Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO₃

Abstract

We demonstrate that solid-state NMR spectra of challenging nuclei with a low gyromagnetic ratio such as yttrium-89 can be acquired quickly with indirect dynamic nuclear polarization (DNP) methods. Proton to 89Y cross polarization (CP) magic angle spinning (MAS) spectra of Y3+ in a frozen aqueous solution were acquired in minutes using the AMUPol biradical as a polarizing agent. Subsequently, the detection of the 89Y and 1H NMR signals from technologically important hydrated yttrium-doped zirconate ceramics, in combination with DFT calculations, allows the local yttrium and proton environments present in these protonic conductors to be detected and assigned to different hydrogen-bonded environments.

Is solid-state NMR enhanced by dynamic nuclear polarization?

“…Indeed, direct hyperpolarization of bulk nuclei that show little spin diffusion, such as 29 Si [59,75], is accordingly very dependent on particle size (average proximity to polarizing agents) [59]. For instance, large particles (diameter o45 μm) of yttrium-doped BaZrO 3 , studied using { 1 H-} 89 Y CP and MAS-DNP, showed small 1H,CP ε DNP through CP ( $ 3), and even larger particles (45 μm odiameter o75 μm) returned negligible 1H,CP ε DNP ( ¼1) for the same experiment, whereas the inter-particle DNP matrix of D 6 -DMSO/H 2 O (60/40; v/v) containing 12 mM AMUPol biradical gave direct 1H ε DNP ¼10 for both [67]. The low proton content ( $ 2-3 mmol cm À 3 ) and large diameter of these potential proton-conducting materials for solid oxide fuel cells prohibited efficient 1 H spin-diffusion of the hyperpolarization from the surface into the bulk.…”

“…Nevertheless, recent studies have shown promise for these two vital sectors with work on inorganic oxides and hydroxides [31] and biologically-relevant small molecules [6] with both direct [6,31] and indirect (via 1 H) [31] MAS-DNP. Since quadrupolar parameters are very sensitive to the local environment, and the low temperatures currently required for efficient DNP have been shown to be exceptionally beneficial to studies of quadrupolar spins [36], as well as low-γ [67] nuclei, it can be envisaged that MAS-DNP has a very appropriate direction. It can also be expected that this will be extended to other demanding, yet key, quadrupolar nuclei, such as 6,7 Li, 14 N [35], 25 Mg, and 43 Ca.…”

“…Nevertheless, as can be seen in Fig. 9, the MAS-DNP experiments were extremely useful in determining the position of trapped protons within the material by combining the experimental results with density functional theory (DFT) calculations [67]. Although the enhancement through DNP was relatively modest, the low temperatures required for the MAS-DNP not only increased the thermal Boltzmann polarization, but the relaxation time of the protons in the rotating frame, 1H T 1ρ ( $ 5 ms at 295 K and 11.7 T, to $ 60 ms at 105 K and 9.4 T, using an rf spin-locking field amplitude of $ 50 kHz), and the heat dissipation in the probe (permitting the long CP contact times required for efficient transfer to the extremely low-γ 89 Y nuclei (γ 89Y E0.05γ 1H )), allowing much improved { 1 H-} 89 Y CP conditions and greater sensitivity, rendering this experiment practicable at the low temperatures; the enhancement from the DNP just made this $ 10 times faster!…”

Dynamic Nuclear Polarization Solid-State NMR in Heterogeneous Catalysis Research

“…Most recently, spectra of other traditionally challenging nuclei were recorded using DNP, including 59 Co MAS spectra (Figure 4C), 52 and wide-line, still unpublished, 195 Pt spectra of Pt 2+ -loaded MOFs in our laboratory. The range of nuclei amenable to DNP SSNMR will undoubtedly expand in the near future through further advances in instrumentation and pulse sequences, and the development of tailored sample formulations.…”

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

“…The advent of Dynamic Nuclear Polarization (DNP), using gyrotron microwave (MW) sources, combined with MAS has opened the door to many advanced studies of solids at high magnetic fields [1]. The development and implementation of high field gyrotrons up to the Terahertz range has enabled the construction of high field MAS-DNP instruments, and led to the commercialization of MAS-DNP spectrometers by Bruker Inc. [2] operating up to 18 T. Theses high field DNP instrument developments have triggered a vast interest of the solid-state NMR community in MAS-DNP experiments, both for biological [3][4][5][6][7][8][9][10][11] and material science [12][13][14][15][16][17][18][19][20][21][22] applications. Most of these experiments performed today are using nitroxide based bi-radicals such as TOTAPOL [23,24,2,8,25,26,9], and recently new nitroxide based biradicals have been introduced with varying longitudinal relaxation times, phase memory times (or transverse relaxation time), electron-electron dipolar couplings, and relative gÀtensor orientations such as AMUPOL, bCTbK, or TEKPOL [27,26,28].…”