Kobayashi, Takeshi, and Marek Pruski. “Indirectly Detected DNP‐Enhanced 17O NMR Spectroscopy: Observation of Non‐Protonated Near‐Surface Oxygen at Naturally Abundant Silica and Silica‐Alumina.” ChemPhysChem 22, no. 14 (July 16, 2021): 1441–45.
https://doi.org/10.1002/cphc.202100290.
Recent studies have shown that dynamic nuclear polarization (DNP) can be used to detect 17O solid-state NMR spectra of naturally abundant samples within a reasonable experimental time. Observations using indirect DNP, which relies on 1H mediation in transferring electron hyperpolarization to 17O, are currently limited mostly to hydroxyls. Direct DNP schemes can hyperpolarize non-protonated oxygen near the radicals; however, they generally offer much lower signal enhancements. In this study, we demonstrate the detection of signals from nonprotonated 17O in materials containing silicon. The sensitivity boost that made the experiment possible originates from three sources: indirect DNP excitation of 29Si via protons, indirect detection of 17O through 29Si nuclei using two-dimensional 29Si {17O} D-HMQC, and Carr-Purcell-Meiboom-Gill refocusing of 29Si magnetization during acquisition. This 29Si-detected scheme enabled, for the first time, 2D 17O 29Si heteronuclear correlation spectroscopy in mesoporous silica and silica-alumina surfaces at natural abundance. In contrast to the silanols showing motionaveraged 17O signals, the framework oxygens exhibit unperturbed powder patterns as unambiguous fingerprints of surface sites. Along with hydroxyl oxygens, detection of these moieties will help in gaining more atomistic-scale insights into surface chemistry.