Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
Pt 6
pubmed:dateCreated
2003-10-28
pubmed:abstractText
Difficulties encountered in modelling the scattering of fluorine in organic compounds have been investigated through refinements of accurate X-ray and neutron diffraction data measured on tetrafluoroterephthalonitrile, TFT, at 122.4 K. Multipole refinements led to a highly contracted octopole on fluorine. The subsequent analysis revealed that fluorine does not possess a valence octopole but exhibits anharmonic thermal motion that can be modelled by the octopole multipole parameters. The scattering contribution from the octopole shows the same cubic dependence in the scattering vector as the Gram-Charlier expansion of the nuclear displacements to third order. The analysis also showed that refinement of third-order Gram-Charlier coefficients on fluorine requires data to at least 0.93 A(-1) resolution in sinthetas/lambda. The X-ray data extending to 1.27 A(-1) were of sufficient resolution to include third-order Gram-Charlier coefficients for N, F and the cyano C atoms in the refinement, whereas the neutron data only enabled refinement of the third-order Gram-Charlier coefficients for nitrogen. The refinements of the neutron and X-ray diffraction data yielded identical atomic displacement parameters for all the atoms. Though inclusion of anharmonic motion for N and F atoms provides the best model, it does not affect the crystal electron density, and all intramolecular bond critical points have identical features. Application of the anharmonic model, however, leads to small differences in the intermolecular interactions, which is illustrated by the electrostatic potential adjacent to the N atom. The characteristics of the C-F bond were elucidated by the topological analysis of the crystal electron density, which also supported the proposed quinonoid structure of the benzene ring.
pubmed:language
eng
pubmed:journal
pubmed:status
PubMed-not-MEDLINE
pubmed:month
Nov
pubmed:issn
0108-7673
pubmed:author
pubmed:issnType
Print
pubmed:volume
59
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
540-50
pubmed:year
2003
pubmed:articleTitle
Simultaneous variation of multipole parameters and Gram-Charlier coefficients in a charge-density study of tetrafluoroterephthalonitrile based on X-ray and neutron data.
pubmed:affiliation
Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
pubmed:publicationType
Journal Article