NMR observation of spin isomerism of the ammonium group protons

The shape of the 40 MHz proton magnetic resonance absorption line of powdered (NH₄)₂Ce(NO₃)₆ at liquid helium temperature is explained by proton spin isomerism as suggested by Tomita for solid methane. In the studied solid, the ammonium ion reorientation is hindered by a very weak crystal field. The presence of a meta isomer (I=2) and a small percentage of ortho isomer (I=1) spin states is inferred from the line shape at 4°K, indicating that the majority of the ions are in the rotational ground state (L=0) and about 10 per cent of the ions are in the L=1 rotational state, which is about 8°K above the ground state.