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 m agic-angle spinning (MAS),experiments. These include the accurate meas urement Of weak dipole-dipole couplings between rare spins, the behavi or of dipolar trajectories: in multiple spin environments, and chemica l shift correlation spectroscopy via polarization exchange. To describ e dipolar trajectories accurately,we adopt an approach to the simulati on df these experiments. which includes finite pulses and the influenc e of coherence decay, The latter effect becomes competitive with the s trength of weak couplings in many experiments, and a simple empirical approach is outlined for the selection of decay parameters, Dipolar tr ajectories are shown to be dominated by the largest couplings in multi ple 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 N-15 C-13-labeled sam ple of the tetrapeptide achatin-II (Gly-L-Phe-L-Ala-L-Asp). In additio n, a frequency-selective approach to recoupling dipolar interactions a mong homonuclear spins is introduced;: selective approaches have possi ble utility in examining weak dipole-dipole couplings in the presence of strong interactions. (C) 1998 American Institute of Physics.