Study of the rotational behaviour of the ammonium ion in several salts by neutron spectrometry

Abstract

The dynamics of the ammonium ion, particularly its rotational behaviour, has been studied in NH₄Cl, ND₄Cl, NH₄Br, ND₄Br, NH₄I, (NH₄)₂SnCl₆and(NH₄)₂SnBr₆ using the techniques of neutron spectrometry. Neutron energy loss data obtained with a modified beryllium detector spectrometer clearly shows that the ammonium ion is performing torsional oscillations in all these salts at 100°K, the torsional frequencies being 389, 280, 335, 254, 293, 240 and 193 cm^-1 respectively. Using the frequencies, the barrier heights for rotation have been derived. Close examination of the torsional peak in NH₄Cl shows that there is a split which reflects on the anharmonicity of the potential. Energy-gain data at 300°K has been obtained for the undeuterated salts using a rotating crystal spectrometer giving neutrons of incident energy 0.0047 eV and the results confirm those of the energy-loss experiments. The angular distributions of elastically scattered neutrons from the undeuterated salts have also been studied, using a triple axis spectrometer. In NH₄Cl (300°K), NH₄Br (300°K) and NH₄I (220°K) the form factor is found to be Gaussian and the mean square displacements of the proton are consistent with those estimated from the observed frequencies. In NH₄I,(NH₄)₂SnCl₆ and (NH₄)₂SnBr₆ (all at 300°K), the form factor besides being non-Gaussian is much sharper indicating greater rotational freedom for the ammonium ion, in agreement with the inelastic scattering results. The form factor for NH₄I (at 300°K) is compared with the calculations based on free rotation and uniaxial rotation models.