Proton magnetic resonance studies of quantum mechanical tunnelling of ammonium ions in several ammonium salts

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Abstract
Proton magnetic resonance spectra were measured at 1·8 K for the compounds NH4Br, NH4I, NH4SCN, (NH4)2CrO4, and NH4NO3. The observed derivative curves cannot be explained on the basis only of the dipolar interactions between protons in the NH4 + ion held rigidly in the crystal lattices. We considered the effect of quantum mechanical tunnelling on the rotation of an NH4 + ion about its C3-axis and calculated the theoretical lineshapes for various assumed values of the tunnelling splitting constants. Comparison between experiment and theory is quite satisfactory, and the tunnelling splitting constants, Δ, in the librational ground state of the ammonium ion in these salts are determined. It is found that there is a linear relation between the Δ values and the potential barrier to reorientation.