Proton magnetic resonance study of classical reorientations and tunneling in LiNH4SO4

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Abstract
Nuclear magnetic resonance studies have been made of the protons and lithiums in LiNH4SO4 at temperatures below 285 K. The proton line shape at 4.2 K has shown that tunneling rotation of the NH+4 ion around one of its C2 axes is favored over the others. Additional tunneling around threefold axes is also present. The ground torsional level structure of the NH+4 ion has been determined. The classical reorientations of the two nonequivalent NH+4 groups can be described as simultaneous two and threefold reorientations with estimated activation energies ranging from 1.0 to 1.6 kcal/mol. Slight differences in activation energies have been observed during cooling and heating cycles below 133 K. The 7Li relaxation time is determined to a large extent by the reorientations of the NH+4 ions. Evidence has been found that the phase transition at 133 K is accompanied by a slight rearrangement of the Li–O–S skeleton.