Spin decoupling dipolar

If no chemical-shift differences existed, an abundant low-y spin system in the absence of further spins (or in the presence of efficient heteronuclear spin decoupling) would behave in the same way as an abundant high-y spin system. In practice, however, the chemical-shift differences tend to be comparable to and often larger than the dipolar couplings and they must be included in the system Hamiltonian... 

Dipolar spin decoupling has been used to study the numbers of contacts of various types and the free energy of formation of a single mixed contact. Sp6vacek et al. used temperature dependence of chemical shifts in H-n.m.r. spectra to examine the specific interactions in poly(methyl methacrylate) solutions. Cabane has used n.m.r. methods to study aggregation in aqueous polymer detergent systems. Compatibility and phase structure in polymer mix-... 

NMR can, in principle, provide complementary information on motional processes in liquid crystals. The dipole-dipole interaction between a C-H pair and the quadrupolar interaction when the proton is replaced with a deuteron share the same principal interaction axis. In the case where the carbon is not directly bonded to a proton, there is still dipole-dipole relaxation by nearby protons, but it is also necessary to include an additional relaxation mechanism, the modulation of the chemical shift anisotropy. Proton spin decoupling is necessary to give well-resolved chemically shifted lines in the mesophase of liquid crystals. Furthermore, it is not practical to determine individual spectral density parameters from measured relaxation rates. Proton-proton dipolar interactions may not be ignored even when observation is exclusively confined to the resonant spin [5.31]. This is because proton relaxation causes population flow among the proton spin levels through dipolar (or scalar) coupling. As a consequence, cross-... 

Figure 13 Magic-angle spinning (MAS) dipolar-decoupled C spectra with cross polarization (a) PEC-urea-inclusion complex (b) PEC.

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