Magnetic Resonance Line Shapes in Solids: The Rotating Three-Spin Group

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Abstract
The density matrix formalism of spin‐exchange theory has been applied to the problem of a rigid equilateral triangle of spin‐1 / 2 nuclei undergoing random hindered rotation about the normal to the plane of the triangle. The resulting equations were solved numerically, and spectra are presented for a variety of rates of rotation for both a single orientation and a sum over orientations. These results are compared with recent calculations for coherent rotation (tunneling) of methyl groups, and the experimental situation is briefly discussed.