Polarization transfer, dipolar coupling homonuclear

Fig. 10.22. Diagram showing the cross-polarization from protons, H, to a heteronucleus, X, such as carbons. Heteronuclear dipolar coupling enables the transfer of magnetization from H to X, such as protons to carbons. Homonuclear dipolar coupling between the abundant protons enables the redistribution of proton spin energy through spin diffusion.

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... 

Relaxation-mediated magnetization transfer is often dominated by the effects of a strongly coupled proton bath. The influence of the corresponding homonuclear and heteronuclear dipolar interactions may indirectly enhance the rate of polarization exchange between rare spins such as by a process called proton-driven spin diffusion... 

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