Magic Angle Turning Research Articles

Structural characterization of tin in toothpaste by dynamic nuclear polarization enhanced 119Sn solid-state NMR spectroscopy

Stannous fluoride (SnF2) is an effective fluoride source and antimicrobial agent that is widely used in commercial toothpaste formulations. The antimicrobial activity of SnF2 is partly attributed to the presence of Sn(II) ions. However, it is challenging to directly determine the Sn speciation and oxidation state within commercially available toothpaste products due to the low weight loading of SnF2 (0.454 wt% SnF2, 0.34 wt% Sn) and the amorphous, semi-solid nature of the toothpaste. Here, we show that dynamic nuclear polarization (DNP) enables 119Sn solid-state NMR experiments that can probe the Sn speciation within commercially available toothpaste. Solid-state NMR experiments on SnF2 and SnF4 show that 19F isotropic chemical shift and 119Sn chemical shift anisotropy (CSA) are highly sensitive to the Sn oxidation state. DNP-enhanced 119Sn magic-angle turning (MAT) 2D NMR spectra of toothpastes resolve Sn(II) and Sn(IV) by their 119Sn chemical shift tensor parameters. Fits of DNP-enhanced 1D 1H → 119Sn solid-state NMR spectra allow the populations of Sn(II) and Sn(IV) within the toothpastes to be estimated. This analysis reveals that three of the four commercially available toothpastes contained at least 80% Sn(II), whereas one of the toothpaste contained a significantly higher amount of Sn(IV).

Structural Role of Phosphate in Metaluminous Sodium Aluminosilicate Glasses As Studied by Solid State NMR Spectroscopy

In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of P2O5 (up to 7.5 mol %) on the network organization of metaluminous sodium aluminosilicate glasses employing a range of advanced solid state NMR methodologies. The combined results from MAS, MQMAS (multiple quantum MAS), or MAT (magic angle turning) NMR spectroscopy and a variety of dipolar based NMR experiments-27Al{31P}-, 27Al{29Si}-, 29Si{31P}-, and 31P{29Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as 31P{27Al}- and 29Si{27Al}-REAPDOR (rotational echo adiabatic passage double resonance) NMR-allow for a detailed analysis of the network organization adopted by these glasses. Phosphate is found as QP2, QP3, and QP4 (with the superscript denoting the number of bridging oxygens), the QP4 units can be safely identified with the help of 31P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO4 units that is needed for charge compensation of the QP2 units necessitates an alternative charge compensation scheme for these AlO4 units via formation of QP4 units or oxygen triclusters. The dipolar NMR experiments suggest a strong preference of P for Al with an average value of ca. 2.4 P-O-Al connections per phosphate tetrahedron. P is thus mainly integrated into the network via P-O-Al bonding, the formation of Si-O-P bonding plays only a minor role.

Magic-angle turning with double acquisition

The double-acquisition scheme for efficient data collection of hypercomplex data (the States method) of a two-dimensional experiment is adopted to magic-angle hopping (MAH) and magic-angle turning (MAT) experiments, which are powerful methods to measure the principal values of the chemical shift anisotropy (CSA) in a powder sample. It is shown that the double acquisition MAT (DAMAT) sequence realizes the S/N ratio comparable to or better than those of other variants of the MAH/MAT sequences. In addition, we show that DAMAT has preferable features that there are no spinning sidebands in the indirect dimension, and no spectral shearing is necessary.

Sideband separation experiments in NMR with phase incremented echo train acquisition

A general approach for enhancing sensitivity of nuclear magnetic resonance sideband separation experiments, such as Two-Dimensional One Pulse (TOP), Magic-Angle Turning (MAT), and Phase Adjust Spinning Sidebands (PASS) experiments, with phase incremented echo-train acquisition (PIETA) is described. This approach is applicable whenever strong inhomogeneous broadenings dominate the unmodulated frequency resonances, such as in non-crystalline solids or in samples with large residual frequency anisotropy. PIETA provides significant sensitivity enhancements while also eliminating spectral artifacts would normally be present with Carr-Purcell-Meiboom-Gill acquisition. Additionally, an intuitive approach is presented for designing and processing echo train acquisition magnetic resonance experiments on rotating samples. Affine transformations are used to relate the two-dimensional signals acquired in TOP, MAT, and PASS experiments to a common coordinate system. Depending on sequence design and acquisition conditions two significant artifacts can arise from truncated acquisition time and discontinuous damping in the T2 decay. Here we show that the former artifact can always be eliminated through selection of a suitable affine transformation, and give the conditions in which the latter can be minimized or removed entirely.

Achieving high resolution dipolar NMR information without fast sample spinning: Combining magic angle turning with dipolar based NMR methods

In this article we present a new approach to high resolution NMR combining the concepts of magic angle hopping (MAH)/magic angle turning (MAT) and dipolar based NMR methods such as SEDOR, REDOR or cross polarization (CP). Employing aluminophosphates as model systems we demonstrate that No MAS needed (NOMAS) is capable of supplying high resolution dipolar information without the need of fast MAS.

High-resolution 13C nuclear magnetic resonance evidence of phase transition of Rb,Cs-intercalated single-walled nanotubes

We present 13 C high-resolution magic-angle-turning (MAT) and magic angle spinning nuclear magnetic resonance data of Cs and Rb intercalated single walled carbon nanotubes. We find two distinct phases at different intercalation levels. A simple charge transfer is applicable at low intercalation level. The new phase at high intercalation level is accompanied by a hybridization of alkali (s) orbitals with the carbon (sp2) orbitals of the single walled nanotubes, which indicate bundle surface sites is the most probable alkali site.

Technical aspects of fast magic-angle turning NMR for dilute spin-1/2 nuclei with broad spectra

For obtaining sideband-free spectra of high-Z spin-1/2 nuclei with large (>1000 ppm) chemical-shift anisotropies and broad isotropic-shift dispersion, we recently identified Gan's modified five-pulse magic-angle turning (MAT) experiment as the best available broadband pulse sequence, and adapted it to fast magic-angle spinning. Here, we discuss technical aspects such as pulse timings that compensate for off-resonance effects and are suitable for large CSAs over a range of 1.8 γB 1; methods to minimize the duration of z-periods by cyclic decrementation; shearing without digitization artifacts, by sharing between channels (points); and maximizing the sensitivity by echo-matched full-Gaussian filtering. The method is demonstrated on a model sample of mixed amino acids and its large bandwidth is highlighted by comparison with the multiple- π-pulse PASS technique. Applications to various tellurides are shown; these include GeTe, Sb 2Te 3 and Ag 0.53Pb 18Sb 1.2Te 20, with spectra spanning up to 190 kHz, at 22 kHz MAS. We have also determined the 125Te chemical shift anisotropies from the intensities of the spinning sidebands resolved by isotropic-shift separation.

On the magic-angle turning and phase-adjusted spinning sidebands experiments

The underlying relation between the magic-angle turning (MAT) and phase-adjusted spinning sidebands (PASS) experiments is examined. The MAT experiment satisfies the PASS conditions for separating spinning sidebands with a non-constant total evolution time and only requires linear t 1 increments of up to one rotor period. The time-domain data of the two experiments are related by a shearing transformation. A combination of the linear evolution-time increments of MAT and simple data processing of PASS are particularly attractive for the implementation of MAT for measuring chemical shift anisotropy.

Broadband “Infinite-Speed” Magic-Angle Spinning NMR Spectroscopy

High-resolution magic-angle spinning NMR of high-Z spin-1/2 nuclei such as (125)Te, (207)Pb, (119)Sn, (113)Cd, and (195)Pt is often hampered by large (>1000 ppm) chemical-shift anisotropies, which result in strong spinning sidebands that can obscure the centerbands of interest. In various tellurides with applications as thermoelectrics and as phase-change materials for data storage, even 22-kHz magic-angle spinning cannot resolve the center- and sidebands broadened by chemical-shift dispersion, which precludes peak identification or quantification. For sideband suppression over the necessary wide spectral range (up to 200 kHz), radio frequency pulse sequences with few, short pulses are required. We have identified Gan's two-dimensional magic-angle-turning (MAT) experiment with five 90 degrees pulses as a promising broadband technique for obtaining spectra without sidebands. We have adapted it to broad spectra and fast magic-angle spinning by accounting for long pulses (comparable to the dwell time in t(1)) and short rotation periods. Spectral distortions are small and residual sidebands negligible even for spectra with signals covering a range of 1.5 gammaB(1), due to a favorable disposition of the narrow ranges containing the signals of interest in the spectral plane. The method is demonstrated on various technologically interesting tellurides with spectra spanning up to 170 kHz, at 22 kHz MAS.

An isotropic chemical shift–chemical shift anisotropic correlation experiment using discrete magic angle turning

An isotropic–anisotropic shift 2D correlation spectroscopy is introduced that combines the advantages of both magic angle turning (MAT) and magic angle hopping (MAH) technologies. In this new approach, denoted DMAT for “discrete magic angle turning”, the sample rotates clockwise followed by an anticlockwise rotation of exactly the same amount with each rotation less or equal than 360° but greater than 240°, with the rotation speed being constant only for times related to the evolution dimension. This back and forth rotation is repeated and synchronized with a special radio frequency (RF) pulse sequence to produce an isotropic–anisotropic shift 2D correlation spectrum. For any spin-interaction of rank-2 such as chemical shift anisotropy, isotropic magnetic susceptibility interaction, and residual homo-nuclear dipolar interaction in biological fluid samples, the projection along the isotropic dimension is a high resolution spectrum. Since a less than 360° sample rotation is involved, the design potentially allows for in situ control over physical parameters such as pressure, flow conditions, feed compositions, and temperature so that true in situ NMR investigations can be carried out.

A comparison of glycans and polyglycans using solid-state NMR and X-ray powder diffraction

Individual polyglycans and their corresponding monomers have been studied separately for several decades. Attention has focused primarily on the modifications of these polyglycans instead of the simple relationship between the polyglycans themselves and their corresponding monomers. Two polyglycans, chitin and chitosan, were examined along with their respective monomeric units, N-acetyl- d-glucosamine (GlcNAc) and (+) d-glucosamine (GlcN) using solid-state proton decoupling Magic Angle Turning (MAT) techniques and X-Ray Powder Diffraction (XRPD). A down-field shift in isotropic 13C chemical shifts was observed for both polymers in Cross Polarization/Magic Angle Spinning (CP/MAS) spectra. An explanation of misleading peak assignments in previous NMR studies for these polyglycans was determined by comparing sideband patterns of the polymers with their corresponding monomers generated in a 2D FIve π REplicated Magic Angle Turning (FIREMAT) experiment processed by Technique for Importing Greater Evolution Resolution (TIGER). Structural changes in the crystalline framework were supported by XRPD diffraction data.

Two-Dimensional 13C Magic Angle Turning NMR Analyses of Dynamics in Poly(2-hydroxypropyl ether of bisphenol A)

Two-dimensional solid-state 13C magic angle turning (MAT) NMR spectroscopy, for measuring and separating the line shapes of chemical shift anisotropy (CSA) in respective 13C species, has been applied to the study of dynamics in amorphous poly(2-hydroxypropyl ether of bisphenol A) (PHR). From the temperature dependence of the CSA line shapes, the flip motion of the phenylene ring is quantitatively analyzed; the phenylene ring undergoes a π-flip motion with standard deviations, σ, of 25° and 30−35° at 27 and 90 °C, respectively. Accompanying the flip motion, the main-chain fluctuational motion is revealed from the temperature dependence of the CSA line shapes of the phenylene ring quaternary carbons. The cone angle of the main-chain fluctuation is 10−15° and 12−20° at 27 and 90 °C, respectively. On the basis of the main-chain motion, we propose a model to account for the coincidences of the frequencies and the activation energies of the phenylene ring π-flip motion and γ-relaxation. In the model, the main-c...

High-resolution 1H NMR spectroscopy in rat liver using magic angle turning at a 1 Hz spinning rate.

It is demonstrated that a high-resolution (1)H NMR spectrum of excised rat liver can be obtained using the technique of magic angle turning (MAT) at a sample spinning rate of 1 Hz. A variant of the phase-corrected MAT (PHORMAT) pulse sequence that includes a water suppression segment was developed for the investigation. The spectral resolution achieved with PHORMAT approaches that obtained from a standard magic angle spinning (MAS) experiment at a spinning rate of several kHz. With such ultra-slow spinning, tissue and cell damage associated with the standard MAS experiment is minimized or eliminated. The technique is potentially useful for obtaining high-resolution (1)H spectra in live animals.

Fluorine-19 Solid-State NMR Magic-Angle-Turning Experiments Using Multiple-Pulse Homonuclear Decoupling

For compounds giving “crowded” 1-dimensional magic-angle-spinning spectra, information about the local atomic environment in the form of the chemical shift anisotropy (CSA) is sacrificed for high resolution of the less informative isotropic chemical shift. Magic-angle-turning (MAT) NMR pulse sequences preserve the CSA information by correlating it to the isotropic chemical shift in a 2-dimensional experiment. For low natural abundance nuclei such as 13C and 15N and under 1H heteronuclear dipolar decoupling conditions, the dominant NMR interaction is the chemical shift. For abundant nuclei such as 1H, 19F, and 31P, the homonuclear dipolar interaction becomes a significant contribution to the observed linewidth in both F1 and F2 dimensions. We incorporate MREV8 homonuclear multiple-pulse decoupling sequences into the MAT experiment to give a multiple-pulse MAT (MP-MAT) experiment in which the homonuclear dipolar interaction is suppressed while maintaining the chemical shift information. Extensive use of computer simulation using GAMMA has guided the pulse sequence development. In particular, we show how the MREV8 pulses can be incorporated into a quadrature-detected sequence such as MAT. The MP-MAT technique is demonstrated for a model two-site system containing a mixture of silver trifluoroacetate and calcium difluoride. The resolution in the isotropic evolution dimension is improved by faster sample spinning, shorter MREV8 cycle times in the evolution dimension, and modifications of the MAT component of the pulse sequence.

Ab initio calculation and experimental determination of the 13C chemical shielding tensors of 4,4′-dimethoxybiphenyl

An ab initio calculation of the principal components of the 13C chemical shielding tensors of 4,4′-dimethoxybiphenyl are reported by employing the ab initio GIAO approach. The geometric parameters were optimized by using the Gaussian-92 program. The observed values of the 13C NMR chemical shifts of a powdered sample were determined by using the recently introduced magic angle turning (MAT) experiment. Theoretical calculations successfully reproduced the MAT experimental resonance peaks. The results presented in this paper demonstrate the importance of using tensor information in assigning the experimental resonances.

Investigation of surface methoxy groups on SAPO-34 A combined magic-angle turning NMR experimental approach with theoretical studies

The formation of stable surface methoxy groups in various zeolite-catalysed reaction processes has been widely accepted as an important intermediate stage and the understanding of such an intermediacy is of great significance. 13C magic-angle-turning (MAT) NMR experiments, in conjunction with theoretical calculations of 13C shielding tensors, are employed to investigate the chemical and electronic nature of methoxy groups on H-SAPO-34.

Solid-State 13C NMR Measurements in Methoxynaphthalenes: Determination of the Substituent Chemical Shift Effects in the Principal Values

The principal values of the 13C chemical shift tensor are measured in 1-methoxy-, 2-methoxy-, 1,4-dimethoxy-, 2,6-dimethoxy-, and 2,7-dimethoxynaphthalene using the triple echo version of the 2D magic angle turning (MAT) experiment. In the case of 1-methoxynaphthalene, a room-temperature liquid, the experiment was performed at low temperature. Assignments of the measured principal values of the chemical shift tensors to the individual carbons are made by both comparison of the solid-state isotropic chemical shift to the known solution values and comparison of the experimental principal values to the results of ab initio calculations of the chemical shielding tensor. The methoxy group substituent chemical shift parameters for each of the three components is determined relative to the parent unsubstituted naphthalene for the protonated and ipso carbons. The substituent parameters are very similar to those previously observed for methoxy substituted benzenes. The bond populations, determined using ab initio ...

Solid State 15N and 13C NMR Study of Several Metal 5,10,15,20-Tetraphenylporphyrin Complexes

The principal values of both the 13C and 15N chemical shift tensors are reported for the Zn, Ni, and Mg 5,10,15,20-tetraphenylporphyrin (TPP) complexes. The principal values of the 15N chemical shift tensors were obtained from static powder patterns of 15N-enriched samples. Due to overlap between the powder patterns of the different carbons, the 13C values were obtained using the recently developed magic angle turning (MAT) 2D experiment on unenriched materials. The measured principal values are presented along with theoretical calculations of the chemical shift tensors and a discussion of the effects that the metal bonding has on the chemical shift tensors in these compounds. Both the isotropic chemical shift and the principal values of the 15N chemical shift tensor are nearly identical for the Mg and Zn complexes. The 15N isotropic chemical shift for the NiTPP, however, changes by nearly 80 ppm relative to the Mg and Zn values, with large changes observed in each of the three principal values. Calculations show that the differences between the 15N chemical shifts are almost entirely determined by the metal−nitrogen separation. In addition, both the experimental data and the calculations show only very minor differences in the 13C chemical shift tensor components as the metal is changed.

Quantum chemical calculation and experimental measurement of the 13C chemical shift tensors of vanillin and 3,4-dimethoxybenzaldehyde

The principal values of the 13C nuclear magnetic resonance chemical shift tensors in vanillin and 3,4-dimethoxybenzaldehyde are reported. Theoretical results of the 13C chemical shift tensors were obtained by employing the gauge included atomic orbital (GIAO) approach. The geometrical parameters were optimized by using the MNDO method. The observed chemical shifts of these two compounds were determined in powders by using the recently introduced magic angle turning (MAT) experiment. The results presented in this paper clearly demonstrate the importance of using tensor information in the study of molecular structures.

Carbon-13 Chemical Shift Tensors and Molecular Conformation of Anisole

The first direct measurement of the ortho steric effect of the methoxy group on the 13C chemical shifts in anisole is reported. The ortho steric effect on the isotropic 13C chemical shifts was obtained from a low-temperature MAS spectrum, and the effect on both the isotropic and the tensor principal components was determined from a low-temperature 2D magic angle turning (MAT) experiment. From the low-temperature MAS spectrum, the 13C chemical shift of the ortho carbon cis to the methoxy carbon is found to be 7.0 ppm lower that of the ortho carbon trans to the methoxy carbon, in good agreement with previous estimates. From the low-temperature MAT experiment, a 6.8 ppm decrease in the chemical shift is observed in the isotropic chemical shift, while the effects on the difference (cis minus trans) between the individual tensor components are measured to be −9 ppm in δ11, 1 ppm in δ22, and −14 ppm in δ33, in reasonable agreement with the results of a previous linear regression substituent analysis on several di- and trimethoxybenzenes. Comparison of the experimental results with calculations, including thermal averaging considerations, further demonstrates that at room temperature the methoxy group in anisole undergoes stochastic jumps between the two equivalent planar configurations. This work demonstrates the feasibility of using the low-temperature MAT experiment at low temperature to measure the principal values of the 13C chemical shift tensors in molecules that are liquids at room temperature.