Homonuclear radio frequency-driven recoupling in rotating solids

We discuss several aspects of homonuclear recoupling and longitudinal exchange using rotor-synchronized spin echo sequences in solid state magic-angle spinning (MAS) experiments. These include the accurate measurement of weak dipole–dipole couplings between rare spins, the behavior of dipolar trajectories in multiple spin environments, and chemical shift correlation spectroscopy via polarization exchange. To describe dipolar trajectories accurately, we adopt an approach to the simulation of these experiments which includes finite pulses and the influence of coherence decay. The latter effect becomes competitive with the strength of weak couplings in many experiments, and a simple empirical approach is outlined for the selection of decay parameters. Dipolar trajectories are shown to be dominated by the largest couplings in multiple spin systems via comparison of two and three interacting spins. Two-dimensional correlation spectroscopy based on dipolar exchange among proximate nuclei is illustrated with a uniformly N15,C13-labeled sample of the tetrapeptide achatin-II (Gly-L-Phe-L-Ala-L-Asp). In addition, a frequency-selective approach to recoupling dipolar interactions among homonuclear spins is introduced; selective approaches have possible utility in examining weak dipole–dipole couplings in the presence of strong interactions.