Robust NMR approaches for the determination of homonuclear dipole-dipole coupling constants in studies of solid...

This review addresses the NMR spectroscopy study of molecular structure and dynamics by way of homonuclear dipole-dipole couplings by relying on their orientation and direct distance dependence. The study of homonuclear couplings as opposed to heteronuclear couplings poses specific challenges. On the one hand, two like spins cannot be independently manipulated easily, which means that simple shift-refocusing concepts by using hard π pulses cannot be used to cope with potentially large chemical-shift dispersions at the high fields used today. On the other hand, the noncommutativity of the different pair Hamiltonians in a multispin system leads to complications associated with the isolation of specific pair couplings while minimizing the influence of the other spins. In particular, the so-called dipolar-truncation effect challenges the observation of weak couplings of interest in the presence of stronger ones. Recent advances in determining homonuclear dipole-dipole coupling constants are reviewed, stressing the use of double-quantum spectroscopy approaches and their similarity to the popular heteronuclear rotational-echo double-resonance experiment. Particular emphasis is put on corrections for the influence of transverse relaxation effects on the measured data, and the handling of distribution effects as well as potential dynamic heterogeneities in complex substances.