Multiple-quantum Magic-angle Spinning Experiment Research Articles

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.

Oxidation Products of NaAlH4 Studied by Solid-State NMR and X-ray Diffraction

The oxidation products of NaAlH4 were studied using solid-state nuclear magnetic resonance experiments combined with X-ray diffraction (XRD) experiments. We studied the coordination of aluminum with oxygen using single-pulse 27Al experiments. 27Al−{1H} rotational echo double-resonance, 23Na−{27Al}/27Al−{23Na} transfer of population in double-resonance (TRAPDOR), and combined 27Al−23Na TRAPDOR−multiple-quantum magic-angle spinning (MQMAS) experiments were used to get qualitative information about Al−H and Al−Na proximities. These NMR experiments show that the intermediate oxidation product is an amorphous sodium aluminum hydroxide with Al in a tetrahedral coordinated site. A 27Al MQMAS experiment shows distributions of chemical shift and quadrupolar coupling parameters. Therefore, the material is probably highly disordered, and multiple compounds may be present. XRD measurements show that this phase indeed lacks long-range order. The end product contains mainly octahedral coordinated aluminum hydroxide. Crystalline aluminum hydroxides, consisting of gibbsite and bayerite, and sodium carbonate hydrates, Na2CO3·H2O and Na3(CO3)2·2H2O, are observed by XRD measurements. Our results show that relatively harmless and nontoxic compounds are formed. However, the materials should handled carefully because of the strong basicity of compounds like sodium carbonate.

Characterisation of different polymorphs of tris(8-hydroxyquinolinato)aluminium(III) using solid-state NMR and DFT calculations

BackgroundOrganic light emitting devices (OLED) are becoming important and characterisation of them, in terms of structure, charge distribution, and intermolecular interactions, is important. Tris(8-hydroxyquinolinato)-aluminium(III), known as Alq3, an organomettalic complex has become a reference material of great importance in OLED. It is important to elucidate the structural details of Alq3 in its various isomeric and solvated forms. Solid-state nuclear magnetic resonance (NMR) is a useful tool for this which can also complement the information obtained with X-ray diffraction studies.ResultsWe report here 27Al one-dimensional (1D) and two-dimensional (2D) multiple-quantum magic-angle spinning (MQMAS) NMR studies of the meridional (α-phase) and the facial (δ-phase) isomeric forms of Alq3. Quadrupolar parameters are estimated from the 1D spectra under MAS and anisotropic slices of the 2D spectra and also calculated using DFT (density functional theory) quantum-chemical calculations. We have also studied solvated phase of Alq3 containing ethanol in its lattice. We show that both the XRD patterns and the quadrupolar parameters of the solvated phase are different from both the α-phase and the δ-phase, although the fluorescence emission shows no substantial difference between the α-phase and the solvated phase. Moreover, we have shown that after the removal of ethanol from the matrix the solvated Alq3 has similar XRD patterns and quadrupolar parameters to that of the α-phase.ConclusionThe 2D MQMAS experiments have shown that all the different modifications of Alq3 have 27Al in single unique crystallographic site. The quadrupolar parameters predicted using the DFT calculation under the isodensity polarisable continuum model resemble closely the experimentally obtained values. The solvated phase of Alq3 containing ethanol has structural difference from the α-phase of Alq3 (containing meridional isomer) from the solid-state NMR studies. Solid-state NMR can hence be used as an effective complementary tool to XRD for characterisation and structural elucidation.

Studies on multiple‐quantum magic‐angle‐spinning NMR of half‐integer quadrupolar nuclei under strong rf pulses with a microcoil

AbstractIn multiple‐quantum magic‐angle‐spinning (MQMAS) NMR experiments, it has been argued that one should use as strong rf irradiation as possible in most situations so as to attain the better sensitivity. In this work, we theoretically derive formulas for the efficiency of MQMAS experiments for spin $ {3 \over 2} $ nuclei under strong rf irradiation, and experimentally examine the validity of the derived formulas. For this purpose, a microcoil was accommodated to magic‐angle‐spinning (MAS) experiments by developing a microcoil‐based, coin‐sized probe, which is attachable to an existing MAS module. The rf intensity of up to ∼1 MHz was realized, and the efficiencies of multiple‐quantum (MQ)‐excitation and conversion were studied for the 23Na (I = 3/2) nucleus in a polycrystalline sample of sodium sulfite, from which the theoretical formulas were confirmed. Moreover, it was found that the effect of rf inhomogeneity is significant for MQ‐excitation when rf irradiation is strong, while it is not appreciable for MQ to single‐quantum conversion. From the insight into the MQ‐excitation and conversion processes, the optimal set of rf intensity and pulse width is proposed for a given quadrupolar coupling constant. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 115–123, 2008

Cation Sublattice Disorder Induced by Swift Heavy Ions in MgAl2O4 and ZnAl2O4 Spinels: 27Al Solid-State NMR Study

MgAl2O4 and ZnAl2O4 spinels have been irradiated by swift heavy ions (86Kr and 36S) simulating the irradiation by fission products for applications in the transmutation targets. The structures of unirradiated and irradiated spinel samples have been studied by NMR spectroscopy, with 27Al magic angle spinning and multiple-quantum magic angle spinning experiments. The parameters of fluence and electronic stopping power have been compared. For 86Kr ions, the obtained spectra are modified by irradiation: we observe a rise of the amount of aluminum in tetrahedral sites and a widening of the lines associated with the different aluminum environments compared with those of the pristine samples. Site exchange in the cationic sublattice is then observed and can be quantified from NMR spectra, determining the inversion parameter. An inversion parameter of 0.77 is estimated for the MgAl2O4 spinel irradiated with 1013 Kr ions/cm2, for a value of 0.275 in the pristine samples. Moreover, a line attributed to aluminum in 5-fold coordination with oxygen is observed in irradiated spinel samples at the maximum fluence for krypton. These new aluminum environments can characterize a transition layer which could change toward an amorphous layer, increasing the electronic stopping power and/or the fluence.

Al speciation in tropical podzols of the upper Amazon Basin: A solid-state 27Al MAS and MQMAS NMR study

In the upper Amazon Basin, aluminum previously accumulated in lateritic formations is massively remobilised in soils by podzolization and exported in waters. We have investigated the speciation of aluminum in the clay-size fractions of eight horizons of waterlogged podzols lying in a depression of a plateau. The horizons illustrate the main steps involved in the podzolization of laterites. They belong to eluviated topsoil A horizons and illuviated subsoil Bhs, Bh and 2BCs horizons of weakly and better-expressed podzols located at the margin and centre of the depression. For the first time, aluminum speciation is quantitatively assessed in soils by spectroscopic methods, namely FTIR, 27Al magic angle spinning (MAS) and multiple-quantum magic angle spinning (MQMAS), nuclear magnetic resonance (NMR). The results thus obtained are compared to chemical extraction data. Solid-state 27Al MAS NMR spectra enable to distinguish Al bound to organic compounds from that incorporated in secondary mineral phases detected by FTIR. MQMAS experiments additionally show that both chemical shifts and quadrupolar constants are distributed for Al nuclei linked with organic compounds. Similar amounts of chelated Al are obtained from NMR spectra and chemical extractions. The study enables to highlight three major steps in the fate of aluminum. (i) Aluminum is first released by mineral weathering, feeds complexing sites of organic matter and accumulates in subsurface Bhs horizons of weakly expressed podzols (acidocomplexolysis). (ii) Complexes of aluminum with organic matter (Al–OM) then migrate downwards in sandy horizons of better-expressed podzols and accumulate at depth in less permeable 2BCs horizons. (iii) The minor amounts of aluminum present in the 2BCs horizon of the downslope podzol show that aluminum is eventually exported towards the river network, either complexed with organic matter or as Al 3+ ions after desorption from organic compounds, due to decreasing pH or biodegradation of organic ligands. The direct spectroscopic determination of Al-speciation during the formation of podzolic soils opens new perspectives to trace metal loads in the rivers of the upper Amazon Basin.

Multiple-quantum magic-angle spinning spectroscopy using nonlinear sampling

NMR spectroscopy is a relatively insensitive technique and many biomolecular applications operate near the limits of sensitivity and resolution. A particularly challenging example is detection of the quadrupolar nucleus 17 O , due to its low natural abundance, large quadrupole couplings, and low gyromagnetic ratio. Yet the chemical shift of 17 O spans almost 1000 ppm in organic molecules and it serves as a potentially unique reporter of hydrogen bonding in peptides, nucleic acids, and water, and as a valuable complement to 13 C and 15 N NMR. Recent developments including the multiple-quantum magic-angle spinning (MQMAS) experiment have enabled the detection of 17 O in biological solids, but very long data acquisitions are required to achieve sufficient sensitivity and resolution. Here, we perform nonlinear sampling in the indirect dimension of MQMAS experiments to substantially reduce the total acquisition time and improve sensitivity and resolution. Nonlinear sampling prevents the use of the discrete Fourier transform; instead, we employ maximum entropy (MaxEnt) reconstruction. Nonlinearly sampled MQMAS spectra are shown to provide high resolution and sensitivity in several systems, including lithium sulfate monohydrate ( LiSO 4 -H 2 17 O) and l-asparagine monohydrate ( H 2 17 O) . The combination of nonlinear sampling and MaxEnt reconstruction promises to make the application of 17 O MQMAS practical in a wider range of biological systems.

Reintroducing anisotropic interactions in magic-angle-spinning NMR of half-integer quadrupolar nuclei: 3D MQMAS.

Selective reintroduction of anisotropic interactions such as the chemical shift anisotropy (CSA) and homonucler dipolar (HMD) coupling were implemented in a high-resolution NMR spectroscopy for half-integer quadrupolar nuclei. Rotary resonance recoupling (R(3)) combined with the multiple-quantum magic-angle spinning (MQMAS) in a three-dimensional (3D) experiment provides not only site-specific high-resolution spectra to yield the quadrupolar interaction parameters but also the CSA or HMD interaction parameters. This 3D experiment provides an avenue for the complete local structural information of half-integer quadrupolar nuclei. Three-dimensional MQMAS experiments incorporating R(3) of HMD and CSA interactions were demonstrated on model compounds containing (11)B, (23)Na, and (87)Rb nuclei.

Optimization of data acquisition and processing in Carr–Purcell–Meiboom–Gill multiple quantum magic angle spinning nuclear magnetic resonance

Data acquisition using the Carr–Purcell–Meiboom–Gill (CPMG) train of π pulses has been recently explored in multiple quantum magic angle spinning (MQMAS) nuclear magnetic resonance of half-integer quadrupolar nuclei [T. Vosegaard, F. H. Larsen, H. J. Jakobsen, P. D. Ellis, and N. C. Nielsen, J. Am. Chem. Soc. 119, 9055 (1997)]. Significant increase of sensitivity can be obtained by using this technique at the expense of spectral definition, as the spectrum transforms into a manifold of narrow sidebands. A detailed analysis of the CPMG method, with emphasis on the MQMAS experiment, is presented. The MQ-QCPMG-MAS approach is adapted to spin-5/2 nuclei. Several numerical methods of data treatment are shown that allow for improvement of the definition of the sideband envelope, reconstruction of the standard line shape, and improvement of S/N ratio via optimized filtering. Theoretical and experimental estimates of signal enhancement through proper acquisition and processing of MQ-QCPMG-MAS data are given.

Theoretical studies of the spin dynamics of quadrupolar nuclei at rotational resonance conditions

A theory of the spin dynamics of I=3/2 quadrupolar nuclei in the sudden-passage limit is discussed in relation to the recently observed rotational resonance (RR) effects on the excitation and conversion of triple-quantum coherence in the FASTER multiple-quantum magic-angle spinning (MQMAS) experiments [T. Vosegaard, P. Florian, D. Massiot, and P. J. Grandinetti, J. Chem. Phys. 114, 4618 (2001)]. A novel interaction frame, which combines the quadrupolar interaction with the central transition radio frequency irradiation, is shown to be useful in understanding the complex spin dynamics at and away from RR conditions. Analytical expressions for the Hamiltonian obtained from bimodal Floquet theory are included in order to provide insight into the spin dynamics observed in the FASTER MQMAS experiments. Numerical simulations have been performed and were found to support the theoretical formalism.

Rotary resonance in multiple-quantum magic-angle spinning

The theory of rotary resonance in multiple-quantum magic-angle spinning (MQMAS) experiment is presented. The rotary resonance effect can enhance the efficiency of MQMAS experiment for high-resolution NMR spectra of half-integer quadrupolar nuclei [J. Chem. Phys. 114 (2001) 4618]. Using a spin-1/2 formalism and Floquet theorem, the spin dynamics of a spin-3/2 under rf irradiation and magic-angle sample spinning is solved analytically in a doubly rotating frame. The results show an oscillatory behavior of MQ excitation with nulls at ω 1= nω r/2, where ω 1 represents the rf field strength and ω r is the spinning frequency. Efficient MQ excitation occurs between these nulls and MQ conversion peaks at ω 1= nω r. The origin of the rotary resonance phenomenon is related to frequency shift of a general two-level system under a randomly modulated periodic perturbation.

Relative orientation of quadrupole tensors from two-dimensional multiple-quantum MAS NMR.

The Multiple-Quantum Magic Angle Spinning (MQMAS) technique enables the resolution of crystallographically inequivalent nuclei and also yield the quadrupolar coupling constant C-Q, quadrupolar asymmetry eta, and isotropic chemical shift delta(CS) of each crystallographic site. However, no information is obtained that relates one quadrupole tensor to another, such as internuclear distances or relative orientations. Here, we discuss a recently developed modification of the MQMAS NMR experiment that utilizes two-dimensional correlation of second-order (rank I = 4) broadened MAS line shapes to obtain the relative orientation of quadrupole tensors. This new method involves the insertion of a mixing time, tau(m), into the MQMAS experiment such that magnetization transfer between two distinct nuclei within a crystallite during this period will result in a two-dimensional cross-peak correlating the two line shapes. The shapes of these cross-peaks are characteristic of the three Euler angles, alpha', beta', and gamma', that describe the relative orientation of the two quadrupole tensors.

Multiple quantum magic-angle spinning using rotary resonance excitation

We have discovered rotary resonances between rf field strength, ω1, and magic-angle spinning (MAS) frequency, ωR, which dramatically enhance the sensitivity of triple quantum preparation and mixing in the multiple-quantum MAS experiment, particularly for quadrupolar nuclei having low gyromagnetic ratios or experiencing strong quadrupole couplings. Triple quantum excitation efficiency minima occur when 2ω1=nωR, where n is an integer, with significant maxima occurring between these minima. For triple quantum mixing we observe maxima when ω1=nωR. In both preparation and mixing the pulse lengths required to reach maxima exceed one rotor period. We have combined these rotary resonance conditions into a new experiment called FASTER MQ-MAS, and have experimentally demonstrated a factor of 3 enhancement in sensitivity in comparison to conventional MQ-MAS.

Influence of glass chemical composition on the Na–O bond distance: a 23Na 3Q-MAS NMR and molecular dynamics study

The sodium environment in oxide glasses was investigated by 23Na multiple-quantum magic-angle spinning (MQ-MAS) NMR spectroscopy and compared with molecular dynamics simulations. In the experimental approach, a spectrum-inversion was employed taking into account the transfer efficiency involved in the MQ-MAS experiment. This allowed the reconstruction of the underlying two-dimensional distribution of the isotropic chemical shift correlated with the quadrupolar interaction. The isotropic chemical shift distributions were extracted from the MQ-MAS spectra to infer Na–O distance distributions. First, a Na 2O–2SiO 2 glass and its crystal analogue were characterized by this method to observe the disorder effect in the glass through the Na–O distance distribution. Thereafter, in order to study the influence of the chemical composition on the Na–O distance and distribution, additional glasses were investigated with NMR and simulation: Na 2O–5SiO 2, Na 2O–2CaO–3SiO 2, Na 2O–Al 2O 3–3SiO 2 and Na 2O–B 2O 3–3SiO 2. The molecular dynamics results are in good agreement with the experimental findings. The mean Na–O distance is higher when network formers are added to the sodium silicate glass. The effects on the Na–O distance distribution are also discussed. The simulation relates these results to the existence of several types of Na: near the non-bridging oxygen of the silicon, or as aluminum or boron charge compensator. This can be explained through charge and geometric effects.

Multiple-quantum MAS NMR of quadrupolar nuclei. Do five-, seven- and nine-quantum experiments yield higher resolution than the three-quantum experiment?

The question of whether or not higher-order (five-, seven- and nine-quantum) multiple-quantum magic angle spinning (MQMAS) experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.

Investigation of Lead Borosilicate Glass Structure With 207Pb and 11B Solid-State NMR

ABSTRACTA series of three lead borosilicate glasses were synthesized and analyzed for structural information with both 11B and 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopic methods. Results showed that increasing lead content caused lead to take a more active role in the network as a former and that the populations in these sites can be approximately quantified. 207Pb phase-adjusted-spinning sidebands (PASS), 11B magic-angle spinning (MAS), and 11B multiple-quantum MAS (MQMAS) experiments were used to determine structural parameters for the two nuclei. The 207Pb PASS experiment showed that at higher lead content, more covalent bonding was present. This principle was demonstrated in both an overall shift of the spectral resonances and a quantitative change in site ratios. The 11B MAS experiment showed that the ratio of BO3 to BO4 units was dependent on the amount of lead and boron, consistent with previous studies. Preliminary 11B MQMAS experiments failed to detect any BO3– units, previously hypothesized to exist in this system.

Very fast MAS and MQMAS NMR studies of the spectroscopically challenging minerals kyanite and andalusite on 400, 500, and 800 MHz spectrometers

The well-characterized minerals kyanite and andalusite have long presented great challenges in using solid state 27 Al NMR to determine the isotropic chemical shift δ CS, quadrupole coupling constant e 2 qQ/ h, and asymmetry parameter η for each of the inequivalent aluminum sites in these minerals. Indeed, these minerals have frequently been used to test advances in instrumentation. Recent advances in magnet technology (up to 18.8 T=800 MHz 1 H ) and in MAS probe technology (spinning up to 35 kHz and considerably stronger rf) and refinements of the two-dimensional, multiple quantum magic angle spinning (MQMAS) technique suggested that these developments could be profitably used to study kyanite and andalusite by solid state 27 Al NMR. The benefit of being able to study kyanite both by MAS and MQMAS techniques on 400, 500, and 800 MHz spectrometers is demonstrated. The two octahedral aluminum sites with the largest (and nearly equal) e 2 qQ/ h values give overlapping 1D MAS or 2D 3QMAS signals at all three field strengths. Nevertheless, quantitatively accurate 3Q signal intensities at 9.4 T for all four octahedral aluminum sites (with e 2 qQ/ h values up to 10 MHz) allow more detailed analysis. Even if the 3Q signal intensities are not quantitative, their isotropic shifts provide an approach (if accurate e 2 qQ/ h and η values are available) other than deconvolution of the MAS spectrum for calculating δ CS values. For andalusite, 34 kHz MAS on the 800 MHz spectrometer significantly narrows the extremely broad signal for the octahedral aluminum, and only slight difficulties are encountered in quantitating the relative amounts of AlO 5 and AlO 6 present. Even with e 2 qQ/ h=15.3 MHz, the octahedral aluminum in andalusite gives a signal in a MQMAS experiment, albeit of reduced intensity. As appropriate, we discuss some of the benefits and limitations of these advances in instrumentation and of different experimental approaches for studying non-integral spin quadrupolar nuclei in solids.

Two-Dimensional Multiple-Quantum MAS NMR of Quadrupolar Nuclei: A Comparison of Methods

Multiple-quantum magic-angle-spinning (MQMAS) NMR experiments have recently been used to remove second-order broadening from the central transition of half-integer quadrupolar nuclei. In this paper, the various methods that have been proposed for obtaining pure absorption-mode lineshapes in MQMAS experiments are described and compared. The methods can be classified according to whether the data are amplitude- or phase-modulated as a function of the evolution period,t1. Both classes of experiment are usually performed in such a way that the inhomogeneous quadrupolar broadening is spread out along a ridge which, for spinsI= 3/2 and 5/2, respectively, has a slope of −7/9 or 19/12 with respect to theF2axis. This paper shows, however, that there are disadvantages associated with recording the data in this fashion and demonstrates, in particular, that a shearing transformation of the final two-dimensional spectrum can lead to distorted lineshapes. Novel amplitude- and phase-modulated “split-t1” MQMAS experiments are introduced which fully refocus the second-order broadening during the evolution period,t1, thereby avoiding the need for a shearing transformation. The considerable practical advantages of these split-t1experiments are discussed, particularly with regard to ease of implementation and processing. In general, the sensitivities achievable using the split-t1MQMAS experiments are predicted to be similar to those obtainable with other methods and, in the special case of the spinI= 3/2 phase-modulated experiments, are even shown to be slightly superior.

A 27Al MQMAS and Off-Resonance Nutation NMR Investigation of Mo−P/γ-Al2O3 Hydrotreating Catalyst Precursors

γ-Al2O3 and γ-Al2O3 impregnated with phosphorus and/or molybdenum, amorphous AlPO4, and Al2(MoO4)3 have been studied by the recently introduced multiple-quantum magic angle spinning (MQMAS) NMR and off-resonance nutation NMR. Average quadrupolar coupling parameters of the resonances in the compounds were determined with 27Al off-resonance nutation spectroscopy. The MQMAS NMR experiment was used to increase the resolution of the spectra in order to gain insight in the distribution of the quadrupole parameters and to resolve overlapping lines from surface species on γ-Al2O3 supports. This combined use of advanced NMR techniques provided information about the bulk and surface structure of γ-Al2O3- and γ-Al2O3-supported catalyst precursors. When phosphorus and molybdenum loadings below “monolayer” coverage are employed, the single pulse spectra did not reveal the formation of new aluminum-containing compounds. At higher phosphorus loading the formation of a new phase was observed that from the MQMAS experiment could be clearly assigned to amorphous AlPO4. In a calcined sample containing both molybdenum and phosphorus, 27Al NMR showed that Al2(MoO4)3 and some AlPO4 had been formed, which could not be detected by XRD and hardly by single-pulse-excitation (SPE) MAS NMR. The fact that the observed distribution of aluminum atoms over octahedral and tetrahedral positions did not change with the loading of the support (except when AlPO4 was formed) led to the conclusion that aluminum atoms with both types of coordination are found not only in the bulk of γ-Al2O3 but also on its surface in about the same ratio.