19F Dynamic Nuclear Polarization and SEM in Suspensions Consisting of Fluorobenzene Derivatives and Asphaltene Extracted from MC-800 Liquid Asphalt

A variety of analytical techniques, such as scanning electron microscopy and ¹⁹F dynamic nuclear polarization (DNP) methods, are applied to characterize asphaltene extracted from MC-800 liquid asphalt in fluorobenzene derivatives at 1.53 mT and at room temperature. Different solvents show variable affinities for the asphaltene surface. The low field EPR spectrum of the asphaltene/hexafluorobenzene sample was recorded. The DNP parameters were determined. Additionally, the interactions between the nuclei of the solvent and the electrons delocalized on the asphaltene are interpreted. Not only dipolar but also scalar interactions between the nuclear spin and the electron spin were found.     

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.     

Sample Ripening through Nanophase Separation Influences the Performance of Dynamic Nuclear Polarization

Mixtures of water and glycerol provide popular matrices for low‐temperature spectroscopy of vitrified samples. However, they involve counterintuitive physicochemical properties, such as spontaneous nanoscopic phase separations (NPS) in solutions that appear macroscopically homogeneous. We demonstrate that such phenomena can substantially influence the efficiency of dynamic nuclear polarization (DNP) by factors up to 20 % by causing fluctuations in local concentrations of polarization agents (radicals). Thus, a spontaneous NPS of water/glycerol mixtures that takes place on time scales on the order of 30–60 min results in a confinement of polarization agents in nanoscopic water‐rich vesicles, which in return affects the DNP. Such effects were found for three common polarization agents, TEMPOL, AMUPol and Trityl.     

Slow Molecular Motions in Ionic Liquids Probed by Cross‐Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization

Solution‐state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron–nuclear cross‐relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross‐relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three‐spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the ¹⁹F/¹H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub‐megahertz (MHz) to tens of MHz range—motions which are not accessed at high fields.     

Dynamic Nuclear Polarization NMR Spectroscopy of Polymeric Carbon Nitride Photocatalysts: Insights into Structural Defects and Reactivity

Metal‐free polymeric carbon nitrides (PCNs) are promising photocatalysts for solar hydrogen production, but their structure–photoactivity relationship remains elusive. Two PCNs were characterized by dynamic‐nuclear‐polarization‐enhanced solid‐state NMR spectroscopy, which circumvented the need for specific labeling with either ¹³C‐ or ¹⁵N‐enriched precursors. Rapid 1D and 2D data acquisition was possible, providing insights into the structural contrasts between the PCNs. Compared to PCN_B with lower performance, PCN_P is a more porous and more active photocatalyst that is richer in terminal N?H bonds not associated with interpolymer chains. It is proposed that terminal N?H groups act as efficient carrier traps and reaction sites.     

On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid‐State and Dissolution 13C NMR Spectroscopy

Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid‐state polarization enhancement at ambient conditions, and the maximization of ¹³C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's ¹³C behavior in nano‐ and micro‐particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid‐state experiments. It was found that ¹³C NMR signals could be boosted by orders of magnitude in either low‐ or room‐temperature solid‐state DNP experiments by utilizing naturally occurring paramagnetic P₁ substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin‐lattice relaxation times characteristic of diamond, coupled with a time‐independent cross‐effect‐like polarization transfer mechanism facilitated by a matching of the nitrogen‐related hyperfine coupling and the ¹³C Zeeman splitting. The efficiency of this solid‐state polarization process, however, is harder to exploit in dissolution DNP‐enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.     

Determining the interaction and characterization of asphaltene in alkylbenzene solvents using nuclear-electron double resonance

This study uses ¹H dynamic nuclear polarization (DNP) methods to determine asphaltene aggregates and the interaction between asphaltene extracted from MC800 asphalt and alkylbenzene solvents, as well as elemental analysis for the characterization of asphaltene. The asphaltene sample was characterized using the elemental analysis of carbon (C), hydrogen (H), nitrogen (N), and sulfur (S). The results show that asphaltenes have the highest carbon content. The sulfur and hydrogen contents are nearly the same and nitrogen content is the smallest. The DNP data provided good results for characterizing asphaltene behavior in alkylbenzene solvents.     

Motional Dynamics of Halogen‐Bonded Complexes Probed by Low‐Field NMR Relaxometry and Overhauser Dynamic Nuclear Polarization

Halogen bonding is a subject of considerable interest owing to wide‐ranging chemical, materials and biological applications. The motional dynamics of halogen‐bonded complexes play a pivotal role in comprehending the nature of the halogen‐bonding interaction. However, not many attempts appear to have been made to shed light on the dynamical characteristics of halogen‐bonded species. For the first time, we demonstrate here that the combination of low‐field NMR relaxometry and Overhauser dynamic nuclear polarization (ODNP) makes it possible to obtain a cogent picture of the motional dynamics of halogen‐bonded species. We discuss here the advantages of this combined approach. Low‐field relaxometry allows us to infer the hydrodynamic radius and rotational correlation time, whereas ODNP probes the molecular translational correlation times (involving the substrate as well as the organic radical) with high sensitivity at low field.     

Experimental Investigation of the Effects of Different Parameters on the Rate of Asphaltene Deposition in Laminar Flow and Its Prediction Using Heat Transfer Approach

In this study, asphaltene deposition from crude oil on the pipe surface has been studied experimentally using a novel designed test loop. Washing technique is used to quantitatively measure the rate of asphaltene deposition during laminar flow in the steel pipe. The effects of oil velocity, asphaltene content, and surface temperature on the thickness of asphaltene deposition are investigated. The results show that the asphaltene deposition rate increases with increasing surface temperature, results in asphaltene content reduction of the flowing crude oil. As the oil velocity increases, less deposition was noticed on the surface of the pipe. Besides, thermal approach was applied to the experimental procedure which shows good agreements between the predicted thickness and the measured value from the test loop.     

超临界流体的共溶剂效应和混合流体研究进展

共溶剂的出现极大地拓展了超临界流体的应用范围,推动了超临界流体科学与技术的发展。本文从相行为和分子间相互作用热力学的角度,对相行为测定、量热技术、光谱技术和分子模拟等在超临界流体中共溶剂效应的研究作了综述,主要介绍超临界流体中共溶剂的作用机理和混合流体在临界点附近热力学性质研究,并对其未来发展方向进行了展望。     

Implementation and Characterization of Flow Injection in Dissolution Dynamic Nuclear Polarization NMR Spectroscopy

The use of dissolution dynamic nuclear polarization (D ‐DNP) offers substantially increased signals in liquid‐state NMR spectroscopy. A challenge in realizing this potential lies in the transfer of the hyperpolarized sample to the NMR detector without loss of hyperpolarization. Here, the use of a flow injection method using high‐pressure liquid leads to improved performance compared to the more common gas‐driven injection, by suppressing residual fluid motions during the NMR experiment while still achieving a short injection time. Apparent diffusion coefficients are determined from pulsed field gradient echo measurements, and are shown to fall below 1.5 times the value of a static sample within 0.8 s. Due to the single‐scan nature of D ‐DNP, pulsed field gradients are often the only choice for coherence selection or encoding, but their application requires stationary fluid. Sample delivery driven by a high‐pressure liquid will improve the applicability of these types of D‐DNP advanced experiments.     

Tailoring of Polarizing Agents in the bTurea Series for Cross‐Effect Dynamic Nuclear Polarization in Aqueous Media

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ? (¹H) in cross‐effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP‐optimized glycerol/water matrix (“DNP juice”) have been studied. We observe that ? (¹H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e–e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e–e distance or too long a T_1e can dramatically limit ? (¹H). Our study also shows that the molecular structure of AMUPol is not optimal and its ? (¹H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better ? (¹H) than AMUPol itself (by a factor of ca. 1.2).     

Interactions of D-Maltose and Sucrose with Some Amino Acids in Aqueous Solutions

Calorimetric titrations have been performed at 298.15 K in aqueous solutions to derive the stability constants and thermodynamic parameters of the interactions of D-maltose and sucrose with some amino acids (glycine, DL-alanine, DL-leucine, and L-serine). The apparent molal volumes of the disaccharides in dilute aqueous solutions of the amino acids have been determined from density measurements at 298.15 K. In contrast to D-maltose, sucrose was found to associate with the amino acids and these associated species are preferentially entropy stabilized. These results are interpreted in terms of the influence of the nature of the solutes, their specific conformations, and hydration, on the ability of the disaccharides to form associated complexes with the amino acids.     

Electronic and Magnetic Transitions in Europium Compounds Studied by Nuclear Forward Scattering of Synchrotron Radiation

Since its observation in 1985, nuclear resonance scattering of synchrotron radiation has become an excellent tool to study hyperfine interactions as well as dynamical effects in solids. It has proven to be a complementary method to Mössbauer spectroscopy. Nuclear resonance scattering combines the advantages of both local probe experiments and scattering techniques. It gives valuable information as well on electronic and magnetic structures and on dynamics in solids. Experiments benefit from the high beam quality of third-generation synchrotron radiation sources, as the small beam size and divergence. Besides the standard isotope ⁵⁷Fe, other Mössbauer isotopes have become more important in nuclear resonant scattering of synchrotron radiation. This article concentrates on the ¹⁵¹Eu isotope.     

Up to 100% Improvement in Dynamic Nuclear Polarization Solid‐State NMR Sensitivity Enhancement of Polymers by Removing Oxygen

High‐field dynamic nuclear polarization (DNP) has emerged as a powerful technique for improving the sensitivity of solid‐state NMR (SSNMR), yielding significant sensitivity enhancements for a variety of samples, including polymers. Overall, depending upon the type of polymer, the molecular weight, and the DNP sample preparation method, sensitivity enhancements between 5 and 40 have been reported. These promising enhancements remain, however, far from the theoretical maximum (>1000). Crucial to the success of DNP SSNMR is the DNP signal enhancement (ε_DNP), which is the ratio of the NMR signal intensities with and without DNP. It is shown here that, for polymers exhibiting high affinity toward molecular oxygen (e.g., polystyrene), removing part of the absorbed (paramagnetic) oxygen from the solid‐state samples available as powders (instead of dissolved or dispersed in a solvent) increases proton nuclear relaxation times and ε_DNP, hereby providing up to a two‐fold sensitivity increase (i.e., a four‐fold reduction in experimental time).

     

A Toolbox for Glutamine Use in Dissolution Dynamic Nuclear Polarization: from Enzymatic Reaction Monitoring to the Study of Cellular Metabolic Pathways and Imaging

Glutamine is under scrutiny regarding its metabolic deregulation linked to energetic reprogramming in cancer cells. Many analytical techniques have been used to better understand the impact of the metabolism of amino acids on biological processes, however only a few are suited to work with complex samples. Here, we report the use of a general dissolution dynamic nuclear polarization (D-DNP) formulation using an unexpensive radical as a multipurpose tool to study glutamine, with insights from enzymatic modelling to complex metabolic networks and fast imaging. First, hyperpolarized [5-¹³C] glutamine is used as molecular probe to study the kinetic action of two enzymes: L-asparaginase that has been used as an anti-metabolic treatment for cancer, and glutaminase. These results are also compared with those acquired with another hyperpolarized amino acid, [1,4-¹³C] asparagine. Second, we explored the use of hyperpolarized (HP) substrates to probe metabolic pathways by monitoring metabolic profiles arising from hyperpolarized glutamine in E. coli extracts. Finally, a highly concentrated sample formulation is proposed for the purpose of fast imaging applications. We think that this approach can be extended to formulate other amino acids as well as other metabolites and provide complementary insights into the analysis of metabolic networks.     

The Optimal Control of Geometry and Voltage Parameters on Electrokinetic Transport to Avoid Sample Leakage in Microfluidic Chips

Computer simulation is used to investigate sample transport phenomena of cross microfluidic chips. In this study, Kirchhoff circuit theory is employed to calculate the electric field strength and approximate electroosmotic flow. It is apparent from the results that both the simulation and the theoretical data show similar trends in the electroosmosis of cross microchips. The main target in this study is to summarize the optimal controlling parameter values for avoiding sample leakage in the transport process. The effects of the applied voltage ratio, the geometry ratio and the zeta potential were simulated using a computational fluid dynamics and multiphysics solver software package (CFD-ACE+). Under our designed conditions, two major conclusions were reached: (1) for high-voltage ratios, the sample leakage can be avoided as the geometry ratio is large enough at 0.5 or greater, and (2) for small geometries, maintaining a smaller voltage ratio, 0.3 or less, is essential for avoiding sample leakage. The key is to govern the sample velocity in the upstream faster than that in the downstream. Although real experimental conditions can be further fine tuned under microscopy monitoring, these conclusions are helpful to design the proper channel geometry and set up suitable voltage parameters to avoid sample leakages in one cross-channel chip.