Quantum Magic Angle Spinning NMR Research Articles

ChemInform Abstract: Magic Angle Spinning NMR of Macromolecular and Supramolecular Systems

AbstractReview: [68 refs.

Magic Angle Spinning NMR of Macromolecular and Supramolecular Systems

AbstractSideband patterns in single quantum and double quantum magic angle spinning NMR spectra are reviewed. Their use in elucidating non‐covalent interactions, such as hydrogen bonds or pi‐pi stacking, is demonstrated by studies of supramolecular systems of current interest. Moreover, the effects of local and collective motions are illustrated by examples from supramolecular and macromolecular science.

Optimized multiple quantum MAS lineshape simulations in solid state NMR

The majority of nuclei available for study in solid state Nuclear Magnetic Resonance have half-integer spin I > 1 / 2 , with corresponding electric quadrupole moment. As such, they may couple with a surrounding electric field gradient. This effect introduces anisotropic line broadening to spectra, arising from distinct chemical species within polycrystalline solids. In Multiple Quantum Magic Angle Spinning (MQMAS) experiments, a second frequency dimension is created, devoid of quadrupolar anisotropy. As a result, the center of gravity of peaks in the high resolution dimension is a function of isotropic second order quadrupole and chemical shift alone. However, for complex materials, these parameters take on a stochastic nature due in turn to structural and chemical disorder. Lineshapes may still overlap in the isotropic dimension, complicating the task of assignment and interpretation. A distributed computational approach is presented here which permits simulation of the two-dimensional MQMAS spectrum, generated by random variates from model distributions of isotropic chemical and quadrupole shifts. Owing to the non-convex nature of the residual sum of squares (RSS) function between experimental and simulated spectra, simulated annealing is used to optimize the simulation parameters. In this manner, local chemical environments for disordered materials may be characterized, and via a re-sampling approach, error estimates for parameters produced. Program summary Program title: mqmasOPT Catalogue identifier: AEEC_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEEC_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 3650 No. of bytes in distributed program, including test data, etc.: 73 853 Distribution format: tar.gz Programming language: C, OCTAVE Computer: UNIX/Linux Operating system: UNIX/Linux Has the code been vectorised or parallelized?: Yes RAM: Example: (1597 powder angles) × (200 Samples) × (81 F2 frequency pts) × (31 F1 frequency points) = 3.5M, SMP AMD opteron Classification: 2.3 External routines: OCTAVE ( http://www.gnu.org/software/octave/), GNU Scientific Library ( http://www.gnu.org/software/gsl/), OPENMP ( http://openmp.org/wp/) Nature of problem: The optimal simulation and modeling of multiple quantum magic angle spinning NMR spectra, for general systems, especially those with mild to significant disorder. The approach outlined and implemented in C and OCTAVE also produces model parameter error estimates. Solution method: A model for each distinct chemical site is first proposed, for the individual contribution of crystallite orientations to the spectrum. This model is averaged over all powder angles [1], as well as the (stochastic) parameters; isotropic chemical shift and quadrupole coupling constant. The latter is accomplished via sampling from a bi-variate Gaussian distribution, using the Box-Muller algorithm to transform Sobol (quasi) random numbers [2]. A simulated annealing optimization is performed, and finally the non-linear jackknife [3] is applied in developing model parameter error estimates. Additional comments: The distribution contains a script, mqmasOpt.m, which runs in the OCTAVE language workspace. Running time: Example: (1597 powder angles) × (200 Samples) × (81 F2 frequency pts) × (31 F1 frequency points) = 58.35 seconds, SMP AMD opteron.

Connectivity and Proximity between Quadrupolar Nuclides in Oxide Glasses: Insights from through-Bond and through-Space Correlations in Solid-State NMR

The connectivity and proximity among framework cations and anions in covalent oxide glasses yields unique information whereby their various transport and thermodynamic properties can be predicted. Recent developments and advances in the reconstruction of anisotropic spin interactions among quadrupolar nuclides (spin > (1)/(2)) in solid-state NMR shed light on a new opportunity to explore local connectivity and proximity in amorphous solids. Here, we report the 2D through-bond (J-coupling) and through-space (dipolar coupling) correlation NMR spectra for oxide glasses where previously unknown structural details about the connectivity and proximity among quadrupolar nuclides ((27)Al, (17)O) are determined. Nonbridging oxygen peaks in Ca-aluminosilicate glasses with distinct connectivity, such as Ca-O-Al and Al-O-(Al, Si) are well distinguished in {(17)O}(27)Al solid HMQC NMR spectra. Both peaks shift to a lower frequency in direct and indirect dimensions upon the addition of Si to the Ca-aluminate glasses. The 2D (27)Al double quantum magic angle spinning NMR spectra for Mg-aluminoborate glasses indicate the preferential proximity between ([4])Al and ([5])Al leading to the formation of correlations peaks such as ([4])Al-([4])Al, ([4])Al-([5])Al, and ([5])Al-O-([5])Al. A fraction of the ([6])Al-([6])Al correlation peak is also noticeable while that of ([4,5])Al-([6])Al is missing. These results suggest that ([6])Al is likely to be isolated from the ([4])Al and ([5])Al species, forming ([6])Al clusters. The experimental realization of through-bond and through-space correlations among quadrupolar nuclides in amorphous materials suggests a significant deviation from the random distribution among framework cations and a spatial heterogeneity due to possible clustering of framework cations in the model oxide glasses.

Electron Paramagnetic Resonance, Scanning Electron Microscopy with Energy Dispersion X-ray Spectrometry, X-ray Powder Diffraction, and NMR Characterization of Iron-Rich Fired Clays

The aim of this study is to clarify the structure of an iron-rich clay and the structural changes involved in the firing process as a preliminary step to get information on ancient ceramic technology. To this purpose, illite-rich clay samples fired at different temperatures were characterized using a multitechnique approach, i.e., by electron paramagnetic resonance, scanning electron microscopy with electron dispersion X-ray spectrometry, X-ray powder diffraction, magic angle spinning and multiple quantum magic angle spinning NMR. During firing, four main reaction processes occur: dehydration, dehydroxylation, structural breakdown, and recrystallization. When the results are combined from all characterization methods, the following conclusions could be obtained. Interlayer H2O is located close to aluminum in octahedral sites and is driven off at temperatures lower than 600 degrees C. Between 600 and 700 degrees C dehydroxylation occurs whereas, between 800 and 900 degrees C, the aluminum in octahedral sites disappears, due to the breakdown of the illite structure, and all iron present is oxidized to Fe3+. In samples fired at 1000 and 1100 degrees C iron clustering was observed as well as large single crystals of iron with the occurrence of ferro- or ferrimagnetic effects. Below 900 degrees C the aluminum in octahedral sites presents a continuous distribution of chemical shift, suggesting the presence of slightly distorted sites. Finally, over the whole temperature range, the presence of at least two tetrahedral aluminum sites was revealed, characterized by different values of the quadrupolar coupling constant.

Site Populations and Short Range Order in Aluminosilicates Investigated by27Al Solid-State NMR

Site populations in Al2Si2O5 kyanite and short range order parameters in CaAl2Si2O8 anorthite have been determined by using 27Al multiple quantum magic angle spinning NMR with quadrupolar Carr−Purcell−Meiboom−Gill echo train and FAM-II triple quantum to single quantum conversion pulses (MQ-QCPMG-MAS NMR). Significant increases in resolution in the isotropic dimension were obtained by using the increased signal intensities to extend t1 acquisition times. Detailed calculations were used to optimize pulse sequences and then evaluate the accuracy of signal intensity to site abundances based on CQ and resonance offset variation in the mineral kyanite. The same approach was used to investigate a well-ordered anorthite and a range of disordered anorthite samples annealed at 1400 °C between 1 min and 179 h. Because of intrinsic disorder accurate site populations were impossible to obtain for these samples but instead upper bounds for the order parameter were determined. Furthermore, the spectra of the annealed sa...

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.

Structural Analysis of ZON-Type Aluminophosphates by Solid State NMR

Solid-state NMR spectroscopy has been used to study the new aluminophosphate material AlPO4-ZON. Highly resolved multiple quantum magic-angle spinning NMR spectra showed all four Al sites in the framework of AlPO4-ZON. Additional 27Al resonances were detected in some samples. They were assigned to an impurity phase of AlPO4-AFR and to six-coordinated aluminum in the extra-lattice positions. High-speed MAS NMR of 1H yielded the OH concentration in the samples synthesized from the fluoride-modified gels of different pH. The location of fluorine in the AlPO4-ZON framework has been verified by measuring the internuclear distances between 19F and all 27Al sites using the MQ-REDOR technique. MQ-REDOR has been also used to measure the distances between 1H and 27Al nuclei in this material.

Structural Analysis of Boron Carbide Using 2D 11B-MQMAS NMR

Abstract 2D 11B-multiple quantum magic angle spinning NMR spectra of boron carbide were measured in order to eliminate the second-order quadrupole interaction of 11B in B4C. Judging from the line width of MQMAS spectrum, not only the second-order quadrupole interaction but also the B-B dipole-dipole interaction was found to cause the line broadening of B4C in 11B-MAS NMR. They affect the line width in the ratio about 1.3 : 1 at spinning rate of 17 kHz.

Resolution enhancement in solid-state MQ-MAS experiments achieved by composite decoupling

It is shown that, in the presence of strong heteronuclear dipolar couplings, resolution can be significantly improved in solid-state multiple quantum magic angle spinning NMR experiments on half integer quadrupolar nuclei by applying composite decoupling schemes during triple quantum evolution and acquisition. Reduction of the effects of heteronuclear dipolar coupling during the multiple quantum evolution period is shown to lead to improved resolution of 27Al sites in crystalline chiolite (Na5Al3F14) and 11B sites in a polyborazilene sample. © John Wiley & Sons, Ltd.

Complete resolution of SiOSi and SiOAl fragments in an aluminosilicate glass by 17O multiple quantum magic angle spinning NMR spectroscopy

Complete NMR resolution of SiOSi and SiOAl species in an aluminosilicate glass (NaAlSi 3O 8) is shown using 17O multiple quantum magic angle spinning (MAS) NMR and indicates that no AlOAl is present at the detection limit of 0.5%.