Isotope induced proton ordering in partially deuterated aspirin

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 a nd fully deuterated methyl groups in aspirin are delocalized at low tempera tures due to quantum mechanical tunneling, we provide here direct evidence that in aspirin-CH2D at 15 K the methyl hydrogens are localized, while rand omly distributed over three sites at 300 K. This is the first observation b y diffraction methods of low-temperature isotopic ordering in condensed mat ter.