The spectra of KHF2, KH2F3, NaH2F3, and of liquid and solid HF were measured between 1200 and 30 cmmdash;1by the inelastic scattering of ``cold neutrons'' (0.005 eV). The spectrum of KHF2is in agreement with previous infrared measurements and shows a peak at 147 meV corresponding to a deformation frequency ngr;2of the Fsngbnd;Hmiddot;middot;middot;F group and a broad peak at 13 meV due to a libration of this group. The observed spectra for KH2F3, NaH2F3, and solid and liquid HF were compared with that for KHF2considering the known results than the Fsngbnd;F distance increases, the position of the H atom between the two fluorines becomes more asymmetrical, and the strength of the hydrogen bond decreases in going from the HF2mdash;ion to H2F3mdash;ion to HF. In this comparison, the known structure of the H2F3mdash;ion is considered as being intermediate between that of HF2mdash;and the zigzag hydrogen bonded chains in solid HF. As a result, the lines observed at 112 and 67 meV in the H2F3mdash;spectra and in HF, respectively, are associated with the deformation frequency ngr;2of the Hsngbnd;Fsngbnd;H group in these compounds. In addition, the spectra of H2F3mdash;ions show a peak at a 15 meV attributed to a libration of the group. Both H2F3mdash;and HF show peaks at 8 and 7 meV, respectively, attributed to a rotational motion of one Fsngbnd;Hmiddot;middot;middot;F unit around the Fsngbnd;F bond. The observed spectrum of HF liquid, showing a high degree of association, is also discussed with respect to these motions.
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