Journal of Physical Chemistry A, Vol.122, No.25, 5635-5643, 2018
Enhancing the Sensitivity of Solid-State NMR Experiments with Very Low Gyromagnetic Ratio Nuclei with Fast Magic Angle Spinning and Proton Detection
Many transition metals commonly encountered in inorganic materials and organometallic compounds possess NMR-active nuclei with very low gyromagnetic ratios (gamma) such as Y-89, Rh-10(3), Ag-109, and( 183)W. A low-gamma leads to poor NMR sensitivity and other experimental challenges. Consequently, nuclei with low-gamma are often impossible to study with conventional solid-state NMR methods. Here, we combine fast magic angle spinning (MAS) and proton detection to enhance the sensitivity of solid-state NMR experiments with very low-gamma nuclei by 1-2 orders of magnitude. Coherence transfer between H-1 and low-gamma nuclei was performed with low-power double quantum (DQ) or zero quantum (ZQ) cross-polarization (CP) or dipolar refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT). Comparison of the absolute sensitivity of CP NMR experiments performed with proton detection with 1.3 mm rotors and direct detection with 4 mm rotors shows that proton detection with a 1.3 mm rotor provides a significant boost in absolute sensitivity, while requiring approximately 1/40' h of the material required to fill a 4 mm rotor. Fast MAS and proton detection were applied to obtain (89)gamma and Rh-103 solid-state NMR spectra of organometallic complexes. These results demonstrate that proton detection and fast MAS represents a general approach to enable and accelerate solid-state NMR experiments with very low-gamma nuclei.