Oxygen-17 dynamic nuclear polarisation enhanced solid-state NMR spectroscopy at 18 . 8 T †

We report O dynamic nuclear polarisation (DNP) enhanced solid-state NMR experiments at 18.8 T. Several formulations were investigated on the Mg(OH)2 compound. A signal enhancement factor of 17 could be obtained when the solid particles were incorporated into a glassy o-terphenyl matrix doped with BDPA using the Overhauser polarisation transfer scheme whilst the cross effect mechanism enabled by TEKPol yielded a slightly lower enhancement but more time efficient data acquisition.

Solid-state nuclear magnetic resonance (NMR) spectroscopy involving quadrupolar nuclei, which account for more than 75% of all active nuclei, has become an essential analytical technique for the atomic-scale characterization of a wide range of inorganic, hybrid and organic materials such as glasses, zeolites, surface catalysts, metal-organic frameworks, battery materials or pharmaceuticals to name but a few. [1][2][3][4] While for spin I = 1/2 nuclei, the dipolar and chemical shift anisotropy (CSA) interactions are fully averaged out by magic angle spinning (MAS), 5 producing sharp lines positioned at the isotropic chemical shifts, the second-order quadrupolar interaction observed for spin I 4 1/2 is not fully removed by MAS, [6][7][8] yielding field-dependent shifts and broadened NMR lines. These are some of the major limitations to the widespread applicability of quadrupolar NMR. [7][8][9][10] The second-order quadrupolar broadening is inversely proportional to the strength of the external field B 0 and therefore very high magnetic fields allow high spectral resolution. 11 In parallel, a range of NMR experiments have been designed to completely average out the second-order broadenings. They are based on technically challenging NMR probes permitting sample double rotation (DOR) 12 or dynamic angle spinning (DAS), 13 or consist of combining conventional MAS with the manipulation of the different nuclear spin transitions in techniques such as multiple quantum MAS (MQMAS) 14 and satellite transition MAS (STMAS). 15 Despite these sophisticated approaches and the increasing availability of high magnetic fields, the sensitivity of quadrupolar NMR remains often low for nuclei of poor natural abundance and/or of low gyromagnetic ratio g, requiring prohibitively long experimental times (days) and/or expensive isotopic enrichment.
A spectacular approach to increase the solid-state NMR sensitivity is dynamic nuclear polarisation (DNP), 16,17 which involves the microwave-driven transfer of the large polarisation of unpaired electrons 18-21 (e.g. added to the samples as paramagnetic polarising agents) [22][23][24][25][26][27][28] to the surrounding nuclei in a glass-forming matrix at cryogenic temperatures, typically 100 K or below. 29,30 The drastic signal enhancements permitted by DNP have opened up ground breaking applications on an ever increasing range of systems [31][32][33][34][35][36][37] and the approach has been reviewed recently. [38][39][40][41][42] Recent developments have extended the temperature range in which the experiments can be conducted (4200 K). 24,43,44 One of the biggest drawbacks to MAS DNP under high magnetic fields results from the unfavorable evolution of the NMR signal enhancements with B 0 which scales as B 0 À1 and B 0 À2 for the two most common polarization transfer mechanisms, the cross effect (CE) and solid effect, respectively. However, large signal enhancements (480) have recently been obtained at 18.8 T using the Overhauser effect (OE) DNP mechanism 16,44,45 and the narrow-line 1,3-bisdiphenylene-2phenylallyl (BDPA) radical. 46 The apparent linear scaling of the OE signal enhancement with B 0 currently represents one of the most attractive approaches for DNP under very high fields and is an exciting opportunity for quadrupolar nuclei. A quadrupolar nucleus of particular interest is 17 O due to the ubiquity of oxygen in materials chemistry and biochemistry. However, its extremely low natural abundance (0.037%) makes its NMR detection near impossible unless samples are 17 O-enriched. 47 The feasibility of MAS DNP on 17 O has been recently reported. [48][49][50][51][52] In particular, we demonstrated that high S/N natural abundance 17 O cross polarization (CP) MAS NMR spectra could be obtained on nanoparticles, 49 while more recently, the detection of 17 O DNP NMR spectra of surface hydroxyl sites on mesoporous silica in natural abundance was reported. 52 All these experiments were recorded at 9.4 T and relied on the CE mechanism with bTbk 23 or TEKPol 24 radicals in tetrachloroethane (TCE) 53  dependency of the CE. It is worth noting that on freshly prepared samples lower enhancements were observed (e H = 16, e O CP = 9) and that the maximum values reported above were obtained after holding the sample at 253 K for 20 h before freeze-thaw cycling (see the ESI †) and inserting it into the probe ( Fig. 1 and Fig. S4, ESI †).
It was recently shown that the use of a PRESTO (phase-shifted recoupling effects a smooth transfer of order) sequence 55 is more efficient for the 1 H-17 O heteronuclear polarization transfer and yields for Mg(OH) 2 , Ca(OH) 2 and silica surfaces 52 line-shapes closer to simulations than with CP, including those in the context of MAS DNP. 59 Fig. 1b shows the corresponding CE DNP 17 O PRESTO Fig. 1 (a-d) 17 (14). However, in our hands and at 18.8 T, the overall signal intensity is lower than with 17 O CP as predicted. 55 We also note that the asymmetric line shape and dephased spinning side bands are exacerbated by the addition of microwave irradiation (Fig. S3, ESI †). The lower intensity of PRESTO vs. CP and line shape disparity is also observed using the OE (Fig. 1e and f). The 17 O signal enhancement factors obtained open the way to obtain natural abundance spectra using the CE at 18.8 T, and we were able to record 17 O CP and PRESTO MAS NMR spectra of Mg(OH) 2 relatively quickly in 82 minutes (Fig. 1c and d), while the corresponding microwave off spectra show no signal (Fig. 1c).
Sample formulation is essential to achieve high DNP enhancement factors, and in particular the choice of the solvent is often critical. 60 It has been shown that OTP forms a highly effective glassy matrix 61 and could be polarized by CE DNP with TEKPol and even more efficiently by OE DNP with BDPA. 44,45,61,62 In the next paragraphs, we report 17 O enhancement factors using OTP with both TEKPol and BDPA as the polarising matrix. The Mg( 17 OH) 2 samples were prepared by grinding them with a mixture of protonated and fully deuterated OTP of various ratios containing TEKPol or BDPA radicals (1.3-1.4 wt% equivalent to 34 mM electron spins) followed by multiple cycles (typically 5) between a melt at B343 K 61 and a frozen state at 77 K (in liquid N 2 ) before the melt was inserted into the precooled NMR probe at B115 K (see the ESI, † Section S1, for detailed sample preparation). Fig. 2 plots the 13 C and 17 O CPMAS CE DNP signal enhancements of the Mg( 17 OH) 2 /TEKPol/OTP glass matrix as a function of the percentage of OTP-d 14 . The 13 C and 17 O enhancements correspond to the NMR signal amplification of, respectively, the OTP matrix and the Mg( 17 OH) 2 particles. The data show that with a fully protonated OTP matrix, the signal enhancements are similar (e C CP = 8 and e O CP = 6), revealing that the polarisation is efficiently transferred from the glassy matrix to Mg( 17 OH) 2 and that the polarisation is evenly distributed across the particles by 1 H-1 H spin diffusion mechanisms. 63 However, the enhancements are lower than with 450% OTP-d 14 matrices (vide infra) and are due to both the large size of the 1 H bath and the short 1 H polarisation time t DNP (B3 s), yielding a fast decay of the enhanced polarization by relaxation before it can be transferred to Mg( 17 OH) 2 . Increasing the content of OTP-d 14 in the matrix improves the signal enhancements of the sample (Table 1 and Fig. S2 Fig. 1e and 2) is substantially larger than with BDPA in a TCE matrix (e O CP = 5, Table 1 and Fig. S5, ESI †). The multiple freeze-melt cycles appear to improve the quality of the glassy matrix, and thus improve the enhancement values on the OTP matrix and sample, compared to directly inserting the sample into the probe (Fig. S5 17 O obtained with the CE (using the same matrix). This is due to the much shorter t DNP in the CE system (B11 s) than that of the OE in OTP-d 14 (B31 s), allowing for more scans to be accumulated per unit time.
In conclusion, we have shown that it is possible to transfer the OE DNP enhanced polarisation of OTP doped with BDPA to the 17 O spins of Mg( 17 OH) 2 hydroxides at 18.8 T with good efficiency by mixing both solids together. We have also demonstrated that despite the lower e values obtained with the CE, TEKPol/OTP and TEKPol/ TCE provide time efficient signal enhancement. This has enabled a large gain in absolute sensitivity, permitting the challenging detection of natural abundance 17 O NMR spectra. This study paves the way to a wider application of 17 O DNP enhanced NMR under a high magnetic field and its transposition to other quadrupolar nuclei in a variety of crystalline and amorphous inorganic materials with strong second-order quadrupolar broadenings hampering the spectral resolution under a low field.
Financial support from the EPSRC for a DTA studentship for N. J. B. and grant EP/M00869X/1 to F. B., ERC Advanced Grant No. 320860 for L. E. and EQUIPEX contract ANR-10-EQPX-47-01 for A. L. and L. E. is acknowledged. We thank Dr Sachin R. Chaudhari (CRMN Lyon), Kenneth K. Inglis (University of Liverpool), and Dr O. Ouari and Prof. P. Tordo (Aix-Marseille Université, CNRS) for valuable technical assistance, sample preparation, and radicals, respectively. F. B. thanks the TGIR-RMN-THC Fr3050 CNRS for access to the 18.8 T DNP NMR facility at the CRMN. The experimental data are provided as a supporting dataset from the University of Liverpool Data Catalogue portal at https:// datacat.liverpool.ac.uk/245/.