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1.  Electrostatic and aspheric influence of the fluoro-substitution of 4-bromodiphenyl ether (PBDE 3) 
Accurate structure determinations by X-ray crystal analysis and computation using semi-empirical self-consistent field molecular orbital calculations are described and compared for 4-bromodiphenyl ether (PBDE 3), the 13C6-isotopic labeled PBDE 3 (13C6-PBDE 3) and its five corresponding monofluorinated analogues (F-PBDEs 3): 2-fluoro-4-bromodiphenyl ether (F-PBDE 3-2F), 2′-fluoro-4-bromodiphenyl ether (F-PBDE 3-2′F), 3-fluoro-4-bromodiphenyl ether (F-PBDE 3-3F), 3′-fluoro-4-bromodiphenyl ether (F-PBDE 3-3′F) and 4′-fluoro-4-bromodiphenyl ether (F-PBDE 3-4′F). The synthesis and full characterization by means of 1H, 13C, 19F nuclear magnetic resonance spectroscopy and mass spectrometry of the F-PBDEs 3 are presented for the first time. Intermolecular interactions for PBDE 3 and the F-PBDEs 3 isomers were dominated by weak C-H(F,Br) ····π and C-H····F interactions. The bond lengths of C-F varied between 1.347(2) Å and 1.362(2) Å, C4-Br between 1.880(3) Å and 1.904(19) Å. Both correlated with electron-density differences as determined by 13C shifts, but not with the strength of C-F couplings. The interior ring angles at ipso-fluoro-substitution increased to 121.95° due to hyperconjugation by p-π-orbital overlapping, a phenomenon that was also computed. An attraction between the vicinal fluoro-and bromo-substituents was not determined, as seen in fluoro-substituted chlorobiphenyls. The torsion angles measured and computed for the series of PBDE 3 and F-PBDEs 3 differed strongly from each other. Since the ether linkage (an average of 2.76 Å) provides more distance and the bonds are flexible up to a certain range, the influence of a fluoro-substituent is only detectable in PBDEs with high ortho-substitution. A concordance of computed and measured torsion angles can be observed with increasing size and/or grade of substitution comparing mono- to tetra- fluoro-, chloro-, bromo- and methyl-substitutions in the ortho-positions of diphenyl ether. Differences between computational versus measured data demonstrates a strong need to evaluate the results against independent techniques to conclude structure receptor activity relationships of PBDEs. Any discussion of the Ah or other biological receptor activity of certain PBDEs should take this in consideration. For the first time a complete overview of known and hypothetical biological activities of PBDEs is presented.
doi:10.1107/S0108768107067079
PMCID: PMC3120100  PMID: 18204217
2.  Orientational disorder and phase transitions in crystals of (NH4)2NbOF5  
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.
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.
doi:10.1107/S0108768108021289
PMCID: PMC2553555  PMID: 18799840
ammonium oxopentafluoroniobate; distorted octahedra; dynamic orientational disorder; phase transitions; twinning; vibrational spectra

Results 1-2 (2)