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 ¹H NMR of rigid solids is now routinely observed under fast magic angle sample spinning (MAS). Nevertheless, the spectral resolution of ¹H nuclei bonded to ¹⁴N is often compromised by residual dipolar splitting (RDS) and scalar coupling between ¹H and ¹⁴N. Given the ubiquity of the NH moiety and the high natural abundance of both nuclei, RDS broadening poses a widespread practical challenge. Heteronuclear ¹⁴N decoupling during ¹H acquisition is therefore essential for enhancing the spectral resolution. Previously, we demonstrated that low-power ¹⁴N CW decoupling via 70 kHz MAS improves the ¹H signal intensity by ∼20% and narrows ¹H line widths by ∼180 Hz but under stringent on-resonance ¹⁴N irradiation. Offset-tolerant ¹⁴N decoupling sequences are necessary to achieve good decoupling in samples with multiple ¹⁴N sites. Here, we assess several amplitude-modulated decoupling schemes, adapted from solution NMR. Experiments and numerical simulations determine that SUSAN1 (with <i>t</i>_p = 10 μs and ν_¹⁴N = 15-23 kHz) provides the best broadband ¹⁴N decoupling performance. Furthermore, an empirical correlation is established between pulse length and selective band-limited and non-selective broadband ¹⁴N decoupling behavior under low-power and fast MAS conditions.