Population transfer HMQC for half-integer quadrupolar nuclei

Advanced solid-state NMR spectroscopy and its applications in zeolite chemistry

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host-guest/guest-guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host-guest/guest-guest interactions, and catalytic reaction mechanisms.

t1-noise elimination by continuous chemical shift anisotropy refocusing

Due to their high gyromagnetic ratio, there is considerable interest in measuring distances and correlations involving protons, but such measurements are compounded by the simultaneous recoupling of chemical shift anisotropy (CSA). This secondary recoupling adds additional modulations to the signal intensities that ultimately lead to t₁-noise and signal decay. Recently, Venkatesh et al. demonstrated that the addition of CSA refocusing periods during ¹H-X dipolar recoupling led to sequences with far higher stability and performance. Herein, we describe a related effort and develop a symmetry-based recoupling sequence that continually refocuses the ¹H CSA. This sequence shows superior performance to the regular and t₁-noise eliminated D-HMQC sequences in the case of spin-1/2 nuclei and comparable performance to the later for half-integer quadrupoles.

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

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 SR 4 1 2 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 titania-supported MoO₃ , Mg(OH)₂ , and l-histidine·HCl·H₂ O. The introduced RINEPT method outperforms the existing methods, both in terms of efficiency and robustness to rf-field inhomogeneity and offset.

35 Cl-1 H Heteronuclear correlation magic-angle spinning nuclear magnetic resonance experiments for probing pharmaceutical salts

Heteronuclear multiple-quantum coherence (HMQC) pulse sequences for establishing heteronuclear correlation in solid-state nuclear magnetic resonance (NMR) between ³⁵ Cl and ¹ H nuclei in chloride salts under fast (60 kHz) magic-angle spinning (MAS) and at high magnetic field (a ¹ H Larmor frequency of 850 MHz) are investigated. Specifically, recoupling of the ³⁵ Cl-¹ H dipolar interaction using rotary resonance recoupling with phase inversion every rotor period or the symmetry-based SR4² ₁ pulse sequences are compared. In our implementation of the population transfer (PT) dipolar (D) HMQC experiment, the satellite transitions of the ³⁵ Cl nuclei are saturated with an off-resonance WURST sweep, at a low nutation frequency, over the second spinning sideband, whereby the WURST pulse must be of the same duration as the recoupling time. Numerical simulations of the ³⁵ Cl-¹ H MAS D-HMQC experiment performed separately for each crystallite orientation in a powder provide insight into the orientation dependence of changes in the second-order quadrupolar-broadened ³⁵ Cl MAS NMR lineshape under the application of dipolar recoupling. Two-dimensional ³⁵ Cl-¹ H PT-D-HMQC MAS NMR spectra are presented for the amino acids glycine·HCl and l-tyrosine·HCl and the pharmaceuticals cimetidine·HCl, amitriptyline·HCl and lidocaine·HCl·H₂ O. Experimentally observed ³⁵ Cl lineshapes are compared with those simulated for ³⁵ Cl chemical shift and quadrupolar parameters as calculated using the gauge-including projector-augmented wave (GIPAW) method: the calculated quadrupolar product (P_Q ) values exceed those measured experimentally by a factor of between 1.3 and 1.9.

Through-space 11 B-27 Al correlation: Influence of the recoupling channel

Through-space heteronuclear correlation (D-HETCOR) experiments based on heteronuclear multiple-quantum correlation (D-HMQC) and refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) sequences have been proven to be useful approaches for the detection of the spatial proximity between half-integer quadrupolar nuclei in solids under magic-angle spinning (MAS) conditions. The corresponding pulse sequences employ coherence transfers mediated by heteronuclear dipolar interactions, which are reintroduced under MAS by radiofrequency irradiation of only one of the two correlated nuclei. We investigate herein using numerical simulations of spin dynamics and solid-state NMR experiments on magnesium aluminoborate glass how the choice of the channel to which the heteronuclear dipolar recoupling is applied affects the transfer efficiency of D-HMQC and D-RINEPT sequences between ¹¹ B and ²⁷ Al nuclei. Experimental results show that maximum transfer efficiency is achieved when the recoupling scheme is applied to the channel, for which the spin magnetization is parallel to the B₀ axis in average.

Proton-detected solid-state NMR spectroscopy of spin-1/2 nuclei with large chemical shift anisotropy

Constant-time (CT) dipolar heteronuclear multiple quantum coherence (D-HMQC) has previously been demonstrated as a method for proton detection of high-resolution wideline NMR spectra of spin-1/2 nuclei with large chemical shift anisotropy (CSA). However, ¹H transverse relaxation and t₁-noise often reduce the sensitivity of D-HMQC experiments, preventing the theoretical gains in sensitivity provided by ¹H detection from being realized. Here we demonstrate a series of improved pulse sequences for ¹H detection of spin-1/2 nuclei under fast MAS, with ¹⁹⁵Pt SSNMR experiments on cisplatin as an example. First, a t₁-incrementation protocol for D-HMQC dubbed Arbitrary Indirect Dwell (AID) is demonstrated. AID allows the use of arbitrary, rotor asynchronous t₁-increments, but removes the constant time period from CT D-HMQC, resulting in improved sensitivity by reducing transverse relaxation losses. Next, we show that short high-power adiabatic pulses (SHAPs), which efficiently invert broad MAS sideband manifolds, can be effectively incorporated into ¹H detected symmetry-based resonance echo double resonance (S-REDOR) and t₁-noise eliminated (TONE) D-HMQC experiments. The S-REDOR experiments with SHAPs provide approximately double the dipolar dephasing, as compared to experiments with rectangular inversion pulses. We lastly show that sensitivity and resolution can be further enhanced with the use of swept excitation pulses as well as adiabatic magic angle turning (aMAT).

t1-Noise eliminated dipolar heteronuclear multiple-quantum coherence solid-state NMR spectroscopy

t₁-Noise eliminated (TONE) heteronuclear multiple quantum correlation (HMQC) solid-state nuclear magnetic resonance pulse sequences improve the sensitivity of 2D ¹H{X} heteronuclear correlation experiments with X = ¹⁷O, ²⁵Mg, ²⁷Al and ³⁵Cl.

The acidic nature of "NMR-invisible" tri-coordinated framework aluminum species in zeolites

The unambiguous characterization of different acid sites in zeolites is of great importance for understanding their catalytic performance and the rational design of highly efficient zeolite catalysts. In addition to various well-characterized extra-framework Al species, a tri-coordinated framework aluminum species can also serve as a Lewis acid site in zeolites, which is "NMR-invisible" owing to its extremely distorted local environment. Here we provide a feasible and reliable approach to elucidate the acidic nature of the tri-coordinated framework Al in dehydrated H-ZSM-5 zeolites via sensitivity-enhanced two-dimensional (2D) multiple nuclear correlation NMR experiments coupled with trimethylphosphine oxide (TMPO) probe molecules. Two types of tri-coordinated framework Al sites have been unambiguously identified, which amount to 11.6% of the total Brønsted and Lewis acid sites. Furthermore, it was found that synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the Brønsted acid site lead to the generation of superacidity (with an acid strength stronger than 100% H₂SO₄) in the zeolite.

Perspectives on high-field and solid-state NMR methods of quadrupole nuclei

High magnetic field can dramatically increase the spectral resolution and sensitivity of quadrupole nuclei S > 1/2 by the reduction of the second-order quadrupole broadening. A brief overview and outlook on spectral acquisition, the importance of high magnetic field, inter-nuclei distance measurement, various 2D separation and correlation methods of quadrupole nuclei are presented. The complications and consequences of spin dynamics under rf irradiation for the (2S + 1) level system and level-crossing with the satellite transition frequencies under magic-angle spinning are discussed. There is a scaling down of (S + 1/2) to the efficiency of many experiments in comparison with a spin-1/2 due to the fact that only two central transition spin states out of the (2S + 1) levels contribute to polarization transfer and spin correlation.

Reducing t1 noise through rapid scanning

The so-called t₁ noise, which arises due to random instabilities in the spectrometer hardware, remains the primary source of noise that limits the sensitivity of most 2D NMR experiments, particularly in the expanding group of solid-state NMR methods that utilize dipolar-recoupling. In this communication we revisit the relationship between the signal intensity and the t₁ noise produced. It is shown that since the latter scales linearly with the signal strength, the use of a conventional relaxation delay of 1.3T₁ may prove far from optimal. In cases where the fluctuations occur on a shorter timescale than the recycle delay, a considerably faster repetition rate should be used to maximize the time sensitivity in a 2D experiment than what is used to maximize the sensitivity in 1D. This is demonstrated with the acquisition of ¹H{¹³C} Dipolar-mediated Heteronuclear Multiple-Quantum Correlation (D-HMQC) type spectra in which the sensitivity could be nearly doubled by choosing a very short relaxation delay corresponding to 0.2T₁.

Uniform signal enhancement in MAS NMR of half-integer quadrupolar nuclei using quadruple-frequency sweeps

We introduce two MAS schemes that allow manipulating the satellite-transition (ST) populations of half-integer quadrupolar nuclei, and which both exhibit improved robustness to the quadrupolar coupling constant (C_Q). These schemes, called quadruple frequency sweep (QFS) or quadruple WURST (QWURST) are the sums of two DFS or four WURST to efficiently invert the ST populations of nuclei subject to large or small quadrupole interactions, simultaneously. These quadruple sweeps methods only require 6% more rf-power than the double sweeps ones. We demonstrate, both numerically and experimentally, that the QFS and QWURST schemes benefit from robustness to C_Q and rf amplitude and offset and hence achieve uniform enhancement of the CT signal for ²⁷Al nuclei subject to different quadrupole interactions. Although the version of QFS with repetitive accumulation can achieve higher enhancement in the S/N of the ²⁷Al MAS spectrum, the final sensitivity gains mainly depend on the longitudinal relaxation time of different ²⁷Al sites. We also confirm that these schemes provide an improved acceleration of the ³¹P-{²⁷Al} coherence transfer in PT-J-HMQC experiments.

New insights into the di-n-propylamine (DPA) molecule as an organic structural directing agent (OSDA) in the crystallization of AlPO4-11 molecular sieve

Di-n-propylamine (DPA) molecules induce the transformation from 4/6-MR chains to a 2D layered structure and then to 3D crystals of AlPO₄-11 molecular sieves.

Proton detection of MAS solid-state NMR spectra of half-integer quadrupolar nuclei

Fast magic angle spinning (MAS) and proton detection has found widespread application to enhance the sensitivity of solid-state NMR experiments with spin-1/2 nuclei such as ¹³C, ¹⁵N and ²⁹Si, however, this approach is not yet routinely applied to half-integer quadrupolar nuclei. Here we have investigated the feasibility of using fast MAS and proton detection to enhance the sensitivity of solid-state NMR experiments with half-integer quadrupolar nuclei. The previously described dipolar hetero-nuclear multiple quantum correlation (D-HMQC) and dipolar refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) pulse sequences were used for proton detection of half-integer quadrupolar nuclei. Quantitative comparisons of signal-to-noise ratios and the sensitivity of proton detected D-HMQC and D-RINEPT and direct detection spin echo and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) solid-state NMR spectra, demonstrate that one dimensional proton detected experiments can provide sensitivity similar to or exceeding that obtainable with direct detection QCPMG experiments. 2D D-HMQC and D-RINEPT experiments provide less sensitivity than QCPMG experiments but proton detected 2D hetero-nuclear correlation solid-state NMR spectra of half-integer nuclei can still be acquired in about the same time as a 1D spin echo spectrum. Notably, the rarely used D-RINEPT pulse sequence is found to provide similar, or better sensitivity than D-HMQC in some cases. Proton detected D-RINEPT benefits from the short longitudinal relaxation times (T₁) normally associated with half-integer quadrupolar nuclei, it can be combined with existing signal enhancement methods for quadrupolar nuclei, and t₁-noise in the indirect dimension can easily be removed by pre-saturation of the ¹H nuclei. The rapid acquisition of proton detected 2D HETCOR solid-state NMR spectra of a range of half-integer quadrupolar nuclei such as ¹⁷O, ²⁷Al, ³⁵Cl and ⁷¹Ga is demonstrated.

Heteronuclear correlation experiments of 23Na-27Al in rotating solids

We demonstrated that the heteronuclear correlation experiments between two quadrupolar nuclei, ²³Na and ²⁷Al, with close Larmor frequencies can be achieved via D-HMQC and D-RINEPT approaches by using a diplexer connected to a conventional probe in magic-angle-spinning solid-state NMR. Low-power heteronuclear dipolar recoupling schemes can be applied on ²³Na or ²⁷Al to establish polarization transfers between the central transitions of ²³Na and ²⁷Al for a model compound, NaAlO₂. Further, we showed a practical implementation of the two dimensional ²³Na-²⁷Al dipolar-based heteronuclear correlation experiment on a heterogeneous catalyst, Na₂CO₃/γ-Al₂O₃. This allows to determine spatial proximities between different ²³Na and ²⁷Al sites, thus the surface Na species adjacent to octahedral-coordination Al can be clearly discriminated.

Solid-State ¹⁷O NMR studies of organic and biological molecules: Recent advances and future directions

This Trends article highlights the recent advances published between 2012 and 2015 in solid-state (17)O NMR for organic and biological molecules. New developments in the following areas are described: (1) new oxygen-containing functional groups, (2) metal organic frameworks, (3) pharmaceuticals, (4) probing molecular motion in organic solids, (5) dynamic nuclear polarization, and (6) paramagnetic coordination compounds. For each of these areas, the author offers his personal views on important problems to be solved and possible future directions.

Quantitative structure parameters from the NMR spectroscopy of quadrupolar nuclei

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important characterization tools in chemistry, however, 3/4 of the NMR active nuclei are underutilized due to their quadrupolar nature. This short review centers on the development of methods that use solid-state NMR of quadrupolar nuclei for obtaining quantitative structural information. Namely, techniques using dipolar recoupling as well as the resolution afforded by double-rotation are presented for the measurement of spin–spin coupling between quadrupoles, enabling the measurement of internuclear distances and connectivities. Two-dimensional J-resolved-type experiments are then presented for the measurement of dipolar and J coupling, between spin-1/2 and quadrupolar nuclei as well as in pairs of quadrupolar nuclei. Select examples utilizing these techniques for the extraction of structural information are given. Techniques are then described that enable the fine refinement of crystalline structures using solely the electric field gradient tensor, measured using NMR, as a constraint. These approaches enable the solution of crystal structures, from polycrystalline compounds, that are of comparable quality to those solved using single-crystal diffraction.

PRESTO polarization transfer to quadrupolar nuclei: implications for dynamic nuclear polarization

We show both experimentally and numerically that in experiments involving transfer of magnetization from ¹H to the quadrupolar nuclei under MAS, the PRESTO technique consistently outperforms the traditionally used CP method, affording more quantitative intensities, improved lineshapes, better sensitivity, and easier optimization.