Solid-State ⁹¹Zr NMR Characterization of Layered and Three-Dimensional Framework Zirconium Phosphates

Layered and open framework zirconium phosphates (ZrPs) have many current and potential applications in the areas of catalysis, sorption, protonic conductors, solar energy storage, crystal engineering, and ion exchange. Characterization of ZrP-based materials is important because understanding the relationship between the properties of these materials and their structures is crucial for developing new uses and for improving their performances in current applications. However, local Zr environments in many ZrPs have not been characterized directly by ⁹¹Zr solid-state NMR (SSNMR). This is because ⁹¹Zn (<i>I</i> = 5/2) is an unreceptive nucleus with many NMR unfavorable characteristics, leading to low sensitivity. In this work, the local environments of the zirconium centers in several ion-exchanged derivatives of layered α-ZrP (K⁺-, Li⁺-, Co­(NH₃)₆³⁺-ZrP) have been probed directly using ⁹¹Zr MAS, static quadrupolar echo, and/or quadrupolar Carr–Purcell–Meiboom–Gill NMR. Several layered and three-dimensional framework zirconium phosphates (ZrPO₄-DES8, ZrPO₄-DES1, ZrPO₄-DES2, ZrPOF-pyr, ZrPOF-Q1, ZrPOF-EA, and ZrPOF-DEA) with novel structures were also examined. Theoretical calculations using the CASTEP and Gaussian model cluster approaches were also performed in order to provide insights into the observed spectra. In addition to ⁹¹Zr SSNMR, ³¹P, ¹³C, and ¹⁹F SSNMR spectroscopy was also utilized to characterize the above-mentioned materials

Oxygen-Induced Ordering in Bulk Polycrystalline Cu₂ZnSnS₄ by Sn Removal

Solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, and Raman spectroscopy were used to show that Cu₂ZnSnS₄ (CZTS) bulk solids grown in the presence of oxygen had improved cation ordering compared to bulk solids grown without oxygen. Oxygen was shown to have negligible solubility in the CZTS phase. The addition of oxygen resulted in the formation of SnO₂, leading to Sn-deficient CZTS. At the highest oxygen levels, other phases such as Cu₉S₅ and ZnS were observed. Beneficial ordering was only observed in samples produced with more than 2 at. % oxygen in the precursor materials but did not occur in samples designed with excess Sn and O. Thus, it is the removal of Sn and formation of Sn-deficient CZTS that improves ordering rather than the presence of SnO₂ or O alone. These results indicate that using oxygen or air annealing to tailor the Sn content of CZTS followed by an etching step to remove SnO₂ may significantly improve the properties of CZTS

Critical Size for Bulk-to-Discrete Transition in 2D Aliphatic Layers: Abrupt Size Effect Observed via Calorimetry and Solid-State ¹³C NMR

Anomalous changes of physical properties are observed in an abrupt bulk-to-discrete transition in layered silver alkanethiolate (AgSC<i>n</i>, <i>n</i> = 1–16). A critical chain length of <i>n</i>_cr = 7 marks the sharp boundary between the bulk (uniform, <i>n</i> ≥ 7) and discrete (individualistic, <i>n</i> ≤ 6) forms of AgSC<i>n</i>. Solid-state ¹³C NMR analysis reveals that none of the carbons share identical chemical environment in the discrete range, making each AgSC<i>n</i> with <i>n</i> = 2–6 uniquely different material, even though the crystal structure is preserved throughout. Extraordinary changes of thermodynamic properties appearing at this bulk-to-discrete transition include ∼500% increases of melting enthalpy (Δ<i>H</i>_m), ∼50 °C increases of melting point (<i>T</i>_m), and an atypical transition between size-dependent <i>T</i>_m depression and <i>T</i>_m enhancement. We develop a new comprehensive Gibbs–Thomson model with piecewise excess free energy (Δ<i>G</i>_excess) to predict the nature of the abrupt size effect melting. A new 3D spatial model is constructed to divide the aliphatic chains of AgSC<i>n</i> into three bulk or discrete segments: (a) tail segment containing three carbons, (b) head segment containing two carbons, and (c) bulk mid-chain segment containing (<i>n</i> – 5) carbons. Odd/even effect of <i>T</i>_m and Δ<i>H</i>_m is described by a constant Δ<i>G</i>_excess over the entire chain length range of AgSC<i>n</i> and is exclusively attributed to the localized tail segment. Bulk-to-discrete transition occurs when material properties are dominated by the discrete head and tail segments at <i>n</i> < <i>n</i>_cr. Values of <i>n</i>_cr are independently measured by both calorimetry and ¹³C NMR. This analysis is generalized to other aliphatic layers including <i>n</i>-alkanes with <i>n</i>_cr ≈ 11. This work is seminal to the design of novel aliphatic layers with tailorable properties (e.g., <i>T</i>_m) and has applications in molecular electronics and biophysics

New Insights into the Short-Range Structures of Microporous Titanosilicates As Revealed by ^47/49Ti, ²³Na, ³⁹K, and ²⁹Si Solid-State NMR Spectroscopy

Seven prototypical microporous titanosilicates have been studied by multinuclear solid-state NMR (SSNMR) spectroscopy, representing four typical Ti environments: square-pyramidal TiO₅ units (natisite, AM-1, ETS-4), edge-shared brookite-type TiO₆ chains (AM-4), cubane-type Ti₄O₁₆ clusters (sitinakite, GTS-1), and corner-shared TiO₆ chains (ETS-10, ETS-4). ^47/49Ti SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in Ti–O bond lengths and O–Ti–O coordination angles are reflected via <i>C</i>_Q(^47/49Ti) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric ^47/49Ti electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses ²⁹Si NMR experiments to assess phase purity and crystallinity. ²³Na NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of ³⁹K SSNMR for investigation of extra-framework counter cations is demonstrated by ETS-10, with increased spectral resolution and enhanced sensitivity to changes in local environment versus ²³Na experiments. Plane-wave DFT calculations predicted ^47/49Ti NMR parameters assisting in spectral assignments and help correlate ²³Na and ²⁹Si NMR resonances to crystallographic sites. The approach described in this work should promote further SSNMR investigations of microporous solids, such as titanosilicates, with unknown or poorly defined structures

Inspecting the Structure and Formation of Molecular Sieve SAPO-34 via ¹⁷O Solid-State NMR Spectroscopy

Silicoaluminophosphates (SAPOs) are microporous frameworks with Brønsted acid sites that can be used as acidic catalysts. A firm understanding of SAPO structure, formation, and crystallinity is necessary for understanding and expanding SAPO applications in heterogeneous catalysis. Solid-state ¹⁷O NMR (SSNMR) spectroscopy is an ideal tool to probe structure and formation of SAPO-based materials; the ¹⁷O quadrupolar and chemical shift interactions are exquisitely sensitive to local electronic and magnetic environments. In this work, a pure trigonal SAPO-34 molecular sieve synthesized via the dry-gel conversion (DGC) method was investigated using a combination of ¹⁷O magic-angle spinning, ¹⁷O triple-quantum magic-angle spinning, ¹⁷O­[²⁷Al] transfer of population in double-resonance, and ¹⁷O­[³¹P] rotational-echo double-resonance SSNMR spectroscopy, complemented by powder X-ray diffraction along with ²⁷Al, ²⁹Si, and ³¹P multinuclear SSNMR experiments. The four observed ¹⁷O resonances were simulated to extract NMR parameters, with each resonance assigned to individual oxygen local environments and connectivities in SAPO-34. The incorporation of oxygen from ¹⁷O-labeled water (i.e., H₂¹⁷O) during the DGC formation of pure trigonal SAPO-34 has also been investigated at various time intervals and stages of crystallization using ¹⁷O SSNMR. There is direct involvement of ¹⁷O-enriched water vapor during the DGC crystallization process of trigonal SAPO-34. The initial dry gel is amorphous, transforming to a crystalline layered AlPO₄ phase during the first hour of heating and then progressing to a semicrystalline phase after 4 h of heating; formation of the crystalline trigonal SAPO-34 product was found to be complete after 2 days

Spies Within Metal-Organic Frameworks: Investigating Metal Centers Using Solid-State NMR

Structural characterization of metal–organic frameworks (MOFs) is crucial, since an understanding of the relationship between the macroscopic properties of these industrially relevant materials and their molecular-level structures allows for the development of new applications and improvements in current performance. In many MOFs, the incorporated metal centers dictate the short- and long-range structure and porosity of the material. Here we demonstrate that solid-state NMR (SSNMR) spectroscopy targeting NMR-active metal centers at natural abundance, in concert with ab initio density functional theory (DFT) calculations and X-ray diffraction (XRD), is a powerful tool for elucidating the molecular-level structure of MOFs. ⁹¹Zr SSNMR experiments on MIL-140A are paired with DFT calculations and geometry optimizations in order to detect inaccuracies in the reported powder XRD crystal structure. ¹¹⁵In and ¹³⁹La SSNMR experiments on sets of related MOFs at two different magnetic fields illustrate the sensitivity of the ¹¹⁵In/¹³⁹La electric field gradient tensors to subtle differences in coordination, bond length distribution, and ligand geometry about the metal center. ^47/49Ti SSNMR experiments reflect the presence or absence of guest solvent in MIL-125­(Ti), and when combined with DFT calculations, these SSNMR experiments permit the study of local hydroxyl group configurations within the MOF channels. ⁶⁷Zn SSNMR experiments and DFT calculations are also used to explore the geometry near Zn within a set of four MOFs as well as local disordering caused by distributions of different linkers around the metal. SSNMR spectroscopy of metal centers offers an impressive addition to the arsenal of techniques for MOF characterization and is particularly useful in cases where XRD information may be ambiguous, incomplete, or unavailable

Synthesis of Pyridine– and Pyrazine–BF₃ Complexes and Their Characterization in Solution and Solid State

Following the discovery of the redox-active 1,4-bis-BF₃-quinoxaline complex, we undertook a structure–activity study with the objective to understand the active nature of the quinoxaline complex. Through systematic synthesis and characterization, we have compared complexes prepared from pyridine and pyrazine derivatives, as heterocyclic core analogues. This paper reports the structural requirements that give rise to the electrochemical features of the 1,4-bis-BF₃-quinoxaline adduct. Using solution and solid-state NMR spectroscopy, the role of aromatic ring fusion and nitrogen incorporation in bonding and electronics was elucidated. We establish the boron atom location and its interaction with its environment from 1D and 2D solution NMR, X-ray diffraction analysis, and ¹¹B solid-state NMR experiments. Crystallographic analysis of single crystals helped to correlate the boron geometry with ¹¹B quadrupolar coupling constant (<i>C</i>_Q) and asymmetry parameter (η_Q), extracted from ¹¹B solid-state NMR spectra. Additionally, computations based on density functional theory were performed to predict electrochemical behavior of the BF₃–heteroaromatic complexes. We then experimentally measured electrochemical potential using cyclic voltammetry and found that the redox potentials and <i>C</i>_Q values are similarly affected by electronic changes in the complexes

Comprehensive Multiphase (CMP) NMR Monitoring of the Structural Changes and Molecular Flux Within a Growing Seed

A relatively recent technique termed comprehensive multiphase (CMP) NMR spectroscopy was used to investigate the growth and associated metabolomic changes of ¹³C-labeled wheat seeds and germinated seedlings. CMP-NMR enables the study of all phases in intact samples (i.e., liquid, gel-like, semisolid, and solid), by combining all required electronics into a single NMR probe, and can be used for investigating biological processes such as seed germination. All components, from the most liquid-like (i.e., dissolved metabolites) to the most rigid or solid-like (seed coat) were monitored <i>in situ</i> over 4 days. A wide range of metabolites were identified, and after 96 h of germination, the number of metabolites in the mobile phase more than doubled in comparison to 0 h (dry seed). This work represents the first application of CMP-NMR to follow biological processes in plants

Comprehensive Multiphase NMR Spectroscopy of Intact ¹³C‑Labeled Seeds

Seeds are complex entities composed of liquids, gels, and solids. NMR spectroscopy is a powerful tool for studying molecular structure but has evolved into two fields, solution and solid state. Comprehensive multiphase (CMP) NMR spectroscopy is capable of liquid-, gel-, and solid-state experiments for studying intact samples where <i>all organic components</i> are studied and differentiated in situ. Herein, intact ¹³C-labeled seeds were studied by a variety of 1D/2D ¹H/¹³C experiments. In the mobile phase, an assortment of metabolites in a single ¹³C-labeled wheat seed were identified; the gel phase was dominated by triacylglycerides; the semisolid phase was composed largely of carbohydrate biopolymers, and the solid phase was greatly influenced by starchy endosperm signals. Subsequently, the seeds were compared and relative similarities and differences between seed types discussed. This study represents the first application of CMP-NMR to food chemistry and demonstrates its general utility and feasibility for studying intact heterogeneous samples