Ammonia tunnelling in Hofmann clathrates: consistent description by internal and centre of mass quantum rotation

The ammonia groups of the NiNiBz(d6) Hofmann clathrate perform almost free tunnelling rotation in a high symmetry environment. We show that a coherent internal and centre of mass rotation describe the inelastic spectra perfectly. The parameters of the potential surface are consistent with usual strengths of intermolecular interactions. No arbitrary rescaling of the rotational constant as required by an analysis within the model of single particle rotation is necessary.