Electron-Diffraction Structural Study of Polymeric Gaseous Hydrogen Fluoride

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Electron diffraction patterns with molecular interference features extending beyond s = 35 Å−1 were obtained for gaseous self-associated HF introduced under its own vapor pressure at −19° and at +22°C through a conventional nozzle system into a 40-kV electron beam. The diffraction patterns and their dependence on temperature are best explained with the hypothesis that the monomer and a puckered, cyclic hexamer were the only appreciable constituents of the scattering vapors. Mean FFF angles in the hexamer were found to be only about 104°, in contrast with the 120° angles reported for the infinite planar zigzag chains in crystalline HF at −125°C. This pucker may simply be a consequence of thermal bending of the extremely flexible ring. Indeed, the experimental radial distribution function is so smeared out by large ring-bending amplitudes that the data cannot distinguish between boatlike and chair conformations. It is likely that the free (HF)6 molecules sweep randomly through both conformations in their thermal undulations. At the two temperatures studied the hydrogen-bonded FF distances were 2.525 and 2.533 Å, with standard errors of 0.003 Å, in comparison with the solid-state result of 2.49 ± 0.01 Å. Corresponding FF amplitudes of vibration were 0.089 and 0.101 Å (±0.003 Å), respectively. Perhaps 70% at −19° and 45% at 22° of the hydrogen bonds were asymmetric with covalent FH distances 0.040 Å longer than those in the monomer molecules. The data suggest, however, that the remainder of the ring protons migrate a substantial distance off-axis to a more symmetrical disposition between the fluorines.