Enhanced symmetry-based dipolar recoupling in solid-state NMR

Symmetry-based RNnν pulse sequences [Chem. Phys. Lett. 321 (2000) 205] provide efficient dipolar recoupling in magic-angle spinning nuclear magnetic resonance. For homonuclear and heteronuclear recouplings, we demonstrate analytically, numerically, and experimentally that the dipolar scaling factors may be increased by replacing previously introduced pulse elements of such sequences by windowed ones. In the case of the sequence R1426, we obtain experimentally 65% faster buildup of 13C double-quantum coherences in uniformly 13C-labelled glycine and alanine powders, and a significantly more broadband recoupling. We also discuss the limitations of the proposed windowed pulse sequences and their use in other recoupling scenarios.