Proton Magnetic Resonance in an Ammonium Chloride Single Crystal

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Abstract
The nuclear resonance absorption spectrum has been calculated theoretically for a tetrahedral 4-spin system, with I of ½, for orientations in which the external magnetic field coincides with a two- or three-fold symmetry axis of the tetrahedron or with a tetrahedral edge. Comparison of the theoretical results with the proton resonance observed at —195°C in a single crystal of ammonium chloride demonstrates that the NH4+ ion is oriented with the N–H bonds pointed essentially towards corners of the unit cube. An N–H distance of 1.032±0.005A is determined from the low-temperature second moments of the resonance line with the applied magnetic field in the (1,0,0) and (1,1,0) directions. At room temperature, the anisotropy of the resonance results from inter —NH4+ interactions, with intra —NH4+ interactions averaged to zero by hindered rotational motions; free rotation of the ammonium ions does not occur. A small decrease with temperature of the second moment of the resonance line at the λ-point agrees with the order-disorder model for the transition. The motion of the NH4+ ion is discussed.