Recoupling and decoupling of nuclear spin interactions at high MAS frequencies: numerical design of CN(n)(nu) symmetry-based RF pulse schemes.

The CN(n)(nu) class of RF pulse schemes, commonly employed for recoupling and decoupling of nuclear spin interactions in magic angle spinning solid state NMR studies of biological systems, involves the application of a basic "C" element corresponding to an RF cycle with unity propagator. In this study, the design of CN(n)(nu) symmetry-based RF pulse sequences for achieving 13C-13C double-quantum dipolar recoupling and through bond scalar coupling mediated 13C-13C chemical shift correlation has been examined at high MAS frequencies employing broadband, constant-amplitude, phase-modulated basic "C" elements. The basic elements were implemented as a sandwich of a small number of short pulses of equal duration with each pulse characterised by an RF phase value. The phase-modulation profile of the "C" element was optimised numerically so as to generate efficient RF pulse sequences. The performances of the sequences were evaluated via numerical simulations and experimental measurements and are presented here.