Solid-state NMR spectroscopy

Analysis of the MODIST Sequence for Selective Proton-Proton Recoupling

Theoretical and simulated analyses of selective homonuclear dipolar recoupling sequences serve as primary tools for understanding and determining the robustness of these sequences under various conditions. In this article, we investigate the recently proposed first-order dipolar recoupling sequence known as MODIST (Modest Offset Difference Internuclear Selective Transfer). We evaluate the MODIST transfer efficiency, assessing its dependence on rf-field strengths and the number of simulated spins, extending up to 10 spins. This helps to identify conditions that enhance polarization transfer among spins that are nearby in frequency, particularly among aliphatic protons. The exploration uncovers a novel effect for first-order selective recoupling sequences that we term "facilitated dipolar recoupling". This effect amplifies the recoupled dipolar interaction between distant spins due to the presence of additional strongly dipolar-coupled spins. Unlike the third spin-assisted recoupling mechanism, facilitated dipolar recoupling only requires a coupling to one of the two distant spins of interest. Experimental demonstration of MODIST, including at different rf-field strengths, was carried out with the membrane protein influenza A M2 in lipid bilayers using 55 kHz magic-angle spinning (MAS). Reducing MODIST rf-field strength by a factor of 2 unveils possibilities for detecting Hα-Hα and HMeth-HMeth correlations with a 3D (H)C(H)(H)CH experiment under fast MAS rates, all achievable without specific spin labeling.

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.

High-throughput calculations and machine learning modeling of 17O NMR in non-magnetic oxides

The only NMR-active oxygen isotope, oxygen-17 (17O), serves as a sensitive probe due to its large chemical shift range, the electric field gradient at the oxygen site, and the quadrupolar interaction. Consequently, 17O solid-state NMR offers unique insights into local structures and finds significant applications in the studies of disorder, reactivity, and host-guest chemistry. Despite recent advances in sensitivity enhancement, isotopic labeling, and NMR crystallography, the application of 17O solid-state NMR is still hindered by low natural abundance, costly enrichment, and challenges in handling spectrum signals. Density functional theory calculations and machine learning techniques offer an alternative approach to mapping the local crystal structures to NMR parameters. However, the lack of high-quality data remains a challenge, despite the establishment of some datasets. In this study, we implement and execute a high-throughput workflow combining AiiDA and CASTEP to evaluate the NMR parameters. Focusing on non-magnetic oxides, we have chosen over 7100 binary, ternary, and quaternary compounds from the Materials Project database and performed calculations. Furthermore, using various descriptors for the local crystalline environments, we model the 17O NMR parameters using machine learning techniques, further enhancing our ability to predict and understand 17O NMR parameters in oxide crystals.

Spiers Memorial Lecture: NMR crystallography

Chemical function is directly related to the spatial arrangement of atoms. Consequently, the determination of atomic-level three-dimensional structures has transformed molecular and materials science over the past 60 years. In this context, solid-state NMR has emerged to become the method of choice for atomic-level characterization of complex materials in powder form. In the following we present an overview of current methods for chemical shift driven NMR crystallography, illustrated with applications to complex materials.

The EFG Rosetta Stone: translating between DFT calculations and solid state NMR experiments

We present a comprehensive study on the best practices for integrating first principles simulations in experimental quadrupolar solid-state nuclear magnetic resonance (SS-NMR), exploiting the synergies between theory and experiment for achieving the optimal interpretation of both. Most high performance materials (HPMs), such as battery electrodes, exhibit complex SS-NMR spectra due to dynamic effects or amorphous phases. NMR crystallography for such challenging materials requires reliable, accurate, efficient computational methods for calculating NMR observables from first principles for the transfer between theoretical material structure models and the interpretation of their experimental SS-NMR spectra. NMR-active nuclei within HPMs are routinely probed by their chemical shielding anisotropy (CSA). However, several nuclear isotopes of interest, e.g.7Li and 27Al, have a nuclear quadrupole and experience additional interactions with the surrounding electric field gradient (EFG). The quadrupolar interaction is a valuable source of information about atomistic structure, and in particular, local symmetry, complementing the CSA. As such, there is a range of different methods and codes to choose from for calculating EFGs, from all-electron to plane wave methods. We benchmark the accuracy of different simulation strategies for computing the EFG tensor of quadrupolar nuclei with plane wave density functional theory (DFT) and study the impact of the material structure as well as the details of the simulation strategy. Especially for small nuclei with few electrons, such as 7Li, we show that the choice of physical approximations and simulation parameters has a large effect on the transferability of the simulation results. To the best of our knowledge, we present the first comprehensive reference scale and literature survey for 7Li quadrupolar couplings. The results allow us to establish practical guidelines for developing the best simulation strategy for correlating DFT to experimental data extracting the maximum benefit and information from both, thereby advancing further research into HPMs.

Crystal structure validation of verinurad via proton-detected ultra-fast MAS NMR and machine learning

The recent development of ultra-fast magic-angle spinning (MAS) (>100 kHz) provides new opportunities for structural characterization in solids. Here, we use NMR crystallography to validate the structure of verinurad, a microcrystalline active pharmaceutical ingredient. To do this, we take advantage of 1H resolution improvement at ultra-fast MAS and use solely 1H-detected experiments and machine learning methods to assign all the experimental proton and carbon chemical shifts. This framework provides a new tool for elucidating chemical information from crystalline samples with limited sample volume and yields remarkably faster acquisition times compared to 13C-detected experiments, without the need to employ dynamic nuclear polarization.

Atomic-level structure of the amorphous drug atuliflapon via NMR crystallography

We determine the complete atomic-level structure of the amorphous form of the drug atuliflapon, a 5-lipooxygenase activating protein (FLAP) inhibitor, via chemical-shift-driven NMR crystallography. The ensemble of preferred structures allows us to identify a number of specific conformations and interactions that stabilize the amorphous structure. These include preferred hydrogen-bonding motifs with water and with other drug molecules, as well as conformations of the cyclohexane and pyrazole rings that stabilize structure by indirectly allowing for optimization of hydrogen bonding.

Resolving Structures of Paramagnetic Systems in Chemistry and Materials Science by Solid-State NMR: The Revolving Power of Ultra-Fast MAS

Ultra-fast magic-angle spinning (100+kHz) has revolutionized solid-state NMR of biomolecular systems but has so far failed to gain ground for the analysis of paramagnetic organic and inorganic powders, despite the potential rewards from substantially improved spectral resolution. The principal blockages are that the smaller fast-spinning rotors present significant barriers for sample preparation, particularly for air/moisture-sensitive systems, and are associated with low sensitivity from the reduced sample volumes. Here, we demonstrate that the sensitivity penalty is less severe than expected for highly paramagnetic solids and is more than offset by the associated improved resolution. While previous approaches employing slower MAS are often unsuccessful in providing sufficient resolution, we show that ultra-fast 100+kHz MAS allows site-specific assignments of all resonances from complex paramagnetic solids. Combined with more reliable rotor materials and handling methods, this opens the way to the routine characterization of geometry and electronic structures of functional paramagnetic systems in chemistry, including catalysts and battery materials. We benchmark this approach on a hygroscopic luminescent Tb3+ complex, an air-sensitive homogeneous high-spin Fe2+ catalyst, and a series of mixed Fe2+/Mn2+/Mg2+ olivine-type cathode materials.

Physiological markers for immunotherapeutics: a review

Immunotherapy has been advanced through multiple approaches, including immunogenic cytokines, monoclonal antibodies, therapeutic vaccinations, adoptive cell transfer, stem cell transplantation, and oncolytic viruses. This review analyses various strategies in genomics, transcriptomics, single-cell techniques, computational analysis, big data, and imaging technologies for the identification of tumour microbiota and microenvironments. Immunotherapy is becoming acknowledged as a feasible cancer treatment method, facilitating innovative cancer medicines and personalized medicine techniques.

Highly efficient heteronuclear polarization transfer using dipolar-echo edited R-symmetry sequences in solid-state NMR

In solid-state NMR, dipolar-based heteronuclear polarization transfer has been extensively used for sensitivity enhancement and multidimensional correlations, but its efficiency often suffers from undesired spin interactions and hardware limitations. Herein, we propose a novel dipolar-echo edited R-symmetry (DEER) sequence, which is further incorporated into the INEPT-type scheme, dubbed DEER-INEPT, for achieving highly efficient heteronuclear polarization transfer. Numerical simulations and NMR experiments demonstrate that DEER-INEPT offers significantly improved robustness, enabling efficient polarization transfer under a wide range of MAS conditions, from slow to ultrafast rates, outperforming existing methods. Its high efficiency leads to noticeably enhanced sensitivity in both 1H → X and X → 1H transfers, applicable to both spin-1/2 and spin-half-integer quadrupolar nuclei. DEER-INEPT is expected to be widely used in various systems, offering advantages in both sensitivity enhancement and structural analysis.

Solid Effect Dynamic Nuclear Polarization Enhancement of >500 at 9.4 T

Efficient polarizing agents for dynamic nuclear polarization (DNP) enhanced magic angle spinning (MAS) NMR spectroscopy are of high current interest due to the potential to significantly boost NMR sensitivity. While most efforts have centered on cross-effect (CE) or Overhauser effect (OE) mechanisms, yielding enhancement factors up to ∼300 at 9.4 T, radicals yielding solid effect (SE) DNP have seen less development. Here we model the comparative performance of OE and SE mechanisms and then measure 1H enhancement factors up to 500 from 1,3-bisdiphenylene-2-phenylallyl (BDPA) in an ortho-terphenyl (OTP) matrix at 9.4 T, 100 K, achieved via increased microwave power across the sample volume. The measured SE and OE performances are in good agreement with the predictions. We note that both the experimental and theoretical analyses indicate that SE DNP remains saturation limited, particularly at elevated temperatures, and we envisage that further improvements in microwave power will further increase SE DNP enhancement factors.

Tailoring Ion Transport in Li3-3yHo1+yCl6-xBrx via Transition-Metal Free Structural Planes and Charge Carrier Distribution

Localized atomistic disorder in halide-based solid electrolytes (SEs) can be leveraged to boost Li+ mobility. In this study, Li+ transport in structurally modified Li3HoCl6, via Br- introduction and Li+ deficiency, is explored. The optimized Li3-3 yHo1+ yCl6- xBrx achieves an ionic conductivity of 3.8 mS cm-1 at 25 °C, the highest reported for holmium halide materials. 6,7Li nuclear magnetic resonance and relaxometry investigations unveil enhanced ion dynamics with bromination, attaining a Li+ motional rate neighboring 116 MHz. X-ray diffraction analyses reveal mixed-anion-induced phase transitions with disproportionate octahedral expansions and distortions, creating Ho-free planes with favorable energetics for Li+ migration. Bond valence site energy analysis highlights preferred Li+ transport pathways, particularly in structural planes devoid of Ho3+ blocking effects. Molecular dynamics simulations corroborate enhanced Li+ diffusion with Br- introduction into Li3HoCl6. Li-Ho electrostatic repulsions in the (001) plane presumably drive Li+ diffusion into the Ho-free (002) layer, enabling rapid intraplanar Li+ motion and exchange between the 2d and 4h sites. Li3-3 yHo1+ yCl6- xBrx also demonstrates good battery cycling stability. These findings offer valuable insights into the intricate correlations between structure and ion transport and will help guide the design of high-performance fast ion conductors for all-solid-state batteries.

ZnH2P4N8: Case Study on Topochemical Imidonitridophosphate High-Pressure Synthesis

Nitridophosphates are subject of current research, as they have a broad spectrum of properties and potential applications, such as ion conductors or luminescent materials. Yet, the subclass of imidonitridophosphates has been studied less extensively. The primary reason is that the controlled N-H functionalization of nitridophosphates is not straight forward, making targeted synthesis more challenging. Inspired by the high-pressure (HP) post-synthetic modification of nitridophosphates, we present the topochemical HP deprotonation of phosphorus nitride imides using the high-pressure polymorph β-PN(NH) as an example. Additional incorporation of Zn2+ results in the first quaternary transition metal imidonitridophosphate ZnH2P4N8. The crystal structure was elucidated by single-crystal X-ray diffraction (SCXRD), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD) and solid-state magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). In addition, the presence of H as part of an imide group was confirmed by IR spectroscopy. The potential of this defunctionalization approach for controlling the N-H content is demonstrated by the preparation of partially deprotonated intermediates ZnxH4-2xP4N8 (x≈0.5, 0.85). This topochemical high-pressure reaction represents a promising way to prepare, control and manipulate new imide-based materials without altering their overall anionic framework.

Mechanistic characterization of fast dissolving PVP-I powder with multipolymer approaches and investigation on their molecular interaction

Povidone-iodine (PVP-I) is widely used as an antiseptic in medical applications. However, its effectiveness is limited by certain drawbacks, such as low solubility in water and high volatility. Therefore, a formulation of a stable solid PVP-I is desirable. In this study, complexes of molecular PVP-I with polyethylene glycol-polyvinyl alcohol copolymer (PEG-PVA copolymer) were considered water-soluble iodophors. Two different methods were used to prepare the solids: physical mixtures and kneading. The physical characteristics of the obtained solids were evaluated using several spectroscopic methods. The presence of iodine was confirmed by a potentiometric titration and antimicrobial activity was tested. The results showed that the PEG-PVA copolymer interacted with povidone primarily through hydrogen bonding between the hydroxyl part of the PEG-PVA copolymer and the amide part of povidone with an estimated binding energy of 3.2 kcal/mol. The amide groups polarity in povidone made them more likely to form hydrogen bonds with the PEG-PVA copolymer. Also, the protonated pyrrolidone bonded to the triiodide anions by intermolecular hydrogen bonds, which increased PVP-I solubility in water. The kneading method provided a faster dissolution rate than physical mixing and pure PVP-I. The iodine contents were within an acceptable range (10-12%), and the antimicrobial activity proved effective against Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus mutans.

Manual and automatic assignment of two different Aβ40 amyloid fibril polymorphs using MAS solid-state NMR spectroscopy

Amyloid fibrils from Alzheimer's amyloid-beta peptides (Aβ) are found to be polymorphic. So far, 14 Aβ40 fibril structures have been determined. The mechanism of why one particular protein sequence adopts so many different three-dimensional structures is yet not understood. In this work, we describe the assignment of the NMR chemical shifts of two Alzheimer's disease fibril polymorphs, P1 and P2, which are formed by the amyloid-beta peptide Aβ40. The assignment is based on 13C-detected 3D NCACX and NCOCX experiments MAS solid-state NMR experiments. The fibril samples are prepared using an extensive seeding protocol in the absence and presence of the small heat shock protein αB-crystallin. In addition to manual assignments, we obtain chemical shift assignments using the automation software ARTINA. We present an analysis of the secondary chemical shifts and a discussion on the differences between the manual and automated assignment strategies.

Dipolar Recoupling in Rotating Solids

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) has evolved significantly over the past three decades and established itself as a vital tool for the structural analysis of biological macromolecules and materials. This review delves into the development and application of dipolar recoupling techniques in MAS NMR, which are crucial for obtaining detailed structural and dynamic information. We discuss a variety of homonuclear and heteronuclear recoupling methods which are essential for measuring spatial restraints and explain in detail the spin dynamics that these sequences generate. We also explore recent developments in high spinning frequency MAS, proton detection, and dynamic nuclear polarization, underscoring their importance in advancing biomolecular NMR. Our aim is to provide a comprehensive account of contemporary dipolar recoupling methods, their principles, and their application to structural biology and materials, highlighting significant contributions to the field and emerging techniques that enhance resolution and sensitivity in MAS NMR spectroscopy.

An Efficient and Stable Polarizing Agent for In-Cell Magic-Angle Spinning Dynamic Nuclear Polarization NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy would be a method of choice to follow biochemical events in cells because it can analyze molecules in complex environments. However, the intrinsically low sensitivity of NMR makes in-cell measurements challenging. Dynamic Nuclear Polarization (DNP) has emerged as a method to circumvent this limitation, but most polarizing agents developed for DNP are unstable in reducing cellular environments. Here, we introduce the use of Gd(III)-based DNP polarizing agents for in-cell NMR spectroscopy. Specifically, we show their persistent stability in cellular formulations, and we investigate the DNP performance of the Gd(III)-based complex [Gd(tpatcn)] in human embryonic kidney cell lysates and intact cells. For cell lysates, DNP enhancements up to -27 are obtained on the cellular signals, reproducible even after storage at room temperature for days. Mixing the [Gd(tpatcn)] solution with intact cells enables the observation of cellular signals with DNP, and DNP enhancement factors of about -40 are achieved.

The Role of Phase Separation and Local Mobility in the Stabilization of a Lyophilized IgG2 Formulation

The utility of employing solid-state NMR (SSNMR) to assess parameters governing the stability of a lyophilized IgG2 protein was the focus of the present work. Specifically, the interaction between the sugar stabilizer (sucrose) and protein component was measured using SSNMR and compared to physical and chemical stability data obtained from thermally stressed samples. 1H T1 and 1H T1⍴ relaxation times were measured by SSMNR for 5 different formulation conditions, and the resultant values were used to examine local mobility and phase separation, respectively. From the SSNMR measurements, it was found local mobility decreased as the sucrose to protein weight ratio increased. The decrease in local mobility corresponded to an increase in storage stability (both chemical and physical) of the lyophilized solids up to a critical weight ratio of sucrose to protein. Additionally, 1H T1⍴ measurements obtained on formulations having higher protein to sucrose weight ratios indicated phase separation of the protein and sucrose phases was occurring, at least on a small scale. Along with an increase in local mobility, phase separation in these specific formulations is thought to have played a role in their decreased storage stability in the solid state.

Insights into Ligand-Mediated Activation of an Oligomeric Ring-Shaped Gene-Regulatory Protein from Solution- and Solid-State NMR

The 91 kDa oligomeric ring-shaped ligand binding protein TRAP (trp RNA binding attenuation protein) regulates the expression of a series of genes involved in tryptophan (Trp) biosynthesis in bacilli. When cellular Trp levels rise, the free amino acid binds to sites buried in the interfaces between each of the 11 (or 12, depending on the species) protomers in the ring. Crystal structures of Trp-bound TRAP show the Trp ligands are sequestered from solvent by a pair of loops from adjacent protomers that bury the bound ligand via polar contacts to several threonine residues. Binding of the Trp ligands occurs cooperatively, such that successive binding events occur with higher apparent affinity but the structural basis for this cooperativity is poorly understood. We used solution methyl-TROSY NMR relaxation experiments focused on threonine and isoleucine sidechains, as well as magic angle spinning solid-state NMR 13C-13C and 15N-13C chemical shift correlation spectra on uniformly labeled samples recorded at 800 and 1200 MHz, to characterize the structure and dynamics of the protein. Methyl 13C relaxation dispersion experiments on ligand-free apo TRAP revealed concerted exchange dynamics on the µs-ms time scale, consistent with transient sampling of conformations that could allow ligand binding. Cross-correlated relaxation experiments revealed widespread disorder on fast timescales. Chemical shifts for methyl-bearing side chains in apo- and Trp-bound TRAP revealed subtle changes in the distribution of sampled sidechain rotameric states. These observations reveal a pathway and mechanism for induced conformational changes to generate homotropic Trp-Trp binding cooperativity.

Selective Methods Promote Protein Solid-State NMR

Solid-state nuclear magnetic resonance (ssNMR) is indispensable for studying the structures, dynamics, and interactions of insoluble proteins in native or native-like environments. While ssNMR includes numerous nonselective techniques for general analysis, it also provides various selective methods that allow for the extraction of precise details about proteins. This perspective highlights three key aspects of selective methods: selective signals of protein segments, selective recoupling, and site-specific insights into proteins. These methods leverage protein topology, labeling strategies, and the tailored manipulation of spin interactions through radio frequency (RF) pulses, significantly promoting the field of protein ssNMR. With ongoing advancements in higher magnetic fields and faster magic angle spinning (MAS), there remains an ongoing need to enhance the selectivity and efficiency of selective ssNMR methods, facilitating deeper atomic-level insights into complex biological systems.

Mechanism of Solid-State 1H Photochemically Induced Dynamic Nuclear Polarization in a Synthetic Donor-Chromophore-Acceptor at 0.3 T

1H photochemically induced dynamic nuclear polarization (photo-CIDNP) has recently emerged as a tool to enhance bulk 1H nuclear magnetic resonance (NMR) signals in solids at magnetic fields ranging from 0.3 to 21.1 T, using synthetic donor-chromophore-acceptor (D-C-A) molecules as optically active polarizing agents (PAs). However, the mechanisms at play for the generation of spin polarization in these systems have not been determined but are essential for an in-depth understanding and further development of the process. Here, we introduce site-selective deuteration to identify the 1H photo-CIDNP mechanisms at 85 K and 0.3 T in D-C-A molecule PhotoPol. We find that the protons on the acceptor moiety are essential for the generation of polarization, establishing differential relaxation as the main mechanism. These results establish selective deuteration as a tool to identify and suppress polarization transfer mechanisms, which opens up pathways for further optimization of the optical PA at both low and high magnetic fields.

Investigating Different Dynamic pHP1α States in Their KCl-Mediated Liquid-Liquid Phase Separation (LLPS) Using Solid-State NMR (SSNMR) and Molecular Dynamic (MD) Simulations

Chromatin phase separation is dynamically regulated by many factors, such as post-translational modifications and effector proteins, and plays a critical role in genomic activities. The liquid-liquid phase separation (LLPS) of chromatin and/or effector proteins has been observed both in vitro and in vivo. However, the underlying mechanisms are largely unknown, and elucidating the physicochemical properties of the phase-separated complexes remains technically challenging. In this study, we detected dynamic, viscous, and intermediate components within the phosphorylated heterochromatin protein 1α (pHP1α) phase-separated system by using modified solid-state NMR (SSNMR) pulse sequences. The basis of these sequences relies on the different time scale of motion detected by heteronuclear Overhauser effect (hetNOE), scalar coupling-based, and dipolar coupling-based transfer schemes in NMR. In comparison to commonly utilized scalar coupling-based methods for studying the dynamic components in phase-separated systems, hetNOE offers more direct insight into molecular dynamics. NMR signals from the three different states in the protein gel were selectively excited and individually studied. Combined with molecular dynamics (MD) simulations, our findings indicate that at low KCl concentration (30 mM), the protein gel displays reduced molecular motion. Conversely, an increase in molecular motion was observed at a high KCl concentration (150 mM), which we attribute to the resultant intermolecular electrostatic interactions regulated by KCl.

Formamidinium Incorporates into Rb-based Non-Perovskite Phases in Solar Cell Formulations

Organic-inorganic hybrid perovskite materials, such as formamidinium lead iodide (FAPbI3), are among the most promising emerging photovoltaic materials. However, the spontaneous phase transition from the photoactive perovskite phase to an inactive non-perovskite phase complicates the application of FAPbI3 in solar cells. To remedy this, alkali metal cations, most often Cs+, Rb+ or K+, are included during perovskite synthesis to stabilize the photoactive phase. The atomic-level mechanisms of stabilization are complex. While Cs+ dopes directly into the perovskite lattice, Rb+ does not, but instead forms an additional non-perovskite phase, and the mechanism by which Rb confers increased stability remains unclear. Here, we use 1H-87Rb double resonance NMR experiments to show that FA+ incorporates into the Rb-based non-perovskite phases (FAyRb1-yPb2Br5 and δ-FAyRb1-yPbI3) for both bromide and iodide perovskite formulations. This is demonstrated by changes in the 1H and 87Rb chemical shifts, 1H-87Rb heteronuclear correlation spectra, and 87Rb{1H} REDOR spectra. Simulation of the REDOR dephasing curves suggests up to ~60 % FA+ incorporation into the inorganic Rb-based phase for the bromide system. In light of these results, we hypothesize that the substitution of FA+ into the non-perovskite phase may contribute to the greater stability conferred by Rb salts in the synthesis of FA-based perovskites.

Light-Induced Metallic and Paramagnetic Defects in Halide Perovskites from Magnetic Resonance

Halide perovskites are promising next-generation solar cell materials, but their commercialization is hampered by their propensity to degrade under operating conditions, particularly under heat, humidity, and light. Identifying degradation products and linking them to the degradation mechanism at the atomic scale is necessary to design more stable perovskite materials. Here we use magnetic resonance methods to identify and characterize the formation of both metallic lead clusters and Pb3+ defects upon light-induced degradation of methylammonium lead halide perovskite using nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements. Paramagnetic relaxation enhancement (PRE) of the 1H NMR resonances demonstrates the presence of localized paramagnetic Pb3+ defects, a large Knight shift of the 207Pb NMR proves the presence of lead metal, and their relative proportions are determined by the differing temperature dependence in variable-temperature EPR. This work reconciles previous conflicting literature results, enabling the use of EPR spectroscopy to monitor photodegradation of perovskite devices.

Molecular architecture of chitin and chitosan-dominated cell walls in zygomycetous fungal pathogens by solid-state NMR

Zygomycetous fungal infections pose an emerging medical threat among individuals with compromised immunity and metabolic abnormalities. Our pathophysiological understanding of these infections, particularly the role of fungal cell walls in growth and immune response, remains limited. Here we conducted multidimensional solid-state NMR analysis to examine cell walls in five Mucorales species, including key mucormycosis causative agents like Rhizopus and Mucor species. We show that the rigid core of the cell wall primarily comprises highly polymorphic chitin and chitosan, with minimal quantities of β-glucans linked to a specific chitin subtype. Chitosan emerges as a pivotal molecule preserving hydration and dynamics. Some proteins are entrapped within this semi-crystalline chitin/chitosan layer, stabilized by the sidechains of hydrophobic amino acid residues, and situated distantly from β-glucans. The mobile domain contains galactan- and mannan-based polysaccharides, along with polymeric α-fucoses. Treatment with the chitin synthase inhibitor nikkomycin removes the β-glucan-chitin/chitosan complex, leaving the other chitin and chitosan allomorphs untouched while simultaneously thickening and rigidifying the cell wall. These findings shed light on the organization of Mucorales cell walls and emphasize the necessity for a deeper understanding of the diverse families of chitin synthases and deacetylases as potential targets for novel antifungal therapies.

Electron diffraction and solid-state NMR reveal the structure and exciton coupling in a eumelanin precursor

Eumelanin, a versatile biomaterial found throughout the animal kingdom, performs essential functions like photoprotection and radical scavenging. The diverse properties of eumelanin are attributed to its elusive and heterogenous structure with DHI (5,6-dihydroxyindole) and DHICA (5,6-dihydroxyindole-2-carboxylic acid) precursors as the main constituents. Despite DHICA being recognized as the key eumelanin precursor, its crystal structure and functional role in the assembled state remain unknown. Herein, we employ a synthesis-driven, bottom-up approach to elucidate the structure and assembly-specifics of DHICA, a critical building block of eumelanin. We introduce an interdisciplinary methodology to analyse the nanocrystalline assembly of DHICA, employing three-dimensional electron diffraction (3D ED), solid-state NMR and density functional theory (DFT), while correlating the structural aspects with the electronic spectroscopic features. The results underscore charge-transfer exciton delocalization as the predominant energy transfer mechanism within the π-π stacked and hydrogen-bonded crystal network of DHICA. Additionally, extending the investigation to the 13C-labelled DHICA-based polymer improves our understanding of the chemical heterogeneity across the eumelanin pigment, providing crucial insights into the structure of eumelanin.

Efficient DNP at high fields and fast MAS with antenna-sensitized dinitroxides

Dynamic Nuclear Polarization (DNP) can significantly enhance the sensitivity of solid-state NMR. In DNP, microwave irradiation induces polarization transfer from unpaired electron spins to 1H nuclear spins via hyperfine couplings and spin-diffusion. The structure of the polarizing agents that host the electron spins is key for DNP efficiency. Currently, only a handful of structures perform well at very high magnetic fields (≥18.8 T), and enhancements are significantly lower than those obtained at lower fields. Here, we introduce a new series of water-soluble nitroxide biradicals with a scaffold augmented by dihydroxypropyl antenna chains that perform significantly better than previous dinitroxides at 18.8 T. The new radical M-TinyPol(OH)4 yields enhancement factors of ∼220 at 18.8 T and 60 kHz MAS, which is a nearly factor 2 larger than for the previous best performing dinitroxides. The performance is understood through 2H ESEEM measurements to probe solvent accessibility, supported by Molecular Dynamics simulations, and by experiments on deuterated samples. We find that the deuterated glycerol molecules in the matrix are located mainly in the second solvation shell of the NO bond, limiting access for protonated water molecules, and restricting spin diffusion pathways. This provides a rational understanding of why the dihydroxypropyl chains present in the best-performing structures are essential to deliver the polarization to the bulk solution.

A metal-free cascaded process for efficient H2O2 photoproduction using conjugated carbonyl sites

Carbon-based metal-free catalysts are promising green catalysts for photocatalysis and electrocatalysis due to their low cost and environmental friendliness. A key challenge in utilizing these catalysts is identifying their active sites, given their poor crystallinity and complex structures. Here we demonstrate the key structure of the double-bonded conjugated carbon group as a metal-free active site, enabling efficient O2 photoreduction to H2O2 through a cascaded water oxidation - O2 reduction process. Using ethylenediaminetetraacetic acid as a precursor, we synthesized various carbon-based photocatalysts and analyzed their structural evolution. Under the polymerization conditions of 260 °C to 400 °C, an N-ethyl-2-piperazinone-like structure was formed on the surface of the catalyst, resulting in high photocatalytic H2O2 photoproduction (2884.7 μmol g-1h-1) under visible light. A series of control experiments and theoretical calculations further confirm that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts.

Nondissociative Activated Dihydrogen Binding on CeO2 Revealed by High-Pressure Operando Solid-State NMR Spectroscopy

Dihydrogen complexes, which retain the H-H bond, have been extensively studied in molecular science and found to be prevalent in homogeneous and enzymatic catalysis. However, their counterparts in heterogeneous catalysis, specifically nondissociative chemisorbed dihydrogen binding on the catalyst surface, are rarely reported experimentally. This scarcity is due to the complexity of typical material surfaces and the lack of effective characterization techniques to prove and distinguish various dihydrogen binding modes. Herein, using high-pressure operando solid-state NMR technology, we report the first unambiguous experimental observation of activated dihydrogen binding on a reduced ceria catalyst through interactions with surface oxygen vacancies. By employing versatile NMR structural and dynamical analysis methods, we establish a proportional relationship between the degree of ceria surface reduction and dihydrogen binding, as evidenced by NMR observations of H-D through-bond coupling (JHD), T1 relaxation, and proton isotropic chemical shifts. In situ NMR analysis further reveals the participation of bound dihydrogen species in a room-temperature ethylene hydrogenation reaction. The remarkable similarities between surface-activated dihydrogen in heterogeneous catalysis and dihydrogen model molecular complexes can provide valuable insights into the hydrogenation mechanism for many other solid catalysts, potentially enhancing hydrogen utilization.

A complete 3D-printed tool kit for Solid-State NMR sample and rotor handling

Solid state NMR (SSNMR) is a highly versatile and broadly applicable method for studying the structure and dynamics of biomolecules and materials. For scientists entering the field of SSNMR, the many quotidian activities required in the workflow to prepare samples for data collection can present a significant barrier to adoption. These steps include transfer of samples into rotors, marking the reflective surfaces for high sensitivity tachometer signal detection, inserting rotors into the magic-angle spinning (MAS) stator, achieving stable spinning, and removing and storing rotors to ensure reproducibility of data collection conditions. Even experienced spectroscopists experience occasional problems with these operations, and the cumulative probability of a delay to successful data collection is high enough to cause frequent disruptions to instrument schedules, particularly in the context of large facilities serving a diverse community of users. These problems are all amplified when utilizing rotors smaller than about 4 mm in diameter. Therefore, to improve the reliability and robustness of SSNMR sample preparation workflows, here we describe a set of tools for rotor packing, unpacking, tachometer marking, extraction and storage. Stereolithography 3D printing was employed as a cost-effective and convenient method for prototyping and manufacturing a full range of designs suitable for several types of probes and rotor geometries.

Studying large biomolecules as sedimented solutes with solid-state NMR

Sedimentation solid-state NMR is a novel method for sample preparation in solid-state NMR (ssNMR) studies. It involves the sedimentation of soluble macromolecules such as large protein complexes. By utilizing ultra-high centrifugal forces, the molecules in solution are driven into a high-concentrated hydrogel, resulting in a sample suitable for solid-state NMR. This technique has the advantage of avoiding the need for chemical treatment, thus minimizing the loss of sample biological activity. Sediment ssNMR has been successfully applied to a variety of non-crystalline protein solids, significantly expanding the scope of solid-state NMR research. In theory, using this method, any biological macromolecule in solution can be transferred into a sedimented solute appropriate for solid-state NMR analysis. However, specialized equipment and careful handling are essential for effectively collecting and loading the sedimented solids to solid-state NMR rotors. To improve efficiency, we have designed a series of loading tools to achieve the loading process from the solution to the rotor in one step. In this paper, we illustrate the sample preparation process of sediment NMR using the H1.4-NCP167 complex, which consists of linker histone H1.4 and nucleosome core particle, as an example.

High-field pulsed EPR spectroscopy under magic angle spinning

In this work, we demonstrate the first pulsed electron paramagnetic resonance (EPR) experiments performed under magic angle spinning (MAS) at high magnetic field. Unlike nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP), commonly performed at high magnetic fields and under MAS to maximize sensitivity and resolution, EPR is usually measured at low magnetic fields and, with the exception of the Spiess group work in the late 1990s, never under MAS, due to great instrumentational challenges. This hampers the investigation of DNP mechanisms, in which electron spin dynamics play a central role, because no experimental data about the latter under DNP-characteristic conditions are available. We hereby present our dedicated, homebuilt MAS-EPR probehead and show the pulsed MAS-EPR spectra of P1 center diamond defect recorded at 7 tesla. Our results reveal unique effects of MAS on EPR line shape, intensity, and signal dephasing. Time-domain simulations reproduce the observed changes in the line shapes and the trends in the signal intensity.

Exploring the complexity of amyloid-beta fibrils: structural polymorphisms and molecular interactions

The aggregation of amyloid-beta (Aβ) peptides into cross-β structures forms a variety of distinct fibril conformations, potentially correlating with variations in neurodegenerative disease progression. Recent advances in techniques such as X-ray crystallography, solid-state NMR, and cryo-electron microscopy have enabled the development of high-resolution molecular structures of these polymorphic amyloid fibrils, which are either grown in vitro or isolated from human and transgenic mouse brain tissues. This article reviews our current understanding of the structural polymorphisms in amyloid fibrils formed by Aβ40 and Aβ42, as well as disease-associated mutants of Aβ peptides. The aim is to enhance our understanding of various molecular interactions, including hydrophobic and ionic interactions, within and among cross-β structures.

Maximizing Relayed 1H Hyperpolarization Transfer by Slow-Fast MAS NMR Spectroscopy

1H-detected dynamic nuclear polarization (DNP)-enhanced fast magic angle spinning (MAS) NMR experiments provide unprecedented sensitivity to study the structure and dynamics in advanced materials and biomolecules. However, in relayed DNP experiments, DNP enhancements decrease with faster MAS rates, which is detrimental for sensitivity. The decrease is because 1H-1H spin diffusion rates are significantly reduced at fast MAS frequencies. To improve sensitivity at these fast MAS rates, here, we propose to combine fast polarization build-up by relay at slow MAS rate with high-resolution 1H NMR at fast MAS for acquisition. We perform experiments on l-histidine·HCl·H2O with MAS rates of up to 65 kHz using a 0.7 mm DNP probe at 18.8 T and 100 K. We obtain a 35% improvement in sensitivity in experiments where the sample is polarized at 20 kHz MAS and where the signal is acquired at 60 kHz MAS.

Stabilization of highly efficient perovskite solar cells with a tailored supramolecular interface

The presence of defects at the interface between the perovskite film and the carrier transport layer poses significant challenges to the performance and stability of perovskite solar cells (PSCs). Addressing this issue, we introduce a dual host-guest (DHG) complexation strategy to modulate both the bulk and interfacial properties of FAPbI3-rich PSCs. Through NMR spectroscopy, a synergistic effect of the dual treatment is observed. Additionally, electro-optical characterizations demonstrate that the DHG strategy not only passivates defects but also enhances carrier extraction and transport. Remarkably, employing the DHG strategy yields PSCs with power conversion efficiencies (PCE) of 25.89% (certified at 25.53%). Furthermore, these DHG-modified PSCs exhibit enhanced operational stability, retaining over 96.6% of their initial PCE of 25.55% after 1050 hours of continuous operation under one-sun illumination, which was the highest initial value in the recently reported articles. This work establishes a promising pathway for stabilizing high-efficiency perovskite photovoltaics through supramolecular engineering, marking a significant advancement in the field.

Comparative Analysis of Polysaccharide and Cell Wall Structure in Aspergillus nidulans and Aspergillus fumigatus by Solid-State NMR

Invasive aspergillosis poses a significant threat to immunocompromised patients, leading to high mortality rates associated with these infections. Targeting the biosynthesis of cell wall carbohydrates is a promising strategy for antifungal drug development and will be advanced by a molecular-level understanding of the native structures of polysaccharides within their cellular context. Solid-state NMR spectroscopy has recently provided detailed insights into the cell wall organization of Aspergillus fumigatus, but genetic and biochemical evidence highlights species-specific differences among Aspergillus species. In this study, we employed a combination of 13C, 15N, and 1H-detection solid-state NMR, supplemented by Dynamic Nuclear Polarization (DNP), to compare the structural organization of cell wall polymers and their assembly in the cell walls of A. fumigatus and A. nidulans, both of which are key model organisms and human pathogens. The two species exhibited a similar rigid core architecture, consisting of chitin, α-glucan, and β-glucan, which contributed to comparable cell wall properties, including polymer dynamics, water retention, and supramolecular organization. However, differences were observed in the chitin, galactosaminogalactan, protein, and lipid content, as well as in the dynamics of galactomannan and the structure of the glucan matrix.

Carboxymethyl cellulose-based rotigotine nanocrystals-loaded hydrogel for increased transdermal delivery with alleviated skin irritation

Transdermal rotigotine (RTG) therapy is prescribed to manage Parkinson's disease (Neupro® patch). However, its use is suffered from application site reactions. Herein, drug nanocrystalline suspension (NS)-loaded hydrogel (NS-HG) employing polysaccharides simultaneously as suspending agent and hydrogel matrix was constructed for transdermal delivery, with alleviated skin irritation. RTG-loaded NS-HG was prepared using a bead-milling technique, employing sodium carboxylmethyl cellulose (Na.CMC) as nano-suspending agent (molecular weight 90,000 g/mol) and hydrogel matrix (700,000 g/mol), respectively. NS-HG was embodied as follows: drug loading: ≤100 mg/mL; shape: rectangular crystalline; crystal size: <286.7 nm; zeta potential: -61 mV; viscosity: <2.16 Pa·s; and dissolution rate: >90 % within 15 min. Nuclear magnetic resonance analysis revealed that the anionic polymers bind to RTG nanocrystals via charge interaction, affording uniform dispersion in the matrix. Rodent transdermal absorption of RTG from NS-HG was comparable to that from microemulsions, and proportional to drug loading. Moreover, NS-HG was skin-friendly; erythema and epidermal swelling were absent after repeated application. Further, NS-HG was chemically stable; >95 % of the drug was preserved up to 4 weeks under long term (25 °C/RH60%), accelerated (40 °C/RH75%), and stress (50 °C) storage conditions. Therefore, this novel cellulose derivative-based nanoformulation presents a promising approach for effective transdermal RTG delivery with improved tolerability.

Quantitative and Structural Characterization of Native Lignin in Hardwood and Softwood Bark via Solid-State NMR Spectroscopy

A major factor limiting bark's industrial use is its greater recalcitrance compared to wood. While lignin is widely recognized as a significant contributor, precise characterization of lignin in bark remains sparse, presenting a crucial gap that impedes understanding of its impact. In this study, we employed advanced solid-state nuclear magnetic resonance (NMR) spectroscopy to analyze bark samples from various species, including willow, poplar, and pine. We established and verified that lignin methoxy peak at 56 ppm serves as a reliable quantitative metric to assess lignin content, with which we calculated the lignin contents in bark are significantly reduced by more than 70% compared to those in wood. Furthermore, in situ characterization revealed significant reduction of β-ether linkage in bark lignin across species, revealing a more condensed and resistant structural configuration. Our results have substantially advanced our comprehension of the composition and structure of native lignin in tree bark.

Effect of Dynamical Motion in ab Initio Calculations of Solid-State Nuclear Magnetic and Nuclear Quadrupole Resonance Spectra

Solid-state nuclear magnetic resonance (SSNMR) and nuclear quadrupole resonance (NQR) spectra provide detailed information about the electronic and atomic structure of solids. Modern ab initio methods such as density functional theory (DFT) can be used to calculate NMR and NQR spectra from first-principles, providing a meaningful avenue to connect theory and experiment. Prediction of SSNMR and NQR spectra from DFT relies on accurate calculation of the electric field gradient (EFG) tensor associated with the potential of electrons at the nuclear centers. While static calculations of EFGs are commonly seen in the literature, the effects of dynamical motion of atoms in molecules and solids have been less explored. In this study, we develop a method to calculate EFGs of solids while taking into account the dynamics of atoms through DFT-based molecular dynamics simulations. The method we develop is general, in the sense that it can be applied to any material at any desired temperature and pressure. Here, we focus on application of the method to NaNO2 and study in detail the EFGs of 14N, 17O, and 23Na. We find in the cases of 14N and 17O that the dynamical motion of the atoms can be used to calculate mean EFGs that are in closer agreement with experiments than those of static calculations. For 23Na, we find a complex behavior of the EFGs when atomic motion is incorporated that is not at all captured in static calculations. In particular, we find a distribution of EFGs that is influenced strongly by the local (changing) bond environment, with a pattern that reflects the coordination structure of 23Na. We expect the methodology developed here to provide a path forward for understanding materials in which static EFG calculations do not align with experiments.

Hydrogen Diffusion in Hybrid Perovskites from Exchange NMR

Ion migration is an important phenomenon affecting the performance of hybrid perovskite solar cells. It is particularly challenging, however, to disentangle the contribution of H+ diffusion from that of other ions, and the atomic-scale mechanism remains unclear. Here, we use 2H exchange NMR to prove that 2H+ ions exchange between MA+ cations on the time scale of seconds for both MAPbI3 and FA0.7MA0.3PbI3 perovskites. We do this by exploiting 15N-enriched MA+ to label the cations by their 15N spin state. The exchange rates and activation energy are then calculated by performing experiments as functions of mixing time and temperature. By comparing the measured exchange rates to previously measured bulk H+ diffusivities, we demonstrate that, after dissociating, H+ ions travel through the lattice before associating to another cation rather than hopping between adjacent cations.

Synergistic Dual-Cure Reactions for the Fabrication of Thermosets by Chemical Heating

Large composite structures, such as those used in wind energy applications, rely on the bulk polymerization of thermosets on an impressively large scale. To accomplish this, traditional thermoset polymerizations require both elevated temperatures (>100 °C) and extended cure durations (>5 h) for complete conversion, necessitating the use of oversize ovens or heated molds. In turn, these requirements lead to energy-intensive polymerizations, incurring high manufacturing costs and process emissions. In this study, we develop thermoset polymerizations that can be initiated at room temperature through a transformative "chemical heating" concept, in which the exothermic energy of a secondary reaction is used to facilitate the heating of a primary thermoset polymerization. By leveraging a redox-initiated methacrylate free radical polymerization as a source of exothermic chemical energy, we can achieve peak reaction temperatures >140 °C to initiate the polymerization of epoxy-anhydride thermosets without external heating. Furthermore, by employing Trojan horse methacrylate monomers to induce mixing between methacrylate and epoxy-anhydride domains, we achieve the synthesis of homogeneous hybrid polymeric materials with competitive thermomechanical properties and tunability. Herein, we establish a proof-of-concept for our innovative chemical heating method and advocate for its industrial integration for more energy-efficient and streamlined manufacturing of wind blades and large composite parts more broadly.

Trigonal Planar [PN3]4- Anion in the Nitridophosphate Oxide Ba3[PN3]O

Nitridophosphates, with their primary structural motif of isolated or condensed PN4 tetrahedra, meet many requirements for high performance materials. Their properties are associated with their structural diversity, which is mainly limited by this specific building block. Herein, we present the alkaline earth metal nitridophosphate oxide Ba3[PN3]O featuring a trigonal planar [PN3]4- anion. Ba3[PN3]O was obtained using a hot isostatic press by medium-pressure high-temperature synthesis (MP/HT) at 200 MPa and 880 °C. The crystal structure was solved and refined from single-crystal X-ray diffraction data in space group R3 ‾ ${\bar 3}$ c (no. 167) and confirmed by SEM-EDX, magic angle spinning (MAS) NMR, vibrational spectroscopy (Raman, IR) and low-cost crystallographic calculations (LCC). MP/HT synthesis reveals great potential by extending the structural chemistry of P to include trigonal planar [PN3]4- motifs.

Solid-State NMR 13C sensitivity at high magnetic field

Sensitivity is the foundation of every NMR experiment, and the signal-to-noise ratio (SNR) should increase with static (B0) magnetic field, by a proportionality that primarily depends on the design of the NMR probe and receiver. In the low B0 field limit, where the coil geometry is much smaller than the wavelength of the NMR frequency, SNR can increase in proportion to B0 to the power 7/4. For modern magic-angle spinning (MAS) probes, this approximation holds for rotor sizes up to 3.2 mm at 14.1 Tesla (T), corresponding to 600 MHz 1H and 151 MHz 13C Larmor frequencies. To obtain the anticipated benefit of larger coils and/or higher B0 fields requires a quantitative understanding of the contributions to SNR, utilizing standard samples and protocols that reproduce SNR measurements with high accuracy and precision. Here, we present such a systematic and comprehensive study of 13C SNR under MAS over the range of 14.1 to 21.1 T. We evaluate a range of probe designs utilizing 1.6, 2.5 and 3.2 mm rotors, including 24 different sets of measurements on 17 probe configurations using five spectrometers. We utilize N-acetyl valine as the primary standard and compare and contrast with other commonly used standard samples (adamantane, glycine, hexamethylbenzene, and 3-methylglutaric acid). These robust approaches and standard operating procedures provide an improved understanding of the contributions from probe efficiency, receiver noise figure, and B0 dependence in a range of custom-designed and commercially available probes. We find that the optimal raw SNR is obtained with balanced 3.2 mm design at 17.6 T, that the best mass-limited SNR is achieved with a balanced 1.6 mm design at 21.1 T, and that the raw SNR at 21.1 T reaches diminishing returns with rotors larger than 2.5 mm.

Quantification of Residual Water in Pharmaceutical Frozen Solutions Via 1H Solid-State NMR

Freezing is essential for the stability of biological drug substances and products, particularly in frozen solution formulations and during the primary drying of lyophilized preparations. However, the unfrozen segment within the frozen matrix can alter solute concentration, ionic strength, and stabilizer crystallization, posing risks of increased biophysical instability and faster chemical degradation. While quantifying the unfrozen water content is important for designing stable biopharmaceuticals, there is a lack of analytical techniques for in situ quantitative measurements. In this study, we introduce a 1H magic angle spinning NMR technique to identify the freezing point (Tice) and quantify mobile water content in frozen biologics, applying this method to analyze the freezing of a commercial high-concentration drug product, Dupixent®. Our results demonstrate that water freezing is influenced by buffer salt properties and formulation composition, including the presence of sugar cryoprotectants and protein concentration. Additionally, the 1H chemical shift can probe pH in the unfrozen phase, potentially predicting the microenvironmental acidity in the frozen state. Our proposed methodology provides fresh insights into the analysis of freeze-concentrated solutions, enhancing our understanding of the stability of frozen and lyophilized biopharmaceuticals.

Mechanisms of the Accelerated Li+ Conduction in MOF-Based Solid-State Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) for lithium metal batteries have garnered considerable interests owing to their low cost, flexibility, lightweight, and favorable interfacial compatibility with battery electrodes. Their soft mechanical nature compared to solid inorganic electrolytes give them a large advantage to be used in low pressure solid-state lithium metal batteries, which can avoid the cost and weight of the pressure cages. However, the application of SPEs is hindered by their relatively low ionic conductivity. In addressing this limitation, enormous efforts are devoted to the experimental investigation and theoretical calculations/simulation of new polymer classes. Recently, metal-organic frameworks (MOFs) have been shown to be effective in enhancing ion transport in SPEs. However, the mechanisms in enhancing Li+ conductivity have rarely been systematically and comprehensively analyzed. Therefore, this review provides an in-depth summary of the mechanisms of MOF-enhanced Li+ transport in MOF-based solid polymer electrolytes (MSPEs) in terms of polymer, MOF, MOF/polymer interface, and solid electrolyte interface aspects, respectively. Moreover, the understanding of Li+ conduction mechanisms through employing advanced characterization tools, theoretical calculations, and simulations are also reviewed in this review. Finally, the main challenges in developing MSPEs are deeply analyzed and the corresponding future research directions are also proposed.

Characterizing the Behavior of Water Interacting with a Nano-Pore Material: A Structural Investigation in Native Environment Using Magnetic Resonance Approaches

The study of fluid absorption, particularly that of water, into nanoporous materials has garnered increasing attention in the last decades across a broad range of disciplines. However, most investigation approaches to probe such behaviors are limited by characterization conditions and may lead to misinterpretations. In this study, a combined MRI and MAS NMR method was used to study a nanoporous silica glass to acquire information about its structural framework and interactions with confined water in a native humid environment. Specifically, MRI was used for a quantitative analysis of water extent. While MAS NMR techniques provided structural information of silicate materials, including interactive surface area and framework packing. Analysis of water spin-spin relaxation times (T2) suggested differences in water confinement within the characterized framework. Subsequent unsuccessful delivery of paramagnetic molecule into the pores enabled a quantitative assessment of the dimensions that "bottleneck" the pores. Finally, pore sizes were derived from the paramagnetic molecular size, density function theory (DFT) simulation and characterizations on standard samples. Our result matches with Brunauer-Emmett-Teller (BET) analysis that the pore size is less than 1.3 nm. The use of a paramagnetic probe for pore size determination introduces a new approach of characterization in the liquid phase, offering an alternative to the conventional BET analysis that uses gas molecule as probes.

Light-Induced 1H NMR Hyperpolarization in Solids at 9.4 and 21.1 T

The inherently low sensitivity of nuclear magnetic resonance (NMR) spectroscopy is the major limiting factor for its application to elucidate structure and dynamics in solids. In the solid state, nuclear spin hyperpolarization methods based on microwave-induced dynamic nuclear polarization (DNP) provide a versatile platform to enhance the bulk NMR signal of many different sample formulations, leading to significant sensitivity improvements. Here we show that 1H NMR hyperpolarization can also be generated in solids at high magnetic fields by optical irradiation of the sample. We achieved this by exploiting a donor-chromophore-acceptor molecule with an excited state electron-electron interaction similar to the nuclear Larmor frequency, enabling solid-state 1H photochemically induced DNP (photo-CIDNP) at high magnetic fields. Through hyperpolarization relay, we obtained bulk NMR signal enhancements εH by factors of ∼100 at both 9.4 and 21.1 T for the 1H signal of o-terphenyl in magic angle spinning (MAS) NMR experiments at 100 K. These findings open a pathway toward a general light-induced hyperpolarization approach for dye-sensitized high-field NMR in solids.

Quantifying Organic Cation Ratios in Metal Halide Perovskites: Insights from X-ray Photoelectron Spectroscopy and Nuclear Magnetic Resonance Spectroscopy

The employment of metal halide perovskites (MHPs) in various optoelectronic applications requires the preparation of thin films whose composition plays a crucial role. Yet, the composition of the MHP films is rarely reported in the literature, partly because quantifying the actual organic cation composition cannot be done with conventional characterization methods. For MHPs, NMR has gained popularity, but for films, tedious processes like scratching several films are needed. Here, we use mechanochemical synthesis of MA1-x FA x PbI3 powders with various MA+: FA+ ratios and combine solid-state NMR spectroscopy (ssNMR) and X-ray photoelectron spectroscopy (XPS) to provide a reference characterization protocol for the organic cations' quantification in either powder form or films. Following this, we demonstrate that organic cation ratio quantification on thin films with ssNMR can be done without scraping the film and using significantly less mass than typically needed, that is, employing a single ∼800 nm-thick MA1-x FA x PbI3 film deposited by pulsed laser deposition (PLD) onto a 1 × 1 in.2, 0.2 mm-thick quartz substrate. While background signals from the quartz substrate appear in the 1H ssNMR spectra, the MA+ and FA+ signals are easily distinguishable and can be quantified. This study highlights the importance of calibrating and quantifying the source and the thin film organic cation ratio, as key for future optimization and scalability of physical vapor deposition processes.

Overhauser enhanced liquid state nuclear magnetic resonance spectroscopy in one and two dimensions

Nuclear magnetic resonance (NMR) is fundamental in the natural sciences, from chemical analysis and structural biology, to medicine and physics. Despite its enormous achievements, one of its most severe limitations is the low sensitivity, which arises from the small population difference of nuclear spin states. Methods such as dissolution dynamic nuclear polarization and parahydrogen induced hyperpolarization can enhance the NMR signal by several orders of magnitude, however, their intrinsic limitations render multidimensional hyperpolarized liquid-state NMR a challenge. Here, we report an instrumental design for 9.4 Tesla liquid-state dynamic nuclear polarization that enabled enhanced high-resolution NMR spectra in one and two-dimensions for small molecules, including drugs and metabolites. Achieved enhancements of up to two orders of magnitude translate to signal acquisition gains up to a factor of 10,000. We show that hyperpolarization can be transferred between nuclei, allowing DNP-enhanced two-dimensional 13C-13C correlation experiments at 13C natural abundance. The enhanced sensitivity opens up perspectives for structural determination of natural products or characterization of drugs, available in small quantities. The results provide a starting point for a broader implementation of DNP in liquid-state NMR.

A Novel Method of Self-Cross-Linking of Syringaldehyde with Activated Methoxy Groups via Cross-Coupling for Lignin-Based Wood Adhesives

As steric hindrance, methoxy groups are limiting the valorization of hardwood lignin. This paper reports a novel method of self-cross-linking of the syringaldehyde with activated methoxy groups (-OCH3) via cross-coupling reaction to obtain thermosetting polymers for lignin-based wood adhesives. The methoxy groups of syringaldehyde have been activated via cross-coupling reaction by substituting Ar-OCH3 with Ar-CH2-SiMe3, and dichloromethane, leading to cross-linking via methylene bridges to build a thermosetting polymer. FTIR spectra showed a decrease in the intensity of a -CH3 and -OH group, owing to the substitution of the methoxy group. 13C NMR spectra also supported these results with the -SiMe3 signal that disappeared after the cross-linking reaction. Furthermore, cross-linking between the activated methoxy groups was confirmed with a strong exothermic peak at 130 °C, resulting in an increase in the adhesion strength as hot-pressing temperature increased from 160 to 180 °C. These results suggest that the cross-linking between the activated methoxy groups of syringaldehyde is an important understanding of valorizing hardwood lignin via building thermosetting polymers for lignin-based adhesives.