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 R4₁ ² (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 (<35 kHz), which restricts its wide applications. To overcome this limitation, herein we introduce a practical modification of SR4 through the strategic incorporation of adjustable window delays between the recoupling π pulses, dubbed windowed SR4 (wSR4). This design introduces the variability of pulse duration and rf amplitude, which simplifies the recoupling optimization procedure, while enabling enhanced recoupling performance and improved tolerance to experimental imperfections. Theoretical analysis and NMR experiments collectively confirm that wSR4 substantially boosts heteronuclear dipolar recoupling efficiency while effectively suppressing undesired spin interactions, particularly under slow MAS conditions. The practical utility of wSR4 is further validated in 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.