The crystal structure and evidence of the phase transition in D-amphetamine sulfate, as studied by X-ray crystallography, DSC and NMR spectroscopy

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 SO42- 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(-1), whereas for the ammonium group it is 27.29 kJ mol(-1).