Variable-Temperature 87Rb Magic-Angle Spinning NMR Spectroscopy of Inorganic Rubidium Salts

On the use of single-frequency versus double-frequency satellite-transition pulses for MQMAS

It has been shown recently that a rotor-period long pulse applied at a frequency selective to the satellite-transitions of half-integer quadrupole nuclei can efficiently interconvert central-transition (CT) and triple-quantum (TQ) coherences for the acquisition of MQMAS spectra [I. Hung, Z. Gan, J. Magn. Reson. 324 (2021) 106913; doi: https://doi.org/10.1016/j.jmr.2021.106913]. By using a pair of such pulses and selecting opposite changes in coherence order, the anisotropic phase of the effective rf field can be refocused. Efficient multiple-quantum interconversion has led to low-power MQMAS pulse sequences capable of obtaining isotropic NMR spectra for the largest quadrupolar couplings to date. In this work, we extend the satellite-transition selective pulses from single- to double-frequency (or cosine) irradiation. By applying average Hamiltonian theory in the quadrupolar jolting frame, it is shown that the phase for TQ/CT conversion converges when the double-frequency irradiation matches the mirror-image symmetry of the satellite-transitions. The coherent conversion explains the mechanism behind the double-frequency sweep (DFS) and fast amplitude modulation (FAM) methods used for MQMAS. However, the strict matching condition limits the bandwidth of such double-frequency pulses to less than one spinning frequency. The use of a pair of identical cosine satellite-transition pulses is proposed to refocus the residual anisotropic phase spread. The refocusing leads to a more efficient MQMAS pulse sequence with a broader bandwidth suitable for large quadrupolar couplings and chemical shift ranges. Comparisons with the recently presented single-frequency lpMQMAS and other MQMAS pulse schemes show that cos-lpMQMAS is more efficient, less susceptible to fluctuations in spinning frequency, and suffers from less distortion in quadrupolar line shapes, as demonstrated with model compounds of moderate and large quadrupolar couplings, ⁸⁷RbNO₃ and β-⁷¹Ga₂O₃. In particular, the results for β-⁷¹Ga₂O₃ show an order of magnitude increase in MQMAS efficiency.

Isotropic solid-state MQMAS NMR spectra for large quadrupolar interactions using satellite-transition selective inversion pulses and low rf fields

Multiple-quantum magic-angle spinning (MQMAS) pulse sequences are presented that are capable of obtaining isotropic NMR spectra for large quadrupolar interactions using lower rf fields. These experiments rely on rotor period long pulses applied at a large offset from the central-transition, making them selective to the satellite-transitions. Each such pulse gives rise to an anisotropic phase, which can be cancelled to obtain coherent signal evolution if a pair of pulses are applied in a symmetric manner. Thus, efficient excitation and conversion of triple-quantum coherences from and to the central-transition is achieved for MQMAS even for large quadrupolar couplings, by selective inversion of the satellite-transitions using such low-power pulses. Low-power multiple-quantum magic-angle spinning (lpMQMAS) pulse sequences are demonstrated on a model compound, RbNO₃, and also applied on β-Ga₂O₃, a sample with the largest quadrupolar interactions for which isotropic NMR spectra have been obtained to date.

Sensitivity-enhanced multiple-quantum MAS NMR for half-integer spin quadrupolar nuclei using WURST-amplitude shaped pulses

Two-dimensional multiple-quantum MAS (MQMAS) NMR spectroscopy is one of the most widely used solid-state NMR techniques for resolving multiple overlapping central-transition lineshapes for half-integer spin quadrupolar nuclei. In particular when relying on nutation-driven MQ coherence transfers, this technique suffers from low sensitivity that can only be improved by increasing the rf-amplitude of the involved radio-frequency (rf) pulses, which are therefore typically operated at the rf-limit. In such situations, frequently encountered for the three-pulse z-filtered and split-t₁ shifted-echo MQMAS NMR sequences, we introduce the advantages of rf-pulses with smoothly truncated amplitude profiles, which we have termed WURST-Amplitude Shaped Pulses (WASPs). When considering the NMR spectrometer hardware, we demonstrate that WASPs feature more suitable properties in comparison to conventional rectangular pulses, enabling a substantial reduction of voltage reflections and transient effects under identical rf-conditions. By employing extensive numerical simulations and experimental validation, we further show that WASPs intrinsically possess a higher potential for nutation-based 3Q excitation involving spin-3/2 and 3Q and 5Q excitation for spin-5/2 quadrupolar nuclei, specifically when large nutation frequencies are available. The concept of smoothly truncating rf-amplitudes is also extended to Fast Amplitude Modulation (FAM) pulses, normally incorporated for rotor-driven 1Q conversion. We additionally evaluate the potential of employing WASPs with peak rf-amplitudes beyond the rf-limit for conventional rectangular rf-pulses.

13C pNMR of "crumple zone" Cu(II) isophthalate metal-organic frameworks

NMR spectroscopy of paramagnetic materials (pNMR) has the potential to provide great structural insight, but many challenges remain in interpreting the spectra in detail. This work presents a study of a series of structurally analogous metal-organic frameworks (MOFs) based on 5-substituted isophthalate linkers and Cu(II) paddlewheel dimers, of interest owing to their "crumple zone" structural rearrangement on dehydration/rehydration. ¹³C MAS NMR spectra reveal a wide variation in the observed resonance position for chemically similar C species in the different MOFs but, despite this, resonances are overlapped in several cases. However, by considering a combination of the integration of quantitative spectra, the resonance position as a function of temperature and T₁ relaxation measurements, the spectra can be fully assigned. It is also demonstrated that the prototypical MOF in this series, STAM-1, displays a crumple zone rearrangement on dehydration, similar to the well-characterised 5-ethoxyisophthalate MOF (STAM-17-OEt) although, while the materials have similar local C environments, dehydrated STAM-1 exhibits less long-range order.

ssNake: A cross-platform open-source NMR data processing and fitting application

For solid-state NMR or for unconventional experiments only a very limited number of modern processing and simulation software packages are available. For this reason, we have developed ssNake, an NMR processing program which provides both interactive and script-based processing tools. ssNake is aimed at solid-state NMR experiments, but can also be used for liquid-state experiments. It can read various data formats, including those from all major spectrometer vendors. It has extensive fitting capabilities, which can be used for spectrum deconvolution. ssNake also provides the unique feature of being able to fit multiple spectra (or curves) simultaneously, where some or all of its parameters are shared. This method can be used, for example, to fit quadrupole spectra at various magnetic fields simultaneously. This allows the quadrupole and chemical shift parameters to be accurately determined. ssNake also provides a method of fitting using external simulation programs, such as SIMPSON. This makes fitting very versatile, as it brings together experimental data and simulation software.

Hydrolytic stability in hemilabile metal-organic frameworks

Highly porous metal-organic frameworks (MOFs), which have undergone exciting developments over the past few decades, show promise for a wide range of applications. However, many studies indicate that they suffer from significant stability issues, especially with respect to their interactions with water, which severely limits their practical potential. Here we demonstrate how the presence of 'sacrificial' bonds in the coordination environment of its metal centres (referred to as hemilability) endows a dehydrated copper-based MOF with good hydrolytic stability. On exposure to water, in contrast to the indiscriminate breaking of coordination bonds that typically results in structure degradation, it is non-structural weak interactions between the MOF's copper paddlewheel clusters that are broken and the framework recovers its as-synthesized, hydrated structure. This MOF retained its structural integrity even after contact with water for one year, whereas HKUST-1, a compositionally similar material that lacks these sacrificial bonds, loses its crystallinity in less than a day under the same conditions.

Polymorphism, Weak Interactions and Phase Transitions in Chalcogen-Phosphorus Heterocycles

A series of P-E-containing heterocycles (E=chalcogen) with aromatic backbones were synthesised and characterised by single-crystal and powder XRD, microanalysis and mass spectrometry. Solution- and solid-state 31 P and 77 Se NMR spectroscopy revealed significant differences between the NMR parameters in solution and in the solid state, related to conformational changes in the molecules. Many compounds were shown to exhibit a number of different polymorphic structures (identified by single-crystal XRD), although the bulk material studied by solid-state NMR spectroscopy often contained just one major polymorph. For the unoxidised heterocycles, the presence of weak intermolecular J couplings was also investigated by DFT calculations.

Phase Segregation in Cs-, Rb- and K-Doped Mixed-Cation (MA)x(FA)1-xPbI3 Hybrid Perovskites from Solid-State NMR

Hybrid (organic–inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use ¹³³Cs, ⁸⁷Rb, ³⁹K, ¹³C, and ¹⁴N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability.

The ambient hydration of the aluminophosphate JDF-2 to AlPO-53(A): insights from NMR crystallography

The aluminophosphate (AlPO) JDF-2 is prepared hydrothermally with methylammonium hydroxide (MAH⁺·HO^-, MAH⁺ = CH₃NH₃⁺), giving rise to a microporous AEN-type framework with occluded MAH⁺ cations and extra-framework (Al-bound) HO^- anions. Despite the presence of these species within its pores, JDF-2 can hydrate upon exposure to atmospheric moisture to give AlPO-53(A), an isostructural material whose crystal structure contains one molecule of H₂O per formula unit. This hydration can be reversed by mild heating (such as the frictional heating from magic angle spinning). Previous work has shown good agreement between the NMR parameters obtained experimentally and those calculated from the (optimized) crystal structure of JDF-2. However, several discrepancies are apparent between the experimental NMR parameters for AlPO-53(A) and those calculated from the (optimized) crystal structure (e.g. four ¹³C resonances are observed, rather than the expected two). The unexpected resonances appear and disappear reversibly with the respective addition and removal of H₂O, so clearly arise from AlPO-53(A). We investigate the ambient hydration of JDF-2 using quantitative ³¹P MAS NMR to follow the transformation over the course of ∼3 months. The structures of JDF-2 and AlPO-53(A) are also investigated using a combination of multinuclear solid-state NMR spectroscopy to characterize the samples, and first-principles density functional theory (DFT) calculations to evaluate a range of possible structural models in terms of calculated NMR parameters and energetics. The published structure of JDF-2 is shown to be a good representation of the dehydrated material, but modification of the published structure of AlPO-53(A) is required to provide calculated NMR parameters that are in better agreement with experiment. This modification includes reorientation of all the MAH⁺ cations and partial occupancy of the H₂O sites.

A 3D experiment that provides isotropic homonuclear correlations of half-integer quadrupolar nuclei

Two 3D experiments, capable of producing enhanced resolution two-spin double-quantum (DQ) homonuclear correlations for half-integer quadrupolar nuclei, are described. The first uses a split-t1 MQMAS sequence followed by a sandwiched oR(3) symmetry-based dipolar recoupling sequence to directly excite DQ coherences. In this case an isotropic single-quantum (SQ) coherence starts the homonuclear DQ excitation. In the second experiment a single strong pulse is used to create triple quantum (TQ) coherence followed by a further single pulse conversion to zero-order before a non-sandwiched oR(3) DQ sequence. The first experiment is demonstrated using (87)Rb in RbNO3, with three Rb sites in a ∼5ppm range, and the second to (11)B in caesium triborate, CsB3O5, with two three-coordinated sites separated by ∼2ppm and one four-coordinated boron site. In both cases, all sites are clearly resolved and their connections observed. The second experiment has higher sensitivity and a good signal to noise is obtained in a reasonable time despite the long T1 relaxation time of (11)B in this material.

EASY-GOING deconvolution: Automated MQMAS NMR spectrum analysis based on a model with analytical crystallite excitation efficiencies

The EASY-GOING deconvolution (EGdeconv) program is extended to enable fast and automated fitting of multiple quantum magic angle spinning (MQMAS) spectra guided by evolutionary algorithms. We implemented an analytical crystallite excitation model for spectrum simulation. Currently these efficiencies are limited to two-pulse and z-filtered 3QMAS spectra of spin 3/2 and 5/2 nuclei, whereas for higher spin-quantum numbers ideal excitation is assumed. The analytical expressions are explained in full to avoid ambiguity and facilitate others to use them. The EGdeconv program can fit interaction parameter distributions. It currently includes a Gaussian distribution for the chemical shift and an (extended) Czjzek distribution for the quadrupolar interaction. We provide three case studies to illustrate EGdeconv's capabilities for fitting MQMAS spectra. The EGdeconv program is available as is on our website http://egdeconv.science.ru.nl for 64-bit Linux operating systems.

Effect of the alkaline cations on the stability of the model polynucleotide poly(dG-dC)·poly(dG-dC)

When the model polynucleotide poly(dG-dC)∙poly(dG-dC) [polyGC] is titrated with a strong acid (HCl) in unbuffered aqueous solutions containing the chlorides of the alkali metals in the concentration range 0.010 M-0.600 M, two transitions in the absorbance vs. pH plots are evidenced, characterized by the constants pK(a(₁)) and pK(a(₂)). The limiting values at infinite saline concentrations of these two constants, namely pK(∞)(a(₁)) and pK(∞)(a(₂)) obtained making use of the "one site saturation constant" equation or, in turn, of the double logarithmic plot: pK(a) vs. log([salt]⁻¹), exhibit a clear dependence on the nature of the cations. The effects of the different alkali cations on the pK(∞)(a) values follow the Hofmeister series. In fact, the pK(∞)(a(₁)) and the pK(∞)(a(₂)) values are smaller for Li+ and Na+ than for Rb+ and Cs+, with K+ at the border between the two, showing that the transitions require higher concentrations of protons to occur in the presence of high concentrations of the cosmotropic ions.

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

Using a two-dimensional multiple-quantum (MQ) double rotation (DOR) experiment the contributions of the chemical shift and quadrupolar interaction to isotropic resonance shifts can be completely separated. Spectra were acquired using a three-pulse triple-quantum z-filtered pulse sequence and subsequently sheared along both the nu(1) and nu(2) dimensions. The application of this method is demonstrated for both crystalline (RbNO(3)) and amorphous samples (vitreous B(2)O(3)). The existence of the two rubidium isotopes ((85)Rb and (87)Rb) allows comparison of results for two nuclei with different spins (I=3/2 and 5/2), as well as different dipole and quadrupole moments in a single chemical compound. Being only limited by homogeneous line broadening and sample crystallinity, linewidths of approximately 0.1 and 0.2 ppm can be measured for (87)Rb in the quadrupolar and chemical shift dimensions, enabling highly accurate determination of the isotropic chemical shift and the quadrupolar product, P(Q). For vitreous B(2)O(3), the use of MQDOR allows the chemical shift and electric field gradient distributions to be directly determined-information that is difficult to obtain otherwise due to the presence of second-order quadrupolar broadening.

Sensitivity enhancement of NMR spectra of half-integer spin quadrupolar nuclei in solids using hyperbolic secant pulses

The experimental factors influencing the enhancements achievable for the central NMR transition, m(I)=1/2-->m(I)=-1/2, of spin-3/2 and spin-5/2 nuclei in the solid state using hyperbolic secant, HS, pulses for population transfer are investigated. In the case of powder samples spinning at the magic angle, it is found that the spinning frequency, the bandwidth and the frequency offset of the HS pulse play a crucial role in determining the maximum enhancements. Specifically, the bandwidth must be set to the spinning frequency for maximum signal enhancements. The (87)Rb NMR enhancement obtained for RbClO(4) using HS pulses was relatively insensitive to the magic angle spinning frequency; however, in the case of Al(acac)(3), the (27)Al enhancement increased with MAS frequency. In order to obtain an adiabatic HS sweep, one should optimize the rf field for a given pulse duration or optimize the pulse duration for a given rf field.

Procedures for labeling the high-resolution axis of two-dimensional MQ-MAS NMR spectra of half-integer quadrupole spins

The increasing development and application of the multiple-quantum MAS NMR for half-integer quadrupole spins has led to various RF pulse sequences for improving the excitation of multiple-quantum coherences and their conversion to single-quantum coherences. As a result, several conventions for labeling the Fl dimension of a 2D MQ-MAS spectrum appear in the literature. The corresponding relations for extracting the isotropic chemical shift, the quadrupole coupling constant, and the asymmetry parameter from experimental data are not always provided. We analyze these various conventions systematically and propose a new one, similar to that introduced by J.-P. Amoureux and C. Fernandez (2000, Solid State NMR 10, 339-343). These various conventions are illustrated with 27Al (I = 5/2) nuclei in aluminum acetylacetonate Al(CH3COCHCOCH3)3. Another experimental problem often met, the aliasing of peaks in the 2D spectrum, is analyzed and illustrated with 27Al (I = 5/2) in NH4Y zeolite and 23Na (I = 3/2) in sodium pyrophosphate Na4P2O7.

59Co chemical shift anisotropy and quadrupole coupling for K3Co(CN)6 from MQMAS and MAS NMR spectroscopy

59Co triple-quantum (3Q) MAS and single-pulse MAS NMR spectra of K3Co(CN)6 have been obtained at 14.1 T and used in a comparison of these methods for determination of small chemical shift anisotropies for spin I = 7/2 nuclei. From the 3QMAS NMR spectrum a spinning sideband manifold in the isotropic dimension with high resolution is reconstructed from the intensities of all spinning sidebands in the 3QMAS spectrum. The chemical shift anisotropy (CSA) parameters determined from this spectrum are compared with those obtained from MAS NMR spectra of (i) the complete manifold of spinning sidebands for the central and satellite transitions and of (ii) the second-order quadrupolar lineshapes for the centerband and spinning sidebands from the central transition. A good agreement between the three data sets, all of high precision, is obtained for the shift anisotropy (delta(sigma) = delta(iso) - delta(zz)) whereas minor deviations are observed for the CSA asymmetry parameter (eta(sigma)). The temperature dependence of the isotropic 59Co chemical shift has been studied over a temperature range from -28 to +76 degrees C. A linear and positive temperature dependence of 0.97 ppm/degree C is observed.

The influence of the radiofrequency excitation and conversion pulses on the lineshapes and intensities of the triple-quantum MAS NMR spectra of I = 3/2 nuclei

A rigorous examination of the various multiple-quantum magic angle spinning sequences is carried out with reference to sensitivity enhancement in the isotropic dimension and the lineshapes of the corresponding MAS peaks in the anisotropic dimension. An echo efficiency parameter is defined here, which is shown to be an indicator of the performance aspects of the various sequences. This can be used in the design of further new experiments in this field. A consequence of such a systematic analysis has been the combination of a spin-lock pulse for excitation of multiple-quantum coherences and an amplitude-modulated pulse for their conversion to observable single-quantum coherences. This approach has resulted in an improved performance over other sequences with respect to both the anisotropic lineshapes and the isotropic intensities.