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Zusammenfassung

We report the nuclear density distribution of partially deuterated aspirin, C8H5O4-CH2D, at 300 and 15 K, as determined by neutron diffraction coupled with maximum entropy method image reconstruction. While fully protonated and fully deuterated methyl groups in aspirin are delocalized at low temperatures due to quantum mechanical tunneling, we provide here direct evidence that in aspirin-CH2D at 15 K the methyl hydrogens are localized, while randomly distributed over three sites at 300 K. This is the first observation by diffraction methods of low-temperature isotopic ordering in condensed matter