Principles of the NMR measurements

A complete introduction to the NMR phenomena described in this chapter can be found in Ref. 6. In the following, we will focus on the measurements that have proved most useful in the case of thermotropic polymer studies. 

For a pair of protons Hi and H2 separated by a distance Thh, the splitting Av between the two spectral lines as a result of dipolar coupling between the nuclear magnetic moments can be written as 

Deuterium, whose spin is equal to 1, is a quadrupolar nucleus. Its NMR parameters are almost exclusively governed by the quadrupolar interaction with the electric field tensor (FGT) at the deuteron site. Although the intermolecular contributions to the electric field gradient can be significant, the field gradient usually originates mainly from the electrons in the C- H bond and is considered to a first approximation to be intramolecular. It is found to be axially symmetric about the C- H bond in aliphatic compounds, and to a good approximation in aromatic compounds as well. Thus, information obtained from H NMR on molecular order and dynamics mostly concerns individual C- H bond directions. 

In the absence of motions, the NMR spectrum is a typical quadrupolar doublet, with a symmetric splitting Av  

High-resolution solid-state NMR is also a very interesting tool for studying non-oriented polymer liquid crystals. Proton dipolar decoupling (DD), which suppresses the dipolar broadening, and rapid 

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