The one-dimensional solid-state NMR sequence called TRAPDOR (TRAnsfer of Populations in DOuble Resonance) has been proposed to probe proximities between quadrupolar nuclei and spin-1/2 isotopes under magic-angle spinning. In that sequence, a single long high-power radiofrequency (rf) pulse is applied to the quadrupolar nuclei, which are the dephaser spins, to reintroduce their dipolar interactions with the spy isotope. However, the signal detected using this sequence is also dephased by the Bloch-Siegert shift and other anisotropic interactions of the dephaser spins, and to limit these dephasings, complicated phase corrections must be applied. Moreover, the rf field inhomogeneity in the sample and the different orientations of the crystallites for a powder lead to a distribution of phases, which results in an apparent reduction of the intensity of the dephased spectrum. This reduced amplitude can be erroneously interpreted as proximity to the dephaser spins. We show here that these issues can be circumvented by applying to the dephaser spins of a pair of identical long high-power rf pulses with relative phase φ. The spin dynamics during this TRAPDOR variant, called TRAPDOR-2φ, and the influence of the delay between the two long pulses along with their relative phase are analyzed using an effective Hamiltonian and numerical simulations. This method is then experimentally demonstrated to probe proximities between protons and 14N or 35Cl nuclei in N-acetyl-l-valine (NAV) and l-histidine·HCl·H2O, as well as between 13C and 14N isotopes in the second compound. This pulse sequence is also applied to observe 27Al-11B proximities in a magnesium aluminoborate glass. Globally, we show that the TRAPDOR-2180 experiment, which is simple to optimize, yields an efficient and robust heteronuclear recoupling between quadrupolar nuclei or between spin-1/2 and quadrupolar isotopes.

Quadrupolar nuclei with half-integer spin, which represent 66 % of the NMR-active isotopes, are present in a wide range of materials with applications in various fields, including heterogeneous catalysis, optoelectronics and energy. The solid-state NMR spectra of these isotopes are affected by quadrupolar interactions, which provide unique information on the local environment of these nuclei, in addition to their chemical shifts. These anisotropic interactions, which are generally larger than other internal spin interactions, split and broaden the NMR transitions, which reduce the sensitivity for the detection of these isotopes. In addition, the large dimensions of their density matrices and the numerous NMR transitions complicate the spin dynamics and can reduce the efficiency of coherence transfers, such as cross-polarization under magic-angle spinning (CPMAS), which is widely employed to boost the sensitivity for the detection of spin-1/2 isotopes. In the last decade, sensitivity gains provided by dynamic nuclear polarization (DNP) have been exploited to detect half-integer quadrupolar nuclei in solids. This review discusses the advantages and limitations of the different DNP-NMR techniques that have been proposed for the detection of these isotopes, including direct excitation and CPMAS, and two more recently introduced methods called PRESTO (Phase-shifted Recoupling Effects by Smooth Transfer of Order) and D-RINEPT (Dipolar-mediated Refocusing Insensitive Nuclei Enhanced by Polarization Transfer). We also show how these techniques can be applied to obtain new insights on the structure of materials, notably of their surfaces, and hence, contribute to extend the range of applications of the surface-enhanced NMR spectroscopy (DNP-SENS).

The majority of chemical elements can be observed by NMR only through half-integer quadrupolar nuclei with spin S = 3/2, 5/2, 7/2 or 9/2. As a result, probing the proximities between these isotopes and protons is crucial for investigating the structure of numerous hydrogen-containing materials, including heterogeneous catalysts, pharmaceuticals, nanostructured materials, etc. These experiments must be conducted under magic-angle spinning (MAS) conditions to resolve the different chemical environments. In particular, different variants of the 1H → S through-space refocused INEPT (D-RINEPT) sequence have been proposed to probe long-range proximities between protons and half-integer quadrupolar nuclei. These pulse sequences differ by the employed heteronuclear dipolar recoupling scheme as well as the method used to refocus the chemical shift anisotropy of protons during the defocusing and refocusing periods. Some of the most efficient D-RINEPT variants that have been reported so far, include (i) that employing the symmetry-based supercycled SR412 recoupling built from adiabatic inversion pulses incorporated into two spin-echoes and (ii) that using wPMRR (windowed Phase-Modulated Rotary Resonance) recoupling combined with two dipolar-echoes. In particular, the former has been used to enhance the sensitivity of NMR detection of quadrupolar isotopes via indirect DNP (Dynamic Nuclear Polarization) via protons. Nevertheless, there is a lack of comparative studies of these D-RINEPT variants in the literature. In the present study, we introduce new variants of this general scheme, exploring the incorporation of windowed basic elements into the R412 recoupling, and the application of these novel recoupling sequences into dipolar-echo instead of spin-echo refocusing. The efficiencies of these novel D-RINEPT sequences are compared with the existing ones as well as to that of CPMAS (Cross-Polarization under MAS). This experimental comparison is performed at 18.8 T with different MAS frequencies, νR = 10, 20, and 50 kHz, for 1H → 27Al and 1H → 35Cl polarization transfers in γ-Al2O3 and l-histidine·HCl·H2O, respectively. The first sample features moderate 1H-1H dipolar interactions, whereas large 1H-1H couplings are present in the second. We demonstrate that at νR ≤ 20-25 kHz, the most efficient D-RINEPT variant is that employing on 1H channel two spin-echoes incorporating (i) the SR412 recoupling built from adiabatic pulses, (ii) two composite pulses and (iii) continuous-wave (CW) irradiation during the windows. However, at νR = 50 kHz, the radiofrequency (rf) field requirement of adiabatic pulses is not compatible with rf power specifications of the probes, and the highest transfer efficiency is then achieved with (270090180) composite π-pulses, instead of adiabatic ones in the SR412 recoupling. However, for γ-Al2O3, in which protons are subject to moderate homonuclear dipolar couplings, similar efficiencies are achieved when employing the R412R41-2 and SR412 recouplings built from windowed pulses with rf irradiation during half of the recoupling delays. These windowed recoupling sequences have the advantage of requiring less average rf power than their counterpart built from (270090180) composite pulses. Compared to CPMAS transfers, the efficiencies of the best D-RINEPT variants are comparable, but they are simpler to optimize, more robust to offsets, and they can transfer to longer distances.

Proximities between spin-1/2, e.g. 1H and 13C, and quadrupolar nuclei can be analyzed using HMQC (Heteronuclear Multiple-Quantum Correlation) experiments, in which two continuous-wave irradiations similar to those used in TRAPDOR (TRAnsfer of Population in DOuble-Resonance) experiments are applied on the indirectly detected quadrupolar isotope during the defocusing and refocusing delays. Here, we demonstrate that this sequence, called T-HMQC (T stands for TRAPDOR), can be applied to probe proximities between distinct half-integer spin quadrupolar isotopes. We introduce two novel variants of this sequence to reduce the number of resonances along the indirect dimension. These selective variants employ either (i) an echo-antiecho quadrature detection to only retain the single-quantum (1Q) coherences or (ii) two π-pulses selective of the central-transition (CT) to observe only the 1Q-CT coherences. We analyze how the effects of various experimental parameters, including the synchronization of the TRAPDOR recoupling pulses with the sample rotation, and their radio-frequency (rf) field amplitude and frequency offset, affect the efficiency of 11B-27Al T-HMQC experiments on a magnesium aluminoborate glass. The performances of these T-HMQC sequences are compared to those of the D-HMQC scheme employing the SPI-R3 (Synchronous Phase-Inversion Rotary-Resonance-Recoupling) or REDOR (Rotational-Echo DOuble-Resonance) symmetry-based heteronuclear dipolar recouplings built from CT-selective pulses. We demonstrate that the two TRAPDOR pulses in the T-HMQC sequence must be separated by an integer number of rotor periods and must employ the maximum rf field strength compatible with the probe specifications. Furthermore, as the TRAPDOR pulses distribute the populations equally to all possible coherences, the sensitivity of the T-HMQC selective variants is lower than that of the D-HMQC techniques. To limit this sensitivity decrease and the number of cross-peaks, it is preferable to detect indirectly the quadrupolar nucleus I with the lowest spin number, and in the case of I = 3/2, the resolution along the indirect dimension can be enhanced with respect to a MAS spectrum (for instance, by a factor of 27/7, without taking into account the quadrupolar-induced shift (QIS), through the sole indirect detection of triple-quantum (3Q) coherences). Moreover, owing to the use of a high-power TRAPDOR recoupling, the T-HMQC technique benefits from a wider excitation bandwidth than the D-HMQC methods, which is advantageous for broad NMR spectra, especially at high magnetic fields.

The observation of half-integer quadrupolar nuclei, which represent 66 % of the NMR-active isotopes, is essential to understand the atomic-level structure of inorganic materials near the surfaces with applications in the field of catalysis, biomaterials and optoelectronics. For that purpose, we have recently introduced an efficient technique, which combines the sensitivity gain provided by indirect DNP (dynamic nuclear polarization) under MAS (magicangle spinning) and the high resolution obtained by refocusing the second-order quadrupolar interaction (H. Nagashima et al., J. Phys. Chem. Lett. 15 (2024) 4858). This technique combines (i) a D-RINEPT (dipolarmediated refocused INEPT) transfer, (ii) an MQMAS (multiple-quantum MAS) filter, and (iii) a QCPMG (quadrupolar Carr-Purcell Meiboom-Gill) detection. We explain the design of several variants of this pulse sequence and notably the selection of the coherence transfer pathways. In particular, the amplitudes of the coherence transfer pathways through the +/- 3Q coherence orders of the quadrupolar isotope can be equalized using a train of it-pulses selective of the central transition, instead of a z-filter. This equalization method has the advantage to limit the length of the phase cycles and to enhance slightly the signal intensity. Moreover, for spin-3/2 nuclei subject to moderate or large quadrupolar interactions, more efficient excitation and conversion of 3Q coherences are achieved using cosine-modulated long-pulses (cos-lp), instead of fast-amplitude-modulated (FAM) pulses. The performances of the different D-RINEPT-MQMAS-QCPMG variants are compared through the observation of 35Cl and 27Al isotopes without DNP in L-histidine hydrochloride and isopropylamine-templated microporous aluminophosphate (ipa-AlPO4-14), respectively, as well as the acquisition of DNP-enhanced high-resolution spectra of 11B and 17O nuclei near the surface of partially oxidized boron nitride supported on dendritic and fibrous nanosilica and gamma-alumina enriched in 17O isotope using a slurrying approach. The spectra recorded for gamma-alumina show that the slurrying method produces less disorder than grinding assisted by 17O-enriched water.

The two-dimensional (2D) refocused INADEQUATE NMR experiment, which correlates double-quantum (DQ) and single-quantum (SQ) coherences, is widely used to probe the chemical connectivities in solids. Nevertheless, the multiplets along the F2 dimension reduce the resolution and sensitivity of this experiment. The Composite-Refocusing (CR) technique with two excitation pulses has been proposed to suppress these multiplets in 2D INADEQUATE spectra of liquids. Recently (Kolyagin et al., J. Phys. Chem. Lett., 13 (2022) 10793), we showed that this technique can also be applied to suppress doublets in 2D 29Si INADEQUATE spectra of 29Si-enriched zeolites, which resulted in improved sensitivity and resolution. We investigate here how this INADEQUATE-CR scheme can also be applied for two other spin-1/2 isotopes: 13C and 31P. We also demonstrate the possibility to accelerate the acquisition of these 2D INADEQUATE-CR spectra with a very simple bi-exponential non-uniform sampling (NUS). For instance, in the case of 31P nuclei in SnP2O7, the use of the INADEQUATE-CR method with NUS yields a 7.5-fold reduction in experimental time with a simultaneous 1.4-1.5 gain in resolution with respect to a conventional INADEQUATE acquisition. Furthermore, we analyze the origins of the possible artifacts in these 2D spectra, including mismatching between J-coupling constants and refocusing delays, differences in relaxation times, coupling with a proton bath, and spin systems containing multiple identical nuclei. Based on this analysis, we introduce a new z-filtered INADEQUATE-CR version, which produces artifact-free 2D spectra, even in the presence of several distinct J-couplings and relaxation times or for multi-spin systems, and notably samples with high proton density.

Through-space heteronuclear correlation experiments under magic-angle spinning (MAS) conditions can provide unique insights into inter-atomic proximities. In particular, it has been shown that experiments based on two consecutive coherence transfers, 1H -> I -> 1H, like D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation), are usually more sensitive for the indirect detection via protons of spin-3/2 quadrupolar nuclei with low gyromagnetic ratio. Nevertheless, the resolution is often decreased by the second-order quadrupolar broadening along the indirect dimension. To circumvent this issue, we incorporate an MQMAS (multiplequantum MAS) quadrupolar filter into the t1 evolution period of the D-HMQC sequence, which results in a novel pulse sequence called D-HMQC-MQ. The triple-quantum coherences evolving during this filter are excited and reconverted using cosine-modulated long-pulses synchronized with the sample rotation to avoid spinning sidebands in the indirect dimension. The desired coherence transfer pathways during this sequence are selected using two nested cogwheel phase cycles with 56 steps. This high-resolution heteronuclear correlation technique is demonstrated experimentally for the indirect detection via 1H of spin-3/2 isotopes, such as 11B, 23Na and 35Cl, in zinc borate hydrate, NaH2PO4 and L-histidine hydrochloride, respectively. We show that this experiment can be applied at high magnetic fields up to 28.2 T for protons subject to chemical shift anisotropies larger than 20 ppm, provided the MAS frequency is sufficiently stable since the D-HMQC-MQ experiment, like the parent D-HMQC, is sensitive to MAS fluctuations, which can produce t1-noise.

The surfaces of nanomaterials with applications in optoelectronics and catalysis control their physicochemical properties. NMR spectroscopy, enhanced by dynamic nuclear polarization (DNP), is a powerful approach to probe the local environment of spin-1/2 nuclei near surfaces. However, this technique often lacks robustness and resolution for half-integer quadrupolar nuclei, which represent more than 66% of the NMR-active isotopes. A novel pulse sequence is introduced here to circumvent these issues. This method is applied to observe with high-resolution Al-27 and O-17 spin-5/2 nuclei on the surface of gamma-alumina. Moreover, we report high-resolution O-17 spectra of ZnO nanoparticles used in optoelectronics. Their assignment using DFT calculations allows the first NMR observation of vacancies near the surfaces. Finally, we employ the introduced NMR technique to observe B-11 spin-3/2 nuclei on the surface of partially oxidized boron nitride supported on silica and to distinguish its different BO2OH active sites.

Various two-dimensional (2D) homonuclear correlation experiments have been proposed to observe proximities between identical half-integer spin quadrupolar nuclei in solids. These experiments select either the single- or double-quantum coherences during the indirect evolution period, t1. We compare here the efficiency and the robustness of the 2D double-quantum to single-quantum (DQ-SQ) and SQ- SQ homonuclear correlations for two half-integer spin quadrupolar isotopes subject to small chemical shift anisotropy (CSA): 11B with a nuclear spin I=3/2 and 27Al with I=5/2. Such a comparison is per- formed using experiments on two model samples: Li2B4O7 for 11B and AlPO4-14 for 27Al. For both iso- topes, the DQ-SQ homonuclear correlations are recommended since they allow probing the proximities between nuclei with close or identical frequencies. In the case of small or moderate isotropic chemical shift differences (e.g. 11B) the [SR212] or [BR212] bracketed DQ-SQ recoupling schemes are recommended; whereas it is the BR212 un-bracketed one otherwise (e.g. 27Al).(c) 2023 Elsevier Inc. All rights reserved.

In MQMAS-based high-resolution solid-state NMR experiments of half-integer spin quadrupolar nuclei, the high radiofrequency (RF) field requirement for the MQ excitation and conversion steps with two hard-pulses is often a sensitivity limiting factor in many practical applications. Recently, the use of two cosine-modulated (cos) low-power (lp) pulses, lasting one-rotor period each, was successfully introduced for efficient MQ excitation and conversion of spin-3/2 nuclei with a reduced RF amplitude. In this study, we extend our previous investigations of spin-3/2 nuclei to systems with higher spin values and discuss the applicability of coslp-MQ excitation and conversion in MQMAS and MQ-HETCOR experiments under slow and fast spinning conditions. For the numerical simulations and experiments we used a moderate magnetic field of 14.1 T. Two spin-5/2 nuclei (85Rb and 27Al) are mainly employed with a large variety of CQ values, but we show that the practical set up is also available for higher spin values, such as spin-9/2 with 93Nb in Cs4Nb11O30. We demonstrate for nuclei with spin value larger than 3/2 a preferential use of coslp-MQ acquisition for low-gamma nuclei and/or large CQ values with a much reduced RF-field with respect to that of hard-pulses used with conventional methods.