Multiple-quantum Magic-angle Spinning Research Articles

Structure and Ionic Interactions of Organic–Inorganic Composite Polymer Electrolytes Studied by Solid-State NMR and Raman Spectroscopy

Solid-state NMR studies of composite polymer electrolytes are reported. The materials consist of polyethylene oxide and an organic–inorganic composite, together with a lithium salt, and are candidates for electrolytes in solid-state lithium ion batteries. Silicon and aluminum MAS and multiple quantum MAS are used to characterize the network character of the organic–inorganic composite, and spin diffusion measurements are used to determine the nanostructure of the polymer/composite blending. Multiple quantum spin counting is used to measure the ion aggregation. The NMR results are supported by Raman spectra, calorimetry, and impedance spectroscopy. From these experiments it is concluded that the composite suppresses polymer crystallization without suppressing its local mobility, and also suppresses the tendency for the ions to aggregate. This polymer composite thus appears very promising for application in lithium ion batteries.

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.

Structural Study on Amorphous AlPO4 by 17O MQMAS and Mutual (31P↔17O) CP/MAS NMR

Abstract The 17O multiple quantum magic angle spinning (MQMAS) and the mutual (31P↔17O) CP/MAS (MCP) NMR was applied to clarify the detailed chemical structure in amorphous AlPO4. An application of the mutual CP/MAS method being capable of mapping out the connectivity with the 17O MQMAS for characterization of oxygen sites provided precious structural information in amorphous AlPO4.

Satellite-Transition MAS NMR of Spin I=3/2, 5/2, 7/2, and 9/2 Nuclei: Sensitivity, Resolution, and Practical Implementation

The satellite-transition MAS (STMAS) experiment offers an alternative approach to established methods such as dynamic angle spinning (DAS), double rotation (DOR), and multiple-quantum MAS (MQMAS) for obtaining high-resolution NMR spectra of half-integer quadrupolar nuclei. Unlike the multiple-quantum experiment, STMAS involves two-dimensional correlation of purely single-quantum coherences; satellite transitions in t 1 (or F 1) and the central transition in t 2 (or F 2). To date, STMAS has primarily been demonstrated for nuclei with spin quantum numbers I=3/2 and, to a lesser extent, I>5/2. However, many chemically relevant nuclei possess I>3/2, such as 17O and 27Al (both I=5/2), 59Co ( I=7/2), and 93Nb ( I=9/2). Here, we discuss the application of STMAS to nuclei with spin quantum numbers from I=3/2 to 9/2. First, we consider the practical implementation of the STMAS experiment using 87Rb ( I=3/2) NMR as an example. We then extend the discussion to include nuclei with higher spin quantum numbers, demonstrating 27Al, 45Sc ( I=7/2), 59Co, and 93Nb STMAS experiments on both crystalline and amorphous samples. We also consider the possibility of experiments involving satellite transitions other than m I =±1/2↔±3/2 and, using 93Nb NMR, demonstrate the correlation of all single-quantum satellite transitions up to and including m I =±7/2↔±9/2. The absolute chemical shift scaling factors in these experiments are discussed, as are the implications for isotropic resolution.

Dealumination of zeolite beta by acid leaching: a new insight with two-dimensional multi-quantum and cross polarization 27Al MAS NMR

The influence of acid leaching on a commercial zeolite beta with concentrated solutions of HCl and HNO3 has been investigated. The parent and modified zeolites are characterized with ICP, BET, XRD, SEM, FTIR, and 27Al solid-state NMR, with conventional MAS, CP/MAS and two-dimensional multiple-quantum MAS (MQMAS) techniques. The first advantage to submitting zeolite beta to an acid leaching procedure is to remove or passivate all kinds of extra-framework impurities from aluminium clusters to silica–alumina debris. These impurities have been shown to be to some extent acidic. However, the most important effect of acid leaching with concentrated acid solutions is to remove selectively framework aluminium from positions close to defect sites and on the external surface area. Thus, the acid treatments markedly reduce the relative activity of the high external surface area so that the intrinsic properties of the beta microporous structure come into effect. The alkylation of biphenyl with propene transforms afterwards into a shape-selective process.

11B-MQMAS and 29Si–{11B} Double-Resonance NMR Studies on the Structure of Binary B2O3–SiO2 Glasses

Glasses in the system x B2O3·(1−x)SiO2 (0.2≤x≤1.0) were studied using 11B multiple quantum magic angle spinning NMR spectroscopy (MQMAS), 29Si–\\{11B\\} rotational echo adiabatic passage double-resonance and 29Si–\\}11B\\{ CP heteronuclear correlation spectroscopy. The results can be quantitatively interpreted in terms of a phase separation of the borosilicate glasses into a virtually SiO2-free B2O3 phase and a mixed borosilicate phase. While the MQMAS spectra allowed the site speciation and resolution of at least two different 11B resonances, attributable to BO3/2 units consumed in boroxol rings, BO3/2 units connecting the boroxol rings and BO3/2 units involved in B–O–Si linkages, the analysis of the double-resonance data further elucidated the structure of the mixed borosilicate phase. The results indicate that only a fraction of 0.48 mol B2O3 can be accommodated per mole SiO2, building a mixed borosilicate network.

Characterization of mesoporous alumina molecular sieves synthesized by nonionic templating

Mesoporous aluminas were produced by nonionic supramolecular templating and characterized to determine the stability of their physical structure during extended calcination at elevated temperatures. The effect of synthesis temperature on the physical structure of these materials and the resulting thermal evolution from synthesized aluminum hydroxide to transitional alumina was studied using thermogravimetric analysis. A high resolution multiple quantum magic angle spinning NMR spectroscopy method was used to determine quantitatively the evolution of the aluminum coordination during the thermal processing. The mesoporous alumina exhibited high stability upon prolonged heating, which is essential for their future applications in catalytic chemistry.

A multiple-quantum 23Na MAS NMR study of amorphous sodium gallium silicate zeolite precursors

Amorphous sodium gallium silicate materials are isolated after short periods of heating a gel of composition Ga2O3 ∶ SiO2 ∶ 4NaOH ∶ 80H2O at 100 °C. On extended heating (periods in excess of 24 hours), the crystalline zeolite gallium hydroxosodalite is produced from the reaction mixture. We have studied the materials using 23Na (I = 3/2) MAS and multiple-quantum MAS (MQMAS) NMR spectroscopy in order to extract information about the local environment of the sodium in the amorphous solids, and thereby gain information about the role of sodium during the crystallisation of the zeolite. The average 23Na isotropic chemical shift, δiso, and the quadrupolar parameter, PQ = CQ(1 + η2/3)½, were evaluated by analysis of the two-dimensional MQMAS spectra. In addition to the sodium gallium silicate materials, a number of crystalline sodium silicates, aluminates and gallates were also studied and their NMR parameters compared with literature values and related to crystallographic data describing the local environment of the sodium atoms. Although no simple correlation is known between 23Na NMR parameters and the coordination number or average sodium–oxygen interatomic distance, a general trend of increasing δiso with decreasing coordination number and interatomic distance is apparent. The data from the amorphous sodium gallium silicates strongly suggest that even after very short periods of heating the local environment of sodium is very similar to that found in the crystalline zeolite product. Additional 71Ga (I = 3/2) MAS NMR spectra were also recorded from the amorphous samples to provide information on the local gallium environment and these show the presence of tetrahedral gallium, as in the crystalline zeolite, even in the samples prepared after the shortest period of heating.

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.

The Sodium Ions Inside a Lipophilic G-Quadruplex Channel as Probed by Solid-State 23Na NMR

We report solid-state 23Na NMR and X-ray crystallographic results for a self-assembled G-quadruplex channel formed by a guanine nucleoside, 5'-tert-butyl-dimethylsilyl-2',3'-O-isopropylidene guanosine (G 1). The study provides an unambiguous 23Na NMR identification for the Na+ ions inside a lipophilic G-quadruplex channel. The crystalline nature of the sample yields a remarkably high resolution in the 23Na multiple-quantum magic-angle spinning (MQMAS) spectrum, making it possible to extract very accurate 23Na NMR parameters for each of the three crystallographically distinct Na sites. The observation of a single Na+ ion from a 9-kDa system demonstrates the potential of solid-state 23Na NMR as a complementary technique to X-ray for detecting Na+ ions in biological structures.

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.

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.

Rotor-encoded heteronuclear MQ MAS NMR spectroscopy of half-integer quadrupolar and spin I=1/2 nuclei.

A new two-dimensional heteronuclear multiple-quantum magic-angle spinning (MQ MAS) experiment is presented which combines high resolution for the half-integer quadrupolar nucleus with information about the dipolar coupling between the quadrupolar nucleus and a spin I=1/2 nucleus. Homonuclear MQ coherence is initially created for the half-integer quadrupolar nucleus by a single pulse as in a standard MQ MAS experiment. REDOR recoupling of the heteronuclear dipolar coupling then allows the creation of a heteronuclear multiple-quantum coherence comprising multiple- and single-quantum coherence of the quadrupolar and spin I=1/2 nucleus, respectively, which evolves during t1. Provided that the t1 increment is not rotor synchronized, rotor-encoded spinning-sideband patterns are observed in the indirect dimension. Simulated spectra for an isolated IS spin pair show that these patterns depend on the recoupling time, the magnitude of the dipolar coupling, the quadrupolar parameters, as well as the relative orientation of the quadrupolar and dipolar principal axes systems. Spectra are presented for Na2HPO4, with the heteronuclear 23Na-1HMQ MAS experiments beginning with the excitation of 23Na (spin I=3/2) three-quantum coherence. Coherence counting experiments demonstrate that four- and two-quantum coherences evolve during t1. The heteronuclear spinning-sideband patterns observed for the three-spin H-Na-H system associated with the Na(2) site are analyzed. For an IS2 system, simulated spectra show that, considering the free parameters, the spinning-sideband patterns are particularly sensitive to only, first, the angle between the two IS internuclear vectors and, second, the two heteronuclear dipolar couplings. It is demonstrated that the proton localization around the Na(2) site according to the literature crystal structure of Na2HPO4 is erroneous. Instead, the experimental data is consistent with two alternative different structural arrangements, whereby either there is a deviation of 10 degrees from linearity for the case of two identical Na-H distances, or there is a linear arrangement, but the two Na-H distances are different. Furthermore, the question of the origin of spinning-sidebands in the (homonuclear) MQ MAS experiment is revisited. It is shown that the asymmetric experimental MQ sideband pattern observed for the low-C(Q) Na(2) site in Na(2)HPO4 can only be explained by considering the 23Na chemical shift anisotropy.

27Al Multiple-Quantum MAS NMR of Mechanically Treated Bayerite (α-Al(OH)3) and Silica Mixtures

Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR experiments are used to study mixtures of bayerite (α-Al(OH)3) with either silicic acid (SiO2.nH2O) or silica gel (SiO2) that have been ground together for varying lengths of time. This mechanical treatment produces changes in the 27Al MAS and MQMAS NMR spectra that correspond to the formation of new Al species. Mean values of the quadrupolar interaction (PQ) and isotropic chemical shift (δCS) are extracted from the two-dimensional 27Al NMR spectra for each of these species. The presence of significant distributions of both 27Al quadrupolar and chemical shift parameters is demonstrated and the effect of grinding duration on the magnitudes of these distributions is discussed.

27Al nuclear magnetic resonance studies of the Y3-xLuxAl5O12 mixed garnets

The character of the substitutional order, the chemical shift, andthe nuclear quadrupole coupling in the mixed yttrium-lutetiumaluminium garnets Y3-xLuxAl5O12 (0⩽x⩽3)were studied for the first time using 27Al magic angle spinning(MAS) and multiple quantum (MQ) MAS nuclear magnetic resonance(NMR). The relatively narrow distribution of the isotropic NMR lineshifts, revealed by MQ MAS NMR, in crystals with compositionsx = 0.75 and x = 1 compared to mixed garnets of other compositionsshowed that these crystals are partially ordered solid solutions.The aluminium quadrupole coupling constants and isotropic chemicalshifts were evaluated for all garnets under study.

Two-Dimensional 17O Multiple Quantum Magic-Angle Spinning NMR of Organic Solids

We report two-dimensional (2D) (17)O multiple-quantum magic-angle spinning (MQMAS) NMR spectra for four (17)O-labeled organic compounds: [(17)O(2)]-D-alanine (1), potassium hydrogen [(17)O(4)]dibenzoate (2), [(17)O(4)]-D,L-glutamic acid.HCl (3) and [2,4-(17)O(2)]uracil (4). The high spectral resolution observed in the 2D (17)O MQMAS NMR spectra allows extraction of precise (17)O NMR parameters for all crystallographically distinct oxygen sites. We demonstrate that rotor synchronization is important in obtaining high-quality (17)O MQMAS spectra for organic compounds. Several issues related to the potential of (17)O MQMAS NMR for large biomolecular systems are also discussed.

17O NMR Studies of Low Silicate Zeolites

Multiple-quantum magic-angle spinning and double-rotation NMR techniques were applied in the high field of 17.6 T to the study of oxygen-17-enriched zeolites A and LSX with the ratio Si/Al=1. A monotonic correlation between the isotropic value of the chemical shift and the Si–O–Al bond angle α (taken from X-ray data) could be found. Hydration of the zeolites causes a downfield 17O NMR chemical shift of about 8 ppm with respect to the dehydrated zeolites. Ion exchange of the hydrated zeolites generates stronger chemical shift effects. The increase of the basicity of the oxygen framework of the zeolite LSX is reflected by a downfield shift of approx. 10 ppm going from the lithium to the cesium form, and the substitution of sodium by thallium in the zeolite A causes a shift of 34 ppm for the O3 signal. 17O DOR NMR spectra are superior to 17O 3QMAS NMR spectra, featuring a resolution increase by a factor of 2 and are about equal with respect to the sensitivity. The residual linewidths of the signals in the 17O DOR and 17O 5QMAS NMR spectra can be explained by a distribution of the Si–O–Al angles in the zeolites.

High-resolution 1H NMR spectroscopy in the solid state: very fast sample rotation and multiple-quantum coherences.

In the past few years, solid-state 1H NMR spectroscopy under fast magic-angle spinning (MAS) has developed into a versatile tool for elucidating structure and dynamics. Dipolar multiple-quantum (MQ), in particular double-quantum (DQ), MAS spectroscopy has been applied to a variety of materials and provided unique insight, e.g., into the structure of hydrogen-bonded systems. This review intends to present solid-state 1H DQ and MQ MAS spectroscopy in a systematic fashion with a particular emphasis on methodological aspects, followed by an overview of applications.

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.

High-resolution NMR of quadrupolar nuclei using mixed multiple-quantum coherences.

A multiple-quantum magic angle spinning (MQMAS) NMR experiment of quadrupolar nuclei is demonstrated, which uses two different multiple quantum coherences in t(1) to refocus the quadrupolar broadening. This experiment has the potential of achieving improved resolution over current techniques.