Broadband 15N-13C dipolar recoupling via symmetry-based RF pulse schemes at high MAS frequencies.

An approach for generating efficient NR(vS, vk)(n) symmetry-based dual channel RF pulse schemes for gamma-encoded broadband (15)N-(13)C dipolar recoupling at high magic angle spinning frequencies is presented. The method involves the numerical optimisation of the RF phase-modulation profile of the basic "R" element so as to obtain heteronuclear double quantum dipolar recoupling sequences with satisfactory magnetisation transfer characteristics. The basic "R" element was implemented as a sandwich of a small number of short pulses of equal duration with each pulse characterised by a RF phase and amplitude values. The performance characteristics of the sequences were evaluated via numerical simulations and (15)N-(13)C chemical shift correlation experiments. Employing such (13)C-(15)N double-quantum recoupling sequences and the multiple receiver capabilities available in the current generation of NMR spectrometers, the possibility to simultaneously acquire 3D NCC and CNH chemical shift correlation spectra is also demonstrated.