Improved NMR transfer of magnetization from protons to half-integer spin quadrupolar nuclei at moderate and high MAS frequencies

Abstract

Abstract. Half-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. Furthermore, this transfer has recently been combined with dynamic nuclear polarization (DNP) in order to enhance the NMR signal of half-integer quadrupolar isotope. Nevertheless, the cross-polarization transfer lacks of robustness in the case of quadrupolar nuclei and we have recently introduced as an alternative technique a through-space refocused insensitive nuclei enhancement by polarization transfer (D-RINEPT) scheme combining hetero-nuclear dipolar recoupling built from adiabatic pulses and continuous wave decoupling. This technique has been demonstrated at 9.4 T with moderate MAS frequencies, νR ≈ 10-15 kHz, in order to transfer the DNP-enhanced 1H polarization to quadrupolar nuclei. Nevertheless, polarization transfers from protons to quadrupolar nuclei are also required at higher MAS frequencies in order to improve the resolution of 1H spectra. We investigate how this transfer can be achieved at νR ≈ 20 and 60 kHz. We demonstrate that the D-RINEPT sequence using adiabatic pulses still produces efficient and robust transfer but requires large rf-fields, which may not be compatible with the specifications of commonly employed MAS NMR probes. As an alternative, we introduce robust and efficient variants of D-RINEPT and PRESTO (phase-shifted recoupling effects a smooth transfer of order) sequences using symmetry-based recoupling schemes built from single and composite π-pulses. Their performances are compared using the average Hamiltonians and experiments at B0 = 18.8 T on γ-alumina and isopropylamine templated microporous aluminophosphate AlPO4-14, featuring low and significant 1H-1H dipolar interactions, respectively. These experiments demonstrate that the 1H magnetization can be efficiently transferred to 27Al nuclei using D-RINEPT with SR412(270090180) recoupling, and PRESTO with R2227(1800) or R1672(270090180) schemes at νR = 20 or 62.5 kHz, respectively. The D-RINEPT and PRESTO recouplings complement each other since the latter is affected by dipolar truncation, whereas the former is not.