Origin of the low temperature excess heat capacity of isotopically substituted acetylsalicylic acid

Abstract The low temperature heat capacities of single crystals of fully protonated acetylsalicylic acid, measured between 2 K  T 3 , CH 2 D and 13 CH 3 . For the partially methyl-deuterated crystal (CH 2 D) a significant excess heat capacity was found below T T  ≈ 14 K. The thermodynamic data are explained on the basis of a Schottky model using results obtained in earlier NMR and neutron spectroscopic experiments. In contrast, the excess heat capacity of the fully deuterated compound can be explained by a change of the phonon density of states.