S. P. Gabuda

Shielding anisotropy of H-bonded protons in Cs2GeF6·4HF

Abstract The chemical shift anisotropy of H-bonded protons in polycrystalline Cs2GeF6·4HF was determined by the analysis of NMR continuous wave spectra narrowed by the HF molecular diffusion in the crystal lattice. High resolution of spectral components was achieved due to “selective averaging” of the intermolecular dipole interaction. The derived value of ΔσH = 39 ± 7 ppm is close to that for bifluoride ion in KHF2. For 19F ΔσF = 86 ± 4 ppm was determined.

Molecular diffusion and spatial symmetry of the arrangement of the HF molecules in crystalline solvates

ConclusionIf diffusion brings about the successive occupation, by the HF molecule, of all nonequivalent positions in the crystal, the intramolecular interaction of the nuclei, averaged by the motion, is a characteristic of the crystal as a whole. In accordance with Neumanns principle, the symmetry of the local field tensor should include the elements of symmetry of the point group of the crystal. From the nonaxial character of the tensor (η≠0), detected experimentally for the crystalline solvates studied, it follows that the symmetry of the three crystals examined belongs to one of the lower classes and cannot be higher than orthorhombic.From the preservation of one of the axes of the tensor with the development of the diffusion process, and published data [7], it follows that all the HF molecules lie in parallel (possibly slightly “corrugated”) planes.The symmetry of the arrangement of the HF molecules in the layers does not have axes of order higher than 2. Otherwise, on diffusion, the local field tensor would have remained axial even in a crystal of low symmetry. If one structure contains not more than two nonequivalent positions for HF molecules, the angle γ between the directions of the H−F vectors can be determined. The value of γ is related to the value of the asymmetry parameter by the expression

Method of individual broadening functions in the NMR structural analysis of the hydrogen bond: The asymmetric hydrogen fluoride ion in N2H6F2·2HF

\gamma = arccos (\eta /3).

Molecular and electronic structure of molybdenum pentafluoride

From this we obtain γ=106° for Cs2GeF6·4HF and 109° for Cs2SnF6·4HF. For comparison it may be noted that for solid hydrogen fluoride, γ=119°.Data on the temperatures of activation of the diffusional mobility were used to find the activation energies of diffusion of HF: 9.7 kcal/mole in Cs2GeF6·4HF, 10.8 kcal/mole in Cs2SnF6·4HF, and 11.2 kcal/mole for the diffusion of HF molecules in K2SnF6·4HF.