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  • 2000-2004
  • 1990-1994  (2)
  • 1994  (2)
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  • 2000-2004
  • 1990-1994  (2)
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  • 1
    Electronic Resource
    Electronic Resource
    Symmetry old and new: Some recent examples of the role of symmetry in X-ray analysis (1994)
    Springer
    Journal of chemical crystallography 24 (1994), S. 51-60 
    Details
    ISSN: 1572-8854
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Two recent examples in X-ray structure analysis at opposite ends of the molecular weight range, in which very careful consideration of space group and diffraction symmetry was necessary to resolve all the associated problems, are described. Case 1 involves the structures of (1) transdiBromo (1,4,8,11-tetraazacyclotetradecane) chromium (III) bromide and (2) the corresponding trans-bromo-chloro compound. Both crystallise in space groupP42/m with unit cell dimensions sufficiently close to suggest that the structures are isomorphous. It is shown however that the structures are not superimposable or even related by simple rotation or inversion, but that it is necessary to apply a hypothetical glide operation in order to bring the structures into coincidence. The organic moieties are thus structural enantiomorphs in spite of existing in a centrosymmetric space group. Case 2 involves studies on a co-crystallised derivative of the enzyme RNase with deoxycytidylyl-3′–5′-deoxyguanosine (dCpdG). Preliminary X-ray precession photographs, from low quality crystals, indicated an orthogonal C-centred unit cell, but were unable to define the true diffraction symmetry, which wasnot orthorhombic. The true symmetry was discovered only after solving the structure in a second (erroneous) cell and careful inspection of the lattice and diffraction symmetry from the measured intensity values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01665345
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Structure, absolute configuration, and conformation of a fused complex cyclic carbohydrate from a cameroon plant extract (CAMT2) (1994)
    Springer
    Journal of chemical crystallography 24 (1994), S. 775-781 
    Details
    ISSN: 1572-8854
    Keywords: Folk Medicine ; Limonoid ; Antimalarial ; Immunomodulator
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The crystal and molecular structure of the natural product C29H38O9, an extract from a Cameroonian plant, has been determined by direct methods. Crystals are orthorhombic, P212121,a=24.571(4),b=16.398(3),c=13.778(4) Å,Z-8,D c=1.270 mg/m3. The final R-factor for 2892 reflections withI〉2σ(I) is 0.0531. The two molecules forming the asymmetric unit have very similar molecular geometry. each molecule comprising a nucleus of six fused rings B to G: where B=pyranyl, C=pyranyl, D=furanyl, E=cyclohexene, F=pyranyl, G=cyclohexyl. Ring B also has a furanyl side-group (ring A). The ring conformations in both molecules are: A (1/2-chair/envelope); B(envelope); C(envelope); D(1/2-chair); E(1/2-chair); F(envelope); G(chair). In the cyclohexene ring E the substitution pattern is 2-hydroxy 3-methyl 5-dimethyl. The two molecules in the asymmetric unit are linked by two OH...O hydrogen bonds to form the crystal structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01668240
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
    Fulltext
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