Interionic potential energy studied by far-infrared spectroscopy

Ionic potential models for tetra-alkylammonium halides are proposed, the repulsive energy being evaluated from crystal properties. By assuming a CsCl type structure, such potentials give a qualitative agreement with far-infrared (FIR) spectroscopic data (lattice bands and ion-pair vibration in solution) for a series of salts. For Pr 4NBr, the true structure is known and a more realistic potential has been elaborated. It is shown that the ionic potential is largely determined by the number of first neighbours and the shortest anion-cation distance in the solid. A possible way to estimate the repulsive energy for any R 4N X salt is suggested by using ion-pair dipole moment and vibration frequency data.