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 NH₃^- groups of the cation about the C-N bond axis was observed. The motional parameters for these motional modes were evaluated.