FHF anion

On page 302 it was stated that although the FHF- anion was usually symmetrical, occasionally ii was found to be unsymmetrical in the solid. Whul physical methods could you use to detect unsymmetrical FHF- ions in a solid-7... 

FIGURE 1.7 The four electron-three center (4e-3c) hypervalent bonding model for [FHF] anion in which the fluoride ions are considered ligands for the central H+. The bonding and nonbonding orbitals are occupied and the antibonding orbital left vacant. 

Limbach et 1 performed DFT calculations on. I(N, 11) and 2hJ(N,N) couplings in the anion [C = N-L-N = C] (L = H, D) and in the cyclic bonded formamide dimer (HCNHNH2)2 to study how such couplings depend on the geometry of the hydrogen bond. To describe such a relationship they employed the valence bond order model and compared the NHN and FHF- hydrogen bonded systems. 

The best, and longest known, case is that of the bifluoride anion in NaHF2 and KHF2. In each the FHF ion lies across a centre of symmetry, and careful neutron-diffraction study finds the proton at the centre. However, neutron diffraction also shows, or appears to show, that the proton vibrates with a particularly large amplitude along the F H F bond. So there is a possibility that this may be partly due to disorder — qualitatively like that in ice (see also section 12), but now with the alternative proton sites not more than 0.1 A from the centre. The neutron-diffraction measurements can be equally well explained by two models the proton vibrates... 

The compounds differ in their Pt—Pt intrachain separations, electrical conductivities, degree of partial oxidation of the platinum atom, and metallic color. These major differences occur even though two pairs of these four compounds contain the same anion, (FHF)-, and cation, Rb+ or Cs+. This provides the unique opportunity to explore only the effects of structural, not elemental, changes on the anisotropic electrical behavior of POTCP salts. 

Spectra of the linear anion FHF in the gas phase were obtained by diode laser spectroscopy P = 583.6539(13) cm, uj = 1286.0284(22) cm , and 1 3 = 1331.1502(7) cm . From the resulting rotational constants an equilibrium F-F intemuclear distance of 227.771(7) pm was calculated (Kawaguchi and Hirota, 1987). In the class of binary hydrides, boron compounds are especially interesting. A recent FTIR investigation revealed the out-of-plane vibration of unstable BH3 1140.8757(39) cm (Kawaguchi et al., 1987). These data confirm the results of a recent matrix study which showed an absorption at 1132 cnr for the same vibration (Kaldor and Porter, 1971). A detailed... 

Kawaguchi, K, and Hirota, E., Infrared diode laser study of the hydrogen hifluoride anion FHF- andFDF-, J. Chem. Phys. 84, 2953-2960 (1986). 

This type of hydrogen bonding is termed symmetric in contrast to the more common unsymmetric form. Symmetrical hydrogen bonds form in only the strongest bonded systems. These are frequently anionic like FHF and the carboxylales mentioned above. An example of a strong, symmetric hydrogen bond in a cation is the bis(A -nitrosopyrrolidine)hydrogen cation (Fig. 8.5). >2... 

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