NUCLEAR-MAGNETIC-RESONANCE METHODS FOR MEASURING DIPOLAR COUPLINGS IN ROTATING SOLIDS

The nuclear dipole-dipole coupling is a strong function of internuclear distance, and can therefore be used as a sensitive probe of local molecular structure. In most cases, however, the simultaneous presence of anisotropic chemical shielding and multiple dipolar interactions prevents isolation of the dipole coupling between individual spin pairs. Under these circumstances, it is customary to improve spectral resolution by employing magic angle spinning (MAS) which preserves the isotropic part of the chemical shift interaction but partially or fully averages the desired dipolar interactions. This article reviews NMR methods designed to selectively restore and measure dipolar couplings between like (homonuclear) spins and unlike (heteronuclear) spins in rotating solids. Applications of these methods rotational resonance (R(2)), rotational-echo double resonance (REDOR), transferred-echo double resonance (TEDOR), dipolar recovery at the magic angle (DRAMA), simple excitation for the dephasing of rotational-echo amplitudes (SEDRA), and rf-driven dipolar recoupling (RFDR) - to investigate the structure of biomolecules are presented.