Use of SPAM and FAM pulses in high-resolution MAS NMR spectroscopy of quadrupolar nuclei

High-resolution heteronuclear correlations between spin-1/2 and half-integer quadrupolar nuclei under fast MAS solid-state NMR

High isotropic resolution is essential for the structural elucidation of samples with multiple sites. In this study, utilizing the benefits of TRAPDOR-based heteronuclear multiple quantum coherence (T-HMQC) and a pair of one rotor period long cosine amplitude modulated low-power (cos-lp) pulse-based symmetric-split-t1 multiple-quantum magic angle spinning (MQMAS) methods, we have developed a proton-detected 2D 35Cl/1H T-HMQC-MQMAS pulse sequence under fast MAS (70 kHz) to achieve high-resolution in the indirect dimension of the spin-3/2 (35Cl) nuclei connected via protons. As T-HMQC polarizes not only single-quantum central transition (SQCT) but also triple-quantum (TQ) coherences, the proposed 2D pulse sequence is implemented via selection of two coherence pathways (SQCT→TQ →SQCT and TQ → SQCT→TQ) resulting in the 35Cl isotropic dimension and is superior to the existing double-quantum satellite-transition (DQST) T-HMQC in terms of resolution.

Accelerating the acquisition of high-resolution quadrupolar MQ/ST-HETCOR 2D spectra under fast MAS via 1H detection and through-space population transfers

Recently, we established an experimental setup protocol to perform the population transfer from half-integer quadrupolar spin to ¹H nuclei under fast MAS in the context of MQ-HETCOR experiments. In this article, we further develop the high-resolution 2D HETCOR methods by ST-based approaches, making use of the sensitivity advantage of STMAS over its MQMAS counterpart. In a similar manner to the previous work, which utilized CP and RINEPT for the population transfer, we also demonstrate the experimental setup protocol for PRESTO. Using {²³Na}-¹H and {²⁷Al}-¹H spin systems of powder samples, we compare a series of MQ/ST-HETCOR 2D spectra to discuss the pros and cons of the distinct MQ/ST-based approaches for spin 3/2 and 5/2 nuclei, respectively. We also incorporate two experimental tricks to reduce the experimental time of such long 2D experiments, the Optimized Rotor-Synchronization (ORS) and the Non-Uniform Sampling (NUS), in the context of high-resolution spectra of half-integer quadrupolar spin nuclei.

Accelerating high-resolution NMR of half-integer quadrupolar nuclei in solids: SPAM-MQMAS and SPAM-STMAS

In solid-state NMR, multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments are well-established techniques to obtain high-resolution spectra of half-integer quadrupolar nuclei. In 2004 and 2005, a soft-pulse-added-mixing (SPAM) concept was introduced by Gan and Amoureux to enhance the S/N ratio of MQMAS and STMAS experiments. Despite their robustness and simplicity, SPAM approaches have not yet been widely applied. Here, we further exploit SPAM concepts for sensitivity enhancement upon acquisition of two-dimensional MQMAS and STMAS spectra and also establish a general procedure upon implementation of SPAM-MQMAS and SPAM-STMAS NMR. Its effectiveness and ease in experimental setup are demonstrated using simulations and experiments performed on I ​= ​3/2 (²³Na, ⁸⁷Rb), 5/2 (²⁷Al, ⁸⁵Rb) and 9/2 (⁹³Nb) nuclei with a variety of quadrupolar coupling constants (C_Q). Compared to the conventional z-filter methods, sensitivity enhancements in between 2 and 4 are achievable with SPAM. We recommend to use SPAM with a ratio of 4:1 for the number of echoes and antiechoes to safely maximize the sensitivity and resolution simultaneously. In addition, a comparison of the experimental approaches is made in the context of SPAM-MQMAS and SPAM-STMAS NMR with respect to repetition delay and spinning frequency, aiming to discuss the precautions upon making a judicious choice of high-resolution NMR methods of half-integer quadrupolar nuclei.

Distinguishing between Structural Models of β'-Sialons Using a Combined Solid-State NMR, Powder XRD, and Computational Approach

β'-Sialons (Si_6-zAl_zO_zN_8-z, where 0 ≤ z ≤ ∼4.2) are studied using a combination of ²⁹Si and ²⁷Al solid-state NMR, using magnetic fields of up to 20 T, powder X-ray diffraction, and density functional theory (DFT) calculations of both the structure and NMR parameters. Four different structural models have been proposed in the literature for the replacement of silicon and nitrogen by aluminum and oxygen within a β-Si₃N₄-structured lattice. Experimental data are presented for the variation with composition (z) of the unit cell parameters from diffraction and the local coordination units present suggested by NMR data. The experimental data are compared to the changes with composition in the DFT calculations of the structure and the NMR parameters according to the four models, allowing the models to be distinguished. It is shown that only one of these, the domain model, is fully consistent with all of the experimental data and is, therefore, a good structural model for β'-sialons. More speculatively, it is suggested that for the domain model, ²⁷Al NMR data might provide a constraint on the thickness of its aluminum-rich layers.

Sensitivity improvement in 5QMAS NMR experiments using FAM-N pulses

The multiple-quantum magic-angle spinning (MQMAS) experiment is a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei with half-integer spin quantum number. However, its inherently poor sensitivity limits its application in more challenging systems. In particular, the use of higher-order multiple-quantum coherences, which have the potential to provide higher resolution in the isotropic spectrum, results in a further decrease in sensitivity. Here we extend our recent work, which introduced an automated, high-throughput approach to generate amplitude-modulated composite pulses (termed FAM-N) to improve the efficiency of the conversion of three-quantum coherences, and explore the use of similar pulses in five-quantum MAS experiments. We consider three different approaches, and are able to demonstrate that all three provide good enhancements over single pulse conversion in all but the most extreme cases, and work well at a range of spinning rates. We show that FAM-N pulses are robust to variation in the quadrupolar coupling and rf nutation rate, demonstrating their applicability in multisite systems and systems where direct experimental optimisation of complex composite pulses is not feasible. This work will ease the implementation of higher-order MQMAS experiments and enable their application to materials and systems that were previously deemed too difficult to study.

Recent advances in solid-state nuclear magnetic resonance spectroscopy of exotic nuclei

We present a review of recent advances in solid-state nuclear magnetic resonance (SSNMR) studies of exotic nuclei. Exotic nuclei may be spin-1/2 or quadrupolar, and typically have low gyromagnetic ratios, low natural abundances, large quadrupole moments (when I > 1/2), or some combination of these properties, generally resulting in low receptivities and/or prohibitively broad line widths. Some nuclides are little studied for other reasons, also rendering them somewhat exotic. We first discuss some of the recent progress in pulse sequences and hardware development which continues to enable researchers to study new kinds of materials as well as previously unfeasible nuclei. This is followed by a survey of applications to a wide range of exotic nuclei (including e.g., ⁹Be, ²⁵Mg, ³³S, ³⁹K, ⁴³Ca, ^47/49Ti, ⁵³Cr, ⁵⁹Co, ⁶¹Ni, ⁶⁷Zn, ⁷³Ge, ⁷⁵As, ⁸⁷Sr, ¹¹⁵In, ¹¹⁹Sn, ^121/123Sb, ^135/137Ba, ^185/187Re, ²⁰⁹Bi), most of them quadrupolar. The scope of the review is the past ten years, i.e., 2007-2017.

Investigating FAM-N pulses for signal enhancement in MQMAS NMR of quadrupolar nuclei

Although a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei, the inherently low sensitivity of the multiple-quantum magic-angle spinning (MQMAS) experiment has limited its application for nuclei with low receptivity or when the available sample volume is limited. A number of methods have been introduced in the literature to attempt to address this problem. Recently, we have introduced an alternative, automated approach, based on numerical simulations, for generating amplitude-modulated pulses (termed FAM-N pulses) to enhance the efficiency of the triple- to single-quantum conversion step within MQMAS. This results in efficient pulses that can be used without experimental reoptimisation, ensuring that this method is particularly suitable for challenging nuclei and systems. In this work, we investigate the applicability of FAM-N pulses to a wider variety of systems, and their robustness under more challenging experimental conditions. These include experiments performed under fast MAS, nuclei with higher spin quantum numbers, samples with multiple distinct sites, low-γ nuclei and nuclei subject to large quadrupolar interactions.

Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR

The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed “too challenging”.

Probing quadrupolar nuclei by solid-state NMR spectroscopy: recent advances

Solid-state nuclear magnetic resonance (NMR) of quadrupolar nuclei has recently undergone remarkable development of capabilities for obtaining structural and dynamic information at the molecular level. This review summarizes the key achievements attained during the last couple of decades in solid-state NMR of both integer spin and half-integer spin quadrupolar nuclei. We provide a concise description of the first- and second-order quadrupolar interactions, and their effect on the static and magic angle spinning (MAS) spectra. Methods are explained for efficient excitation of single- and multiple-quantum coherences, and acquisition of spectra under low- and high-resolution conditions. Most of all, we present a coherent, comparative description of the high-resolution methods for half-integer quadrupolar nuclei, including double rotation (DOR), dynamic angle spinning (DAS), multiple-quantum magic angle spinning (MQMAS), and satellite transition magic angle spinning (STMAS). Also highlighted are methods for processing and analysis of the spectra. Finally, we review methods for probing the heteronuclear and homonuclear correlations between the quadrupolar nuclei and their quadrupolar or spin-1/2 neighbors.

Improving sensitivity and resolution of MQMAS spectra: a 45Sc-NMR case study of scandium sulphate pentahydrate

To efficiently obtain multiple-quantum magic-angle spinning (MQMAS) spectra of the nuclide 45Sc (I=7/2), we have combined several previously suggested techniques to enhance the signal-to-noise ratio and to improve spectral resolution for the test sample, scandium sulphate pentahydrate (ScSPH). Whereas the 45Sc-3QMAS spectrum of ScSPH does not offer sufficient resolution to clearly distinguish between the 3 scandium sites present in the crystal structure, these sites are well-resolved in the 5QMAS spectrum. The loss of sensitivity incurred by using MQMAS with 5Q coherence order is partly compensated for by using fast-amplitude modulated (FAM) sequences to improve the efficiency of both 5Q coherence excitation and conversion. Also, heteronuclear decoupling is employed to minimise dephasing of the 45Sc signal during the 5Q evolution period due to dipolar couplings with the water protons in the ScSPH sample. Application of multi-pulse decoupling schemes such as TPPM and SPINAL results in improved sensitivity and resolution in the F(1) (isotropic) dimension of the 5QMAS spectrum, the best results being achieved with the recently suggested SW(f)-TPPM sequence. By numerical fitting of the 45Sc-NMR spectra of ScSPH from 3QMAS, 5QMAS and single-quantum MAS at magnetic fields B(0)=9.4 T and 17.6 T, the isotropic chemical shift delta(iso), the quadrupolar coupling constant chi, and the asymmetry parameter eta were obtained. Averaging over all experiments, the NMR parameters determined for the 3 scandium sites, designated (a), (b) and (c) are: delta(iso)(a)=-15.5+/-0.5 ppm, chi(a)=5.60+/-0.10 MHz, eta(a)=0.06+/-0.05; delta(iso)(b)=-12.9+/-0.5 ppm, chi(b)=4.50+/-0.10 MHz, eta(b)=1.00+/-0.00; and delta(iso)(c)=-4.7+/-0.2 ppm, chi(c)=4.55+/-0.05 MHz, eta(c)=0.50+/-0.02. The NMR scandium species were assigned to the independent crystallographic sites by evaluating their experimental response to proton decoupling, and by density functional theory (DFT) calculations using the PAW and GIPAW approaches, in the following way: Sc(1) to (c), Sc(2) to (a), and Sc(3) to (b). The need to compute NMR parameters using an energy-optimised crystal structure is once again demonstrated.

Practical comparison of sensitivity and resolution between STMAS and MQMAS for 27Al

An experimental comparison of sensitivity and resolution of satellite transition (ST) MAS and multiple quantum (MQ) MAS was performed for (27)Al (I=5/2) using several pulse sequences with a z-filter and SPAM, and two inorganic samples of kaolin (Al(2)Si(2)O(5)(OH)(4)) and glass (43.1CaO-12.5Al(2)O(3)-44.4SiO(2)). Six pulse sequences of STMAS (double-quantum filter-soft pulse added mixing=DQF-SPAM, double-quantum filter=DQF, double-quantum=DQ) and MQMAS (3QMAS-z-filter=3Qz, 3QMAS-SPAM=3Q-SPAM, 5QMAS-z-filter=5Qz) are employed. All experiments have been conducted utilizing a static field of 16.4T (700MHz for (1)H) and a rotor spinning frequency of 20kHz. Dependence of S/N ratios as a function of radio frequency (r.f.) field strengths indicates that strong r.f. fields are essential to obtain a better S/N ratio in all experiments. High sensitivity is obtained in the following order: DQF-SPAM, DQF, DQ, 3QSPAM, and 3Qz, although the degree of sensitivity enhancement given by STMAS for glass is slightly smaller than that for kaolin. This might be due to the different excitation and conversion efficiencies of ST and MQ coherences as a function C(q) values because quadrupolar interaction of the glass are widely distributed, or to motional broadening caused by framework flexibility in the structure of glass. With respect to resolution, the full widths at half maximum (FWHM) of F(1) projections of DQF-STMAS and 3QMAS spectra for kaolin are found to be comparable, which agrees with a simulated result reported in a literature. For glass, the STMAS possess slightly wider line widths than 3QMAS. However, because such a difference in line widths of STMAS and 3QMAS spectra is substantially small, we have concluded that STMAS and 3QMAS have comparable resolution for crystalline and non-crystalline materials.

Correlation NMR spectroscopy involving quadrupolar nuclei

We review the recent developments proposed for integer or half-integer quadrupolar nuclei, focussing on the methods to observe them under high-resolution and to analyze their through-space and through-bond connectivities.

Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in solid-state NMR of spin-7/2 nuclei

We here investigate the sensitivity enhancement of central-transition NMR spectra of quadrupolar nuclei with spin-7/2 in the solid state, generated by fast amplitude-modulated RF pulse trains with constant (FAM-I) and incremented pulse durations (SW-FAM). Considerable intensity is gained for the central-transition resonance of single-quantum spectra by means of spin population transfer from the satellite transitions, both under static and magic-angle-spinning (MAS) conditions. It is also shown that incorporation of a SW-FAM train into the excitation part of a 7QMAS sequence improves the efficiency of 7Q coherence generation, resulting in improved signal-to-noise ratio. The application of FAM-type pulse trains may thus facilitate faster spectra acquisition of spin-7/2 systems.