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

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Abstract
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 tp = 10 μs and ν14N = 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.
Funding Information
  • Department of Atomic Energy, Government of India (RTI 4007)
  • Grantov? Agentura Cesk? Republiky (24-13437L)

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