Fluorinated Bridge Structures

Explanation of the dramatic effects on physical properties of perfluorinated bridges in contrast with hydrocarbon-based bridges probably lies in the larger van der Waals radius (147 pm compared with 120 pm for hydrogen, i.e. fluorine is 23% larger than hydrogen) and the low polarizability of fluorine. Molecular 

If pushed to an extreme the perfluorinated bridge concept can be used to obtain liquid crystals with no cyclic moieties in their mesogenic core structure [4-8]. Since the beginning of the 1980s it has been known that semi-fluorinated n-alkanes, so-called diblock compounds, F(CF2) (CH2) H, form smectic phases [49], because of microphase separation as a result of separate, layer-like aggregation of the hydrocarbon and fluorocarbon moieties. Nevertheless, if introduced into a nematic host mixture, even small quantities of these diblocks cause gelation of the mixture. Their solubility is also limited to a few percent by weight. 

---

TeF4 and SbF5 form a 1 1 adduct (16). The Raman spectrum of this complex in the molten state has been recorded (65). It was not possible, however, to decide between the most plausible alternatives, i.e., ionization into the ions (TeF3)+ and (SbF ), or the formation of a fluorine-bridged structure F3TeFSbF5. 

The polymeric structure of the hquid SbF5 has been established by 19F NMR spectroscopy38 and is shown to have the frameworks depicted in Figure 2.2. A cis-fluorine bridged structure is found in which each antimony atom is surrounded by six fluorine atoms in an octahedral arrangement. 

Conductometry and F NMR spectroscopy have been used to study the systems of Sep4 with BF3 in FiF. " Fluorine-bridged structures such as in SeF4-NbF5 and SeF4-2NbF5 with polymeric molecules and ions are present. The simplest equilibrium of this type involves the dimer (SeF4 BF3 )2, which may be either linear or cyclic. " ... 

The formation of [BrF2]+ and [BrF4] -ions may also be explained by assuming a fluorine bridged structure in the liquid. 

---