Structural Investigations by Means of Nuclear Magnetism. III. Ammonium Halides

The proton magnetic resonance absorption has been measured in NH4Cl, NH4Br, and NH4I crystal powders from —195°C to room temperature, and for NH4Cl to 200°C. Line width transitions were found at —144, —171, and about —198°C in the chloride, bromide, and iodide, respectively. The N–H distance in the ammonium ion was determined to be 1.035±0.01A from the second moments of the broad, low-temperature absorption lines observed in NH4Cl and NH4Br, with allowance for broadening by modulation effects and narrowing by zero-point torsional oscillations. The relatively narrow room temperature line shapes are consistent with the broadening estimated for inter-NH4+ interactions, the intra-NH4+ interactions averaging to zero over the hindered rotational motions of the NH4+ ions. No appreciable line shape changes were found at the λ temperatures. An electrostatic calculation is made of the potential barriers to rotation of the NH4+ ion giving values comparing favorably with experimental inferences. The results clarify the motions which narrow the absorption line at low temperatures and emphasize the dynamic aspects of the order-disorder process.