# Improved NMR transfer of magnetization from protons to half-integer spin quadrupolar nuclei at moderate and high magic-angle spinning frequencies

page448Half-integer spin quadrupolar nuclei are the only magnetic isotopes for the majority of the chemical elements. Therefore, the transfer of polarization from protons to these isotopes under magic-angle spinning (MAS) can provide precious insights into the interatomic proximities in hydrogen-containing solids, including organic, hybrid, nanostructured and biological solids. This transfer has recently been combined with dynamic nuclear polarization (DNP) in order to enhance the NMR signal of half-integer quadrupolar isotopes. However, the cross-polarization transfer lacks robustness in the case of quadrupolar nuclei, and we have recently introduced as an alternative technique a inline-formulaD-RINEPT (through-space refocused insensitive nuclei enhancement by polarization transfer) scheme combining a heteronuclear dipolar recoupling built from adiabatic pulses and a continuous-wave decoupling. This technique has been demonstrated at 9.4 T with moderate MAS frequencies, inline-formulaνR≈10–15 kHz, in order to transfer the DNP-enhanced inline-formula1H polarization to quadrupolar nuclei. Nevertheless, polarization transfers from protons to quadrupolar nuclei are also required at higher MAS frequencies in order to improve the inline-formula1H resolution. We investigate here how this transfer can be achieved at inline-formulaνR≈20 and 60 kHz. We demonstrate that the inline-formulaD-RINEPT sequence using adiabatic pulses still produces efficient and robust transfers but requires large radio-frequency (rf) fields, which may not be compatible with the specifications of most MAS probes. As an alternative, we introduce robust and efficient variants of the inline-formulaD-RINEPT and PRESTO (phase-shifted recoupling effects a smooth transfer of order) sequences using symmetry-based recoupling schemes built from single and composite inline-formulaπ pulses. Their performances are compared using the average Hamiltonian theory and experiments at inline-formulaB0=18.8 T on inline-formulaγ-alumina and isopropylamine-templated microporous aluminophosphate (AlPOinline-formula4-14), featuring low and significant inline-formula1H–inline-formula1H dipolar interactions, respectively. These experiments demonstrate that the inline-formula1H magnetization can be efficiently transferred to inline-formula27Al nuclei using inline-formulaD-RINEPT with inline-formula $M17inlinescrollmathml\mathrm{normal SR}{normal 4}_{normal 1}^{normal 2}$ 28pt16ptsvg-formulamathimgbfb1b3a831bd4cb203459286d6469a1c mr-2-447-2021-ie00001.svg28pt16ptmr-2-447-2021-ie00001.png (270inline-formula090inline-formula180) recoupling and using PRESTO with inline-formula $M20inlinescrollmathml\mathrm{normal R}{normal 22}_{normal 2}^{normal 7}$ 27pt16ptsvg-formulamathimg1467477ecea38809b6b6f06c82d8832a mr-2-447-2021-ie00002.svg27pt16ptmr-2-447-2021-ie00002.png (180inline-formula0) or inline-formula $M22inlinescrollmathml\mathrm{normal R}{normal 16}_{normal 7}^{normal 6}$ 27pt16ptsvg-formulamathimg77936c715f231774d5e767150eec791c mr-2-447-2021-ie00003.svg27pt16ptmr-2-447-2021-ie00003.png (270inline-formula090inline-formula180) schemes at inline-formulaνR=20 or 62.5 kHz, respectively. The inline-formulaD-RINEPT and PRESTO recoupling schemes complement each other since the latter is affected by dipolar truncation, whereas the former is not.

We also analyze the losses during these recoupling schemes, and we show how these magnetization transfers can be used at inline-formulaνR=62.5 kHz to acquire in 72 min 2D HETCOR (heteronuclear correlation) spectra between inline-formula1H and quadrupolar nuclei, with a non-uniform sampling (NUS).

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Gómez, Jennifer S. / Rankin, Andrew G. M. / Trébosc, Julien / et al: Improved NMR transfer of magnetization from protons to half-integer spin quadrupolar nuclei at moderate and high magic-angle spinning frequencies. 2021. Copernicus Publications.

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