Magic Angle Spinning Nuclear Magnetic Resonance Spectra Research Articles

Chemical exchange at the ferroelectric phase transition of lead germanate revealed by solid state 207Pb nuclear magnetic resonance.

The nature of the dynamics and structural changes that take place at the ferroelectric phase transition in lead oxides is a rich field of study. Solid-state nuclear magnetic resonance (NMR) of 207Pb is well suited to study the local structure and disorder in lead oxide ferroelectric transitions at the atomic level. However, very large 207Pb shielding anisotropy results in poor resolution in 1D static and magic angle spinning (MAS) NMR spectra. We address this problem by using short high-power adiabatic pulses (SHAPs) with magic-angle-turning sequences to correlate the isotropic and anisotropic parts of the 207Pb chemical shift tensor in a 2D NMR experiment, yielding resolved 207Pb NMR spectra of the nine distinct lead sites in uniaxial ferroelectric lead germanate (Pb5Ge3O11). Using this technique we detect the magnetic environments of displaced Pb2+ ions and unambiguously identify the nature of the phase transition as mixed displacive and order-disorder. We also observe that the atomic-level process responsible for the phase transition in ferroelectric lead germanate is chemical exchange on the kilohertz timescale. We derive an activation energy of 103.4 ± 1.7 kJ mol-1 and compare it to dielectric spectroscopy studies on similar materials. These results show that this method can be used to characterize ferroelectric phase transitions of complex materials with high resolution using nuclei that are typically inaccessible due to their large shielding anisotropy.

SiAlOC glasses derived from sol-gel synthesized ladder-like silsesquioxanes

The main aim of this study was to describe both the formation process of SiOC glasses doped with Al3+ cations and the resulting differences in their structures in comparison with undoped silicon oxycarbide glasses. A novel material was formed by means of the sol-gel method with the application of alkyl-substituted alkoxysilanes and a butoxy derivative of aluminum (Al(OC4H9)3) as a source of Al3+ ions. After the synthesis of a sol, the material was treated with a two-step thermal processing including aging (70 °C) and annealing (800 °C). As a result, yellowish xerogel (aging) and black glasses (annealing) were obtained and subjected to structural investigation, including X-ray diffraction (XRD), middle infrared spectroscopy (MIR), Raman, electron dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR) methods. The obtained results were compared to corresponding findings for SiOC glasses, obtained with the same experimental procedure. An additional thermogravimetric analysis (TGA) study was carried out for the xerogel in order to determine its thermal behavior in a protective atmosphere up to 1000 °C. The resultant mass loss curve was much smoother for the Al-doped material, indicating the beneficial impact of Al3+ addition on the formation process. XRD and spectroscopic (MIR, Raman, magic angle spinning (MAS) NMR) research revealed the formation of an amorphous silsesquioxane xerogel with a slightly modified ladder-like structure, and silicon oxycarbide glasses successfully doped with Al3+ ions, after aging and annealing, respectively. Moreover, NMR studies confirmed the introduction of aluminum into the glass network, based on the presence of Q4(1Al) and [AlO4] species in the 29Si and 27Al MAS NMR spectra, respectively. Additionally, the results from EDS and Raman spectroscopic studies suggested the partial release of free carbon phase from the glass network because of cationic substitution and ordering influence of Al3+ cations on the glass structure.

Probing the structure of Fe-free model basaltic glasses: A view from a solid-state 27Al and 17O NMR study of Na-Mg silicate glasses, Na2O-MgO-Al2O3-SiO2 glasses, and synthetic Fe-free KLB-1 basaltic glasses

The extent of structural disorder in multi-component basaltic (mafic) melts with varying compositions (particularly, Mg content) and the formation depth has not been well understood. Here, we explore the structure of Mg-rich silicate glasses including Na-Mg silicate glasses, NaO-MgO-Al2O3-SiO2 (NMAS) glasses in a nepheline (NaAlSiO4)-forsterite (Mg2SiO4)-quartz (SiO2) eutectic composition, and synthetic equivalents of basaltic glasses generated by partial melting of upper mantle peridotite (KLB-1) in a multi-component CaO-MgO-Na2O-K2O-Al2O3-TiO2-SiO2 system using high-resolution solid-state nuclear magnetic resonance (NMR). The 2-dimensional (2D) 17O 3Q (triple quantum) magic angle spinning (MAS) NMR spectra for those multi-component glasses revealed the nature of diverse aspects of structural disorder. Non-bridging oxygens (NBOs) apparently prefer proximity toward Ca2+ and Mg2+, whereas bridging oxygens (BOs) prefer to be near Na+, confirming a stronger interaction between NBOs and high field-strength cations in multi-component basaltic melts. The changes of oxygen configurations with increasing Mg contents provide implications for the structural evolution of basaltic melts with varying formation depth. Thus, while the pressure effect remains to be explored, considering the changes in melt composition alone, the NBO fraction of the KLB-1 basaltic melts increases with increasing melting depth of KLB-1 peridotite.The 27Al 3QMAS NMR spectra for multi-component KLB-1 basaltic glasses reveal non-negligible fraction of [5]Al that increases with increasing Mg content. The fraction of [5]Al in natural basaltic magma is expected to increase with melting depth of KLB-1 peridotite. The 27Al NMR quadrupolar coupling constant for [4]Al in multi-component silicate glasses systematically increases with Mg content, indicating an increase of the topological distortion (variation of bond angle and length distribution and network distortion) around [4]Al in the basaltic glasses. The observed increase in the extent of structural disorder in basaltic melts with Mg content may increase the overall configurational and topological entropy. While further confirmation is necessary, the changes in melt polymerization observed from 17O NMR spectra and network disorder around Al imply the decrease in melt viscosity in the complex silicate melts with varying formation depth of KLB-1 basaltic melts.

Ultrahigh-Resolution 7Li Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy by Isotopic Dilution

The functional importance of lithium in many types of materials requires reliable methods for studying its local structure and dynamics. Nuclear magnetic resonance (NMR) spectroscopy should be ideal for this purpose but suffers from poor resolution due to intrinsic dipolar broadening. We overcome this problem by diluting the 7Li spins with 6Li, producing ultrahigh-resolution 7Li magic-angle spinning (MAS) NMR spectra of lithium borates without appreciably sacrificing sensitivity. Resolution of up to six distinct lithium environments permits assignments, aided by density functional theory, which indicate separate chemical shift regions for different coordination numbers. With high sensitivity and site-specific Li resolution, two-dimensional NMR methods for probing Li hopping become more informative, revealing preferential site exchange in a complex lithium borate phase. The success of this approach will make 7Li MAS NMR an attractive method for probing lithium coordination environments and mobility in diamagnetic lithium-bearing materials.

Revealing long- and short-range structural modifications within phosphorus-treated HZSM-5 zeolites by atom probe tomography, nuclear magnetic resonance and powder X-ray diffraction.

The average and the local structure of phosphorus-treated HZSM-5 zeolites were investigated by means of atom probe tomography, powder X-ray diffraction (at ambient and cryogenic temperatures) and 1H, 29Si, 27Al, and 31P magic angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy. Phosphatation to yield a product with P/Al ≤ 1 followed by thermal treatment leads to breaking of the Si-OH-Al bridging groups, and subsequent partial dealumination of the zeolite framework, as shown by the contraction of the orthorhombic unit-cell volume and by the loss of tetrahedral framework Al, as observed in the 27Al Multiple Quantum (MQ) MAS NMR spectrum. Most of the framework Al is present in an electronic environment distorted by the presence of phosphorus and appears not to be involved in classic Si-OH-Al Brønsted acid sites. The 31P MAS NMR signals indicate that phosphorus interacts with the zeolitic framework to locally form silico-aluminophosphate (SAPO) domains and the presence of a new kind of acidic site was confirmed by the resonance at ∼8.6 ppm in the 1H MAS NMR spectra, attributed to P-OH groups. Increasing the phosphorus loading (P/Al ≫ 1) promotes further dealumination of the framework and cross-dehydroxylation between P-OH and Si-OH species, leading to the formation of a crystalline silicon orthophosphate phase. With decreasing Al content, the monoclinic HZSM-5 structure becomes preferred, especially at 85 K where the strain relaxation is higher. However, the presence of a higher amount of silicophosphate impurities hinders the low-temperature strain release of the framework, indicating that some of these species are localized in the zeolite pores and contribute to the strain build up.

Structural properties of two inequivalent Cs(1) and Cs(2) sites in perovskite tricaesium pentahalogencobaltate, Cs3CoX5 (X = Cl, Br)

The local structures of Cs3CoX5 (X = Cl, Br) were examined using nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR experiments. The two inequivalent Cs(1) and Cs(2) sites in two compounds were clearly distinguished based on static NMR and MAS NMR spectra. We found that the structural geometry for Cs(2) changed more abruptly than that for Cs(1) with increasing temperature. The Cs(1) ions surrounded by 10X− ions exhibited long relaxation times, whereas the Cs(2) ions surrounded by 8X− ions exhibited short relaxation times. This is consistent with the bond lengths of Cs(1)-X and Cs(2)-X. The halogen species in Cs3CoX5 enabled an examination of the effect of Cl and Br ions, which was found to influence the quadrupole parameter but not the relaxation time.

Understanding the Formation of CaAl2Si2O8 in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies.

An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO–9.7MgO–12.9Al2O3–38.7SiO2 mol %), used as a sealing material in solid oxide fuel cells, in view of the detrimental effect of anorthite on the sealing properties. Several advanced characterization techniques are employed to assess the material after prolonged heat treatment, including neutron powder diffraction (ND), X-ray powder diffraction (XRD), 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy. ND, 29Si MAS-NMR, and 27Al MAS-NMR results revealed that both Si and Al adopt tetrahedral coordination and participate in the formation of the network structure. In situ XRD measurements for the CMAS glass demonstrate the thermal stability of the glass structure up to 850 °C. Further heat treatment up to 900 °C initiates the precipitation of melilite, a solid solution of akermanite/gehlenite crystalline phase. Qualitative XRD data for glass-ceramics (GCs) produced after heat treatment at 850 °C for 500 h revealed the presence of anorthite along with the melilite crystalline phase. Rietveld refinement of XRD data indicated a high fraction of glassy phase (∼67%) after the formation of crystalline phases. The 29Si MAS-NMR spectra for the CMAS-GC suggest the presence of structural units in the remaining glassy phase with a polymerization degree higher than dimer units, whereas the 27Al MAS-NMR spectra revealed that most Al3+ cations exhibit a 4-fold coordination. In situ Raman spectroscopy data indicate that the formation of anorthite crystalline phase initiated after 240 h of heat treatment at 850 °C owing to the interaction between the gehlenite crystals and the remaining glassy phase.

The investigation of the crystalline phases development in Macor® glass-ceramic

The commercially available, machinable, fluormica glass-ceramic, Macor®, was remelted to produce a glass. This glass was then characterized by differential scanning calorimetry (DSC), combined differential thermal analysis/thermal gravimetric analysis (DTA/TGA), X-ray powder diffraction (XRD) and 19F magic angle spinning −nuclear magnetic resonance (MAS-NMR). The Macor® glass was shown to crystallize to chondrodite (Mg5F2(SiO4)2), followed by the conversion of chondrodite to norbergite (Mg3SiO4F2) and the conversion of norbegite to a potassium fluorphlogopite phase (KMg3AlSi3O10F2). The glass exhibited an optimum nucleation temperature just above its glass transition temperature, which is indicative of a nucleation route involving amorphous phase separation. The 19F MAS-NMR spectra showed the fluorine environments being present as F-Mg(3) in the original glass, the chondrodite, norbergite and fluorphlogopite phases, indicating that the fluorine structure is conserved throughout the crystallization process. The activation energies for crystallization were found to be 215, 431 and 251kJmol−1 for chondrodite, norbergite and fluorphlogopite, respectively.

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.

Solid-state NMR studies for the development of non-woven biomaterials based on silk fibroin and polyurethane

Silk fibroin (SF) possesses several characteristics that are favorable for tissue engineering. However, the mechanical properties must be modified in some cases. For instance, soft tissues such as those of the circulatory system are more elastic than tissue-engineered materials. The development of polymer blends is a simple method with the potential to provide materials with extended useful properties. In this study, we developed a non-woven SF and thermoplastic polyurethane-blend sheet from a polymer solution. The structure and miscibility of the blend sheet was evaluated using solid-state nuclear magnetic resonance (NMR) methods. In the 13C cross-polarization and magic angle spinning NMR spectra, no peak shift was observed between the pure and blended samples. The miscibility of the sample was investigated using proton spin-lattice relaxation times in the laboratory frame (T1H). The T1H measurement clearly revealed that the molecular chains of SF and Pellethane exist in close proximity of several tens of nanometers. The development of polymer blends is a simple method with the potential to provide materials with extended useful properties. Here, the SF and Pellethane were observed to be highly miscible with each other even though SF maintained the strong β-sheet crystal structure. This result suggests that SF/Pellethane-blended sheets possess strength that is derived from SF and elasticity that is derived from Pellethane. Therefore, this blended sheet should be a good medical material for soft tissue engineering.

Solid-state NMR and computational insights into the crystal structure of silicocarnotite-based bioceramic materials synthesized mechanochemically

In this work, we report the results of a detailed structural study of a promising bioceramic material silicocarnotite Ca5(PO4)2SiO4 (SC) synthesized from mechanochemically treated nanosized silicon-substituted hydroxyapatite by annealing at 1000°C. This novel synthetic approach represents an attractive and efficient route towards large-scale manufacturing of the silicocarnotite-based bioceramics. A combination of solid-state nuclear magnetic resonance (NMR), powder X-ray crystallography and density function theory (DFT) calculations has been implemented to characterize the phase composition of the prepared composite materials and to gain insight into the crystal structure of silicocarnotite. The phase composition analysis based on the multinuclear solid-state NMR has been found in agreement with X-ray powder diffraction indicating the minority phases of CaO (5–6wt%) and residual silicon-apatite (7–8wt%), while the rest of the material being a fairly crystalline silicocarnotite phase (86–88wt%). A combination of computational (CASTEP) and experimental methods was used to address the anionic site disorder in the silicocarnotite crystal structure. Distorted [OPO3] pyramids have appeared as an important structural motif in the SC crystal structure. The ratio between regular [PO4] and distorted [OPO3] tetrahedra is found between 2:1 and 3:1 based on XRD experiments and CASTEP calculations. The natural abundance 43Ca magic angle spinning NMR spectra of silicocarnotite are reported for the first time.

Combining 27Al Solid-State NMR and First-Principles Simulations To Explore Crystal Structure in Disordered Aluminum Oxynitride.

The nuclear magnetic resonance (NMR) technique gives insight into the local information in a crystal structure, while Rietveld refinement of powder X-ray diffraction (PXRD) sketches out the framework of a crystal lattice. In this work, first-principles calculations were combined with the solid-state NMR technique and Rietveld refinement to explore the crystal structure of a disordered aluminum oxynitride (γ-alon). The theoretical NMR parameters (chemical shift, δiso, quadrupolar coupling constants, CQ, and asymmetry parameter, η) of Al22.5O28.5N3.5, predicted by the gauge-including projector augmented wave (GIPAW) algorithm, were used to facilitate the analytical investigation of the 27Al magic-angle spinning (MAS) NMR spectra of the as-prepared sample, whose formula was confirmed to be Al2.811O3.565N0.435 by quantitative analysis. The experimental δiso, CQ, and η of 27Al showed a small discrepancy compared with theoretical models. The ratio of aluminum located at the 8a to 16d sites was calculated to be 0.531 from the relative integration of peaks in the 27Al NMR spectra. The occupancies of aluminum at the 8a and 16d positions were determined through NMR investigations to be 0.9755 and 0.9178, respectively, and were used in the Rietveld refinement to obtain the lattice parameter and anion parameter of Al2.811O3.565N0.435. The results from 27Al NMR investigations and PXRD structural refinement complemented each other. This work provides a powerful and accessible strategy to precisely understand the crystal structure of novel oxynitride materials with multiple disorder.

Nuclear magnetic resonance study of the ferroelastic phase transition of order-disorder type in [N(C2H5)4]2CdCl4

This study uses nuclear magnetic resonance (NMR) techniques to examine the detailed changes in [N(C2H5)4]2CdCl4 around its phase transition at the temperature TC = 284 K. The chemical shifts and spin-lattice relaxation times in the rotating frame (T1ρ) were determined from 1H magic angle spinning (MAS) NMR and 13C cross-polarization (CP)/MAS NMR spectra. The two sets of inequivalent 1H and 13C nuclei in CH3 and CH2 were distinguished. A ferroelastic phase transition was observed at TC, without structural symmetry change. The phase transition is mainly attributed to the orientational ordering of the [N(C2H5)4]+ cations, and the spectral splitting at low temperature is associated with different ferroelastic domains.

Understanding the symmetric line shape in the 17O MAS spectra for hexagonal ice

Solid-state 17O Magic-Angle Spinning (MAS) nuclear magnetic resonance (NMR) spectra of 17O-enriched hexagonal ice, [17O]-Ih, between 173 and 253 K were presented. Marked changes in the line shape were clearly observed, indicating water molecular reorientation in the crystal structure. At 173 K, molecular motions were considered to be frozen and analysis of the 1D MAS spectrum yielded the following parameters: quadrupole coupling constant (CQ) = 6.6 ± 0.2 MHz and asymmetry parameter (ηQ) = 0.95 ± 0.05. At 232 K and above, contrary to the conventional explanation, pseudo-symmetric line shapes appeared in the 17O MAS NMR spectra arising from the contribution of second-order quadrupole interactions. As a chemical exchange model to describe these isotropic 17O MAS NMR spectra, a modified Ratcliffe model, which consider the effects of proton disorder, was proposed, and the resulting theoretical spectra could well reproduce the experimental spectra.

Relationship between structure and mobility of proton carriers injected by electrochemical substitution of sodium ions with protons in 35NaO1/2-1 WO3-8NbO5/2-5LaO3/2-51PO5/2-based glasses

Using Raman and 31P magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies, we studied the structural changes in 35NaO1/2-1WO3-8NbO5/2-5LaO3/2-51PO5/2 glass upon introducing Al2O3 and/or Y2O3 in order to understand the reduced mobility of proton carriers in glasses with Al2O3 and/or Y2O3 in which proton carriers were injected by alkali-proton substitution (APS). The Raman and 31P MAS-NMR spectra showed that phosphate chains were shortened by the Al2O3 and/or Y2O3 introduced into 1W glass. This structural change increased the fraction of protons bound to oxygen atoms in the terminal PO4 (i.e., the Q1 unit) of the phosphate chains. Because the protons bound to terminal Q1 units in phosphate chains tightly bind to oxygen atoms, as opposed to the protons bound to inner Q2 units in phosphate chains, we attributed the reduced mobility of proton carriers upon introducing AlO3/2 and/or YO3/2 into 1W glass to the shortening of phosphate chains. From these results we propose that to obtain a highly proton-conducting glass after APS, the glass must have a composition of O/P <3.5 and thus a sufficiently high fraction of Q2 units.

Study of chemically inequivalent N(CH3)4 ions in [N(CH3)4]2ZnBr4 near the phase transition temperature using 1H MAS NMR, 13C CP/MAS NMR, and 14N NMR

The temperature dependences of the chemical shifts and intensities of 1H, 13C, and 14N nuclei in tetramethylammonium tetrabromozincate, [N(CH3)4]2ZnBr4, were investigated using single-crystal nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR spectroscopy to elucidate the structural geometry near the phase transition temperature. Based on the analysis of the 13C cross-polarization (CP)/MAS NMR and 14N NMR spectra, the two chemically inequivalent N(1) (CH3)4 and N(2) (CH3)4 ions were distinguished. Furthermore, the 14N NMR spectrum at the phase transition temperature indicated the existence of the ferroelastic characteristics of the N(CH3)4 ions.

From crystalline to amorphous calcium pyrophosphates: A solid state Nuclear Magnetic Resonance perspective

Hydrated calcium pyrophosphates (CPP, Ca2P2O7·nH2O) are a fundamental family of materials among osteoarticular pathologic calcifications. In this contribution, a comprehensive multinuclear NMR (Nuclear Magnetic Resonance) study of four crystalline and two amorphous phases of this family is presented. 1H, 31P and 43Ca MAS (Magic Angle Spinning) NMR spectra were recorded, leading to informative fingerprints characterizing each compound. In particular, different 1H and 43Ca solid state NMR signatures were observed for the amorphous phases, depending on the synthetic procedure used. The NMR parameters of the crystalline phases were determined using the GIPAW (Gauge Including Projected Augmented Wave) DFT approach, based on first-principles calculations. In some cases, relaxed structures were found to improve the agreement between experimental and calculated values, demonstrating the importance of proton positions and pyrophosphate local geometry in this particular NMR crystallography approach. Such calculations serve as a basis for the future ab initio modeling of the amorphous CPP phases. Statement of significanceThe general concept of NMR crystallography is applied to the detailed study of calcium pyrophosphates (CPP), whether hydrated or not, and whether crystalline or amorphous. CPP are a fundamental family of materials among osteoarticular pathologic calcifications. Their prevalence increases with age, impacting on 17.5% of the population after the age of 80. They are frequently involved or associated with acute articular arthritis such as pseudogout. Current treatments are mainly directed at relieving the symptoms of joint inflammation but not at inhibiting CPP formation nor at dissolving these crystals. The combination of advanced NMR techniques, modeling and DFT based calculation of NMR parameters allows new original insights in the detailed structural description of this important class of biomaterials.

Investigating the effect of TiO2 on the structure and biocompatibility of bioactive glass.

Titanium (Ti4+ ) containing materials have been widely used in medical applications due to its associated bioactivity in vivo. This study investigates the replacement of Si4+ with Ti4+ within the system SiO2 -Na2 O-CaO-P2 O5 to determine its influence on glass structure. This strategy was conducted in order to control the glass solubility to further improve the cellular response. Ti4+ incorporation was found to have little influence on the glass transition temperature (Tg = 520 ± 8°C) and magic angle spinning-nuclear magnetic resonance (MAS-NMR) shifts (-80 ppm) up to additions of 18 wt %. However, at 30 wt % the Tg increased to 600°C and MAS-NMR spectra shifted to -88 ppm. There was also an associated reduction in glass solubility as a function of Ti4+ incorporation as determined by inductively coupled plasma optical emission spectroscopy where Si4+ (1649-44 mg/L) and Na+ (892-36 mg/L) levels greatly reduced while Ca2+ (3-5 mg/L) and PO43- (2-7 mg/L) levels remained relatively unchanged. MC3T3 osteoblasts were used for cell culture testing and it was determined that the Ti4+ glasses increased cell viability and also facilitated greater osteoblast adhesion and proliferation to the glass surface compared to the control glass. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1703-1712, 2016.

23Na Nuclear Magnetic Resonance Study of the Structure and Dynamic of Natrolite

Abstract The temperature dependences of nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR spectra of 23Na nuclei in natrolite (Na2Al2Si3O10·2H2O) have been studied. The temperature dependences of the spin-lattice relaxation times T1 in natrolite have also been studied. It has been shown that the spin-lattice relaxation of the 23Na is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H2O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the 23Na nuclei only at low temperature (&lt;270 K).

Structural Study of Mg-Based Metal–Organic Frameworks by X-ray Diffraction, 1H, 13C, and 25Mg Solid-State NMR Spectroscopy, and First-Principles Calculations

Understanding the formation principles, structure, and stability of magnesium-based metal–organic frameworks is a difficult task that requires application of a set of experimental and theoretical methods. In this work, three polycrystalline Mg–benzene–1,3,5-tricarboxylates, differing in the dimensionality of their frameworks, were studied by X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles calculations based on the density functional theory (DFT). XRD provided information on the average periodic structures of the three materials. Natural abundance 25Mg, 13C, and 1H magic-angle spinning NMR spectra confirmed the proposed structural models, elucidated hydrogen bonds within two of the materials, and detected the strong effect of the anisotropy of the bulk magnetic susceptibility in one. The calculations of 13C and 25Mg chemical shifts and 25Mg quadrupolar coupling parameters with the gauge-including projector-augmented wave approach enabled assignment of carbon and magnesium NMR signals to carbon and magnesium sites within the three metal–organic framework structures. The detected 13C and 25Mg NMR parameters could not be simply related to the geometry of the environments of these nuclei. DFT-based calculation of formation energies of the materials enabled the prediction of the thermodynamical stability of their structures.