An analysis of the NMR line shapes of the ammonium ion undergoing composite tunneling and reorientational motions at low temperatures

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Abstract
An analysis is made of the NMR line shapes of the rotationally tunneling ammonium (NH4+) ion which also undergoes rapid reorientataional jumps around its C3 or C2 symmetry axis. The proposed composite motion, believed to be brought about by the coupling of rotor states of the ion with lattice phonons, leads to line narrowing and offers an interpretation of experimental line shape and second moment data not explained previously. In particular, the proton resonance line shapes of (NH4)2SnCl6, NH4BF4, NH4ClO4, (NH4)2GeF6, (NH4)2PbCl6, and (NH4)2PtCl6 at liquid helium temperatures are discussed. The analysis suggests that proton magnetic resonance line shape measurements on single crystals may provide an assessment on the state of ordering of the tunneling and reorienting NH4+ ions.