Indirectly Detected DNP‐Enhanced ¹⁷O NMR Spectroscopy: Observation of Non‐Protonated Near‐Surface Oxygen at Naturally Abundant Silica and Silica‐Alumina

Abstract

Recent studies have shown that dynamic nuclear polarization (DNP) can be used to detect 17 O solid-state NMR spectra of naturally abundant samples within a reasonable experimental time. Observations using indirect DNP, which relies on 1 H mediation in transferring electron hyperpolarization to 17 O, 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 17 O in materials containing silicon. The sensitivity boost that made the experiment possible originates from three sources: indirect DNP excitation of 29 Si via protons, indirect detection of 17 O through 29 Si nuclei using two-dimensional 29 Si { 17 O} D-HMQC, and Carr-Purcell-Meiboom-Gill refocusing of 29 Si magnetization during acquisition. This 29 Si-detected scheme enabled, for the first time, 2D 17 OÀ 29 Si heteronuclear correlation spectroscopy in mesoporous silica and silica-alumina surfaces at natural abundance. In contrast to the silanols showing motionaveraged 17 O 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.

17O Solid-State NMR Spectroscopy of Lipid Membranes

“…Generally, the observed 2 H NMR spectra of water exhibited a much smaller quadrupolar coupling (a few kHz) than that of rigid water (∼220 kHz), indicating that water molecules in the lipid environments are highly mobile, 34,35 although they can be characterized as two different types: trapped water with no quadrupolar coupling and "bound" water with a small residual quadrupolar coupling. However, recent 17 O NMR experiments indicated that the dynamics of water molecules in a lipid environment is much slower than what had been known from molecular dynamics simulations 36 and fast-time scale experimental methods, 37 and even the rigidly bound water with a significantly large quadrupolar coupling was found in the lipid headgroup region after selectively suppressing signals from mobile water. 38 A list of 17 O NMR methods that have been developed for use under different sample conditions is given in Table 1.…”

“…It was theoretically predicted and experimentally confirmed 7,8 that the line width for the quadruple central transition (QCT) is narrower in the slow-motion region (i. ω 0 τ c ≫ 1) than in the fast-motion region (i.e., ω 0 τ c ≪ 1). In addition, the relaxation process of the 17 O quadrupolar interaction induces the so-called dynamic frequency shift away from its true isotropic chemical shift. This dynamic frequency shift is inversely proportional to the square of the Larmor frequency.…”

“…This dynamic frequency shift is inversely proportional to the square of the Larmor frequency. Thus, by extrapolation in a plot of signal frequency obtained at different magnetic fields versus (ω 0 ) −2 , the real 17 O isotropic chemical shift can be obtained, provided the molecular motion falls into the slow-motion region in these fields. Although the 17 O relaxation process in biological macromolecules is complicated, especially when the chemical shift anisotropy provides another relaxation mechanism, 17 O QCT NMR is considered to be very promising and powerful.…”

“…As a result, as in the standard quadrupolar-echo experiments, the second-order quadrupolar line shapes can be refocused thus rendering the high-resolution 17 O NMR spectra. Similar to this refocusing idea but without dynamically reorienting the rotation angles, the multiple-quantum MAS (MQMAS) 16,17 and satellite transition MAS (STMAS) 18,19 experiments were proposed to obtain the correlation between the high-resolution 17 O spectral lines and their respective second-order quadrupolar line shapes. The MQMAS method correlates the multiple-quantum transitions with the central transitions in such a way that the quadrupolar line-broadening can be refocused after evolution during the given times, while the STMAS method is to evolve the satellite transitions for a given time, forming the quadrupolar echo with the central transitions.…”

“…B. 17 O-X Heteronuclear Correlation (HETCOR). In this method, the 17 O NMR spectrum is correlated with the highresolution spectrum of other spin-1/2 nuclei (X).…”