Structure of manganese diacetate tetrahydrate and low-temperature methyl-group dynamics

We have determined the crystal structure of manganese(II) diacetate tetrahy drate at 300 and 14 K by single-crystal neutron diffraction. Proton density distributions for each of the three crystallographically distinct methyl g roups have been calculated by Fourier difference. At room temperature the o bserved densities are those of quasi-free rotors. At low temperature rather well localized protons are observed. Inelastic neutron scattering measurem ents performed with single crystals allow us to assign each of the three tu nnelling lines to a particular crystal site. Classical molecular dynamics s imulations give density distributions in qualitative agreement with the obs ervations. With quantum mechanics proton distributions can be represented w ith rotational wavefunctions convoluted with static distributions of librat ional coordinates. The effective rotational potentials are temperature depe ndent.