Recoupling second-order

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... 

I. Scholz, B.H. Meier, M. Ernst, NMR polarization transfer by second-order resonant recoupling RESORT, Chem. Phys. Lett. 485 (2010) 335—342. 

Hou et al. have recently described a family of experiments for R2 Driven Spin Diffusion (RDSD) spectroscopy suitable for homonuclear correlation experiments under fast MAS conditions. In these RDSD experiments, since the broadened second-order rotational resonance conditions are dominated by the rf field strength and the phase shifts, as well as the size of reintroduced dipolar couplings, the different R2 sequences display unique polarization transfer behaviours and different recoupling frequency bandwidths. In the subsequent report, Hou et al. present a series of modified R2/ sequences, dubbed combined R2 -Driven (CORD), that yield broadband homonuclear dipolar recoupling and give rise to uniform distribution of cross peak intensities... 

Proton-Driven) Spin Diffusion and Second-Order Recoupling for Polarization... 

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