Nuclear Magnetic Resonance Investigation Research Articles

Imbibition models quantifying interfacial interactions: Based on nuclear magnetic resonance investigation and coupled structural characteristics

An investigation into spontaneous imbibition in porous media is of paramount scientific significance in various projects. However, a precise understanding of the interaction mechanisms between media structural characteristics and imbibition remains elusive, and quantitative analysis of the interfacial interaction is lacking. Therefore, to mitigate the influence of dispersion, this study first investigates cyclic imbibition experiments of coal samples to explore the interaction mechanism between pore-fracture structure (PFS) and imbibition. Nuclear magnetic resonance is used to visualize water transport during imbibition across all scales. Subsequently, the slake durability index is suggested to clarify the coupling relationship between water–coal interactions and imbibition. Two more comprehensive and accurate imbibition models are established, based on pore size and comprehensive seepage parameters, respectively. The results demonstrate that both new models exhibit superior conformity with experimental data compared to traditional models. The memory factor quantifies interface interaction within these models. Sensitivity analysis reveals that strong interface interaction diminishes the effective imbibition ratio, while the structural characteristics of porous media significantly influence the interaction. Furthermore, the fractal dimension quantitatively characterizes the PFS features of coal samples. An exploration of the relationship between fractal dimension and memory factor indicates the influence of porous media heterogeneity on imbibition.

Recent advances in liquid-phase NMR of the coal-derived products.

Present review focuses on the most recent advances in a liquid-phase nuclear magnetic resonance (NMR) of the coal-derived products-coal tar pitches, asphaltenes, and humic and fulvic acids, covering exclusively the results in the liquid-phase NMR studies leaving apart an overwhelming amount of publications dealing with the solid-state NMR investigations in this field (which are comprehensively reviewed elsewhere). Owing to the complexity of the coal-derived products, their 1H and 13C NMR spectra consist of a number of overlapping signals belonging to different hydrocarbon types. Comprehensive studies of coal tar pitches, asphaltenes, and humic and fulvic acids by means of NMR over the past several decades revealed characteristic functional groups of those fractions together with spectral regions in which they resonate. Quantitative 1H and 13C NMR spectra characterize aromatic and saturated carbons spread over many structural moieties, which provides a solid guideline into molecular structure of the coal-derived products. Nowadays, quantitative 13C NMR measurements yield information about a variety of structural parameters such as functional group distribution, aromaticity, degree of condensation of aromatic rings, and medium chain lengths together with many other more specific parameters. The structural NMR studies of coal and coal-derived products are developing on a backdrop of a marked progress in computational NMR. At present, we are witnessing an unprecedentedly fast development of theoretical and computational methods in the field of NMR spectroscopy. Discussed in the present review are the most recent advances in the NMR studies of the processing products of peat, lignite or brown coal, anthracite or hard coal, and graphite in solution, like coal tar pitches, asphaltenes, and humic and fulvic acids.

Nuclear magnetic resonance investigation of superconducting and normal state Nb3Sn

The superconductor Nb3Sn has important applications for construction of very high-field superconducting magnets. In this work we investigate its microscopic electronic structure with 93Nb nuclear magnetic resonance (NMR). The high-quality Nb3Sn powder sample was studied in both 3.2 T and 7 T magnetic fields in the temperature range from 4 K to 300 K. From measurement of the spectrum and its theoretical analysis, we find evidence for anisotropy despite its cubic crystal structure. Magnetic alignment of the powder grains in the superconducting state was also observed. The Knight shift and spin-lattice relaxation rate, T1−1 , were measured and the latter compared with BCS theory for the energy gap Δ(0)=2.7±0.3kBTc at 3.2 T and Δ(0)=2.33±0.07kBTc at 7 T, indicating suppression of the order parameter by magnetic field.

A chiral fluorescent probe for molecular recognition of basic amino acids in solutions and cells

A novel and readily available binaphthyl-based fluorescent probe (S)-1 was designed and synthesized. (S)-1 can be used to not only chemoselectively discriminate 3 basic amino acids out of common amino acids, but also enantioselectively recognize histidine. Encouragingly, enantioselective imaging of histidine in cells was achieved for the first time by the probe (S)-1. These performances endowed it potential application in the chiral analysis of basic amino acids in asymmetric synthesis and cell imaging for diagnosis of diseases caused by racemization of histidine. Nuclear magnetic resonance (NMR) and mass spectrometry investigations suggested that different reaction extent of (S)-1 with l/d-histidine and different product structures generated the observed enantioselective fluorescent response. The molecular structures and thermodynamic stability of the complexes, formed from (S)-1 + Zn2+ and enantiomers of histidine, were calculated by Gaussian 16 based on density functional theory (DFT) to validate the above action mechanism.

Fractal and Multifractal Characteristics on Pore Structure of Coal-Based Sedimentary Rocks Using Nuclear Magnetic Resonance

Summary Unconventional reservoirs have nanoscale pores, complex pore structures, and heterogeneity that directly affect reservoir storage performance and fluid transport capacity. In this study, shale, mudstone, and sandstone, three typical coal sedimentary rocks from the Daqiang coal mine in the Tifa Basin, were selected for nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) investigation, with the aim to investigate the pore structure and multifractal characteristics of the coal sedimentary reservoirs and to qualitatively analyze the effects of the physical property parameters and the mineralogical compositions on the multifractal parameters. The distribution data of the NMR T2 spectra were analyzed. The results showed that (1) SEM analysis concluded that the pore system of the three different lithological samples (mudstone, shale, and sandstone) was dominated by mineral matrix pores (i.e., intergranular and intragranular pores) and in the sandstone samples, there were only a few biological pores found. (2) The distribution of the NMR T2 spectrum peaks indicates that the sandstone and shale T2 spectra are bimodal, dominated by micropores, and contain a small number of transitional pores; most of the T2 spectra of mudstone are single peaks in distribution, mainly dominated by micropores. (3) Multifractal parameters are positively correlated with porosity and significantly negatively correlated with permeability; multifractal parameters are significantly positively correlated with the content of clay minerals and kaolinite, which suggests that the increase in clay minerals and kaolinite content enhances the heterogeneity of the pore space. The negative correlation with the content of quartz suggests that the enrichment of quartz reduces the irregularity of the pore space.

Comprehensive NMR Investigation of Imidazolium-Based Ionic Liquids [BMIM][OSU] and [BMIM][Cl] Impact on Binding and Dynamics of the Anticancer Drug Doxorubicin Hydrochloride.

For the design of an efficient drug delivery system utilizing an ionic liquid (IL) as a carrier, it is prudent to gain molecular/atomistic level insights of a drug with IL in terms of binding and dynamics. In this scenario, the influence of anionic counterpart of imidazolium-based ILs, namely, 1-butyl-3-methyl-imidazolium octyl sulfate [BMIM][OSU] and 1-butyl-3-methyl-imidazolium chloride [BMIM][Cl] in their submicellar region ([IL] = 20 mM) on the model water-soluble anticancer drug doxorubicin hydrochloride (DOX) was probed by employing an arsenal of nuclear magnetic resonance (NMR) approaches. The salient feature of the present study includes the significant interaction of DOX with [BMIM][OSU], whereas the lack of such an interaction with [BMIM][Cl] is gauged by 1H NMR translation self-diffusometry and is further corroborated by 13C chemical shift perturbation. The two-step model was utilized to estimate the bound fraction (pb) and equivalent partition coefficient (K) of DOX with [BMIM][OSU]. A combination of selective and nonselective spin-lattice relaxation rates (R1SEL and R1NS, respectively) enables to gauze the significant interaction of DOX with [BMIM][OSU] over [BMIM][Cl]. Furthermore, 1D transient and truncated driven nuclear Overhauser enhancement (NOE) data analyses in the initial rate limit permits the evaluation of the cross-relaxation efficacy of DOX with the investigated ILs. An Arrhenius-type temperature dependence of the drug's self-diffusion was observed for DOX, DOX-[BMIM][OSU], and DOX-[BMIM][Cl] aqueous mixtures and the corresponding activation energies were evaluated.

Quantitative Operando 7Li NMR Investigations of Silicon Anode Evolution during Fast Charging and Extended Cycling.

The development and optimization of fast battery charging protocols require detailed information regarding lithium speciation inside a battery. Nuclear magnetic resonance (NMR) spectroscopy has the unique capability of identifying the Li phases formed in an anode during Li-ion cell operation and quantifying their relative amounts. In addition, both Li metal films and dendrites are readily detected and quantified. Here, our recently reported parallel-plate resonator radio frequency (RF) probe and the cartridge-type single-layer full cell were used to track the behavior of Si electrodes during cycling and during fast charging. The LixSi compounds formed during electrochemical cycling exhibit an unexpected intrinsic nonequilibrium behavior at both slow and fast rates, evolving toward increasingly disordered local environments. The evolution with time of lithiated phases is nonlinear during both charging and discharging at constant current, unlike the case for pure graphite, and asymmetric between charge and discharge. During charging at rates of 1C, 2C, and 3C, metallic Li in both films and (to a lesser extent) dendritic forms are deposited on the Si anode. Part of the Li metal film formation is reversible, but a fraction remains on the electrode surface as dead Li, while all of the dendritic Li, even though formed in a considerably smaller amount, is entirely irreversible. Such performance-governing properties are critical to the development of fast-charging protocols for lithium-ion batteries (LIBs) and are exceptionally well evaluated and quantified by 7Li magnetic resonance strategies such as those presented here.

Role of imperfect coordination of amorphous metal oxide in its chemical reaction with epoxy resin in metal − polymer hybrid material

The chemical bonding between laser − induced metal oxide and polymer highly determines the interfacial bonding performance of metal − polymer hybrid materials. To achieve reliable interfacial chemical bonding, it is crucial to gain a molecular/atomic level understanding of the chemical reaction mechanism between the metal oxide and epoxy resin (main components of epoxy polymer). Previous research has demonstrated that laser ablation induces the formation of amorphous aluminium oxide on aluminium alloy surfaces. Our solid − state nuclear magnetic resonance (ssNMR) investigation reveals that this amorphous aluminium oxide is consisted mainly of 4 − fold and 5 − fold coordinated Al sites. Through an innovative combination of ssNMR and density functional theory calculations, this aluminium oxide was confirmed to form Al − O − C bonds with epoxy resin, preferentially at imperfectly coordinated Al sites due to active donor − acceptor interactions. It is clarified that imperfect coordination of amorphous aluminium oxide plays a more critical role than its amorphous nature in forming stable adsorption structure with epoxy resin. Furthermore, compared to on crystalline surface, 5 − fold coordinated Al site on amorphous surface exhibits higher reactivity, and the origin of activity is revealed through distortion/interaction analysis. This study provides a research guidance for further regulation and optimization of interfacial reactivity and bonding characteristics of metal − polymer hybrid materials.

Low-Field Nuclear Magnetic Resonance Investigation on Early Hydration Characterization of Cement Paste Mixed with Mineral Admixtures

Mineral admixtures (MA), like fly ash (FA), silica fume (SF), and slag (S), are usually added to cement-based materials to improve their compactness and further enhance their mechanical properties, permeability resistance, and durability. In this study, low-field nuclear magnetic resonance (LF-NMR) is adopted to explore the evolution of the early hydration characterization of cement-based materials with MA by testing the transverse relaxation time T2. Meanwhile, the effect of MA on mechanical properties is analyzed by measuring compressive and flexural strength. The results show that, in the early hydration (0–7 days), the T2 distribution shows a trend of gradually moving to a short relaxation time and changes from a double peak to a single main peak. The decrease in T2i (main peak vertex) means that the evaporated water is gradually distributed in smaller pores with more motion constraints. However, the type and content of MA have little effect on T2i. Porosity gradually decreases in the period of early hydration. The addition of MA causes the porosity to decrease, and the order influence is FA > S > SF, i.e., the porosities of cement paste with 0%MA, 10%FA + 10%SF, 10%FA + 10%S, and 20%FA at 7 days are 48%, 44.5%, 40.7%, and 40.2%, respectively. Additionally, the addition of MA to cement-based materials also decreases the early strength, and the influence order is FA > S > SF, i.e., the compression strength values of cement paste with 0%MA, 10%FA + 10%SF, 10%FA + 10%S, and 20%FA at 7 days are 47.8 MPa, 40.1 MPa, 38.6 MPa, and 37 MPa, respectively.

Atomic Layer Deposition of the Geometry Separated Lewis and Brønsted Acid Sites for Cascade Glucose Conversion.

Solid acid catalysts with bi-acidity are promising as workhouse catalysts in biorefining to produce high-quality chemicals and fuels. Herein, we report a new strategy to develop bi-acidic cascade catalysts by separating both acid sites in geometry via the atomic layer deposition (ALD) of Lewis acidic alumina on Brønsted acidic supports. Visualized by transmission electron microscopy and electron energy loss spectroscopy mapping, the ALD-deposited alumina forms a conformal alumina domain with a thickness of around 3 nm on the outermost surface of mesoporous silica-alumina. Solid state nuclear magnetic resonance investigation shows that the dominant Lewis acid sites distribute on the outermost surface, whereas intrinsic Brønsted acid sites locate inside the nanopores within the silica-rich substrate. In comparison to other bi-acidic solid catalyst counterparts, the special geometric distance of Lewis and Brønsted acid sites minimized the synergetic effect, leading to a cascade reaction environment. For cascade glucose conversion, the designed ALD catalyst showed a highly enhanced catalytic performance.

(Invited) Investigating Polymorphism in Na2Fe2F7 and Its Effect on Electrochemical Properties

With the present materials supply crunch caused by the burgeoning Li-ion battery-based electric vehicle market, more sustainable battery chemistries are needed. When considering possible chemistries, sodium (Na) and iron (Fe) stand out as promising candidates for the intercalant ions and redox centers, respectively. Recently, weberite-type sodium metal fluorides (Na2 M 2+ Mʹ 3+F7) have emerged as potential high performance Na cathode materials1-4 with predicted energy densities in the 600 to 800 Wh/kg range (higher than any known Na cathode material) and high ionic conductivity and structural stability. One of the few weberites that has been electrochemically tested so far is Na2Fe2F7 and while one study has shown it to have great, high-rate performance,2 there have been inconsistent reports regarding the structure and electrochemical properties of Na2Fe2F7.In this study, we endeavor to reconcile these characteristics using a combined experimental-computational approach. Density functional theory calculations reveal the inherent metastability of the weberite phase and the close energetics of several Na2Fe2F7 polymorphic forms, as well as the predicted (de)intercalation behavior for each Na2Fe2F7 polymorph. Experimentally, we find that Na2Fe2F7 samples inevitably contain a mixture of weberite polymorphs as characterized using a broad suite of long-range and local probes sensitive to both amorphous and crystalline phases. Notably, we present the first ever 23Na nuclear magnetic resonance (NMR) investigation into weberites and showcase NMR’s ability to parse out different local environments despite the presence of polymorphism. Electrochemically, our Na2Fe2F7 material shows respectable early capacities yet exhibits steady capacity fade, which ex situ synchrotron X-ray diffraction and 23Na NMR reveal to be associated with transformation of the weberite phase upon cycling. However, fine tuning of electrode composition and design appears to be effective at reducing this effect.Overall, this study presents an in-depth characterization of Na2Fe2F7 and provides key insights that pave the way to unlocking weberites as high energy density Na-ion cathode materials.This work made use of the shared facilities of the UC Santa Barbara MRSEC (DMR 1720256) and Center for Scientific Computing (CNS 1725797, DMR 1720256). E. E. Foley and V. C. Wu were supported by the NSF Graduate Research Fellowship Program under Grant No. DGE 1650114. This work is supported by an NSF CAREER award under Grant No. DMR 2141754. Euchner, O. Clemens, and M.A. Reddy, Npt Comput. Mater. 5 31 (2019).Park, Y. Lee, M.-K. Cho, J. Kang, W. Ko, Y.H. Jung, T.-Y. Jeon, J. Hong, H. Kim, S.-T. Myung, and J. Kim, Energy Environ. Sci. 14 1469-1479 (2021).K. Dey, N. Barman, S. Ghosh, S. Sarkar, S.C. Peter, P. Senguttuvan, Chem. Mater. 31 295-299 (2019).Kang, J. Ahn, H. Park, W. Ko, Y. Lee, S. Lee, S. Lee, S.-K. Jung, J. Kim, J. Kang, J. Ahn, H. Park, W. Ko, Y. Lee, S. Lee, J. Kim and S.-K. Jung, Adv. Funct. Mater. 32 2201816 (2022). Fig. 1 , Charge- discharge curve for Na2Fe2F7 and the weberite structure. Figure 1

Chain Dynamics in a Polyelectrolyte Solution Under Shear: A Rheological NMR Investigation

Rheological nuclear magnetic resonance (NMR) has been applied to study the effect of shear on the chain dynamics in a solution of a linear poly(sodium 4-styrenesulfonate). Information on the chain dynamics of poly(sodium 4-styrenesulfonate) sheared in a narrow-gap concentric double cylinder cell is inferred from the NMR transverse relaxation T2 for a concentration in the semidilute-entangled and in the concentrated regime. In the semidilute-entangled concentration enhanced relaxation is observed. In the concentrated regime signatures of a loss of entanglements are seen with increasing T2 and a decreasing fraction of the polymer showing restricted motion. The effect is enhanced when the electrostatic interaction along the polymer chain is reduced by the addition of NaCl.

Site-Specific Introduction of Alanines for the Nuclear Magnetic Resonance Investigation of Low-Complexity Regions and Large Biomolecular Assemblies.

Nuclear magnetic resonance (NMR) studies of large biomolecular machines and highly repetitive proteins remain challenging due to the difficulty of assigning frequencies to individual nuclei. Here, we present an efficient strategy to address this challenge by engineering a Pyrococcus horikoshii tRNA/alanyl-tRNA synthetase pair that enables the incorporation of up to three isotopically labeled alanine residues in a site-specific manner using in vitro protein expression. The general applicability of this approach for NMR assignment has been demonstrated by introducing isotopically labeled alanines into four distinct proteins: huntingtin exon-1, HMA8 ATPase, the 300 kDa molecular chaperone ClpP, and the alanine-rich Phox2B transcription factor. For large protein assemblies, our labeling approach enabled unambiguous assignments while avoiding potential artifacts induced by site-specific mutations. When applied to Phox2B, which contains two poly-alanine tracts of nine and twenty alanines, we observed that the helical stability is strongly dependent on the homorepeat length. The capacity to selectively introduce alanines with distinct labeling patterns is a powerful tool to probe structure and dynamics of challenging biomolecular systems.

Planar Waveguide NMR Gradients (B0 and B1) for spatiotemporal Investigations of Anticancer Drug Concentration in 3D Cancer Cell Cultures

Despite the progression in cancer therapeutic approaches, cancer is still the leading cause of death worldwide with a steadily increasing incidence rate. In Jordan, CRC is the most common type of cancer among men and the second most common among women, while being one of the most accruing cancers in Germany.
 The difficulties in cancer treatments arise from the poor correlation between in vivo and in in vitro anti-cancer drug testing results and cancer cells resistance to chemotherapy, which are attributed to the heterogeneity of cancer cells, and the interaction of cancer cells with non-cancerous cells in the tumor microenvironment (TME). In addition to, the several mechanisms, that governs DNA damages repair, epigenetics, epithelial to mesenchymal transition (EMT).
 Cell culture techniques have been developed to construct in vitro multicellular cancer spheroids (MTCS) that mimic cell-cells interaction and the cellular organization in the TME. They also illustrate the proliferation gradient of solid tumors, and the nutrition and gas supply to cancer cells at different depth in the spheroid. Therefore, they are considered an excellent in vitro cancer model to study the efficiency and to investigate the penetration depth of the anti-cancer materials.
 Nuclear Magnetic Resonance (NMR) spectroscopy is a reliable analytical method that gives comprehensive and rich chemical information, along with high speciation performance. It is one of the major noninvasive tools for metabolomics, therefore it has been utilized to study the metabolic profiling of freshly isolated- non-destructed tissues, and living in vitro cellular models.
 A micro-imaging technique for real-time NMR investigations is developed to study the penetration depth, rate, and effective diffusion coefficients of anti-cancer materials through cancer spheroids. This technique is based on our patented invention of planar waveguide micro slot NMR detector with on board thermal regulator integrated and a microfluidic device. Based on this innovative development, we are able to perform real-time magnetic resonance spectroscopic imaging (MRSI) on a living synthetic tissue constructed in micrometer scale to monitor drug penetration and diffusion, while providing spectroscopic information about the production and degradation of metabolites in picomoles concentrations.

Composite anion exchange membranes based on polysulfone and silica nanoscale ionic materials for water electrolyzers

Anion exchange membrane water electrolysis (AEMWE) offers the ambition of combining the advantages of alkaline electrolysers, i.e. the use of cheap and plentiful catalytic materials, with those of proton exchange membrane electrolysis of water, i.e. high performance and fast response to changing operating conditions. However, the development of performing and durable anion exchange membranes is still the major challenge for the ultimate industrial adoption of AEMWE. Here, we introduce an innovative nanocomposite AEM based on trimethylammonium functionalized silica nanoscale ionic materials (NIM-N+) incorporated in quaternized polysulfone (qPSU). The presence of NIM-N+ in the hydrophilic clusters of qPSU produces a remarkable enhancement in the dimensional and thermo-mechanical stability of the composite AEM. Furthermore, Nuclear Magnetic Resonance (NMR) and Electrochemical Impedance Spectroscopy (EIS) investigations highlighted that the nanoparticles are directly involved in the transport process of hydroxide ions. This enabled qPSU/NIM-N+ composite membrane to achieve impressive anionic conductivity values, e.g. about 110 mS cm−1 at 80 °C and 95% relative humidity (RH). The AEMWE single cell, equipped with this membrane, operated properly both with 1 or 0.5 M KOH solution, displaying a current density larger than 3.5 A cm−2 at 2.2 V and 80 °C.

13C Nuclear Magnetic Resonance Investigations of Molecular Mesogens with a Terminal Phenyl Ring Linked via a Spacer: Odd-Even Effect.

A set of mesogens considered as model molecules to the technologically important twist-bend (NTB) nematogens are investigated. They consist of a three-ring core connected to a phenyl ring via a flexible spacer and displayed enantiotropic nematic and smectic C mesophases. In such systems, odd or even number of atoms present in the spacer could influence the orientation of the terminal phenyl ring and thus have a bearing on designing the NTB phase, considered as intermediate between the nematic and the cholesteric phases. One-dimensional (1D) and two-dimensional (2D) 13C NMR spectra have been recorded in the liquid crystalline phases and the alignment-induced chemical shifts (AIS) and the 13C-1H dipolar couplings obtained. The order parameters of the phenyl rings reveal features relatable to the odd/even number of atoms of the flexible spacer and the type of linkage. The AIS plots of the phenyl rings of the even spacer-based mesogen showed the usual behavior for all of the phenyl rings with a decrease in AIS with increasing temperature. However, for the odd-spacer mesogens, unusual behaviors are noted for the terminal phenyl ring. Thus, two of the mesogens showed an increase of AIS in the smectic C phase that continued till the middle of the nematic phase temperature range, followed by a decrease. The other two odd-spacer mesogens also showed different behaviors. These observations indicate that the terminal phenyl ring is oriented at an angle with respect to the long molecular axis for the odd-spacer mesogens that changes as a function of temperature. The angles have been found to depend on the nature of the atom/group connecting the spacer to the terminal ring and the spacer length. Thus, the present study provides critical information on the design of the odd dimers that are recognized to generate fascinating NTB mesophases.

Diruthenium Catalyst for Hydrogen Production from Aqueous Formic Acid

Diruthenium complexes [{(η6-arene)RuCl}2(μ-κ2:κ2-benztetraimd)]2+ containing the bridging bis-imidazole methane-based ligand {1,4-bis(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)benzene} (benztetraimd) are synthesized for catalytic formic acid dehydrogenation in water at 90 °C. Catalyst [{(η6-p-cymene)RuCl}2(μ-κ2:κ2-benztetraimd)]2+ [1-Cl2] exhibited a remarkably high turnover frequency (1993 h-1 per Ru atom) and long-term stability over 60 days for formic acid dehydrogenation, while the analogous (η6-benzene)diruthenium and mononuclear catalysts displayed low activity with poor long-term stability. Notably, catalyst [1-Cl2] also displayed an appreciably high turnover number of 93 200 for the bulk-scale reaction. In addition, the in-depth mass and nuclear magnetic resonance investigations under the catalytic and control experimental conditions revealed the active involvement of several crucial catalytic intermediate species, such as Ru-aqua species [{(η6-p-cymene)Ru(H2O)}2(μ-L)]2+ [1-(OH2)2], Ru-formato species [{(η6-p-cymene)Ru(HCOO)}2(μ-L)] [1-(HCOO)2], and Ru-hydrido species [{(η6-p-cymene)Ru(H)}2(μ-L)] [1-(H)2], in the catalytic formic acid dehydrogenation reaction.

A critical mini-review on the low-field nuclear magnetic resonance investigation of pore coupling effects in near-surface environments

The assessment and monitoring of groundwater resources is of increasing importance to ensure the continuous supply of fresh water for human activity and endangered ecosystems. These groundwater resources include fully saturated aquifers, water in unsaturated soil, and water trapped as rock moisture in weathered bedrocks. Low-field nuclear magnetic resonance (NMR) is a method with unique sensitivity to pore water, as it is based on the magnetization and relaxation behavior of the spin magnetic moment of hydrogen atoms forming water molecules. It is a cost-effective and minimally-invasive technology that can help characterize the pore structures and the groundwater distribution and transport in different types of subsurface materials. However, the interpretation of NMR data from samples with complex bimodal or multimodal porous geometries requires the consideration of pore coupling effects. A pore-coupled system presents significant magnetization exchange between macro- and micropores within the measurement time, making the independent characterization of each pore environment difficult. Developing a better understanding of pore coupling is of great importance for the accurate estimation of hydrogeological parameters from NMR data. This mini-review presents the state-of-art in research exploring the two factors controlling pore coupling: surface geochemistry and network connectivity, summarizes existing experimental and numerical modeling approaches that have been used to study pore coupling and discusses the pore coupling effects in fully and partially saturated conditions. At the end of this review, we outline major knowledge gaps and highlight the research needs in the vadose zone.

Solvent-derived defects suppress adsorption in MOF-74

Defects in metal-organic frameworks (MOFs) have great impact on their nano-scale structure and physiochemical properties. However, isolated defects are easily concealed when the frameworks are interrogated by typical characterization methods. In this work, we unveil the presence of solvent-derived formate defects in MOF-74, an important class of MOFs with open metal sites. With multi-dimensional solid-state nuclear magnetic resonance (NMR) investigations, we uncover the ligand substitution role of formate and its chemical origin from decomposed N,N-dimethylformamide (DMF) solvent. The placement and coordination structure of formate defects are determined by 13C NMR and density functional theory (DFT) calculations. The extra metal-oxygen bonds with formates partially eliminate open metal sites and lead to a quantitative decrease of N2 and CO2 adsorption with respect to the defect concentration. In-situ NMR analysis and molecular simulations of CO2 dynamics elaborate the adsorption mechanisms in defective MOF-74. Our study establishes comprehensive strategies to search, elucidate and manipulate defects in MOFs.

Enhanced and suppressed phosphorus mineralization by Ca complexation: NMR and CD spectroscopy investigation

With the increasing demand for P fertilizer for world food production, the use of soil organic P fraction via mineralization could become an important P resource in agricultural soils. However, the predominant organic P species, phytic acid, has been considered rather recalcitrant to mineralization due to its active interaction with dissolved metals like Ca2+ in soil pore water. Calcium ions can be an inhibitor to many phytases, yet the mechanism was not clear. The objective of this study was to understand the effects of Ca2+(aq) on the phytase activity and inhibitory mechanisms using batch degradation kinetic experiments, Nuclear Magnetic Resonance (NMR) spectroscopy, Saturation Transfer Difference (STD) NMR, and Circular dichroism (CD) spectroscopy. The phytase activity followed Michaelis-Menten kinetics and increased Michaelis constant Km and decreased Vmax with Ca2+ addition were observed at pH 6. Therefore, mixed inhibition was the inhibition mechanism which was likely a result of the allosteric effect of Ca2+. The near-UV CD spectra supported phytase secondary conformational change upon the interaction between Ca2+ and the enzyme. It was found that phytase initially reacted with the D/L-3 phosphate of phytic acid at pH 6. At pH 8, the overall phytase activity decreased, yet the effect of Ca2+ on phytase activity was the opposite of that of pH 6. Enhanced phytase activity with Ca2+ addition was attributed to the structural change of phytic acid upon the Ca2+ complexation, which was confirmed by NOE spectra. The Ca2+-phytic acid complex might be a more favorable substrate than the free phytic acid. Unlike the findings from pH 6, Ca2+ didn't induce significant changes in either the near- or far-UV region of the CD spectra at pH 8. Furthermore, P5 was found to be the target of phytase at pH 8. The study revealed the pH-specific effects of Ca2+ on the mineralization of phytic acid