X-Ray crystallography, DSC and NMR spectroscopy methods have been used to study the phase transition of D-amphetamine sulfate. The phase transition temperature occurs at about 325 K, and belongs to the discontinuous type with a large temperature hysteresis. A change in the space group is observed from monoclinic P2 1 in the low temperature phase to monoclinic C2 in the high temperature phase. The transformation from phase I to phase II is a result of the ordering of the SO 4 2À groups, allowing a T-shape interaction between D-amphetamine molecules. Proton relaxation rate data obtained for the studied compound have been analysed by assuming two dynamically-inequivalent methyl groups below the phase transition and only one type of ammonium group above the phase transition. The activation energies for the C 3 reorientation of the methyl groups have been determined to be 7.31 and 11.89 kJ mol À1 , whereas for the ammonium group it is 27.29 kJ mol À1 .
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