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

摘要

The proton magnetic resonance absorption has been measured in NH4Cl, NH4Br, and NH4I crystal powders from mdash;195deg;C to room temperature, and for NH4Cl to 200deg;C. Line width transitions were found at mdash;144, mdash;171, and about mdash;198deg;C in the chloride, bromide, and iodide, respectively. The Nsngbnd;H distance in the ammonium ion was determined to be 1.035plusmn;0.01A from the second moments of the broad, lowhyphen;temperature absorption lines observed in NH4Cl and NH4Br, with allowance for broadening by modulation effects and narrowing by zerohyphen;point torsional oscillations. The relatively narrow room temperature line shapes are consistent with the broadening estimated for interhyphen;NH4+interactions, the intrahyphen;NH4+interactions averaging to zero over the hindered rotational motions of the NH4+ions. No appreciable line shape changes were found at the lgr; 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 orderhyphen;disorder process.