Correlation Nuclear Magnetic Resonance Spectroscopy Research Articles

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.

Dynamics and Microstructures of Nicotine/Water Binary Mixtures near the Lower Critical Solution Temperature.

The orientational dynamics and microscopic structures of nicotine/water binary mixtures near the system's lower critical solution temperature (LCST) were elucidated using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy, nuclear magnetic resonance correlation spectroscopy (NMR COSY), first-principles calculations, and molecular dynamics simulations. Water concentrations were investigated from zero to close to pure water. At temperatures below the LCST, OHD-OKE experiments measured an anomalous slowing as the phase transition concentration was approached. At moderate concentrations and low temperatures, intermolecular cross-peaks between nicotine and water molecules were observed in the COSY spectra, demonstrating the formation of structures that persist for milliseconds. These results suggest that pair correlations contribute to the slowdown in the OHD-OKE data at moderate water concentrations. First-principles calculations revealed that intermolecular hydrogen bonding coordination between nitrogen atoms in pyridine moieties and water lowers the energy barriers for the reorientations of the two nicotine rings. Atomistic simulations demonstrate that with increasing water concentration, hydrogen bonding interactions between pyridine moieties and water molecules first increase and then decrease with a maximum at moderate water concentrations. These experimental and computational characterizations of the dynamics of nicotine molecules are attributed to the distinct configurations of water molecules around the pyridine ring moieties in nicotine molecules.

Accelerating two-dimensional nuclear magnetic resonance correlation spectroscopy via selective coherence transfer.

Nuclear magnetic resonance (NMR) spectroscopy serves as an important tool for both qualitative and quantitative analyses of various systems in chemistry, biology, and medicine. However, applications of one-dimensional 1H NMR are often restrained by the presence of severe overlap among different resonances. The advent of two-dimensional (2D) 1H NMR constitutes a promising alternative by extending the crowded resonances into a plane and thereby alleviating the spectral congestions. However, the enhanced ability in discriminating resonances is achieved at the cost of extended experimental duration due to necessity of various scans with progressive delays to construct the indirect dimension. Therefore, in this study, we propose a selective coherence transfer (SECOT) method to accelerate acquisitions of 2D correlation spectroscopy by converting chemical shifts into spatial positions within the effective sample length and then performing an echo planar spectroscopic imaging module to record the spatial and spectral information, which generates 2D correlation spectrum after 2D Fourier transformation. The feasibility and effectiveness of SECOT have been verified by a set of experiments under both homogeneous and inhomogeneous magnetic fields. Moreover, evaluations of SECOT for quantitative analyses are carried out on samples with a series of different concentrations. Based on these experimental results, the SECOT may open important perspectives for fast, accurate, and stable investigations of various chemical systems both qualitatively and quantitatively.

TG/DTG, FT-ICR Mass Spectrometry, and NMR Spectroscopy Study of Heavy Fuel Oil

There is an increasing interest in the comprehensive study of heavy fuel oil (HFO) due to its growing use in furnaces, boilers, marines, and recently in gas turbines. In this work, the thermal combustion characteristics and chemical composition of HFO were investigated using a range of techniques. Thermogravimetric analysis (TGA) was conducted to study the nonisothermal HFO combustion behavior. Chemical characterization of HFO was accomplished using various standard methods in addition to direct infusion atmospheric pressure chemical ionization Fourier transform ion cyclotron resonance mass spectrometry (APCI-FTICR MS), high resolution 1H nuclear magnetic resonance (NMR), 13C NMR, and two-dimensional heteronuclear multiple bond correlation (HMBC) spectroscopy. By analyzing thermogravimetry and differential thermogravimetry (TG/DTG) results, three different reaction regions were identified in the combustion of HFO with air, specifically, low temperature oxidation region (LTO), fuel deposition (FD), and hig...

Effects of interaction between a polycation and a nonionic polymer on their cross-assembly into mixed micelles.

In this paper, to investigate the effects of interactions between poly(quaternary ammonium) salts (PQAs) and poly(ethylene glycol) on their mixed micellar surface structures and properties under spontaneous conditions, a series of PQAs were first designed and synthesized by atom transfer radical polymerization (ATRP) using 2-(dimethylamino) ethyl methacrylate (DMAEMA) quaternized by bromobutane, bromooctane, and bromododecane, respectively. Poly(poly(ethylene glycol) methyl ether methacrylate) (PPEG) with a similar degree of polymerization was also prepared using poly(ethylene glycol) methyl ether methacrylate by ATRP. Next, these PQAs were mixed with an equal weight of PPEG in water to cross-assemble into mixed micelles. The structures and features of these mixed micelles were characterized by fluorescence measurements, transmission electron microscopy (TEM), dynamic light scattering (DLS), phase analysis light scattering (PALS), proton nuclear magnetic resonance ((1)H NMR), and hydrogen-hydrogen correlation spectroscopy nuclear magnetic resonance (H-H COSY NMR). These results suggest that PQAs and PPEG mixtures can cross-assemble into mixed micelles with low CMC. The surface structures, particle sizes, size distributions, and zeta potentials of PQAs and PPEG mixtures can be tailored by varying the alkyl chain length in quaternary ammonium salts, and the alkyl chain length also influences the distribution and the alkyl chain orientation of quaternary ammonium salts on mixed micelle surfaces. In addition, cytotoxicity of these mixed micelles can be markedly reduced by PPEG compared with their corresponding PQAs, but their good antibacterial activities are still maintained to a certain degree, as evaluated by methyl tetrazolium assay (MTT) and minimum inhibitory concentration (MIC). Our present work provides a new avenue for the preparation of biocompatible and antibacterial materials for biomedical applications.

Metabolomic tools for secondary metabolite discovery from marine microbial symbionts.

Marine invertebrate-associated symbiotic bacteria produce a plethora of novel secondary metabolites which may be structurally unique with interesting pharmacological properties. Selection of strains usually relies on literature searching, genetic screening and bioactivity results, often without considering the chemical novelty and abundance of secondary metabolites being produced by the microorganism until the time-consuming bioassay-guided isolation stages. To fast track the selection process, metabolomic tools were used to aid strain selection by investigating differences in the chemical profiles of 77 bacterial extracts isolated from cold water marine invertebrates from Orkney, Scotland using liquid chromatography-high resolution mass spectrometry (LC-HRMS) and nuclear magnetic resonance (NMR) spectroscopy. Following mass spectrometric analysis and dereplication using an Excel macro developed in-house, principal component analysis (PCA) was employed to differentiate the bacterial strains based on their chemical profiles. NMR 1H and correlation spectroscopy (COSY) were also employed to obtain a chemical fingerprint of each bacterial strain and to confirm the presence of functional groups and spin systems. These results were then combined with taxonomic identification and bioassay screening data to identify three bacterial strains, namely Bacillus sp. 4117, Rhodococcus sp. ZS402 and Vibrio splendidus strain LGP32, to prioritize for scale-up based on their chemically interesting secondary metabolomes, established through dereplication and interesting bioactivities, determined from bioassay screening.

Functional Groups Determine Biochar Properties (pH and EC) as Studied by Two-Dimensional 13C NMR Correlation Spectroscopy

While the properties of biochar are closely related to its functional groups, it is unclear under what conditions biochar develops its properties. In this study, two-dimensional (2D) 13C nuclear magnetic resonance (NMR) correlation spectroscopy was for the first time applied to investigate the development of functional groups and establish their relationship with biochar properties. The results showed that the agricultural biomass carbonized to biochars was a dehydroxylation/dehydrogenation and aromatization process, mainly involving the cleavage of O-alkylated carbons and anomeric O-C-O carbons in addition to the production of fused-ring aromatic structures and aromatic C-O groups. With increasing charring temperature, the mass cleavage of O-alkylated groups and anomeric O-C-O carbons occurred prior to the production of fused-ring aromatic structures. The regression analysis between functional groups and biochar properties (pH and electrical conductivity) further demonstrated that the pH and electrical conductivity of rice straw derived biochars were mainly determined by fused-ring aromatic structures and anomeric O-C-O carbons, but the pH of rice bran derived biochars was determined by both fused-ring aromatic structures and aliphatic O-alkylated (HCOH) carbons. In summary, this work suggests a novel tool for characterising the development of functional groups in biochars.

Microstructural Characterization and Thermal Analysis of Block Copolymer of Methyl Methacrylate and n-Butyl Acrylate

Microstructural characterization of synthetic copolymers can be employed to establish structure-property relationships for various practical applications. Acrylic copolymers are economical and suitable for processing into interesting materials for diverse applications. The relative composition, sequence distribution, and configuration of block copolymer monomeric units of poly(methyl methacrylate-b-n-butyl acrylate) (PMMA-b-nBA) was studied by the combination of Fourier transform-infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy, and modulated differential scanning calorimetry (MDSC). Configurational assignments of MMA and BA blocks were determined from 13C NMR and heteronuclear single-quantum correlation spectroscopy (HSQC), respectively. The α-carbon resonances from methacrylic units appear from downfield to upfield in the order mm, rm/mr, rr, whereas they appear in the opposite order in acrylic units.

Two-Dimensional NMR Evidence for Cleavage of Lignin and Xylan Substituents in Wheat Straw Through Hydrothermal Pretreatment and Enzymatic Hydrolysis

Solution-state two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy of plant cell walls is a powerful tool for characterizing changes in cell wall chemistry during the hydrothermal pretreatment process of wheat straw for second-generation bioethanol production. One-bond 13C–1H NMR correlation spectroscopy, via an heteronuclear single quantum coherence experiment, revealed substantial lignin β-aryl ether cleavage, deacetylation via cleavage of the natural acetates at the 2-O- and 3-O-positions of xylan, and uronic acid depletion via cleavage of the (1 → 2)-linked 4-O-methyl-α-d-glucuronic acid of xylan. In the polysaccharide anomeric region, decreases in the minor β-d-mannopyranosyl, and α-l-arabinofuranosyl units were observed in the NMR spectra from hydrothermally pretreated wheat straw. The aromatic region indicated only minor changes to the aromatic structures during the process (e.g., further deacylation revealed by the depletion in ferulate and p-coumarate structures). Supplementary chemical analyses showed that the hydrothermal pretreatment increased the cellulose and lignin concentration with partial removal of extractives and hemicelluloses. The subsequent enzymatic hydrolysis incurred further deacetylation of the xylan, leaving approximately 10 % of acetate intact based on the weight of original wheat straw.

The COSY and DQF-COSY NMR spectra for systems of three spins I = 7/2

The two-dimensional nuclear magnetic resonance (NMR) correlation spectroscopy (COSY) and double quantum-filtered COSY (DQF-COSY) spectra of an AMX and an AX2 systems of spins I = 7/2 have been calculated by numerical density matrix calculations. The results are presented in three extreme cases, namely in the slow relaxation regime, the intermediate relaxation regime, and in the fast relaxation regime. It is concluded for such spin systems, as was recently shown in the case of an AX system of spins I = 3/2, that in addition to the usual advantages that the DQF-COSY NMR experiment offer compared to the conventional COSY NMR experiment (observation of only-coupled spins, antiphase character of both diagonal and cross-peaks, and intensity of the cross-peaks comparable to the intensity of the diagonal peaks), the DQF-COSY NMR experiment appear to be more powerful for observation, interpretation, and analysis than the conventional COSY NMR experiment, and this is particularly true for quadrupolar nuclei relaxing usually very quickly and/or small coupling constants. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part A 38A: 74–83, 2011.

Measurement of the exchange rate of waters of hydration in elastin by 2D T2–T2 correlation nuclear magnetic resonance spectroscopy

We report on a direct measurement of the exchange rate of waters of hydration in elastin by T2–T2 exchange spectroscopy. The exchange rates in bovine nuchal ligament elastin and aortic elastin at temperatures near, below and at the physiological temperature are reported here. Using an inverse Laplace transform (ILT) algorithm, we are able to identify four components in the relaxation times. While three of the components are in good agreement with previous measurements that used multi-exponential fitting, the ILT algorithm distinguishes a fourth component having relaxation times close to that of free water and is identified as water between fibers. With the aid of scanning electron microscopy, a model is proposed that allows for the application of a two-site exchange analysis between any two components for the determination of exchange rates between reservoirs. The results of the measurements support a model (described by Urry and Parker 2002 J. Muscle Res. Cell Motil.23 543–59) wherein the net entropy of waters of hydration should increase with increasing temperature in the inverse temperature transition.

Quantifying the transfer and settling in NMR experiments with sample shuttling

Nuclear magnetic resonance (NMR) in combination with pulsed magnetic field gradients has proven very successful for measuring molecular diffusion, where the correlation time of the motion is much shorter than the timescale of the experiment. In this article, it is demonstrated that a single-scan NMR technique to measure molecular diffusion can be employed to also study incoherent random motions over macroscopic length scales that show correlation times similar to the timescale of the experiment. Such motions are observed, for example, after the mixing of two components or after transferring a sample from one container into another. To measure the fluid settling, a series of magnetization helices were encoded onto a sample. Stimulated gradient echo trains were then generated after different mixing times, which enabled the determination of an effective dispersion coefficient for the fluid. This technique was used to optimize the timing of NMR experiments combined with dissolution dynamic nuclear polarization, where a sample was shuttled between two magnets. In addition to the decay of fluid turbulences, the presence of microbubbles in the sample tube at the end of the shuttling step was identified as another contribution to the NMR linewidth. Microbubbles could be indirectly observed through the line broadening effect on the NMR signal due to their different susceptibility compared to the solvent, which induced field gradients near the interfaces. Using these data, the signal attenuation caused by sample motion in single-scan two-dimensional correlation spectroscopy NMR experiments could be predicted with reasonable accuracy.

The theory of cosy NMR experiments revisited: Application to an AX spin system of quadrupolar nuclei

AbstractThe two‐dimensional nuclear magnetic resonance correlation spectroscopy (2D NMR COSY) spectrum of an AX spin system of spin‐1 has been calculated by numerical density matrix calculations. The mathematical expressions found are valid for an AX spin system of any spins. These expressions should be used to calculate the 2D NMR COSY spectrum of an AX spin system of high spin nuclei as their use will significantly simplify the calculations. More precisely, one needs to calculate only one set of coefficients despite the need of two steps in the phase cycling to achieve phase modulation during t1. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 170–177, 2010.

Diffusion in Model Networks as Studied by NMR and Fluorescence Correlation Spectroscopy

We have studied the diffusion of small solvent molecules (octane) and larger hydrophobic dye probes in octane-swollen poly(dimethyl siloxane) linear-chain solutions and end-linked model networks, using pulsed-gradient nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS), respectively, focusing on diffusion in the bulk polymer up to the equilibrium degree of swelling of the networks, that is, 4.8 at most. The combination of these results allows for new conclusions on the feasibility of different theories describing probe diffusion in concentrated polymer systems. While octane diffusion shows no cross-link dependence, the larger dyes are increasingly restricted by fixed chemical meshes. The simple Fujita free-volume theory proved most feasible to describe probe diffusion in linear long-chain solutions with realistic parameters, while better fits were obtained assuming a stretched exponential dependence on concentration. Importantly, we have analyzed the cross-link specific effect on probe diffusion independently of any specific model by comparing the best-fit interpolation of the solution data with the diffusion in the networks. The most reasonable description is obtained by assuming that the cross-link effect is additive in the effective friction coefficient of the probes. The concentration dependences as well as the data compared at the equilibrium degrees of swelling indicate that swelling heterogeneities and diffusant shape have a substantial influence on small-molecule diffusion in networks.

Thermal history effects and methyl tunneling dynamics in a supramolecular complex of calixarene and para-xylene

The low-temperature structure and dynamics of guest molecules of p-xylene incorporated in the isopropyl-calix[4] arene(2:1) p-xylene complex have been investigated by solid state nuclear magnetic resonance (NMR). Using one-dimensional 1H-decoupled 13C cross-polarization magic-angle-spinning (MAS) NMR and two-dimensional 1H-13C correlation spectroscopy, a full assignment of the 13C and 1H chemical shifts has been made. Using 1H NMR relaxometry, the effects of thermal history on the structure of the system have been investigated. Rapidly cooled samples have 1H spin-lattice relaxation times T1, which at low temperature (T<60 K) are typically two orders of magnitude faster than those observed in annealed samples which have been cooled slowly over many hours. In both forms, the low-temperature relaxation is driven by the dynamics of the weakly hindered methyl rotors of the p-xylene guest. The substantial difference in T1 is attributed in the rapidly cooled sample to disorder in the structure of the complex leading to a wide distribution of correlation times and methyl barrier heights. A comparison of the linewidths and splittings in the high resolution 13C MAS spectra of the two forms provides structural insight into the nature of the disorder. Using 1H field-cycling NMR relaxometry, the methyl dynamics of the p-xylene guest in the annealed sample have been fully characterized. The B-field dependence of the 1H T1 maps out the spectral density from which the correlation times are directly measured. The methyl barrier heights are determined from an analysis of the temperature dependence.

Evidence for cleavage of lignin by a brown rot basidiomycete

Biodegradation by brown-rot fungi is quantitatively one of the most important fates of lignocellulose in nature. It has long been thought that these basidiomycetes do not degrade lignin significantly, and that their activities on this abundant aromatic biopolymer are limited to minor oxidative modifications. Here we have applied a new technique for the complete solubilization of lignocellulose to show, by one-bond (1)H-(13)C correlation nuclear magnetic resonance spectroscopy, that brown rot of spruce wood by Gloeophyllum trabeum resulted in a marked, non-selective depletion of all intermonomer side-chain linkages in the lignin. The resulting polymer retained most of its original aromatic residues and was probably interconnected by new linkages that lack hydrogens and are consequently invisible in one-bond (1)H-(13)C correlation spectra. Additional work is needed to characterize these linkages, but it is already clear that the aromatic polymer remaining after extensive brown rot is no longer recognizable as lignin.

Fast nuclear magnetic resonance correlation spectroscopy without diagonal peaks: The “2-1” correlation spectroscopy

A fast NMR experiment is proposed for measuring correlation spectroscopy (COSY) spectra with diagonal peaks completely removed or largely suppressed. This new pulse sequence is based on double quantum spectroscopy but with delayed detection, so that the double quantum coherence is effectively transferred to single quantum coherence. Therefore, the pulse sequence can be named "2-1" COSY.

Comparison of 1D and 2D NMR Spectroscopy for Metabolic Profiling

High-resolution, liquid state nuclear magnetic resonance (NMR) spectroscopy is a popular platform for metabolic profiling because the technique is nondestructive, quantitative, reproducible, and the spectra contain a wealth of biochemical information. Because of the large dynamic range of metabolite concentrations in biofluids, statistical analyses of one-dimensional (1D) proton NMR data tend to be biased toward selecting changes in more abundant metabolites. Although two-dimensional (2D) proton-proton experiments can alleviate spectral crowding, they have been mainly used for structural determination. In this study, 2D total correlation spectroscopy NMR was used to compare the global metabolic profiles of urine obtained from wild-type and Abcc6-knockout mice. The 2D data were compared to an improved 1D experiment in which signal contributions from macromolecules and the urea peak have been spectroscopically removed for more accurate quantitation of low-abundance metabolites. Although statistical models from both 1D and 2D data could differentiate samples acquired from the two groups of mice, only the 2D spectra allowed the characterization of statistically relevant changes in the low-abundance metabolites. While acquisition of the 2D data require more time, the data obtained resulted in a more meaningful and comprehensive metabolic profile, aided in metabolite identifications, and minimized ambiguities in peak assignments.

On the use of 2D correlation and exchange NMR spectroscopy in organic porous materials

Two-dimensional (2D) nuclear magnetic resonance (NMR) methods for the investigation of correlation and exchange have been introduced in recent years and have been applied to a range of different systems. Here, we report on the use of 2D NMR diffusion-diffusion correlation spectroscopy for the investigation of diffusion anisotropy in cellular plant tissues and of diffusion-diffusion exchange spectroscopy for the study of the diffusive exchange of dextran in a dispersion of polyelectrolyte multilayer hollow capsules. Furthermore, diffusion-relaxation correlation spectroscopy was applied to both systems.