Suppressing ¹H Spin Diffusion in Fast MAS Proton Detected Heteronuclear Correlation Solid-State NMR Experiments

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Materials characterization and structural analysis.

Background:

• Fast magic angle spinning (MAS) and indirect detection enhance 2D heteronuclear correlation (HETCOR) solid-state NMR spectra.
• Cross-polarization (CP) based HETCOR sequences suffer from proton (¹H) spin diffusion, causing relayed correlations.
• This issue is critical for low-gyromagnetic ratio (γ) nuclei (e.g., ⁸⁹Y, ¹⁰³Rh, ¹⁰⁹Ag, ¹⁸³W) requiring long CP contact times.

Purpose of the Study:

• To develop and validate methods for eliminating relayed correlations in CP HETCOR solid-state NMR.
• To enable more reliable distance information acquisition from 2D HETCOR spectra.
• To improve HETCOR experiments for both moderate- and low-γ nuclei.

Main Methods:

• Implementation of Lee-Goldburg (LG) cross-polarization during the X→¹H back-transfer step to suppress ¹H spin diffusion.
• Determination of LG conditions compatible with fast MAS (40-95 kHz) at various effective radiofrequency (RF) fields.
• Application of D-RINEPT (dipolar refocused insensitive nuclei enhanced by polarization transfer) during the X→¹H back-transfer step.

Main Results:

• Efficient suppression of ¹H spin diffusion was achieved using LG-CP under fast MAS conditions.
• Modified Hartmann-Hahn LG-CP conditions were established for indirect detection of ¹³C and ⁸⁹Y.
• Demonstrated effectiveness of both LG-CP and D-RINEPT in reducing relayed signal intensities in ¹H{¹³C} and ¹H{⁸⁹Y} HETCOR experiments.

Conclusions:

• The proposed LG-CP and D-RINEPT methods effectively eliminate relayed correlations in HETCOR solid-state NMR.
• These techniques provide more reliable distance constraints, particularly for challenging low-γ nuclei.
• Enhanced HETCOR experiments facilitate more accurate structural investigations using solid-state NMR.