High-Resolution Proton NMR Spectra of NH Moieties in Solids Enabled by Offset-Tolerant Nitrogen-14 Decoupling via Fast Magic Angle Spinning at 70 kHz

High-resolution 1 H NMR of rigid solids is now routinely observed under fast magic angle sample spinning (MAS). Nevertheless, the spectral resolution of 1 H nuclei bonded to 14 N is often compromised by residual dipolar splitting (RDS) and scalar coupling between 1 H and 14 N. Given the ubiquity of the NH moiety and the high natural abundance of both nuclei, RDS broadening poses a widespread practical challenge. Heteronuclear 14 N decoupling during 1 H acquisition is therefore essential for enhancing the spectral resolution. Previously, we demonstrated that low-power 14 N CW decoupling via 70 kHz MAS improves the 1 H signal intensity by ∼20% and narrows 1 H line widths by ∼180 Hz but under stringent on-resonance 14 N irradiation. Offset-tolerant 14 N decoupling sequences are necessary to achieve good decoupling in samples with multiple 14 N sites. Here, we assess several amplitude-modulated decoupling schemes, adapted from solution NMR. Experiments and numerical simulations determine that SUSAN1 (with t p = 10 μs and ν 14 N = 15–23 kHz) provides the best broadband 14 N decoupling performance. Furthermore, an empirical correlation is established between pulse length and selective band-limited and non-selective broadband 14 N decoupling behavior under low-power and fast MAS conditions.