1H Nuclear magnetic resonance studies on cationic reorientation and translational self-diffusion in two solid phases, including a new high-temperature phase of methylammonium sulphate

The temperature dependences of the proton spin–lattice and spin–spin relaxation times, and also the second moment of the 1H n.m.r. absorptions of (CH3NH3)2SO4 and its partially deuterated analogue, have been observed in a wide temperature range, including a new high-temperature solid phase obtainable above 428 K. X-Ray powder patterns obtained at ca. 450 K revealed that the crystal in this phase has a hexagonal structure isomorphous with the α-form of K2SO4. From 1H n.m.r. experiments it was found that the cations in the high-temperature phase undergo overall reorientation as well as translational self-diffusion. The presence of these motions suggests that this new phase is in a state resembling the plastic phase of molecular crystals. In the low-temperature phase a reorientation of the CH3 and NH+3 groups of the cation about the C—N bond axis was observed. The motional parameters for these motional modes were evaluated.