Windowed Symmetry Pulses for Enhanced Heteronuclear Dipolar Recoupling in Solid-State MAS NMR

Dipolar recoupling techniques play a fundamental role in solid-state NMR spectroscopy, enabling precise structural characterization of solid materials under magic-angle spinning (MAS). Among these, the supercycled R412 (SR4) stands out for its efficiency and stability in recoupling heteronuclear dipolar interactions under fast MAS conditions (≥40 kHz). However, its performance significantly degrades at slow-to-moderate MAS rates (in situ NMR experiments on SAPO zeolite, where the markedly improved 2D correlation spectroscopy provides deeper insights into local structures. This approach enables precise dipolar measurement, efficient polarization transfer, and the establishment of high-fidelity heteronuclear correlations, all of which are critical for structural elucidation in complex solid systems. Moreover, the windowed SR4 design strategy introduced here is expected to provide a generalizable framework for optimizing symmetry-based recoupling sequences across a wide range of MAS conditions.