Correlation Nuclear Magnetic Resonance Spectroscopy Research Articles

Characterization of elusive rhamnosyl dioxanium ions and their application in complex oligosaccharide synthesis

Attaining complete anomeric control is still one of the biggest challenges in carbohydrate chemistry. Glycosyl cations such as oxocarbenium and dioxanium ions are key intermediates of glycosylation reactions. Characterizing these highly-reactive intermediates and understanding their glycosylation mechanisms are essential to the stereoselective synthesis of complex carbohydrates. Although C-2 acyl neighbouring-group participation has been well-studied, the reactive intermediates in more remote participation remain elusive and are challenging to study. Herein, we report a workflow that is utilized to characterize rhamnosyl 1,3-bridged dioxanium ions derived from C-3 p-anisoyl esterified donors. First, we use a combination of quantum-chemical calculations and infrared ion spectroscopy to determine the structure of the cationic glycosylation intermediate in the gas-phase. In addition, we establish the structure and exchange kinetics of highly-reactive, low-abundance species in the solution-phase using chemical exchange saturation transfer, exchange spectroscopy, correlation spectroscopy, heteronuclear single-quantum correlation, and heteronuclear multiple-bond correlation nuclear magnetic resonance spectroscopy. Finally, we apply C-3 acyl neighbouring-group participation to the synthesis of complex bacterial oligosaccharides. This combined approach of finding answers to fundamental physical-chemical questions and their application in organic synthesis provides a robust basis for elucidating highly-reactive intermediates in glycosylation reactions.

Characterization of the Pb Coordination Environment and Its Connectivity in Lead Silicate Glasses: Results from 2D 207Pb NMR Spectroscopy.

The Pb-O coordination environment in binary (PbO)x(SiO2)100-x glasses with 30 ≤ x ≤ 70 is probed by using two-dimensional 207Pb nuclear magnetic resonance (NMR) isotropic-anisotropic correlation spectroscopy. The isotropic 207Pb NMR spectra show little composition-dependent evolution of the Pb-O nearest-neighbor coordination environment. The systematic variation of the chemical shift tensor parameters offers a unique insight into their local site symmetry and suggests the presence of pyramidal PbO3 and PbO4 sites with sterically active electron lone pairs and with Pb-O bond lengths ranging between 0.23 and 0.25 nm. The PbO3/PbO4 ratio shows a small but monotonic increase from ∼70:30 to 80:20 as the PbO content increases from 30 to 70 mol %. When taken together, the isotropic and anisotropic 207Pb NMR spectra suggest that the majority of the PbOn (3 ≤ n ≤ 4) pyramids in these glasses are connected to the SiO4 tetrahedra via Pb-O-Si linkages. A significant fraction of Pb-O-Pb linkages, where the oxygen is linked only to Pb atoms, appears only in glasses with PbO ≥ 60 mol %. These oxygen atoms appear to be corner-shared between the PbOn pyramids in the structure, and no evidence for edge-sharing between these pyramids is observed in this composition range. We hypothesize that a substantial fraction of the constituent PbOn pyramids start to participate in edge-sharing only at higher PbO contents (>70 mol %), which diminishes the glass-forming ability of the network. This work illustrates the potential of isotropic-anisotropic correlation NMR spectroscopy in structural studies involving nuclides with large chemical shift ranges and anisotropy.

Solvents Influence 1H NMR Chemical Shifts and Complete 1H and 13C NMR Spectral Assignments for Florfenicol

Florfenicol (FFC) is an important and widely used veterinary drug, and its structure has been characterized by nuclear magnetic resonance (NMR) spectroscopy. The study aimed to investigate the influences of solvent type, solvent concentration, and temperature on the chemical shifts of the 1H NMR of FFC. The results showed that different types of solvents significantly affected the chemical shifts, especially the chemical shifts of 2-H, 3-H, 5-H, and the active protons. When DMSO-d 6 is used as the solvent, there is no significant difference in the chemical shifts of FFC with a concentration ranging from 20 to 250 mmol/L; however, as the temperature increases, the chemical shifts of the active protons move to a higher field. Besides, the NMR spectroscopic data and structural analysis of FFC were refined by 1H, 13C, distortionless enhancement by polarization transfer-135 (DEPT-135), 1H–1H correlation spectroscopy (1H–1H COSY), phase-sensitive gradient heteronuclear singular quantum correlation (gHSQC), and heteronuclear multiple bond correlation (gHMBC) NMR spectroscopy using DMSO-d 6 as a solvent. The study will help with qualitative and quantitative analysis of FFC in the future.

Application of 15N-Edited 1H-13C Correlation NMR Spectroscopy─Toward Fragment-Based Metabolite Identification and Screening via HCN Constructs.

Many key building blocks of life contain nitrogen moieties. Despite the prevalence of nitrogen-containing metabolites in nature, 15N nuclei are seldom used in NMR-based metabolite assignment due to their low natural abundance and lack of comprehensive chemical shift databases. However, with advancements in isotope labeling strategies, 13C and 15N enriched metabolites are becoming more common in metabolomic studies. Simple multidimensional nuclear magnetic resonance (NMR) experiments that correlate 1H and 15N via single bond 1JNH or multiple bond 2-3JNH couplings using heteronuclear single quantum coherence (HSQC) or heteronuclear multiple bond coherence are well established and routinely applied for structure elucidation. However, a 1H-15N correlation spectrum of a metabolite mixture can be difficult to deconvolute, due to the lack of a 15N specific database. In order to bridge this gap, we present here a broadband 15N-edited 1H-13C HSQC NMR experiment that targets metabolites containing 15N moieties. Through this approach, nitrogen-containing metabolites, such as amino acids, nucleotide bases, and nucleosides, are identified based on their 13C, 1H, and 15N chemical shift information. This approach was tested and validated using a [15N, 13C] enriched Daphnia magna (water flea) metabolite extract, where the number of clearly resolved 15N-containing peaks increased from only 11 in a standard HSQC to 51 in the 15N-edited HSQC, and the number of obscured peaks decreased from 59 to just 7. The approach complements the current repertoire of NMR techniques for mixture deconvolution and holds considerable potential for targeted metabolite NMR in 15N, 13C enriched systems.

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM‐5 Zeolite Identified by 2D 1H−95Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy

AbstractMethane dehydroaromatization (MDA) over Mo‐modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D 95Mo NMR and 2D 1H‐95Mo heteronuclear correlation solid‐state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM‐5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoOxCy species, with the more carbidic one (MoOxCy‐II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM‐5 pores are related to the MDA activity.

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM-5 Zeolite Identified by 2D 1 H-95 Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy.

Methane dehydroaromatization (MDA) over Mo-modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D 95 Mo NMR and 2D 1 H-95 Mo heteronuclear correlation solid-state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM-5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoOx Cy species, with the more carbidic one (MoOx Cy -II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM-5 pores are related to the MDA activity.

A fast and efficient tool for the structural characterization of marine dissolved organic matter: Nonuniform sampling 2D COSY NMR

AbstractAn in‐depth structural characterization of marine dissolved organic matter (DOM) is crucial for a better understanding of its connection to marine and global biogeochemical cycles. High‐field nuclear magnetic resonance (NMR) spectroscopy in general and two‐dimensional (2D) correlation spectroscopy (COSY) in particular are powerful tools for the molecular level structural analysis of marine DOM. These 2D NMR experiments demand prolonged experimental times of days to weeks per sample due to the requirement of a large number of experiments to record the second dimension (t1) of the 2D NMR experiment. Herein, we demonstrate the efficacy of nonuniform sampling (NUS) in 2D COSY, which (i) reduces the measurement time by half without compromising spectral quality and (ii) enhances the signal intensity for the given experiment time. This approach can lead to substantial progress in the structural analysis of previously poorly characterized marine DOM. NUS COSY has been exemplified on two solid‐phase extracted DOM samples from the surface and deep ocean at 800 MHz and 1.2 GHz instruments. A dramatic improvement in sensitivity and spectral resolution is observed in NUS COSY spectra recorded at 1.2 GHz instrument when compared to 800 MHz instrument. NUS COSY NMR is versatile and anticipated to have significant potential for uncovering the hidden molecular diversity of DOM from various aquatic environments within a reasonable timeframe. The introduction of NUS into the environmental sciences was long overdue, and our study now opens the door for a wide field of new applications of NMR in the marine and aquatic sciences.

Transfer of phase coherence by the dipolar field in total correlation liquid state nuclear magnetic resonance spectroscopy.

In this work, it is shown that radio-frequency pulse trains designed for total correlation nuclear magnetic resonance spectroscopy can mediate inter-molecular transfer of phase coherence from one spin species to another by the dipolar field. In contrast to previous studies (where short pulses interleaved with long free precession delays were used), here the transverse component of the dipolar field plays an important role. The transfer is so efficient that signal intensities close to those of a single pulse experiment can be achieved. The results are rationalized by numerical simulations that take into account relaxation and diffusion.

Probing the homogeneous distribution of sodium atoms in silicate glasses

Sodium, silicon, and oxygen—the major constituents of magmatic silicate melts in the Earth's crust and mantle—are also common ingredients in commercial flexible and scratch-proof glasses for advanced optoelectronic devices. The distribution of alkali metal cations in these amorphous networks is known to control their overall strength, flexibility, and chemical resistance. While the exact nature of their distribution remains a longstanding problem in understanding the glass structures, metal cations have often been expected to form heterogeneous nano-scale percolation channels where they have exclusive proximity towards nonbridging oxygens (NBOs; Na–O–Si species). Contrary to this picture, we show that nuclear magnetic resonance correlation spectroscopy between sodium and oxygen atoms demonstrates the presence of bridging oxygens (BOs; Si–O–Si species), as well as NBOs, in the immediate vicinity of sodium atoms. Our results indicate a metal distribution which is more homogeneous than the previously envisaged Modified Random Network (MRN) model, which represents the extreme end of heterogeneity with metal percolation channels encapsulated exclusively by nonbridging oxygens, by placing qualitative limits on the extent of segregation of alkali channels from silica-enriched regions as suggested by the MRN model. A refined structural model of oxide glass is proposed in terms of a “perturbed” metal ion distribution, which features a rather homogeneous metal dispersion with differences in Na affinity towards the bridging and non-bridging oxygen species. Such perturbed alkali distribution may be used to account for the anomalous viscosity of magmatic melts, as well as revealing the origins of pronounced toughness in complex oxide glasses.

A scalable solid-state nanoporous network with atomic-level interaction design for carbon dioxide capture.

Carbon capture and sequestration reduces carbon dioxide emissions and is critical in accomplishing carbon neutrality targets. Here, we demonstrate new sustainable, solid-state, polyamine-appended, cyanuric acid–stabilized melamine nanoporous networks (MNNs) via dynamic combinatorial chemistry (DCC) at the kilogram scale toward effective and high-capacity carbon dioxide capture. Polyamine-appended MNNs reaction mechanisms with carbon dioxide were elucidated with double-level DCC where two-dimensional heteronuclear chemical shift correlation nuclear magnetic resonance spectroscopy was performed to demonstrate the interatomic interactions. We distinguished ammonium carbamate pairs and a mix of ammonium carbamate and carbamic acid during carbon dioxide chemisorption. The coordination of polyamine and cyanuric acid modification endows MNNs with high adsorption capacity (1.82 millimoles per gram at 1 bar), fast adsorption time (less than 1 minute), low price, and extraordinary stability to cycling by flue gas. This work creates a general industrialization method toward carbon dioxide capture via DCC atomic-level design strategies.

Regio- and stereoselectivity of the 1,3-dipolar cycloaddition of azomethine ylides to (E)-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-ones: A combined experimental and theoretical study

Functionalized oxindoles and pyrrolizidines form the central structural framework for numerous natural products with extensive biological and pharmacological applications. The requirement for high regio- and stereoselectivity is the main obstacle in the synthesis of such five-membered heterocycles. Multicomponent cycloaddition reactions often provide an efficient and straightforward approach for the preparation of specific regio- and stereoisomers. In this article, the regio- and stereochemistry of the polar [3 + 2]-cycloaddition (32CA) reaction of azomethine ylides prepared by the reaction of isatin derivatives and L-proline with a series of ( E )-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-ones was investigated by experimental and theoretical methods. Among the isatin and ( E )-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-one derivatives, a remarkable inversion of regioselectivity was observed in the 32CA reaction of azomethine ylide generated by the reaction of L-proline and 5-chloroisatin or N -methyl-5-chloroisatin with ( E )-5-chloro-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-one. The regio- and stereochemical assignment of the structures of the cycloaddition products was determined by one- and two-dimensional (1D&2D) homonuclear and heteronuclear correlation nuclear magnetic resonance spectroscopy. The molecular mechanism as well as the regio- and stereoselectivity of the cycloaddition were investigated by means of global and local reactivity indices and a density functional theory (DFT) and explained in detail on the basis of the transition state stabilities of the reactants.

Synthesis and characterization of the novel 4-(1-(pyridin-4-yl) ethoxyl) substituted bis(phthalocyaninato) rare earth complexes and investigation of their two-photon absorption-based third-order non-linear optical properties

Novel 4-(1-(pyridin-4-yl) ethoxyl) substituted double-decker rare earth phthalocyanine complexes were synthesized starting from (R)-4-(1-(pyridin-4-yl) ethoxy) phthalonitrile with proper rare earth metal acetates in octanol catalyzed by 1,8-diazabicyclo[5.4.0]undec‑7-ene at 300 °C. The new starting phthalonitrile compound was obtained from (R)-1-(pyridin-4-yl) ethan-1-ol and 4-nitrophthalonitrile in acetonitrile at reflux temperature in the presence of potassium carbonate as a catalyst. The characterization of the synthesized and isolated compounds was performed by elemental analysis, infrared, ultraviolet-visible, proton and correlation spectroscopy nuclear magnetic resonance, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopic methods. Nonlinear optical responses of all complexes are investigated using the Z-scan method. The nonlinear absorption coefficient, two-photon absorption cross-section, and the imaginary part of the nonlinear susceptibility of complexes are calculated for each discrete concentration. All these nonlinear parameters of our rare-earth coordinated double-decker phthalocyanines present a decrement due to the aggregation under high optic powers, which result in sequential two-photon absorption for smaller ionic radii.

Direct observation of the active sites in methane dehydroaromatization by NMR

Direct observation of the active sites in methane dehydroaromatization by NMR

Identifying the Unknown Content of an Ancient Egyptian Sealed Alabaster Vase from Kha and Merit’s Tomb Using Multiple Techniques and Multicomponent Sample Analysis in an Interdisciplinary Applied Chemistry Course

This article highlights the multianalytical study of exuded liquid from an ancient Egyptian sealed alabaster vase by Master’s students in an applied chemistry for cultural heritage course. Master students are introduced to the field of Archaeometry that see the collaboration of experts in different areas of research such as conservators, curators of museums, physicists, chemists, etc. The sample is a residue exuded on the linen strip sealing an ancient Egyptian alabaster vase (inventory number S.8448) from the collection of the Museo Egizio in Turin (Italy). The students start to plan the noninvasive investigation by X-ray fluorescence (XRF), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) for the inorganic compounds characterization, followed by the extraction of the organic components (such as oils, fats, and waxes) to be analyzed by high-resolution 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and 2D NMR correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC), and long-range heteronuclear correlation (HMBC) techniques and by gas chromatography coupled with mass spectrometry (GC/MS). Reference standards, spectral databases, and published data on similar artifacts served as the basis for the interpretation of the instrumental results. The approach was introduced in the course of Applied Chemistry for Cultural Heritage for Master students in Archaeology (University of Palermo, Italy), where the need is to know how to approach the scientific investigation together with the conservation scientists and how to manage with a very low amount of sample. Pedagogically, the approach introduces students to the main techniques currently used in the field of Archaeometry while reinforcing fundamental concepts in sample collecting and multicomponent microsample analysis. This interdisciplinary approach provides a unique experience that demonstrates chemistry’s broad applicability outside of the traditional laboratory. Students are guided to identify the inorganic and organic components of the exudate liquid: the first one is ascribable to clay minerals iron oxides, which could impart the brown color to the sample; the second one is ascribed to triglycerides of various kinds, which probably comes from vegetable oil.

A single‐step synthesis of 1,3,4,6‐tetraaryl‐5‐aryliminopiperazin‐2‐one

AbstractChemical diversity is a strong driver in drug discovery, and chemical reactions that produce multi‐substituted heterocycles can result in significant chemical diversity. Here, we report a reaction that leads to highly substituted piperazinone derivatives in a single step. This was discovered accidently while reacting phenylglyoxal and anilines under Povarov conditions. We elucidated the structure of the compound formed when 2‐(4‐trifluoromethyphenyl)‐2‐oxoacetaldehyde was reacted with 4‐methoxyaniline as (1,4‐bis(4‐hydroxyphenyl)‐5‐[(4‐hydroxyphenyl)imino]‐3,6‐diphenylpiperazin‐2‐one) (5e). The structure was confirmed after extensive spectral analyses, including correlation spectroscopy, nuclear Overhauser effect spectroscopy, heteronuclear single quantum correlation spectroscopy, and heteronuclear multiple‐bond correlation nuclear magnetic resonance spectroscopy, in addition to high‐resolution mass spectrometry. This reaction is unprecedented and can potentially be exploited as a short route to highly diversified heterocycles.

Benzimidazole-based fluorophores for the detection of amyloid fibrils with higher sensitivity than Thioflavin-T.

Protein aggregation into amyloid fibrils is a key feature of a multitude of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Prion disease. To detect amyloid fibrils, fluorophores with high sensitivity and better efficiency coupled with the low toxicity are in high demand even to date. In this pursuit, we have unveiled two benzimidazole-based fluorescence sensors ([C15 H15 N3 ] (C1) and [C16 H16 N3 O2 ] (C2), which possess exceptional affinity toward different amyloid fibrils in its submicromolar concentration (8×10-9 M), whereas under a similar concentration, the gold standard Thioflavin-T (ThT) fails to bind with amyloid fibrils. These fluorescent markers bind to α-Syn amyloid fibrils as well as amyloid fibrils forming other proteins/peptides including Aβ42 amyloid fibrils. The 1 H-15 N heteronuclear quantum correlation spectroscopy nuclear magnetic resonance data collected on wild-type α-Syn monomer with and without the fluorophores (C1 and C2) reveal that there is weak or no interactions between C1 or C2 with residues in α-Syn monomer, which indirectly reflects the specific binding ability of C1 and C2 to the α-Syn amyloid fibrils. Detailed studies further suggest that C1 and C2 can detect/bind with the α-Syn amyloid fibril as low as 100×10-9 M. Extremely low or no cytotoxicity is observed for C1 and C2 and they do not interfere with α-Syn fibrillation kinetics, unlike ThT. Both C1/C2 not only shows selective binding with amyloid fibrils forming various proteins/peptides but also displays excellent affinity and selectivity toward α-Syn amyloid aggregates in SH-SY5Y cells and Aβ42 amyloid plaques in animal brain tissues. Overall, our data show that the developed dyes could be used for the detection of amyloid fibrils including α-Syn and Aβ42 amyloids with higher sensitivity as compared to currently used ThT.

Quantitative density operator analysis of correlation spectroscopy NMR experiments

Nuclear magnetic resonance (NMR) spectroscopy, also known as magnetic resonance spectroscopy, is a preeminent and noninvasive analytical technique that provides detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. The development of NMR spectroscopy has led to the awarding of many Nobel Prizes, and today NMR spectroscopy serves as an important and irreplaceable tool in physics and chemistry. Two-dimensional (2D) NMR is effective at separating resonances which have similar chemical shifts, although the interpretation of 2D spectra can be challenging. A systematic density operator-based derivation will aid the understanding of the quantitative mechanism of 2D NMR spectroscopy and the interpreting of outcomes of 2D NMR experiments. Therefore, in this study, we systematically analyzed and compared the quantitative basis of 2D and 1D NMR. Meanwhile, as a proof of principle, simulations using the FID Appliance software toolkit were performed and interpreted using a brain phantom, a popular model for studying brain metabolites. The scheme shown in this paper will facilitate the understanding of quantitative 2D NMR spectroscopic analyses in chemistry and biology.

Codon Harmonization of a Kir3.1-KirBac1.3 Chimera for Structural Study Optimization

The expression of functional, folded, and isotopically enriched membrane proteins is an enduring bottleneck for nuclear magnetic resonance (NMR) studies. Indeed, historically, protein yield optimization has been insufficient to allow NMR analysis of many complex Eukaryotic membrane proteins. However, recent work has found that manipulation of plasmid codons improves the odds of successful NMR-friendly protein production. In the last decade, numerous studies showed that matching codon usage patterns in recombinant gene sequences to those in the native sequence is positively correlated with increased protein yield. This phenomenon, dubbed codon harmonization, may be a powerful tool in optimizing recombinant expression of difficult-to-produce membrane proteins for structural studies. Here, we apply this technique to an inward rectifier K+ Channel (Kir) 3.1-KirBac1.3 chimera. Kir3.1 falls within the G protein-coupled inward rectifier K+ (GIRK) channel family, thus NMR studies may inform on the nuances of GIRK gating action in the presence and absence of its G Protein, lipid, and small molecule ligands. In our hands, harmonized plasmids increase protein yield nearly two-fold compared to the traditional ‘fully codon optimized’ construct. We then employ a fluorescence-based functional assay and solid-state NMR correlation spectroscopy to show the final protein product is folded and functional.

Probing the Surface Structure of Semiconductor Nanoparticles by DNP SENS with Dielectric Support Materials.

Surface characterization is crucial for understanding how the atomic-level structure affects the chemical and photophysical properties of semiconducting nanoparticles (NPs). Solid-state nuclear magnetic resonance spectroscopy (NMR) is potentially a powerful technique for the characterization of the surface of NPs, but it is hindered by poor sensitivity. Dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) has previously been demonstrated to enhance the sensitivity of surface-selective solid-state NMR experiments by 1-2 orders of magnitude. Established sample preparations for DNP SENS experiments on NPs require the dilution of the NPs on mesoporous silica. Using hexagonal boron nitride (h-BN) to disperse the NPs doubles DNP enhancements and absolute sensitivity in comparison to standard protocols with mesoporous silica. Alternatively, precipitating the NPs as powders, mixing them with h-BN, and then impregnating the powdered mixture with radical solution leads to further 4-fold sensitivity enhancements by increasing the concentration of NPs in the final sample. This modified procedure provides a factor of 9 improvement in NMR sensitivity in comparison to previously established DNP SENS procedures, enabling challenging homonuclear and heteronuclear 2D NMR experiments on CdS, Si, and Cd3P2 NPs. These experiments allow NMR signals from the surface, subsurface, and core sites to be observed and assigned. For example, we demonstrate the acquisition of DNP-enhanced 2D 113Cd-113Cd correlation NMR experiments on CdS NPs and natural isotropic abundance 2D 13C-29Si HETCOR of functionalized Si NPs. These experiments provide a critical understanding of NP surface structures.

Separation of Benzoic and Unconjugated Acidic Components of Leonardite Humic Material Using Sequential Solid-Phase Extraction at Different pH Values as Revealed by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and Correlation Nuclear Magnetic Resonance Spectroscopy.

Here, we report on sequential solid-phase extraction of leonardite hymatomelanic acid (CHM) on a non-ionic sorbent at four steadily lowered pH values: 7, 5, 3, and 2, yielding fractions with different acidic properties. Using nuclear magnetic resonance (NMR) spectroscopy and ultrahigh-resolution mass spectrometry, we revealed a gradual shift of dominating scaffolds in the fractions of CHM from reduced saturated to oxidized aromatic compounds. An increase on the average aromaticity of the CHM fractions was accompanied by a red shift in fluorescence spectra. These results were supported by heteronuclear single quantum coherence and heteronuclear multiple bond correlation NMR experiments. We have demonstrated that the CHM fraction isolated at pH 5 was dominated by aliphatic carboxyl carriers, while the pH 3 fraction was dominated by aromatic carboxyl acids. The developed fractionation technique will enable deeper insight on structure-property relationships and the design of the humic-based materials with tailored reactive properties.