Nuclear magnetic resonance study of ionic motions in ammonium bifluoride

Publisher Website
Google Scholar
Abstract
Proton and fluorine second moments in powdered NH4HF2 have been measured over the temperature range 77 <T2− and NH4+ reorientations. Comparison of these values with the experimentally observed plateau’s shows that the lattice is rigid at 77 K and that the first motion affecting the second moments is the isotropic reorientation of the NH4+ ion. At higher temperatures one of the nonequivalent HF2− ions starts executing 180° flips followed by the other HF2− ion with a similar reorientation. Proton line shapes for powdered NH4HF2 and KHF2 have been calculated by substituting for each fine structure component of an isolated HF2− ion a Gaussian line weighed by isotropic angular distribution functions. Good agreement between experimental and calculated line shapes have been obtained. Proton and fluorine spin‐lattice relaxation times in the laboratory frame, T1, and the rotating frame, T1ρ, have been measured over the temperature range 167<T4+ ion is the dominant relaxation mechanism. The activation energy is 6.0±0.2 kcal/mole. At higher temperatures the unixial 180° reorientations of the two nonequivalent HF2− ions modulate the interionic H–F and F–F dipole–dipole interactions. Activation energies of 5.9±1 and 15.5±2 kcal/mole have been obtained for the reorientations of the HF2− ions. These values are discussed in terms of the environments of the nonequivalent HF2− ions. The influence of the reorientations on the strength of the hydrogen bonds is discussed and in terms thereof an explanation is offered for the absence of the phase transition observed in other bifluoride compounds.

This publication has 35 references indexed in Scilit: