Double-quantum Magic-angle Spinning Research Articles

Assessing the Detection Limit of a Minority Solid-State Form of a Pharmaceutical by 1H Double-Quantum Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy

The lower detection limit for 2 distinct crystalline phases by 1H magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) is investigated for a minority amount of cimetidine (anhydrous polymorph A) in a physical mixture with the anhydrous HCl salt of cimetidine. Specifically, 2-dimensional 1H double-quantum (DQ) MAS NMR spectra of polymorph A and the anhydrous HCl salt constitute fingerprints for the presence of each of these solid forms. For solid-state NMR data recorded at a 1H Larmor frequency of 850 MHz and a MAS frequency of 30 kHz on ∼10 mg of sample, it is shown that, by following the pair of cross-peaks at a 1H DQ frequency of 7.4 + 11.6 = 19.0 ppm that are unique to polymorph A, the level of detection for polymorph A in a physical mixture with the anhydrous HCl salt is a concentration of 1% w/w.

Resolution enhancement in proton double quantum magic-angle spinning spectra by constant-time acquisition

Although very fast MAS rate (>60 kHz) paves a way to obtain a sufficient resolution in the 1H double-quantum magic-angle spinning (DQMAS) experiments to probe 1H proximities, the 1H resolution still limits wider applications below its potential use. Here, the combination of the DQMAS experiment with the constant-time (CT) acquisition approach is demonstrated, giving an increased peak-separation power in the DQ dimension. The advantages and disadvantages in terms of sensitivity and resolution of the conventional and CT approaches are discussed.

Glycerin-Induced Conformational Changes in Bombyx mori Silk Fibroin Film Monitored by (13)C CP/MAS NMR and ¹H DQMAS NMR.

In order to improve the stiff and brittle characteristics of pure Bombyx mori (B. mori) silk fibroin (SF) film in the dry state, glycerin (Glyc) has been used as a plasticizer. However, there have been very limited studies on the structural characterization of the Glyc-blended SF film. In this study, 13C Cross Polarization/Magic Angle Spinning nuclear magnetic resonance (CP/MAS NMR) was used to monitor the conformational changes in the films by changing the Glyc concentration. The presence of only 5 wt % Glyc in the film induced a significant conformational change in SF where Silk I* (repeated type II β-turn and no α-helix) newly appeared. Upon further increase in Glyc concentration, the percentage of Silk I* increased linearly up to 9 wt % Glyc and then tended to be almost constant (30%). This value (30%) was the same as the fraction of Ala residue within the Silk I* form out of all Ala residues of SF present in B. mori mature silkworm. The 1H DQMAS NMR spectra of Glyc-blended SF films confirmed the appearance of Silk I* in the Glyc-blended SF film. A structural model of Glyc-SF complex including the Silk I* form was proposed with the guidance of the Molecular Dynamics (MD) simulation using 1H–1H distance constraints obtained from the 1H Double-Quantum Magic Angle Spinning (DQMAS) NMR spectra.

Parallel β-Sheet Structure of Alanine Tetrapeptide in the Solid State As Studied by Solid-State NMR Spectroscopy.

The structural analysis of alanine oligopeptides is important for understanding the crystalline region in silks from spiders and wild silkworms and also the mechanism of cellular toxicity of human diseases arising from expansion in polyalanine sequences. The atomic-level structures of alanine tripeptide and tetrapeptide with antiparallel β-sheet structures (AP-Ala3 and AP-Ala4, respectively) together with alanine tripeptide with parallel β-sheet structures (P-Ala3) have been determined, but alanine tetrapeptide with a parallel β-sheet structure (P-Ala4) has not been reported yet. In this article, first, we established the preparation protocol of P-Ala4 from more stable AP-Ala4. Second, complete assignments of the (13)C, (15)N, and (1)H solid-state NMR spectra were performed with (13)C- and (15)N-labeled Ala4 samples using several solid-state NMR techniques. Then, the structural constraints were obtained, for example, the amide proton peaks of P-Ala4 in the (1)H double-quantum magic-angle spinning NMR spectrum were heavily overlapped and observed at about 7.4 ppm, which was a much higher field than that of 8.7-9.1 ppm observed for AP-Ala4, indicating that the intermolecular hydrogen-bond lengths across strands (N-H···O═C) were considerably longer for P-Ala4, that is, 2.21-2.34 Å, than those reported for AP-Ala4, that is, 1.8-1.9 Å. The structural model was proposed for P-Ala4 by NMR results and MD calculations.

Acidic Properties and Structure-Activity Correlations of Solid Acid Catalysts Revealed by Solid-State NMR Spectroscopy.

Solid acid materials with tunable structural and acidic properties are promising heterogeneous catalysts for manipulating and/or emulating the activity and selectivity of industrially important catalytic reactions. On the other hand, the performances of acid-catalyzed reactions are mostly dictated by the acidic features, namely, type (Brønsted vs Lewis acidity), amount, strength, and local environment of acid sites. The latter is relevant to their location (intra- vs extracrystalline), and possible confinement and Brønsted-Lewis acid synergy effects that may strongly affect the host-guest interactions, reaction mechanism, and shape selectivity of the catalytic system. This account aims to highlight some important applications of state-of-the-art solid-state NMR (SSNMR) techniques for exploring the structural and acidic properties of solid acid catalysts as well as their catalytic performances and relevant reaction pathway invoked. In addition, density functional theory (DFT) calculations may be exploited in conjunction with experimental SSNMR studies to verify the structure-activity correlations of the catalytic system at a microscopic scale. We describe in this Account the developments and applications of advanced ex situ and/or in situ SSNMR techniques, such as two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) homonuclear correlation spectroscopy for structural investigation of solid acids as well as study of their acidic properties. Moreover, the energies and electronic structures of the catalysts and detailed catalytic reaction processes, including the identification of reaction species, elucidation of reaction mechanism, and verification of structure-activity correlations, made available by DFT theoretical calculations were also discussed. Relevant discussions will focus primarily on results obtained from our laboratories in the past decade, including (i) quantitative and qualitative acidity characterization utilizing assorted probe molecules, (ii) probing the spatial proximity and synergy effect of acid sites, and (iii) influence of acid features and pore confinement effect on catalytic activity, transition-state stability, reaction pathway, and product selectivity of solid acid catalysts such as zeolites, metal oxides, and heteropolyacids. It is conclusive that a synergy of acidity (local effect) and pore confinement (environmental effect) tend to strongly dictate the formations of intermediates and transition states, hence, the reaction pathways and catalytic performance of solid acid catalysts. We hope that these information can provide additional insights toward our understanding in heterogeneous catalysis, especially the roles of structural and acidic properties on catalytic performances and reaction mechanism of acid-catalyzed systems, which should be beneficial for rational design of solid acid catalysts.

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.

Understanding the High Photocatalytic Activity of (B, Ag)-Codoped TiO2 under Solar-Light Irradiation with XPS, Solid-State NMR, and DFT Calculations

The origin of the exceptionally high activity of (B, Ag)-codoped TiO(2) catalysts under solar-light irradiation has been investigated by XPS and (11)B solid-state NMR spectroscopy in conjunction with density functional theory (DFT) calculations. XPS experimental results demonstrated that a portion of the dopant Ag (Ag(3+)) ions were implanted into the crystalline lattice of (B, Ag)-codoped TiO(2) and were in close proximity to the interstitial B (B(int.)) sites, forming [B(int.)-O-Ag] structural units. In situ XPS experiments were employed to follow the evolution of the chemical states of the B and Ag dopants during UV-vis irradiation. It was found that the [B(int.)-O-Ag] units could trap the photoinduced electron to form a unique intermediate structure in the (B, Ag)-codoped TiO(2) during the irradiation, which is responsible for the photoinduced shifts of the B 1s and Ag 3d peaks observed in the in situ XPS spectra. Solid-state NMR experiments including (11)B triple-quantum and double-quantum magic angle spinning (MAS) NMR revealed that up to six different boron species were present in the catalysts and only the tricoordinated interstitial boron (T*) species was in close proximity to the substitutional Ag species, leading to formation of [T*-O-Ag] structural units. Furthermore, as demonstrated by DFT calculations, the [T*-O-Ag] structural units were responsible for trapping the photoinduced electrons, which prolongs the life of the photoinduced charge carriers and eventually leads to a remarkable enhancement in the photocatalytic activity. All these unprecedented findings are expected to be crucial for understanding the roles of B and Ag dopants and their synergistic effect in numerous titania-mediated photocatalytic reactions.

Brønsted/Lewis Acid Sites Synergy in H-MCM-22 Zeolite Studied by 1H and 27Al DQ-MAS NMR Spectroscopy

Brønsted/Lewis acid sites synergy in H-MCM-22 zeolite was studied by solid-state nuclear magnetic resonance (NMR). Two-dimensional 1H double quantum-magic angle spinning (DQ-MAS) NMR revealed the details of the spatial relationship between the Lewis and the Brønsted acid sites in a dealuminated H-MCM-22 zeolite, which implied the existence of a Brønsted/Lewis acid sites synergy. Two-dimensional 27Al DQ-MAS NMR was used to give the details of the spatial proximities of various aluminum species. The Brønsted/Lewis acid sites synergy occurred in the supercage of the H-MCM-22 zeolite between a T6 site Al and extra-framework Al species. 13C CP/MAS NMR of adsorbed acetone demonstrated that the spatial proximities of the Brønsted and Lewis acid sites led to a synergy that enhanced the Brønsted acid strength of the dealuminated zeolites. 1H MAS NMR of adsorbed deuterated pyridine confirmed that the Brønsted/Lewis acid sites synergy occurred in the supercage of H-MCM-22. This finding is important for understanding the mechanism of acid-catalyzed reactions in H-MCM-22 zeolites. 采用各种固体核磁共振 (NMR) 技术详细研究了 H-MCM-22 分子筛中 Brønsted/Lewis 酸的协同效应. 二维 1H 双量子魔角旋转 (DQ-MAS) NMR 结果表明, 在脱铝 H-MCM-22 分子筛中 Brønsted 酸位 (骨架桥式羟基) 和 Lewis 酸位 (非骨架铝羟基) 之间是空间邻近的, 暗示着可能存在 B/L 酸协同效应. 二维 27Al DQ-MAS NMR 结果揭示了各种铝物种之间的空间邻近性, 表明 B/L 酸协同效应优先发生在 H-MCM-22 分子筛超笼中的骨架 T6 位铝和非骨架铝物种之间. 2-13C-丙酮探针分子实验发现, 因 B/L 酸协同效应而导致脱铝 H-MCM-22 分子筛酸性明显增强, 氘代吡啶探针分子实验也证实在 H-MCM-22 分子筛的超笼中发生了 B/L 酸协同效应. 上述结果将有助于我们理解在脱铝 H-MCM-22 分子筛上发生的多相催化机理.

Brønsted/Lewis Acid Synergy in H–ZSM-5 and H–MOR Zeolites Studied by 1H and 27Al DQ-MAS Solid-State NMR Spectroscopy

The spatial proximities of acid sites in dealuminated H-MOR and H-ZSM-5 zeolites have been comprehensively studied by two-dimensional H-1 and Al-27 double-quantum magic-angle spinning (DQ-MAS) NMR spectroscopy. H-1 DQ-MAS NMR revealed the detailed spatial proxirnities among Bronsted and Lewis acid sites, while Al-27 DQ-MAS NMR probed the detailed spatial proximities among various aluminum species in the dealuminated zeolites. It was found that all of the extra-framework aluminum species (EFAL, Lewis acid sites) are always in close proximity to the framework aluminum (Bronsted acid sites) in both H-MOR and H-ZSM-5 zeolites. The result from C-13 CP/MAS NMR of adsorbed acetone demonstrated that the spatial proximities between Bronsted and Lewis acid sites lead to a synergy effect that can remarkably enhance the Bronsted acid strength of the dealuminated zeolites. On the basis of these experimental results, we proposed a model to describe the Bronsted/Lewis acid synergy in the dealuminated zeolites.

Sensitivity enhancement of 29Si double-quantum dipolar recoupling spectroscopy by Carr–Purcell–Meiboom–Gill acquisition method

An enhancement in the detection sensitivity of dipolar recoupled 29Si double-quantum magic-angle spinning experiment is shown with a Carr–Purcell–Meiboom–Gill (CPMG) train of π pulses during the acquisition period. Symmetry-adapted pulse schemes, such as POST-C7 and SR26 4 11 , are used for the double-quantum excitation. Application of POST-C7-CPMG method for framework characterisation is demonstrated in the disordered and catalytically important ZSM-5 molecular sieve. Based on the observed double-quantum dipole–dipole correlations and the known T-site Si connectivities, the assignment of all the signals is made for the orthorhombic phase of the as-synthesised (CN form) material.

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.

Brønsted/Lewis Acid Synergy in Dealuminated HY Zeolite: A Combined Solid-State NMR and Theoretical Calculation Study

The Brønsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The 1H double quantum magic-angle spinning (DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis and Brønsted acid sites. The results from 13C NMR of adsorbed acetone as well as DFT calculation demonstrated that the Brønsted/Lewis acid synergy considerably enhanced the Brønsted acid strength of dealuminated HY zeolite. Two types of Brønsted acid sites (with enhanced acidity) in close proximity to extra-framework aluminum (EFAL) species were identified in the dealuminated HY zeolite. The NMR and DFT calculation results further revealed the detailed structures of EFAL species and the mechanism of Brønsted/Lewis acid synergy. Extra-framework Al(OH)3 and Al(OH)2+ species in the supercage cage and Al(OH)2+ species in the sodalite cage are the preferred Lewis acid sites. Moreover, it is the coordination of the EFAL species to the oxygen atom nearest the framework aluminum that leads to the enhanced acidity of dealuminated HY zeolite though there is no direct interaction (such as the hydrogen-bonding) between the EFAL species and the Brønsted acid sites. All these findings are expected to be important in understanding the roles of Lewis acid and its synergy with the Brønsted acid in numerous zeolite-mediated hydrocarbon reactions.

Description of depolarization effects in double-quantum solid state nuclear magnetic resonance experiments using multipole-multimode Floquet theory

Using an analytical model based on multipole-multimode Floquet theory (MMFT), we describe the polarization loss (or depolarization) observed in double-quantum (DQ) dipolar recoupling magic angle spinning (MAS) experiments. Specifically, the factors responsible for depolarization are analyzed in terms of higher order corrections to the spin Hamiltonian in addition to the usual phenomenological decay rate constant. From the MMFT model and the effective Hamiltonians, we elucidate the rationale behind the inclusion of a phenomenological damping term in DQ recoupling experiments. As a test of this theoretical approach, the recoupling efficiency of one class of (13)C-(13)C and (13)C-(15)N resonance width dipolar recoupling experiments are investigated at different magnetic field strengths and compared with the more exact numerical simulations. In contrast to existing analytical treatments, the role of higher order corrections is clearly explained in the context of the MMFT approach leading to a better understanding of the underlying spin physics. Furthermore, the analytical model presented herein provides a general framework for describing coherent and incoherent effects in homonuclear and heteronuclear DQ MAS recoupling experiments.

Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa2O·B2O3·KSiO2·xP2O5 (0.5<R<2; 0.86<K<3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). 31P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and 31P–11B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with 11B, 29Si and 23Na MAS NMR. Ab initio calculations of the 31P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na3PO4 (monophosphate), Na4P2O7 (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in Tg, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.

Study of Molecular Interactions and Dynamics in Thin Silica Surface Layers by Proton Solid-State NMR Spectroscopy

We present results from proton double-quantum magic-angle spinning NMR spectroscopy in support of tight molecular contacts of the modifier bis(triethoxysilylpropyl) tetrasulfane with the surface of precipitated, amorphous silica. While chemical bonding cannot be proven directly, its presence is indicated by through-space dipolar contacts between surface-OH groups and all aliphatic protons of the modifier, as well as strongly anisotropic motions of the surface-bound molecules. Silica−modifier contacts are further only found when the samples were heated during preparation. Only small amounts of ethanol were seen to leave the silica surface upon modification. A good part of the ethanol set free upon condensation becomes bound to the silica surface, which is supported by the presence of tight contacts in silica treated with pure ethanol. The anisotropy of molecular motions of the surface-bound molecules is characterized by the estimation of proton dipole−dipole coupling constants from double-quantum sideband patterns and build-up curves. Self-condensed samples of the modifier either exhibit very different dynamic properties or show no indication of contacts between hydroxyl and aliphatic protons.

Advanced 1 H Solid-State NMR Spectroscopy on Hydrogels, 1

Summary: The nature of the pH dependent collapse of poly(methacrylic acid) (PMAA) hydrogels is investigated using recent 1H solid-state NMR methods. In aqueous solution, PMAA changes from an expanded conformation at high pHs to a compact contracted form at low pHs, where hydrogen bonds play a central role. In solid-state 1H NMR spectra, recorded under fast magic angle spinning (MAS), dried PMAA samples previously collapsed at low pHs show characteristic signals in the spectral region of the carboxylic acid protons. With the aid of 2D 1H-1H double-quantum (DQ) MAS NMR spectra, three signals can be distinguished at 8, 10.5 and 12.5 ppm, which are attributed to free carboxylic groups and two different types of hydrogen bonded forms, respectively. The 12.5 ppm signal arises from the hydrogen bond with the shortest HH distance, corresponding to the form that is most stable with respect to increasing temperature and pH. The weaker hydrogen-bonded form (with a signal at 10.5 ppm) requires a slightly lower pH, while the free acid signal (at 8 ppm) emerges under the most acidic medium. Moreover, the stabilities of the hydrogen-bonded carboxylic acid dimers can be inferred from the proton-proton distances within the dimers, i.e. (275 ± 5) pm and (295 ± 15) pm for the protons at 12.5 and 10.5 ppm, respectively, which are determined by means of DQ MAS sideband patterns. Both the stability of the hydrogen bonds and the acidity of the protons may be related to the stereochemistry and the conformation of the PMAA chains.

Distribution effects on 1H double-quantum MAS NMR spectra

The effect of a distribution in the 1H– 1H dipolar coupling on 1H double-quantum (DQ) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spinning sideband patterns is considered. In disordered or amorphous materials a distribution in the magnitude of the 1H– 1H dipolar coupling is a realistic possibility. Simulations of the 1H DQ MAS NMR spinning sideband spectra were performed with the two-spin approximation. These simulations reveal that a dipolar coupling distribution can greatly affect the DQ spectral shape and behavior of the DQ build-up. The spectral line shapes are quantified by measurement of the relative intensities of the DQ sidebands. These variations in the 1H DQ NMR spectra are evaluated as a function of the width of the dipolar coupling distribution. As an example, the experimental DQ spinning sideband spectrum for a hydrated polyoxoniobate containing 15 H 2O molecules per hexaniobate cluster, are better simulated with a distribution of dipolar couplings opposed to a single coupling constant.

Double-quantum solid-state NMR of 13C spin pairs coupled to 14N

We examine the double-quantum magic angle spinning NMR spectra of pairs of 13C nuclei coupled to one or more 14N nuclei. The experimental spectra of 13 C 2 -glycine and glycyl- [ 13 C 2] -glycyl-glycine are used to demonstrate the sensitivity of the spectra to the orientation of 14N quadrupole interaction tensors and to the molecular torsional angles.

Methyl groups as local probes for polymer dynamics as investigated by [formula omitted] double-quantum magic-angle spinning NMR spectroscopy

We illustrate the application of high-resolution homonuclear 1 H double-quantum magic-angle spinning solid-state NMR recoupling experiments for an efficient measurement of segmental order parameters for polymers with methyl groups. A theoretical treatment of the spin dynamics of methyl groups during a corresponding pulse sequence including experimental imperfections is given. The applicability of the analytical result for the extraction of dipole–dipole coupling constants from experimental data is illustrated for a model substance. Experiments on entangled polymer melts indicate that double-quantum spectroscopy on methyl groups is well-suited for the extraction of very weak coupling constants, which permit the calculation of well-defined order parameters.