Enabling Natural Abundance ¹⁷O Solid-State NMR by Direct Polarization from Paramagnetic Metal Ions

Abstract

Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing its applications and the quality of NMR spectroscopy as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample, enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low-sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal-ion-based DNP, the polarization agents are distributed in the bulk sample and act as a source of both relaxation and polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted, and the entire sample can be directly polarized. We exploit this finding to measure high-quality NMR spectra of 17 O in the electrode material Li 4 Ti 5 O 12 doped with Fe(III) despite its low abundance and long relaxation time.

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

“…Recently, dynamic nuclear polarization (DNP) NMR has opened the way for detecting signals of unreceptive nuclides which suffer from low gyromagnetic ratios, low natural abundance, dilution, line broadening or any combination of these challenges. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] As a matter of course, researchers have applied this emerging technique to the 17 O nuclide, and demonstrated that 17 O SS NMR spectra could be acquired at natural abundance from nanoparticles [4,9,[12][13][15][16][17] and even from surfaces. [9,[18][19][20][21] In the majority of studies, however, the hyperpolarization of 17 O nuclei has been achieved via protons using the route e À !…”

“…Indeed, the natural abundance 17 O DNP NMR has been mostly restricted to protonated oxygens, i. e. hydroxyls, [4,9,12,[18][19] with one exception of a recent report on the use of D-RINEPT. [21] The use of advanced radical molecules [20,[22][23][24] or suitable metal ions [13,15] has enabled a direct, e À ! 17 O DNP route; however, the absence of 17 OÀ 17 O spin diffusion in naturally abundant samples keeps most spins unpolarized.…”

Oxygen Vacancy Distribution in Yttrium-Doped Ceria from 89Y–89Y Correlations via Dynamic Nuclear Polarization Solid-State NMR

“…Because spin diffusion is not expected to be efficient for any of the present nuclei, the large DNP enhancements can be related to direct polarization, in accordance with our previous results. 31 Consequently, DNP will yield homogeneous enhancements throughout the entire bulk of the sample, and the results will have quantitative value. In agreement, no qualitative changes in the line shape are observed in the enhanced spectra.…”

Structure and Functionality of an Alkylated LixSiyOz Interphase for High-Energy Cathodes from DNP-ssNMR Spectroscopy

“…Recently, we have shown that direct polarization transfer in the bulk of Fe­(III)-doped Li 4 Ti 5 O 12 is distance independent in cases where the dominant nuclear relaxation mechanism is the paramagnetic dopant . In the current system, because of the presence of strong dipole moments of 1 H and 7 Li in the coating and overall heterogeneity and disorder in the coating layer, it is unlikely that the Fe­(III) dopants in the bulk are the only source of relaxation.…”

“…The development of highly efficient polarizing agents, based on nitroxide biradicals introduced into the sample of interest, results in 10–10 4 -fold increase in sensitivity. Such a boost in sensitivity enables the detection of otherwise extremely challenging surfaces and interfaces, including the SEI and interphases formed on various electrode materials. − Another DNP approach is to transfer endogenous polarization from paramagnetic metal ion dopants. − This approach has been successfully applied to gain NMR sensitivity in the bulk of oxides. − To date, this approach has not been used to polarize interphases.…”