High-resolution solid-state NMR

We have already mentioned in Chapter 4 that, in the solid state, the relaxation time T ₁ is long due to the lack of modulation of the dipole–dipole interaction and T ₂ is short due to mutual spin flips occurring between pairs of spins. In a static solid, each nucleus produces a rotating magnetic field as it precesses in the applied magnetic field, and this can cause direct exchange of energy between nuclei. The lifetimes of the spin states are thus reduced and so T ₂. In addition, each spin has a static field component that influences the Larmor frequencies of its neighbours. An individual nucleus will experience the fields of several neighbours, but their spin directions will vary randomly, so that there will be a range of frequencies that will add to the line broadening due to the rapid rate of relaxation. Finally, particularly for the heavier nuclei, including ¹³C, there will exist a chemical shift anisotropy, which will also contribute to the broadening, assuming that the sample is a powder or a glass and not a single crystal, because the chemical shift varies with orientation relative to the B ₀ direction. Thus solid materials, particularly if they contain nuclei with high magnetic moments such as ¹H or ¹⁹F, will have broad, structureless resonances, which will not permit the type of investigation that we have shown can be carried out in the liquid phase. This state of affairs has proved a challenge to the NMR community, who have over the last two and a half decades found means to render ineffective the apparent physical restraints to the spectroscopic examination of solids at high resolution.