Proton-assisted recoupling

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

Second-order recoupling may be less sensitive to dipolar truncation, and several methods have been developed, based on the assistance of strongly coupled nuclei. However, many proton-driven spin diffusion experiments fail at ultrafast MAS. Among those that have been tested at ultrafast MAS, we can mention proton-assisted recoupling (PAR), efficient for providing long-range distances in the GBl protein at v = 65 kHz [153] (PAR... 

J.R. Lewandowski, G. De Paepe, M.T. Eddy,J. Struppe, W. Maas, R.G. Griffin, Proton assisted recoupling at high spinning frequencies, J. Phys. Chem. B 113 (2009) 9062-9069. 

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