Cross-polarization-dipole decoupling

Schaefer and coworkers, in another chapter in this text, used 1h - 13(j dipole-dipole "line shapes" obtained in a very clever way to investigate rotational motion of the aromatic rings in polystyrene. The method used a WAHUHA pulse sequence to decouple proton-proton dipolar interactions, cross polarization to enhance signal acquisition and an overall sampling technique synchronous with the sample rotation. The dipole-dipole interaction was mapped in rotational sideband spectra obtained from 16 "normal" CP/MAS spectra. The method, though somewhat involved, provided a measure of dipole-dipole line-shapes which can be interpreted in terms of side-chain rotation in the polymer. 

Signal enhancement due to NOE is an example of cross-polarization, in which a polarization of the spin states in one type of nucleus causes a polarization of the spin states in another nucleus. Cross-polarization will be explained in Section 4.6. In the current example (proton-decoupled spectra), when the hydrogens in the molecule are irradiated, they become saturated and attain a distribution of spins very different from their equilibrium (Boltzmann) state. There are more spins than normal in the excited state. Due to the interaction of spin dipoles, the spins of the carbon nuclei sense the spin imbalance of the hydrogen nuclei and begin to adjust themselves to a new equilibrium state that has more spins in the lower state. This increase of population in the lower spin state of carbon increases the intensity of the NMR signal. 

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