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

Fast magic angle spinning (MAS) and indirect detection by high gyromagnetic ratio (γ) nuclei such as proton or fluorine are increasingly utilized to obtain 2D heteronuclear correlation (HETCOR) solid-state NMR spectra of spin-1/2 nuclei by using cross polarization (CP) for coherence transfer. However, one major drawback of CP HETCOR pulse sequences is that H spin diffusion during the back X→H CP transfer step may result in relayed correlations. This problem is particularly pronounced for the indirect detection of very low-γ nuclei such as Y, Rh, Ag and W where long contact times on the order of 10-30 ms are necessary for optimal CP transfer. Here we propose two methods that eliminate relayed correlations and allow more reliable distance information to be obtained from 2D HETCOR NMR spectra. The first method uses Lee-Goldburg (LG) CP during the X→H back-transfer step to suppress H spin diffusion. We determine LG conditions compatible with fast MAS frequencies (ν) of 40-95 kHz and show that H spin diffusion can be efficiently suppressed at low effective radiofrequency (RF) fields (ν ≪ 0.5ν) and also at high effective RF fields (ν ≫ 2ν). We describe modified Hartmann-Hahn LG-CP match conditions compatible with fast MAS and suitable for indirect detection of moderate-γ nuclei such as C, and low-γ nuclei such as Y. The second method uses D-RINEPT (dipolar refocused insensitive nuclei enhanced by polarization transfer) during the X→H back-transfer step of the HETCOR pulse sequence. The effectiveness of these methods for acquiring HETCOR spectra with reduced relayed signal intensities is demonstrated with H{C} HETCOR NMR experiments on l-histidine⋅HCl⋅HO and H{Y} HETCOR NMR experiments on an organometallic yttrium complex.