Fast MAS Research Articles

Benzoxazine oligomers: evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations.

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MAS (1)H NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and (1)H chemical-shift calculations. Also supporting these proposed structures are homonuclear (1)H[bond](1)H double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative (15)N[bond](1)H distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.

Changes in Structural and Electronic Properties of the Zeolite Framework Induced by Extraframework Al and La in H-USY and La(x)NaY: A29Si and27Al MAS NMR and27Al MQ MAS NMR Study

A 27Al 3Q MAS, a quantitative 27Al MAS, and a 29Si MAS NMR study has been carried out on La(x)NaY and H-USY. Assignment of the different types of Al coordinations has been done using the results of the MQ MAS experiments. The 29Si MAS and 27Al MAS NMR results obtained at high fields (14.1 T) and fast MAS up to 27 kHz made a quantitative assignment possible of the framework and nonframework Al coordinations. It is shown that highly charged octahedrally coordinated extraframework Al causes a polarization inducing a quadrupolar broadening of part of the framework Al in ultrastable Y. A similar interaction is found for the framework of La(x)NaY due to the La3+ cations. In both cases, charged extraframework species polarize the framework Al. In La(x)NaY, La3+ in a cation position and in H-USY extraframework octahedral Al are the origin of the polarization. Besides this short-range polarization effect, an additional long-range effect is observed in La(x)NaY. This long-range geometrical effect causes an increase...

Newly Applied Two-Dimensional Solid-State NMR Correlation Techniques for the Characterization of Organically Modified Silicates

This paper highlights the use of two-dimensional (2D) solid-state NMR correlation techniques to probe the chemical homogeneity of organically modified silicate networks. Specifically, 29Si{1H} heteronuclear correlation (HETCOR) NMR experiments have revealed the spatial proximity of the two types of Si units present in a gel obtained from co-hydrolysis of methyldiethoxysilane and triethoxysilane. Similar information has also been obtained by using 2D 1H homonuclear correlation NMR spectroscopy. Such experiments were only possible by combining the use of high magnetic field (14.10 T) with fast MAS spinning rate (30 kHz).

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.

Semicrystallinity and polymorphism in PVDF: A solid-state 19F n.m.r. investigation

Solid-state 19F nuclear magnetic resonance spectroscopy has been employed to investigate the semicrystallinity and polymorphism of poly(vinylidene fluoride) (PVDF). Fast MAS and high-power proton decoupling were applied to separate the spinning sidebands from the relevant spectral range and to remove the effects of dipolar coupling between the protons and the fluorine nuclei. Amorphous and crystalline regions provide distinct chemically-shifted signals. The application of relaxation filters can suppress the signal either of the amorphous or of the crystalline part. The polymorphism, which characterizes the PVDF crystallites, is reflected in spectra of the crystalline part. Some of the variety of possible conversions between the modifications have been exemplified. Variable-temperature experiments demonstrate both crystallization effects and changes in the modifications.

Rotation-Synchronized Homonuclear Dipolar Decoupling

Rotor-synchronized pulse sequences for homonuclear dipolar decoupling are analyzed for the particular case of the magic-sandwich pulse sequence. The decoupling efficiency of this sequence in the presence of sample spinning is considered for the slow- and the fast-spinning regime and compared with the results of the experiments. For arbitrary ratios of the cycle time of the pulse sequence tC and the rotor period τR, the decoupling properties of the pulse sequence are largely reduced by the influence of the sample spinning. However, the decoupling efficiency is recovered, if special synchronization conditions tC/τR are met and a rotational dipolar magic echo is formed. These synchronization conditions are derived by secular averaging theory for weak and strong dipolar solids and confirmed experimentally for pulse sequences consisting of one or four magic-sandwich pulse trains. For the pulse sequence with one magic sandwich, the first synchronization condition is met for tC/τR = 3, whereas for the sequence consisting of four magic sandwiches, the synchronization condition is t(c)/τR = 14. These results form the basis for the application of combined rotation and multiple-pulse sequences at high spinning speeds. Similar synchronization conditions are expected to apply to other sequences for decoupling, recoupling, multiple-quantum excitation, and dipolar-filter experiments, when performed under fast MAS.

Assignment of 13C NMR spectra of solids using fast MAS with cross-polarization

Assignment of 13C NMR spectra of solids using fast MAS with cross-polarization

Observation and accurate quantification of 27Al MAS NMR spectra of some Al 2SiO 5 polymorphs containing sites with large quadrupole interactions

27Al MAS NMR spectra from two polymorphs of Al 2SiO 5, kyanite and andalusite, are presented. Unambiguous extraction of quadrupole interactions of up to 15.5 MHz (= C q= e 2 qQ/h) are possible from fast MAS( > 18 kHz) NMR spectra obtained at an applied magnetic field of 11.7 T. It is shown from a knowledge of the magnitude of C q combined with some recently introduced analysis that correction of the integrated intensity of the centreband to reflect accurately the aluminium distribution in these samples is possible.

Synthesis of zeolite Na-A from metakaolinite revisited

29 Si and 27Al NMR, XRD, IR, SEM and EDAX have been used to examine the synthesis of zeolite Na-A from metakaolinite. When metakaolinite is treated with 5 mol dm–3 NaOH, five- and six-coordinated Al in the aluminous matrix is rapidly converted into four-coordination. The siliceous matrix of kaolinite is partially preserved in metakaolinite and is responsible for the residual long-range order in the ab plane. The siliceous matrix is much more stable to the base than the aluminous matrix. 27Al NMR spectra recorded with fast MAS (> 12 kHz) allow quantification of Al and show that metakaolinite is most reactive after treatment at ca. 700 °C, when the populations of four- and five-coordinated Al are at a maximum and the population of six-coordinated Al at a minimum. Upon treatment with alkali, metakaolinite slowly dissolves. SiO4+4 and Al(OH)4–4 ions in solution condense to an amorphous phase with Si/Al = 1. This direct precursor transforms into zeolite A by a solid-state rearrangement.