Electrostatic model for the rotational potential in ammonium halides

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Abstract
An electrostatic model for the rotational potential of an ammonium tetrahedron in a cubic field is proposed. The rotational potential can be written as a rapidly converging series of kubic rotator functions. The NX4 ion, where X is hydrogen or deuterium, performs small angular motions about the minimum of the potential in both the ordered and disordered states, giving rise to librational energy levels. The anharmonic portion of the energy levels, both from the kinetic and potential energies, accounts for good agreement with infrared absorption data in NH4Cl, ND4Cl, NH4Br, and ND4Br. We use a one parameter fit to these data and then compute the potential barriers for rotations about threefold and fourfold axes of the tetrahedron. The difference between the transition temperatures for ND4Cl and NH4Cl is also explained.