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Quadrupolar Nuclei Hyperpolarization by 18.8 T DNP

More than 75% of all NMR active nuclei have a spin larger than 1/2. The detection of these quadrupolar nuclei have been essential for the characterization of new solid materials, such as zeolites, surface catalysts, metal-organic frameworks and battery materials or pharmaceuticals,1 in order to obtain structure-function relationships and rationalize their properties.

Nevertheless, the NMR spectra of these nuclei are often broad and poorly resolved due to the presence of the strong quadrupolar interaction which is not fully averaged to zero by the routinely available magic angle spinning (MAS) technique. The residual broadening is however inversely proportional to the strength of the external field and a brute force approach towards high resolution often consists of recording spectra at very high magnetic fields.2

In contradiction, the most spectacular approach to enhancing the NMR sensitivity of dynamic nuclear polarization (DNP),3–5 which consists of transferring the high electron polarization to the weak nuclear polarization, is usually fairly inefficient at very high magnetic fields (> 18 T) and therefore of limited use to quadrupolar nuclei.

A team of scientists at the University of Liverpool, the Centre de RMN à Très Hauts Champs de Lyon and the Ecole Polytechnique Fédérale de Lausanne, and lead by Prof. Frédéric Blanc, has recently showed, in the RSC journal Chemical Communications, that the use of a rigid matrix doped with a radical provides efficient electron – nuclei polarisation transfer at 18.8 T and permits the detection of 17O, a quadrupolar nuclei with spin 5/2.6

hyperpolarization of quadrupolar nuclei

The research, which was funded by the UK Engineering and Physical Sciences Research Council, was performed on one of the three 527 GHz 18.8 T DNP NMR spectrometer in the world installed in Lyon, manufactured by Bruker, and made available to the Blanc’s group as part of the french national infrastructure network in NMR (TGIR-RMN).

The work explores the hyperpolarization of 17O using both the cross effect and Overhauser3 electron – nuclei polarisation transfer mechanisms, the latter having recently been identified as a possible new approach for DNP at very high magnetic fields.7,8 The authors found that although the Overhauser mechanism provides a slightly larger enhancement than the cross effect, this mechanism still offers the most time effective pathway for rapid data acquisition for 17O NMR spectra in layered hydroxides.

Very high field DNP enhanced solid state NMR is evolving rapidly, alongside instrumentation development, and the results described here will pave the way for expanding the range of applications of quadrupolar DNP NMR for the study of materials.

References

1.  MacKenzie, K. J. D. et al. Multinuclear Solid-State NMR of Inorganic Materials; Pergamon Press: Oxford, 2002.
2.  Gan, Z. et al. J. Am. Chem. Soc. 2002, 124, 5634.
3.  Carver, T. R. et al. Phys. Rev. 1956, 102, 975.
4.  Hall, D. A. et al. Science  1997, 276, 930.
5.  Zhe Ni, Q. et al. Acc. Chem. Res. 2013, 46, 1933.
6.  Brownbill, N. J. et al. Chem. Commun. 2017, 53, 2563.
7.  Can, T. V et al. J. Chem. Phys. 2014, 141, 64202.
8.  Lelli, M. et al. J. Am. Chem. Soc. 2015, 137, 14558.

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