Dipolar recoupling in magic angle spinning solid-state nuclear magnetic resonance

Solid-state nuclear magnetic resonance (SSNMR) magic angle spinning (MAS) can be used to record high-resolution data dominated by site-specific information. Although MAS introduces high resolution by attenuating the anisotropic broadening, it also suppresses the nuclear dipole-dipole distance information that is the source of most structural data in the spectra. Such information can be reintroduced coherently and thus selectively by the application of a carefully chosen sequence of radiofrequency pulses, an approach that was introduced 20 years ago and is referred to as dipolar recoupling. This review presents the establishment of recoupling techniques in SSNMR and recalls the major steps achieved by the community throughout the last two decades. This review also presents emerging techniques and their corresponding new concepts. Finally, we present some recent developments based on second-order recoupling mechanisms and discuss their implications regarding dipolar truncation and the possibility to extract structural constraints in uniformly labeled systems.