The tunneling frequencies of the ammonium ion in its compounds

The tunneling frequencies of the ammonium ion in a tetrahedral crystalline field have been computed up to a librational frequency of 320 cm−1. The tunneling frequencies determined from analyzing NMR results are used to compute the librational frequencies and barriers to rotation of a number of ammonium compounds. The librational frequencies and barriers to rotation determined from the tunneling frequencies are in reasonable agreement with the experimental results for the cubic compounds NH4I, NH4Br, and (NH4)2PbCl6. The results are less satisfactory for cubic (NH4)2SnCl6. The librational frequencies for noncubic compounds have been determined from the tunneling frequencies by assuming tetrahedral site symmetry for the ammonium ion. For the noncubic compounds NH4SCN, (NH4)2Cd(SO4)3, NH4NO3, and (NH4)2CrO4, the librational frequencies determined from the tunneling frequencies are lower than those observed experimentally, while the barriers to rotation determined from the tunneling frequencies are higher than the experimentally determined activation energies. The tunneling frequency for NH4ClO4 is discussed.