Efficient transfer of DNP‐enhanced ¹H magnetization to half‐integer quadrupolar nuclei in solids at moderate spinning rate

Abstract

We show herein how the proton magnetization enhanced by dynamic nuclear polarization (DNP) can be efficiently transferred at moderate magic-angle spinning (MAS) frequencies to half-integer quadrupolar nuclei, S ≥ 3/2, using the Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (D-RINEPT) technique, in which a symmetry-based SR4 2 1 recoupling scheme built from adiabatic inversion 1 H pulses reintroduces the 1 H-S dipolar couplings, while suppressing the 1 H-1 H ones. The use of adiabatic pulses also improves the robustness to offsets and radiofrequency (rf)-field inhomogeneity. Furthermore, the efficiency of the polarization transfer is further improved by using 1 H composite pulses and continuous-wave irradiations between the recoupling blocks, as well as by manipulating the S satellite transitions during the first recoupling block. Furthermore, in the case of large 1 H-S dipolar couplings, the D-RINEPT variant with two pulses on the quadrupolar channel results in an improved transfer efficiency. We compare here the performances of this new adiabatic scheme with those of its parent version with single π pulses, as well as with those of PRESTO and CPMAS transfers. This comparison is performed using simulations as well as DNP-enhanced 27 Al, 95 Mo, and 17 O NMR experiments on isotopically unmodified γ-alumina, hydrated titaniasupported MoO 3 , Mg(OH) 2 , and L-histidineÁHClÁH 2 O. The introduced RINEPT method outperforms the existing methods, both in terms of efficiency and robustness to rf-field inhomogeneity and offset.

Ultra-High-Field 67Zn and 33S NMR Studies Coupled with DFT Calculations Reveal the Structure of ZnS Nanoplatelets Prepared by an Organometallic Approach

“…In order to observe the Zn and S sites located near the surface of ZnS·DDA NPls, we also transferred the 1 H magnetization to 67 Zn and 33 S nuclei using the dipolar-mediated refocused INEPT scheme ( D -RINEPT) , and enhanced the signal using QCPMG detection along with DNP. ,, For the DNP-NMR experiments performed at 9.4 T and 105 K, the unpaired electrons were introduced in the sample by impregnating ZnS·DDA NPls with a solution of 13 mM TEKPol nitroxide biradicals in 1,1,2,2-tetrachloroethane (TCE).…”

Advances in the characterization of inorganic solids using NMR correlation experiments

“…Conversely when the spin-1/2 nuclei are subject to large homonuclear dipolar interactions, like in the case of protons, recoupling reintroducing the |m| = 2 space component of the heteronuclear dipolar interaction, such as those based on SR4 1 2 symmetry 220 , are required since they suppress the contribution of homonuclear dipolar interactions to the first-order average Hamiltonian 239,244,245 . Furthermore, we have recently demonstrated that the efficiency of the D-RINEPT transfer from protons to half-integer quadrupolar nuclei can be improved by employing SR4 1 2 recoupling built from composite or adiabatic tanh/tan inversion pulses, continuous wave irradiation during the windows and composite 90° and 180° pulses on the 1 H channel [245][246][247] . These modifications limit the losses due to 1 H-1 H dipolar interactions and improve the robustness to rf field inhomogeneity.…”

43Ca MAS-DNP NMR of Frozen Solutions for the Investigation of Calcium Ion Complexation

“…The latter is typically low in the case of a quadrupolar nucleus such as 43 Ca because a proper magnetization spin lock, required in CP, can be prevented by the time dependence of the quadrupolar interaction. A viable option to partially circumvent this issue is to use low radio frequency (RF) fields. − In DNP NMR ,− applications, RF fields comprised between 0.8 and 20 kHz have been reported on quadrupolar nuclei for proton-to-quadrupolar nucleus CPMAS. In the case of 43 Ca, MAS-DNP NMR was successfully used by Lee et al to understand the chemical environment of hydroxyapatite by recording 1 H– 43 Ca dipolar-based 2D correlation spectra that enabled the distinction of core and surface Ca 2+ sites.…”