Probing Local Structure of Layered Double Hydroxides with (1)H Solid-State NMR Spectroscopy on Deuterated Samples

Ultrafast magic angle spinning NMR characterization of pharmaceutical solid polymorphism: A posaconazole example

Protons represent the most NMR-sensitive nucleus in pharmaceutical compounds. Therefore, proton-detected solid-state NMR techniques under fast magic angle spinning are among the few solutions to overcome the challenge of low sensitivity to analyze natural abundant drug substances and products. In this study, we report the structural characterization of crystal polymorphs of a commercial drug molecule, posaconazole, with a relatively large molecular weight of 700.8 g·mol^-1 and at the natural abundance. The enhanced sensitivity and resolution at 100 kHz MAS enables the exploration of the distinct intermolecular packing in posaconazole forms I, III, and γ. These results demonstrate that proton-detected homo- and heteronuclear correlation methods can probe the structural details of pharmaceutical polymorphism.

Insights into memory effect mechanisms of layered double hydroxides with solid-state NMR spectroscopy

Layered double oxides (LDOs) can restore the parent layered double hydroxides (LDHs) structure under hydrous conditions, and this “memory effect” plays a critical role in the applications of LDHs, yet the detailed mechanism is still under debate. Here, we apply a strategy based on ex situ and in situ solid-state NMR spectroscopy to monitor the Mg/Al-LDO structure changes during recovery at the atomic scale. Despite the common belief that aqueous solution is required, we discover that the structure recovery can occur in a virtually solid-state process. Local structural information obtained with NMR spectroscopy shows that the recovery in aqueous solution follows dissolution-recrystallization mechanism, while the solid-state recovery is retro-topotactic, indicating a true “memory effect”. The amount of water is key in determining the interactions of water with oxides, thus the memory effect mechanism. The results also provide a more environmentally friendly and economically feasible LDHs preparation route.

The “memory effect” of layered double hydroxides (LDHs) plays a critical role in their applications, yet the details of the mechanism are still under debate. Here authors reveal the nature of the “memory effect” with ex situ and in situ solid-state NMR spectroscopy.

Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed ¹⁹F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six ¹⁹F NMR peaks centered at -140, -148, -156, -163, -176, and -183 ppm was resolved. Combining quantum chemical calculations based on density function theory (DFT) and ¹⁹F{²⁷Al} transfer of populations in double resonance (TRAPDOR) analysis, we could assign the peaks at -140, -148, -156, and -163 ppm to Al-F (F coordinated to surface Al) and those at -176 and -183 ppm to Mg-F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al-F is the predominant mode of F^- sorption at low pH, whereas the formation of Mg-F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F^- uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F^- removal under alkaline conditions.

Recent Advances of Solid-State NMR Spectroscopy for Microporous Materials

Microporous materials have attracted a rapid growth of research interest in materials science and the multidisciplinary area because of their wide applications in catalysis, separation, ion exchange, gas storage, drug release, and sensing. A fundamental understanding of their diverse structures and properties is crucial for rational design of high-performance materials and technological applications in industry. Solid-state NMR (SSNMR), capable of providing atomic-level information on both structure and dynamics, is a powerful tool in the scientific exploration of solid materials. Here, advanced SSNMR instruments and methods for characterization of microporous materials are briefly described. The recent progress of the application of SSNMR for the investigation of microporous materials including zeolites, metal-organic frameworks, covalent organic frameworks, porous aromatic frameworks, and layered materials is discussed with representative work. The versatile SSNMR techniques provide detailed information on the local structure, dynamics, and chemical processes in the confined space of porous materials. The challenges and prospects in SSNMR study of microporous and related materials are discussed.

Favorable Intercalation of Nitrate Ions with Fluorine-Substituted Layered Double Hydroxides

Understanding and controlling confined nanospace to accommodate substrates and promote high ion conduction are essential to various fields. Layered double hydroxides (LDHs) have emerged as promising candidates for anion exchangers using the interlayer nanospace in their crystal structures. Miyata reported in 1983 that the affinity of anions for intercalation with most major Mg-Al LDHs increased in the following order: NO₃^- < Br^- < F^- < SO₄^2- < HPO₃^2-. Attempts to alter the affinity with different metal cations (M²⁺ and M³⁺) have been unsuccessful. Analyses of the crystalline structures of LDHs, positively charged host layers, interlayer anions, and interlayer water molecules indicate that they inevitably interact through hydrogen bonding. In other words, the affinity of LDHs for anions is controlled by tuning the hydrogen bonding. In this study, we prepared fluorine-substituted LDHs (F-LDHs) with different Mg/Al ratios by partially replacing the OH structural groups, which originated from the host layer, with fluorine atoms; the resulting change in affinity was investigated. The distribution coefficient, which is a useful indicator of the affinity of an LDH for a particular anion, was examined. The results showed that only F-LDHs with Mg/Al ratios of 3.5 exhibited high affinity, especially for NO₃^- ions, and the affinity increased in the following order: HPO₄^2- < SO₄^2- < F^- < Br^- < NO₃^-. The separation factors of these specific F-LDHs with respect to both NO₃^-/F^- and NO₃^-/SO₄^2- were higher than that of LDHs with other compositions by 1 order of magnitude. Raman spectroscopy above 3000 cm^-1 revealed that the fluorine substitution of LDHs significantly changed the hydrogen bonding nature in the interlayer space. Highly electronegative fluorine atoms significantly decrease the extent of hydrogen bonding interactions between OH structural groups and both interlayer water molecules and anions, wherein steric effects are induced by the shrunken interlayer space, and van der Waals forces are revealed to be the predominant interaction with anions. Therefore, the highest affinity was observed for NO₃^- ions in F-LDHs.

A solid state NMR study of layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug

Para-amino salicylate (PAS), a tuberculosis drug, was intercalated in three different layered double hydroxides (MgAl, ZnAl, and CaAl-LDH) and the samples were studied by multi-nuclear ((1)H, (13)C, and (27)Al) solid state NMR (SSNMR) spectroscopy in combination with powder X-ray diffraction (PXRD), elemental analysis and IR-spectroscopy to gain insight into the bulk and atomic level structure of these LDHs especially with a view to the purity of the LDH-PAS materials and the concentration of impurities. The intercalations of PAS in MgAl-, ZnAl-, and CaAl-LDH's were confirmed by (13)C SSNMR and PXRD. Moreover, (13)C MAS NMR and infrared spectroscopy show that PAS did not decompose during synthesis. Large amounts (20-41%) of amorphous aluminum impurities were detected in the structure using (27)Al single pulse and 3QMAS NMR spectra, which in combination with (1)H single and double quantum experiments also showed that the M(II):Al ratio was higher than predicted from the bulk metal composition of MgAl-PAS and ZnAl-PAS. Moreover, the first high-resolution (1)H SSNMR spectra of a CaAl LDH is reported and assigned using (1)H single and double quantum experiments in combination with (27)Al{(1)H} HETCOR.