| 1. |
- Abrahamsson, Jonas, 1954, et al.
(författare)
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The crystal structure of O-ethyl-S-(11-carboxyundecyl)dithiocarbonate
- 1976
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Ingår i: Acta Crystallographica Section B: Structural Science. - 0108-7681. ; 32:10, s. 2745-2749
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Bergmann, Justin, et al.
(författare)
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Can the results of quantum refinement be improved with a continuum-solvation model?
- 2021
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Ingår i: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 77:6, s. 906-918
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Tidskriftsartikel (refereegranskat)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.
(författare)
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High-pressure transformations of NbO2F
- 2000
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Ingår i: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 56:2, s. 189-196
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Vacancy Ordering Effects in AlB2-type ErGe2-x(0.4 < x < or = 0.5)
- 2008
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 64:3, s. 272-280
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Topological studies of three related metal-organic frameworks of GdIII and 5-nitroisophthalate
- 2012
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Ingår i: Acta Crystallographica Section B: Structural Science. - : International Union of Crystallography (IUCr). - 0108-7681 .- 1600-5740. ; 68:5, s. 528-535
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Tidskriftsartikel (refereegranskat)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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| 9. |
- Dinnebier, R., et al.
(författare)
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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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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 62, s. 467-473
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Proton-transfer dynamics in the (HCO3-)(2) dimer of KHCO3 from Car-Parrinello and path-integrals molecular dynamics calculations
- 2010
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 66, s. 222-228
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Tidskriftsartikel (refereegranskat)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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