disorder of the room-temperature phase is significantly decreased and only a two- or threefold rotational disorder of the molecules remains at 105 K. First-order phase transitions have been observed at pressures of 0.13-0.28 GPa for Bu1 and 0.20-0.24 GPa for Bu2. The high-pressure structures are isostructural to the low-temperature structures. The pressure dependencies of the unit-cell volumes were fitted with Vinet equations of state and the bulk moduli were obtained. At still higher pressures further anomalies in the pressure dependencies of the lattice parameters were observed. These anomalies are explained as additional disorder-order phase transitions.
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