Magic-angle spinning, dipolar decoupling and cross polarisation

4 Magic-angle spinning, dipolar decoupling and cross polarisation 

The effects of the anisotropy of the chemical shift and of the spin-spin interaetions in the spectra of solids can be eliminated by the use of a teehnique ealled magic-angle spinning (MAS). Equation (2.9) shows that the frequeney of the transition for a particular spin S2 interacting with another spin Si can be written 

Although the above discussion shows only that magic-angle spinning can remove the elfects of spin-spin interactions, it can be shown that it also removes the elfects of the anisotropy of the shielding that gives rise to chemical shifts. 

Another method that is used to remove the spin-spin coupling in experiments on NMR is called dipolar decoupling (DD). The natural abundance of nuclei is very low, so the spins of such nuclei are well 

In any pulsed NMR experiment the pulse sequenee is repeated many times and the results are added to increase the signal-to-noise ratio. The repetition must not take plaee until thermal equilibrium has been restored, whieh is usually regarded as implying a clear time of about 3 Ti between sequenees. The value of T for is often very long, so that the simple DD 

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