In Situ Single-crystal X-ray Diffraction Studies of Physisorption and Chemisorption of SO2 within a Metal-Organic Framework and Its Competitive Adsorption with Water.

Living on an increasingly polluted planet, the removal of toxic pollutants such as sulfur dioxide (SO2) from the troposphere and power station flue gas is becoming more and more important. The CPO-27/MOF-74 family of metal-organic frameworks (MOFs) with their high densities of open metal sites is well suited for the selective adsorption of gases that, like SO2, bind well to metals and have been extensively researched both practically and through computer simulations. However, until now, focus has centered upon the binding of SO2 to the open metal sites in this MOF (called chemisorption, where the adsorbent-adsorbate interaction is through a chemical bond). The possibility of physisorption (where the adsorbent-adsorbate interaction is only through weak intermolecular forces) has not been identified experimentally. This work presents an in situ single-crystal X-ray diffraction (scXRD) study that identifies discrete adsorption sites within Ni-MOF-74/Ni-CPO-27, where SO2 is both chemisorbed and physisorbed while also probing competitive adsorption of SO2 of these sites when water is present. Further features of this site have been confirmed by variable SO2 pressure scXRD studies, DFT calculations, and IR studies.

Concluding remarks: Faraday Discussion on NMR crystallography.

This Faraday Discussion explored the field of NMR crystallography, and considered recent developments in experimental and theoretical approaches, new advances in machine learning and in the generation and handling of large amounts of data. Applications to a wide range of disordered, amorphous and dynamic systems demonstrated the range and quality of information available from this approach and the challenges that are faced in exploiting automation and developing best practice. In these closing remarks I will reflect on the discussions on the current state of the art, questions about what we want from these studies, how accurate we need results to be, how we best generate models for complex materials and what machine learning approaches can offer. These remarks close with thoughts about the future direction of the field, who will be carrying out this type of research, how they might be doing it and what their focus will be, along with likely possible challenges and opportunities.

An NMR crystallographic characterisation of solid (+)-usnic acid.

We use a combination of one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations to obtain a full assignment of the 1H and 13C signals for solid (+)-usnic acid, which contains two molecules in the asymmetric unit. By combining through-space 1H-1H correlation data with computation it is possible to assign signals not just to the same molecules (relative assignment) but to assign the signals to specific crystallographic molecules (absolute assignment). Variable-temperature measurements reveal that there is some variation in many of the 13C chemical shifts with temperature, likely arising from varying populations of different tautomeric forms of the molecule. The NMR spectrum of crystalline (+)-usnic acid is then compared with that of ground Usnea dasopoga lichen (the source material of the usnic acid). The abundance of usnic acid is so great in the lichen that this natural product can be observed directly in the NMR spectrum without further purification. This natural sample of usnic acid appears to have the same crystalline form as that in the pure commercial sample.

13C pNMR shifts of MOFs based on Cu(II)-paddlewheel dimers - DFT predictions for spin-1/2 defects.

We present DFT predictions (CAM-B3LYP/II level) for the paramagnetic Nuclear Magnetic Resonance (pNMR) spectra of small molecular models based on the Cu(II)-paddlewheel dimer motif that is present in metal-organic frameworks (MOFs, notably the HKUST and STAM families). We explore potential point defects with spin-1/2 discovered through electron paramagnetic resonance (EPR) experiments. We consider defects through substitution of one Cu(II) centre in the dimer with protons, or through one-electron reduction, affording a mixed-valence dimer. While most of the defects have predicted pNMR shifts at room temperature in the range of those for the non-defective MOFs, their detection and assignment should be possible based on their distinct temperature dependence.

Exploring cation distribution in ion-exchanged Al,Ga-containing metal-organic frameworks using 17O NMR spectroscopy.

A mixed-metal metal-organic framework, (Al,Ga)-MIL-53, synthesised by post-synthetic ion exchange has been investigated using solid-state nuclear magnetic resonance (NMR) spectroscopy, microscopy and energy dispersive X-ray (EDX) spectroscopy. 17O enrichment during the ion-exchange process enables site specific information on the metal distribution to be obtained. Within this work two ion-exchange processes have been explored. In the first approach (exchange of metals in the framework with dissolved metal salts), 17O NMR spectroscopy reveals the formation of crystallites with a core-shell structure, where the cation exchange takes place on the surface of these materials forming a shell with a roughly equal ratio of Al3+ and Ga3+. For the second approach (exchange of metals between two frameworks), no core-shell structure is observed, and instead crystallites containing a majority of Al3+ are obtained with lower levels of Ga3+. Noticeably, these particles show little variation in the metal cation distribution between crystallites, a result not previously observed for bulk (Al,Ga)-MIL-53 materials. In all cases where ion exchange has taken place NMR spectroscopy reveals a slight preference for clustering of like cations.

Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions.

Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins. Given the complexity of advanced NMR experiments required, and the potential need for expensive and difficult isotopic enrichment, the computational work is invaluable in predicting which experiments and approaches are likely to give the most information on the disorder, local structure and pore forms of these mixed-metal MOFs.

Synthesis of FeAPO-34 Molecular Sieve under Ionothermal Condition.

FeAPO-34 with a chabazite (CHA) topology structure was successfully synthesized under ionothermal conditions using 1-ethyl-3-methylimidazoliumchloride (EMIMCl) ionic liquid in the presence of ethylenediamine (EDA). The material was characterized using powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), elemental analyses, and solid-state NMR spectroscopy. Incorporation of iron within the covalent framework of the material was confirmed by the presence of broad signals between 1000 and 14,000 ppm in the 31P NMR spectrum, corresponding to the P(OFe)x(OAl)4-x species.

Exploring cation disorder in mixed-metal pyrochlore ceramics using 17 O NMR spectroscopy and first-principles calculations.

Characterising the local structures (e.g., the cation distribution) of mixed-metal ceramics by NMR spectroscopy is often challenging owing to the unfavourable properties (low γ, large quadrupole moment and/or low abundance) of many metal nuclei. 17 O is an attractive option owing to the prevalence of oxygen within ceramics. The moderate γ and small quadrupole moment of 17 O mean that the greatest barrier to accessing the information available from this nucleus is isotopic enrichment. We explore the challenges of ensuring uniform isotopic enrichment with 17 O2 (g) for the pyrochlore solid solutions, Y2 Snx Ti2-x O7 , La2 Snx Zr2-x O7 and La2 Snx Hf2-x O7 , demonstrating that high enrichment temperatures (900 °C for 12 hr) are required. In addition, for sites with very high symmetry (such as the tetrahedral OY4 and OLa4 sites with CQ ≈ 0 present here), we demonstrate that quantitative 17 O NMR spectra require correction for the differing contributions from the centreband of the satellite transitions, which can be as high as a factor of ~3.89. It is common to use first-principles calculations to aid in interpreting NMR spectra of disordered solids. Here, we use an ensemble modelling approach to ensure that all possible cation arrangements are modelled in the minimum possible number of calculations. By combining uniform isotopic enrichment, quantitative NMR spectroscopy and a comprehensive computational approach, we are able to show that the cation distribution in Y2 Snx Ti2-x O7 is essentially random, whereas in La2 Snx Zr2-x O7 and La2 Snx Hf2-x O7 , OLa2SnZr and OLa2SnHf sites are slightly energetically disfavoured, leading to a weak preference for clustering of like cations.

Facile, Room-Temperature 17O Enrichment of Zeolite Frameworks Revealed by Solid-State NMR Spectroscopy.

A new approach for room-temperature 17O enrichment of zeolites reveals a surprisingly dynamic and labile framework, where rapid and reversible bond breaking takes place. 17O NMR spectroscopy shows that although O sites in both framework Si-O-Al and Si-O-Si linkages are enriched simply on exposure to H217O(l), the enrichment of Si-O-Al species is more rapid, with a more uniform framework enrichment observed at longer durations. We demonstrate that this unexpected enrichment can be observed for two different framework topologies and for Na-exchanged (i.e., nonacidic) zeolites, as well as their protonic forms, confirming that the Brønsted acid proton is not necessary for isotopic exchange into the framework. This work not only offers new opportunities for structural characterization of these chemically and industrially important materials using NMR spectroscopy but suggests that further investigation of the rate and position of enrichment in zeolite frameworks could provide new insight into their chemical reactivity and their stability in aqueous-based applications such as ion exchange and catalysis.

Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid.

The synthesis of chiral metal-organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N'-dimethylformamide, proline coordinates monodentate to the Zn2+ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.

Synthesis and Polymorphism of Mixed Aluminum-Gallium Oxides.

The synthesis of a new solid solution of the oxyhydroxide Ga5-xAlxO7(OH) is investigated via solvothermal reaction between gallium acetylacetonate and aluminum isopropoxide in 1,4-butanediol at 240 °C. A limited compositional range of 0 ≤ x ≤ 1.5 is produced, with the hexagonal unit cell parameters refined from powder X-ray diffraction (XRD) showing a linear contraction in unit cell volume with an increase in Al content. Solid-state 27Al and 71Ga nuclear magnetic resonance (NMR) spectroscopies show a strong preference for Ga to occupy the tetrahedral sites and Al to occupy the octahedral sites. Using isopropanol as the solvent, γ-Ga2-xAlxO3 defect spinel solid solutions with x ≤ 1.8 can be prepared at 240 °C in 24 h. These materials are nanocrystalline, as evidenced by their broad diffraction profiles; however, the refined cubic lattice parameter shows a linear relationship with the Ga:Al content, and solid-state NMR spectroscopy again shows a preference for Al to occupy the octahedral sites. Thermal decomposition of Ga5-xAlxO7(OH) occurs via poorly ordered materials that resemble ε-Ga2-xAlxO3 and κ-Ga2-xAlxO3, but γ-Ga2-xAlxO3 transforms above 750 °C to monoclinic ß-Ga2-xAlxO3 for 0 ≤ x ≤ 1.3 and to hexagonal α-Ga2-xAlxO3 for x = 1.8, with intermediate compositions of 1.3 < x < 1.8 giving mixtures of the α- and ß-polymorphs. Solid-state NMR spectroscopy shows only the expected octahedral Al for α-Ga2-xAlxO3, and for ß-Ga2-xAlxO3, the ∼1:2 tetrahedral:octahedral Al ratio is in good agreement with the results of Rietveld analysis of the average structures against powder XRD data. Relative energies calculated by periodic density functional theory confirm that there is an ∼5.2 kJ mol-1 penalty for tetrahedral rather than octahedral Al in Ga5-xAlxO7(OH), whereas this penalty is much smaller (∼2.0 kJ mol-1) for ß-Ga2-xAlxO3, in good qualitative agreement with the experimental NMR spectra.

Application of NMR Crystallography to Highly Disordered Templated Materials: Extensive Local Structural Disorder in the Gallophosphate GaPO-34A.

We present an NMR crystallographic investigation of two as-made forms of the recently characterized gallophosphate GaPO-34A, which has an unusual framework composition with a Ga:P ratio of 7:6 and contains both hydroxide and fluoride anions and either 1-methylimidazolium or pyridinium as the structure-directing agent. We combine previously reported X-ray crystallographic data with solid-state NMR spectroscopy and periodic density functional theory (DFT) calculations to show that the structure contains at least three distinct types of disorder (occupational, compositional, and dynamic). The occupational disorder arises from the presence of six anion sites per unit cell, but a total occupancy of five of these, leading to full occupancy of four sites and partial occupancy of the fifth and sixth (which are related by symmetry). The mixture of OH and F present leads to compositional disorder on the occupied anion sites, although the occupancy of some sites by F is calculated to be energetically unfavorable and signals relating to F on these sites are not observed by NMR spectroscopy, confirming that the compositional disorder is not random. Finally, a combination of high-field 71Ga NMR spectroscopy and variable-temperature 13C and 31P NMR experiments shows that the structure directing agents are dynamic on the microsecond time scale, which can be supported by averaging the 31P chemical shifts calculated with the SDA in different orientations. This demonstrates the value of an NMR crystallographic approach, particularly in the case of highly disordered crystalline materials, where the growth of large single crystals for conventional structure determination may not be possible owing to the extent of disorder present.

Phosphorus-Bismuth Peri-Substituted Acenaphthenes: A Synthetic, Structural, and Computational Study.

A series of acenaphthene species with a diisopropylphosphino group and a variety of bismuth functionalities in the peri positions were synthesized and fully characterized, including single-crystal X-ray diffraction. The majority of the reported species feature a relatively rare interpnictogen P-Bi bond. The series includes the phosphine-bismuthine Acenap(PiPr2)(BiPh2) (2; Acenap = acenaphthene-5,6-diyl), which was subjected to a fluorodearylation reaction to produce Acenap(PiPr2)(BiPhX) (5-8 and 10; X = BF4-, Cl, Br, I, SPh), displaying varying degrees of ionicity. The geminally bis(acenaphthyl)-substituted [Acenap(PiPr2)]2BiPh (3) shows a large through-space coupling of 17.8 Hz, formally 8TSJPP. Coupling deformation density calculations confirm the double through-space coupling pathway, in which the P and Bi lone pairs mediate communication between the two 31P nuclei. Several synthetic routes toward the phosphine-diiodobismuthine Acenap(PiPr2)(BiI2) (9) have been investigated; however, the purity of this, surprisingly thermally stable potential synthon, remains poor.

Following the unusual breathing behaviour of 17O-enriched mixed-metal (Al,Ga)-MIL-53 using NMR crystallography.

The breathing behaviour of 17O-enriched (Al,Ga)-MIL-53, a terephthalate-based metal-organic framework, has been investigated using a combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (PXRD) and first-principles calculations. These reveal that the behaviour observed for as-made, calcined, hydrated and subsequently dehydrated mixed-metal MIL-53 materials differs with composition, but cannot be described as the compositionally weighted average of the breathing behaviour seen for the two end members. Although the form of MIL-53 adopted by the as-made material is independent of metal composition, upon calcination, materials with higher levels of Al adopt an open pore (OP) form, as found for the Al end member, but substitution of Ga results in mixed pore materials, with OP and narrow pore (NP) forms co-existing. Although the Ga end member is prone to decomposition under the calcination conditions used, a low level of Al in the starting synthesis (5%) leads to an OP mixed-metal MOF that is stable to calcination. Upon hydration, all materials almost exclusively adopt a closed pore (CP) structure, with strong hydrogen bonding interactions with water leading to two distinct resonances from the carboxylate oxygens in 17O NMR spectra. When dehydrated, different framework structures are found for the two end members, OP for Al-MIL-53 and NP for Ga-MIL-53, with the proportion of NP MOF seen to increase systematically with the Ga content in mixed-metal materials, in contrast to the forms seen upon initial calcination. 17O NMR spectra of mixed-metal MIL-53 materials show an increased preference for clustering of like cations as the Ga content increases. This is not a result of the small-scale dry gel conversion reactions used for enrichment, as a similar cation distribution and clustering is also observed for (Al0.5,Ga0.5)-MIL-53 synthesised hydrothermally and enriched with 17O via post-synthetic steaming.

Site-Specific Iron Substitution in STA-28, a Large Pore Aluminophosphate Zeotype Prepared by Using 1,10-Phenanthrolines as Framework-Bound Templates.

An AlPO4 zeotype has been prepared using the aromatic diamine 1,10-phenanthroline and some of its methylated analogues as templates. In each case the two template N atoms bind to a specific framework Al site to expand its coordination to the unusual octahedral AlO4 N2 environment. Furthermore, using this framework-bound template, Fe atoms can be included selectively at this site in the framework by direct synthesis, as confirmed by annular dark field scanning transmission electron microscopy and Rietveld refinement. Calcination removes the organic molecules to give large pore framework solids, with BET surface areas up to 540 m2 g-1 and two perpendicular sets of channels that intersect to give pore space connected by 12-ring openings along all crystallographic directions.

Kinetics and Mechanism of the Hydrolysis and Rearrangement Processes within the Assembly-Disassembly-Organization-Reassembly Synthesis of Zeolites.

The hydrolysis (disassembly, D) and rearrangement (organization, O) steps of the assembly-disassembly-organization-reassembly (ADOR) process for the synthesis of zeolites have been studied. Germanium-rich UTL was subjected to hydrolysis conditions in water to understand the effects of temperature (100, 92, 85, 81, 77, and 70 °C). Samples were taken periodically over an 8-37 h period, and each sample was analyzed by powder X-ray diffraction. The results show that the hydrolysis step is solely dependent on the presence of liquid water, whereas the rearrangement is dependent on the temperature of the system. The kinetics have been investigated using the Avrami-Erofeev model. With increasing temperature, an increase in the rate of reaction for the rearrangement step was observed, and the Arrhenius equation was used to ascertain an apparent activation energy for the rearrangement from the kinetic product of the disassembly (IPC-1P) to the thermodynamic product of the rearrangement (IPC-2P). From this information, a mechanism for this transformation can be postulated.

Ensemble-Based Modeling of the NMR Spectra of Solid Solutions: Cation Disorder in Y2(Sn,Ti)2O7.

The sensitivity of NMR to the local environment, without the need for any long-range order, makes it an ideal tool for the characterization of disordered materials. Computational prediction of NMR parameters can be of considerable help in the interpretation and assignment of NMR spectra of solids, but the statistical representation of all possible chemical environments for a solid solution is challenging. Here, we illustrate the use of a symmetry-adapted configurational ensemble in the simulation of NMR spectra, in combination with solid-state NMR experiments. We show that for interpretation of the complex and overlapped lineshapes that are typically observed, it is important to go beyond a single-configuration representation or a simple enumeration of local environments. The ensemble method leads to excellent agreement between simulated and experimental spectra for Y2(Sn,Ti)2O7 pyrochlore ceramics, where the overlap of signals from different local environments prevents a simple decomposition of the experimental spectral lineshapes. The inclusion of a Boltzmann weighting confirms that the best agreement with experiment is obtained at higher temperatures, in the limit of full disorder. We also show that to improve agreement with experiment, in particular at low dopant concentrations, larger supercells are needed, which might require alternative simulation approaches as the complexity of the system increases. It is clear that ensemble-based modeling approaches in conjunction with NMR spectroscopy offer great potential for understanding configurational disorder, ultimately aiding the future design of functional materials.

A Picture of Disorder in Hydrous Wadsleyite-Under the Combined Microscope of Solid-State NMR Spectroscopy and Ab Initio Random Structure Searching.

The Earth's transition zone, at depths of 410-660 km, while being composed of nominally anhydrous magnesium silicate minerals, may be subject to significant hydration. Little is known about the mechanism of hydration, despite the vital role this plays in the physical and chemical properties of the mantle, leading to a need for improved structural characterization. Here we present an ab initio random structure searching (AIRSS) investigation of semihydrous (1.65 wt % H2O) and fully hydrous (3.3 wt % H2O) wadsleyite. Following the AIRSS process, k-means clustering was used to select sets of structures with duplicates removed, which were then subjected to further geometry optimization with tighter constraints prior to NMR calculations. Semihydrous models identify a ground-state structure (Mg3 vacancies, O1-H hydroxyls) that aligns with a number of previous experimental observations. However, predicted NMR parameters fail to reproduce low-intensity signals observed in solid-state NMR spectra. In contrast, the fully hydrous models produced by AIRSS, which enable both isolated and clustered defects, are able to explain observed NMR signals via just four low-enthalpy structures: (i) a ground state, with isolated Mg3 vacancies and O1-H hydroxyls; (ii/iii) edge-sharing Mg3 vacancies with O1-H and O3-H species; and (iv) edge-sharing Mg1 and Mg3 vacancies with O1-H, O3-H, and O4-H hydroxyls. Thus, the combination of advanced structure searching approaches and solid-state NMR spectroscopy is able to provide new and detailed insight into the structure of this important mantle mineral.

Nuclear Magnetic Resonance Spectroscopy as a Dynamical Structural Probe of Hydrogen under High Pressure.

An unambiguous crystallographic structure solution for the observed phases II-VI of high pressure hydrogen does not exist due to the failure of standard structural probes at extreme pressure. In this work we propose that nuclear magnetic resonance spectroscopy provides a complementary structural probe for high pressure hydrogen. We show that the best structural models available for phases II, III, and IV of high pressure hydrogen exhibit markedly distinct nuclear magnetic resonance spectra which could therefore be used to discriminate amongst them. As an example, we demonstrate how nuclear magnetic resonance spectroscopy could be used to establish whether phase III exhibits polymorphism. Our calculations also reveal a strong renormalization of the nuclear magnetic resonance response in hydrogen arising from quantum fluctuations, as well as a strong isotope effect. As the experimental techniques develop, nuclear magnetic resonance spectroscopy can be expected to become a useful complementary structural probe in high pressure experiments.

Sensitivity improvement in 5QMAS NMR experiments using FAM-N pulses.

The multiple-quantum magic-angle spinning (MQMAS) experiment is a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei with half-integer spin quantum number. However, its inherently poor sensitivity limits its application in more challenging systems. In particular, the use of higher-order multiple-quantum coherences, which have the potential to provide higher resolution in the isotropic spectrum, results in a further decrease in sensitivity. Here we extend our recent work, which introduced an automated, high-throughput approach to generate amplitude-modulated composite pulses (termed FAM-N) to improve the efficiency of the conversion of three-quantum coherences, and explore the use of similar pulses in five-quantum MAS experiments. We consider three different approaches, and are able to demonstrate that all three provide good enhancements over single pulse conversion in all but the most extreme cases, and work well at a range of spinning rates. We show that FAM-N pulses are robust to variation in the quadrupolar coupling and rf nutation rate, demonstrating their applicability in multisite systems and systems where direct experimental optimisation of complex composite pulses is not feasible. This work will ease the implementation of higher-order MQMAS experiments and enable their application to materials and systems that were previously deemed too difficult to study.