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Dipolar Powder Patterns

The observed NMR spectrum of a solid powder (no MAS, no molecular motion) is the summed signal from crystallites over all possible orientations relative to the applied field Bo. In a system of two spins that are strongly dipolar coupled (relative to J, shift anisotropies, quadrupolar couplings) one will observe a typical dipolar powder pattern as shown below.

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By changing the orientation relative to Bo (set to point along +z) one can visualize how an individual crystallite contributes to the powder pattern. The vertical lines above represent the observed transitions at a particular orientation. All orientations contribute to the observed powder spectrum. All transitions from spin(s) of type specified by Spin 1 are detected. Note what happens at the "magic angle", where q=54.7. Also note how rotations about the +z axis don't affect the spectrum because dipolar interactions are symmetric.


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