Nuclear multiple-pulse narrowing

For powdered or amorphous samples, in order to avoid line broadening due to chemical shift anisotropy, MAS is employed (which also averages various hetero-nuclear dipolar couplings). Gerstein and co-workers [25] were the first to combine a multiple pulse homonuclear line-narrowing technique with MAS, and introduced the acronym CRAMPS for Combined Rotation And Multiple-Pulse Spectroscopy [4,25]. 

Instead of averaging away the anisotropic nuclear spin interactions by MAS or multiple pulse sequences, it is possible to take advantage of the anisotropy of these interactions, provided that macro-scopically oriented samples are available. This kind of static solid state NMR approach is entirely different from the experiments discussed above. It can lead to highly resolved H and spectra with narrow lines, whose position carries information about... 

Fig. 1. The basic line-narrowing m.p. sequence and definition of the pulse spacing t, the pulsewidth t, and the cycle time The pulses embraced by t constitute a cycle because they impose a zero net rotation on the nuclear magnetization. The cycle is repeated over and over and the NMR signal is sampled at integer multiples of t. ...

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