Proton Magnetic Resonance in an Ammonium Chloride Single Crystal

摘要

The nuclear resonance absorption spectrum has been calculated theoretically for a tetrahedral 4hyphen;spin system, withIof half;, for orientations in which the external magnetic field coincides with a twohyphen; or threehyphen;fold symmetry axis of the tetrahedron or with a tetrahedral edge. Comparison of the theoretical results with the proton resonance observed at mdash;195deg;C in a single crystal of ammonium chloride demonstrates that the NH4+ion is oriented with the Nsngbnd;H bonds pointed essentially towards corners of the unit cube. An Nsngbnd;H distance of 1.032plusmn;0.005A is determined from the lowhyphen;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 mdash;NH4+interactions, with intra mdash;NH4+interactions averaged to zero by hindered rotational motions; free rotation of the ammonium ions does not occur. A smalldecreasewith temperature of the second moment of the resonance line at the lgr;hyphen;point agrees with the orderhyphen;disorder model for the transition. The motion of the NH4+ion is discussed.