Multiple-quantum Magic-angle Spinning NMR Research Articles

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.

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.

Solid-State 27Al NMR Spectroscopy of the γ-Al13 Keggin Containing Al Coordinated by a Terminal Hydroxyl Ligand.

We report solid-state 27Al NMR spectroscopic results for the sulfate salt of the γ-Al13 Keggin cluster, γ-[AlO4Al12(OH)25(OH2)11][SO4]3·[H2O]14, that provide a spectroscopic signature for partial hydrolysis of this Keggin-type cluster. In 27Al multiple-quantum magic-angle spinning NMR spectra, all 13 Al positions of the cluster are at least partially resolved and assigned with the aid of density functional theory (DFT) calculations of the 27Al electric field gradients. The isotropic chemical shift of the single tetrahedral site, 75.7 ppm, is nearly identical to that reported for solutions from which the cluster crystallizes. Reflecting broadly similar coordination environments, the octahedral Al show mostly small variations in isotropic chemical shift (+7 to +11 ppm) and quadrupolar coupling constant (CQ; 6-7.5 MHz), except for one resonance that exhibits a much smaller CQ and another site with a larger value. DFT calculations show that deprotonation of a terminal water ligand, to form an η-OH group, causes a large reduction in the 27Al CQ, allowing assignment of a distinct, narrow peak for octahedral Al to this hydroxyl-terminated site. This result suggests a relationship between octahedral 27Al NMR line width and hydrolysis for solids prepared from Keggin-type clusters.

23Na magic-angle spinning and double-rotation NMR study of solid forms of sodium valproate

Sodium valproate is a pharmaceutical with applications in the treatment of epilepsy, bipolar disorder, and other ailments. Sodium valproate can exist in many hydrated and acid-stabilized forms in the solid state, and it can be difficult to obtain precise structural information about many of these. Here, we present a 13C and 23Na solid-state NMR study of several forms of sodium valproate, only one of which has been previously structurally characterized by single-crystal X-ray diffraction. 23Na magic-angle spinning (MAS), double-rotation (DOR), and multiple-quantum magic-angle spinning (MQMAS) NMR spectra are shown to provide useful information on the number of molecules in the asymmetric unit, the local coordination geometry of the sodium cations, and the presence of amorphous phases. Two previously identified forms are shown to be highly similar, or identical, according to the 23Na NMR data. The utility of carrying out both DOR and MQMAS NMR experiments to identify all crystallographically unique sites is demonstrated. 13C cross-polarization MAS NMR spectra also provide complementary information on the number of molecules in the asymmetric unit and the crystallinity of the sample.

Network connectivity in cesium borosilicate glasses: 17O multiple-quantum MAS and double-resonance NMR

17O solid-state NMR was used to study oxygen connectivity in ternary cesium borosilicate glasses. 17O multiple-quantum magic-angle spinning (MAS) NMR and ultrahigh-field 17O MAS NMR provide resolution of non-bridging and bridging oxygens, permitting their quantification for modeling short-range order. 17O{11B} rotational-echo double-resonance (REDOR) NMR is used to verify peak assignments for B–O–B, Si–O–B and Si–O–Si species, which agree well with a body of data from other alkali borosilicate glasses. The resulting bridging oxygen populations are in poor agreement with a model based on random network mixing, suggesting some degree of macroscopic segregation into silica-rich and borate-rich phases. Electron microprobe analysis reveals significant compositional variation between the batch and final products due to cesium volatilization at high temperatures. Both phase separation and elemental boil-off must be considered to account fully for the 17O MAS NMR results.

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.

23Na double-rotation NMR of sodium nucleotides leads to the discovery of a new dCMP hendecahydrate

Obtaining definitive information concerning the coordination environment of sodium ions which balance the negative charges found in nucleotides is a challenging task. We show that high resolution 1D and 2D (23)Na NMR spectra of sodium nucleotides obtained in the solid state with the use of double-rotation (DOR) provide valuable structural information. Sensitive spin diffusion homonuclear correlation experiments are used to establish the relative proximities of various pairs of crystallographically distinct Na sites and to assign the spectral resonances. Additionally, the DOR sidebands are simulated to obtain coordination information which is complementary to that obtained using multiple-quantum magic-angle spinning NMR spectra. These experiments led us to discover a new hendecahydrate of deoxycytidine monophosphate (dCMP), the structure of which is confirmed via single-crystal X-ray diffraction. This hydrate crystallizes reproducibly when deuterated water is used exclusively in the preparation process.

Inhomogeneous distribution of Na+ in alkali silicate glasses

We investigated the Na+ environment in sodium silicate glasses and mixed alkali silicate glasses using 23Na multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy, ab initio molecular orbital (MO) calculations, and Na+ elution analysis. The 23Na MQMAS NMR spectra of Na2O–xSiO2 and (1 − y)Na2O–yM2O–2SiO2 (M = Li, K) glasses showed an inhomogeneous distribution of local structures around Na+, even though spectral deconvolution was impossible. The quantum chemical calculations indicated that the alkali silicate glasses contained both aggregated and isolated Na+ sites. The elution behavior also supported the local structure distribution described above. These results indicated that a cation with a high cation field strength tends to aggregate in mixed silicate glasses.

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

Temperature dependencies of the 87Rb quadrupole coupling constants (CQ), asymmetry parameters (ηQ), and isotropic chemical shifts (δiso) have been observed and determined for RbCl, RbClO4, Rb2SO4, and RbNO3 from variable-temperature (VT) 87Rb magic-angle spinning (MAS) NMR spectra for the temperature range from about −100 to +165 °C. VT 87Rb multiple-quantum MAS NMR spectra of the trigonal form for RbNO3 have assisted the retrival of CQ, ηQ, and δiso for the three Rb sites of this phase. The results demonstrate that CQ(87Rb) and δiso(87Rb) are highly temperature-dependent for all samples over the studied temperature range. Linear correlations of δiso with temperature, corresponding to temperature coefficients in the range −0.014 to −0.036 ppm/°C (i.e., increasing shielding with increasing temperature), are observed for all samples. An exception is the Rb(2) site in Rb2SO4 for which a small linear decrease in shielding is observed with increasing temperature. For RbClO4 and Rb2SO4 the three CQ(87Rb) values...

Q-shear transformation for MQMAS and STMAS NMR spectra

The multiple-quantum magic-angle spinning (MQMAS) and satellite-transition magic-angle spinning (STMAS) experiments refocus second-order quadrupolar broadening of half-integer quadrupolar spins in the form of two-dimensional experiments. Isotropic shearing is usually applied along the indirect dimension of the 2D spectra such that an isotropic projection free of anisotropic quadrupolar broadening can be obtained. An alternative shear transformation by a factor equal to the coherence level (quantum number) selected during the evolution period is proposed. Such a transformation eliminates chemical shift along the indirect dimension leaving only the second-order quadrupolar-induced shift and anisotropic broadening, and is expected to be particularly useful for disordered systems. This transformation, dubbed Q-shearing, can help avoid aliasing problems due to large chemical shift ranges and spinning sidebands. It can also be used as an intermediate step to the isotropic representation for expanding the spectral window of rotor-synchronized experiments.

Application of shaped adiabatic pulses to MQMAS NMR spectroscopy of spin 3/2 nuclei

Competition between nutation (r.f. driven) and adiabatic (rotor-driven) multi-quantum coherence transfer mechanisms in spin 3/2 systems results in diminished performance of rotation induced adiabatic coherence transfer (RIACT) in isotropic multiple-quantum magic-angle spinning (MQMAS) experiments for small e 2 qQ / h (<2 MHz) and high radio-frequency powers. We present a simple shaped RIACT pulse consisting of a truncated sine wave (spanning 0–0.8 π) that corrects the sensitivity losses, phase twist and relative intensity errors that can arise in MQMAS spectra utilizing constant-amplitude RIACT pulses. The shaped RIACT pulse may enhance the study of metals in biomolecules where quadrupole couplings of S = 3/2 nuclei such as 23Na tend to be small.

Dynamics on the Microsecond Timescale in Microporous Aluminophosphate AlPO-14 as Evidenced by 27Al MQMAS and STMAS NMR Spectroscopy

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.

Solid-State Structural Characterization of a Rigid Framework of Lacunary Heteropolyniobates

In our ongoing investigations of heteropolyniobate chemistry, a phase featuring decorated, A-type trivacant alpha-Keggin ions linked by their charge-balancing sodium cations has been isolated and structurally characterized. This is the first heteropolyniobate reported that has a true lacunary structure type. Na15[(PO2)3PNb9O34] x 22 H2O (1) [triclinic space group P1 (No. 2); a = 12.242 (2) A, b = 12.291 (3) A, c = 22.056 (4) A; alpha = 93.12 (3) degrees, beta = 99.78 (3) degrees, gamma = 119.84 (3) degrees; Z = 4, V = 2799.2 (10) A3] is composed of bilayers of the heteropolyanions alternating with layers of hydrated Na+ cations. Sodium cations also bridge the clusters within their layers through Na-O(t)-Nb, Na-O(b)-Nb2, and Na-O(t)-P bonds (t = terminal and b = bridging). This phase is poorly soluble in water, suggesting that it is more characteristic of a framework of linked heteropolyanions rather than a water-soluble heteropolyanion salt. Two-dimensional solid-state 23Na multiple-quantum magic angle spinning (MAS) NMR of 1 reveals five distinctive chemical and structural environments for sodium, which agrees with the crystallographic data. The 23Na and 1H MAS NMR studies further illustrate the rigid and immobile nature of this framework of cations and anions.

Improved Excitation Schemes for Multiple-Quantum Magic-Angle Spinning for Quadrupolar Nuclei Designed Using Optimal Control Theory

We have investigated the use of optimal control theory for the design of improved multiple-quantum excitation schemes for the popular multiple-quantum magic-angle spinning NMR experiment for quadrupolar nuclei with half-integer quadrupolar spin. The advantage of the new low-power experiments, termed OCFASTER, is demonstrated by sensitivity improvements approaching 50% for 87Rb in RbClO4 and RbNO3 as compared to FASTER and standard strong-pulse excitation schemes.

Structure of Duplex Oxide Layer in Porous Alumina Studied by 27Al MAS and MQMAS NMR

Abstract A local structure analysis using 27Al multiple-quantum magic-angle-spinning (MQMAS) and MAS NMR techniques has been performed on porous alumina fabricated by the anodic oxidation of aluminum. It is suggested that the main constituents of the inner and outer oxide layers composing porous alumina are [AlO4 and AlO5] and [AlO6], respectively, and that both of the layers adopt the amorphous structure.

Natural Abundance Solid-state 25Mg MQMAS NMR Studies on Inorganic Solids at a High Magnetic Field of 16.4 T

Abstract An application of a high magnetic field (16.4 T) with a specialized high power radio frequency (rf)-resistant probe that produces a very short 90° pulse enabled measurement of natural abundance 25Mg multiple quantum magic angle spinning (MQMAS) spectra of complex inorganic materials, such as minerals and steel-making slag with the large quadrupolar coupling constant (CQ) (≈10 MHz).

Investigation on chemical structure of minerals in coal using 27Al MQMAS NMR

High-resolution two-dimensional 27Al multiple-quantum magic-angle-spinning (MQMAS) NMR, being capable of averaging the second-order quadrupolar interaction, was applied in order to characterize the mineral matter in coals that have been spotlighted recently in new approach to the coal utilization. Octahedral aluminum compounds which consist of kaolin and other mineral phases could be clearly distinguished by 27Al MQMAS method. As the total carbon percent increases, octahedral aluminum species tend to vary from kaolin. A combination of cross polarization and MQMAS, it being called as 1H→ 27Al CP/MQMAS, was also employed to evaluate chemical bonding between aluminum atom and hydrogen atom. As a result, the octahedral components containing the linkage of Al–O–H such as kaolin and excluding the Al–O–H bond were separated by this technique. MQMAS and CP/MQMAS are very effective in characterization of inorganic matter including quadrupolar nuclei, which are particularly well suited to the analysis of clay minerals with noncrystalline materials and present in coals while XRD is difficult to analyze the amorphous compounds.

Probing through bond connectivities with MQMAS NMR

Polarization transfer from quadrupolar ( 27Al) to spin-1/2 ( 31P) nuclei via J-coupling is employed to measure two-dimensional 27Al– 31P heteronuclear correlation spectra with isotropic resolution. The proposed experiment, MQ- J-HETCOR, uses multiple quantum magic angle spinning (MQMAS) NMR for elimination of the second-order quadrupolar broadening and INEPT, INEPTR, INEPT+ and DEPT sequences for the polarization transfer. The experimental conditions leading to best sensitivity and resolution are detailed using AlPO 4-14 as a test sample.

Chemical order in mixed-cation silicate glasses and melts

Cation ordering in covalent oxide glasses and melts profoundly affects the macroscopic properties, such as viscosity, diffusivity, and thermodynamic potentials. It is commonly assumed that in glasses and melts nonframework cations such as ${\mathrm{Na}}^{+},$ ${\mathrm{Ca}}^{2+},$ and ${\mathrm{Ba}}^{2+}$ distribute randomly around nonbridging oxygen (NBO). Several macroscopic studies on the melting of silicates and thermodynamic data have suggested that a possible nonrandomness may exist among cations around NBO in mixed-cation silicate glasses. Here, we report unambiguous experimental evidence of chemical ordering of nonframework cations and demonstrate a clear preference for certain types of cation-NBO complexes in mixed-cation silicate glasses using ${}^{17}\mathrm{O}$ magic angle spinning (MAS) and multiple quantum MAS NMR. Particularly, complete bonding preference and cation ordering occurs in Ba-Mg silicate glasses $({\mathrm{BaMgSi}}_{2}{\mathrm{O}}_{6})$ glass in such a way that nonbridging oxygen either only has ${\mathrm{Ba}}^{2+}$ as a nearest neighbor (Ba-NBO) or exists as a complex containing one ${\mathrm{Ba}}^{+}$ and two ${\mathrm{Mg}}^{2+}$ as nearest neighbors while no detectable fraction of Mg-NBO is observed. Ba-Na silicate glasses, on the other hand, show a wide distribution of configurations for two types of cations around NBO, forming Ba- and Na-NBO as well as substantial intensity of mixed NBO peaks, which indicates a prevalence of dissimilar pairs around NBO or a stronger preference to $\mathrm{Ba}\ensuremath{-}\mathrm{O}{\ensuremath{-}}^{[4]}\mathrm{Si}$ over $\mathrm{Na}\ensuremath{-}\mathrm{O}{\ensuremath{-}}^{[4]}\mathrm{Si}.$ The present results, combined with the previous results on Na-Ca silicate glasses, highlight the tendency for chemical ordering upon cation mixing in oxide glasses and may provide an atomistic explanation for diffusivity anomalies as well as activity-composition relationship of silicate melts.

Solid-state 27Al NMR investigation of thermal decomposition of LiAlH 4

Solid-state nuclear magnetic resonance is used to study the thermal decomposition of lithium tetrahydroaluminate into metallic aluminum, hydrogen and trilithium hexahydroaluminate. Aluminum sites in LiAlH 4 and Li 3AlH 6 were characterized using static, magic angle spinning (MAS) and multiple-quantum MAS NMR. By applying the in situ NMR method, it has been demonstrated that melting is not a prerequisite for the decomposition of LiAlH 4. Based on the observed data, a decomposition path has been established that is consistent with the concentrations of observed Al species at various stages of the thermally induced reaction.