Recoupling of Homo- and Heteronuclear Dipolar Interactions in Rotating Solids

The measurement of homo- and heteronuclear dipolar couplings by nuclear magnetic resonance (NMR) techniques is an important tool for the determination of molecular structure in solids. In a static polycrystalline solid, the dipolar coupling between two magnetically dilute spins results in the characteristic “Pake pattern” [1], first observed in the 1H spectrum of gypsum, CaSo₄-2H₂O, which arises from the interaction between the two protons in the water molecules of hydration. The splitting between the singularities provides a straightforward measurement of the dipolar coupling constant and therefore the internuclear distance between the two spins. Unfortunately, in the more general case, the structural information revealed by internuclear distances cannot be obtained directly from the static 1H NMR spectrum because of the multiplicity of couplings. In situations involving other nuclei, such as 13C, 15N, and 31P, large chemical shift anisotropics, as well as other line-broadening mechanisms, obscure the lineshape perturbations from the through-space dipolar couplings.