Heterogeneous samples and MAS

Despite the fact that most synthetic organic chemistry takes place in solution, interest remains in solid-phase organic synthesis protocols for the production of combinatorial libraries of new molecules [92, 93]. A challenging factor in this approach is the analysis of the newly synthesised materials for which, ideally, direct structural analysis of the material would be carried out whilst still attached to the solid support and thus still available for further chemistry. The direct NMR analysis of such materials, even when solvated, is complicated by two principal factors, which can severely degrade spectrum resolution  

The restricted motion of the tethered analytes relative to that of free molecules in solution may mean that dipolar couplings, D, are not fully averaged to zero. These have a time-averaged angular dependence 

Magnetic susceptibility has a similar angular dependence to that above, and line broadening from both dipolar couplings and susceptibility discontinuities can be eliminated by rapid rotation of the sample about the so-called magic angle 6 = Sd. , such that the above 

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