Abstract
The following activation energies for NH4+ reorientation have been derived from proton spin-lattice relaxation T1 and T1 rho : Ea=9.4 kcal mol-1 in NH4F, Ea=8.0 kcal mol-1 in NH4CH3COO, Ea=6.8 kcal mol-1 in (NH4)2(COO)2.H2O, Ea=6.1 kcal mol-1 in (NH4)2CO3, Ea=3.7 kcal mol-1 in (NH4)2CrO4, and Ea=2.6 kcal mol-1 in (NH4)2S2O3. The following frequencies for NH4+ torsional oscillations have been found from inelastic neutron scattering: nu v=450 cm-1 for (NH4)2(COO)2.H2O, nu v=330 cm-1 for (NH4)2S2O3 and nu v=250 cm-1 for (NH4)2Ce(NO3)6. The relation between Ea and nu v has been calculated for a cubic potential with an improved zero-point energy correction. Comparisons between Ea and nu v for these and other ammonium salts show that the reorientation is by 90 degrees around twofold axes in most high-barrier salts.
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