Natural Abundance ¹⁷O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy

Due to its extremely low natural abundance and quadrupolar nature, the ¹⁷O nuclide is very rarely used for spectroscopic investigation of solids by NMR without isotope enrichment. Additionally, the applicability of dynamic nuclear polarization (DNP), which leads to sensitivity enhancements of 2 orders of magnitude, to ¹⁷O is wrought with challenges due to the lack of spin diffusion and low polarization transfer efficiency from ¹H. Here, we demonstrate new DNP-based measurements that extend ¹⁷O solid-state NMR beyond its current capabilities. The use of the PRESTO technique instead of conventional ¹H–¹⁷O cross-polarization greatly improves the sensitivity and enables the facile measurement of undistorted line shapes and two-dimensional ¹H–¹⁷O HETCOR NMR spectra as well as accurate internuclear distance measurements at natural abundance. This was applied for distinguishing hydrogen-bonded and lone ¹⁷O sites on the surface of silica gel; the one-dimensional spectrum of which could not be used to extract such detail. Lastly, this greatly enhanced sensitivity has enabled, for the first time, the detection of surface hydroxyl sites on mesoporous silica at natural abundance, thereby extending the concept of DNP surface-enhanced NMR spectroscopy to the ¹⁷O nuclide