The derivation of the rotational potential function from atom–atom potentials. II. Ammonium–fluorine compounds

The rotational potential functions of a number of ammonium–fluorine compounds have been derived from atom–atom potentials of the Buckingham (exp-6) type. The potential functions for the ammonium pervoskites NH4ZnF3, NH4MnF3, NH4MgF3, and NH4CoF3 with Oh site symmetry for the ammonium ion are derived. The hexagonal ammonium hexafluorides (NH4)2SiF6, (NH4)2GeF6, and (NH4)2TiF6 are analyzed including the J=7 term in the potential function. Of all the compounds studied, the potential function converges most slowly for these compounds. The effect of the J=6 and J=7 terms in the potential function are discussed in terms of their effect on the librational and tunneling frequencies. The J=7 and J=8 terms are included in the potential function of cubic (NH4)2SiF6. Three ammonium compounds with strong hydrogen bonds are studied, NH4F, NH4SnF3, and NH4HF2. The computed librational frequencies of NH4HF2 are compared with the librational frequency derived from the heat capacity. It is shown that the librational frequencies of the tetrahedral ions NH+4, BH−4, AlH−4, and GaH−4 derived from heat capacity results are in good agreement with the librational frequencies determined by spectroscopic studies. The potential functions for NH4AlF4 and NH4CrF4 are also derived. For all the compounds studied, the activation energies, librational frequencies, and when applicable, the tunneling frequencies are computed for the ammonium ion.