Rotational dynamics of the NH+4 tetrahedron in NH+4Cl−

Abstract

The ammonium tetrahedron in NH⁺₄Cl⁻ experiences an orientational potential of considerable strength. Changes of orientation occur through thermally activated jumps across the potential barriers (rotational jumps). Whereas the experimentally determined overall jump rate is in general agreement with deductions from model potentials, the detailed jump geometry is not. The experimental ratio of jump rates ν₃/ν₄ around the threefold and fourfold symmetry axes of the molecule differs by roughly a factor 40 from its theoretical value. We have performed a detailed numerical study of the rotational Langevin equation in a theoretical model potential. Following the rotational path of an individual ammonium tetrahedron, it turns out that multiple jumps are crucial for the jump dynamics and that their inclusion can explain the experimental results. The probability of multiple jumps depends on the friction which couples a single particle to the many body system. The friction constant has been determined from the comparison with experiment.