Distances Determined by SSNMR

The strength of the dipolar coupling between two nuclei is proportional to the inverse cube of the intemuclear distance according to the following equation  

The dipolar coupling allows one to obtain distance information that can be used to confirm signal assignment or to elucidate phase separation. Solid state NMR is then a powerful method for investigating molecular conformation and geometry in large molecular systems. The technique is able to provide information about protein structure of membrane, biomaterials and polymers that are not accessible with X-Ray or liquid state NMR spectroscopy. 

In RR the recoupling can be achieved by setting the spinning rate wr to multiples of the frequency difference between the NMR resonances. 

The various techniques have some intrinsic limitations and cannot be universally applied, for example in the case of a polycrystalline powder obtained from solvent-free reactions such as those occurring in the solid state between molecules, or between two solids, or between a solid and a gas. For these kinds of reaction, which are important from both the environmental and topochemical viewpoints, the alternative investigation techniques are mainly IR and NMR spectroscopy. 

Since in the solid state the signals are not averaged by solvent effects or by rapid exchange processes often present in solution, the solid state NMR approach allows a more accurate study. 

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