Literatura académica sobre el tema "DQ-NMR"
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Artículos de revistas sobre el tema "DQ-NMR"
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Howse, Samantha, Christopher Porter, Tesfaldet Mengistu, Ivan Petrov y Richard J. Pazur. "EXPERIMENTAL DETERMINATION OF THE QUANTITY AND DISTRIBUTION OF CHEMICAL CROSSLINKS IN UNAGED AND AGED NATURAL RUBBER. II: A SULFUR DONOR SYSTEM". Rubber Chemistry and Technology 92, n.º 3 (1 de julio de 2019): 513–30. http://dx.doi.org/10.5254/rct.19.81473.
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ABSTRACT A series of unfilled and stabilized natural rubber compounds varying in concentration of tetramethylthiuram disulfide (TMTD) was analyzed using rheometry, hardness, dynamic mechanical properties, stress–strain (Mooney–Rivlin), equilibrium solvent swell (Flory–Rhener), and low-field nuclear magnetic resonance (NMR) by the double quantum (DQ) technique. Crosslinking level increased proportionately with TMTD concentration, and the reaction ratio of three TMTD molecules producing one crosslink was generally upheld. Unreacted TMTD acted as a pseudo-plasticizer and lowered the chain entanglement density with increasing TMTD content. DQ NMR confirmed that the elastic network was homogeneous and that the absolute chemical crosslink distributions broaden with increasing curative level. Upon mild heat aging, zinc complexes based on TMTD/ZnO are likely responsible for causing additional crosslinking, explaining the rise in crosslink density by equilibrium solvent swell and DQ NMR. The amine-based antioxidant, the generation of thiocarbamate radicals from TMTD, and the heat stability of the predominant monosulfide crosslinking system helped to limit network breakdown through chain scission. The chain entanglement increase is likely due to reduction of the plasticizing effect caused by unreacted curative. The distribution of crosslinks slightly broadens toward higher total crosslink density because of the generation of additional chemical crosslinks and chain entanglement densification.
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Soubaneh, Youssouf D., Steeven Ouellet, Caroline Dion y Jonathan Gagnon. "Formation of highly quaternized N,N,N-trimethylchitosan: a chemoselective methodology in aqueous media". Pure and Applied Chemistry 91, n.º 3 (26 de marzo de 2019): 489–96. http://dx.doi.org/10.1515/pac-2018-0924.
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Abstract N,N,N-Trimethylchitosan (TMC) represents a rare example of cationic polysaccharides and numerous studies have shown its potential in biological and biomedical applications. TMC with high degrees of quaternization (DQ) were synthesized from N-methylation of N,N-dimethylchitosan (DMC), which was obtained by reductive alkylation of high molecular weight chitosan in a simple step process and in good yields. The effects of base and solvents were evaluated on the quaternization reaction. The N-methylation of DMC was performed selectively by CH3I and carbonate in water where quaternization was achieved quantitatively with a low degree of O-methylation (17 %). Moreover, the greener procedure allows easy recovery and purification by conventional filtration as a carbonate salt, in which the anion can be exchanged by an acid-base reaction. Quantification of DQ involving 1H NMR integration of methyl peaks must be performed on protonated TMC. High field NMR spectra of TMC showed two specific chemical shifts for anomeric peaks (5.0 and 5.4 ppm) that can also be used for the determination of DQ. This latter method avoids the superimposition problems with other pyranosyl peaks.
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Zhao, Yan, Yuling Liang, Yingjie Yao, Hao Wang, Tong Lin, Yun Gao, Xiaoliang Wang y Gi Xue. "Chain Dynamics of Partially Disentangled UHMWPE around Melting Point Characterized by 1H Low-Field Solid-State NMR". Polymers 15, n.º 8 (16 de abril de 2023): 1910. http://dx.doi.org/10.3390/polym15081910.
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Melts of ultrahigh molecular weight polyethylene (UHMWPE) entangled significantly, suffering processing difficulty. In this work, we prepared partially disentangled UHMWPE by freeze-extracting, exploring the corresponding enchantment of chain mobility. Fully refocused 1H free induction decay (FID) was used to capture the difference in chain segmental mobility during the melting of UHMWPE with different degrees of entanglement by low-field solid-state NMR. The longer the polyethylene (PE) chain is in a less-entangled state, the harder the process of merging into mobile parts after detaching from crystalline lamella during melting. 1H double quantum (DQ) NMR was further used to obtain information caused by residual dipolar interaction. Before melting, the DQ peak appeared earlier in intramolecular-nucleated PE than in intermolecular-nucleated PE because of the strong constraints of crystals in the former one. During melting, less-entangled UHMWPE could keep disentangled while less-entangled high density polyethylene (HDPE) could not. Unfortunately, no noticeable difference was found in DQ experiments between PE melts with different degrees of entanglement after melting. It was ascribed to the small contribution of entanglements compared with total residual dipolar interaction in melts. Overall, less-entangled UHMWPE could reserve its disentangled state around the melting point long enough to achieve a better way of processing.
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Tsutsumi, Hiroyuki, Tomonori Ohata, Rie Nakashima y Hirohito Ikeda. "Inclusion complexation and self-association of cucurbit[n]uril (n = 6, 7) and diquat under pseudo-physiological conditions". New Journal of Chemistry 46, n.º 13 (2022): 6150–57. http://dx.doi.org/10.1039/d1nj06170d.
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The formation and steric structure of the inclusion complex of cucurbit[n]uril (CB[n]; n = 6, 7) and diquat (DQ) were investigated through NMR measurements under the pH conditions of human pseudo-gastric or body fluids, in physiological saline.
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Wang, Shijun, You-Lee Hong, Shichen Yuan, Wei Chen, Wenxuan Zhou, Zhen Li, Kun Wang, Xu Min, Takashi Konishi y Toshikazu Miyoshi. "Chain Trajectory, Chain Packing, and Molecular Dynamics of Semicrystalline Polymers as Studied by Solid-State NMR". Polymers 10, n.º 7 (15 de julio de 2018): 775. http://dx.doi.org/10.3390/polym10070775.
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Chain-level structure of semicrystalline polymers in melt- and solution-grown crystals has been debated over the past half century. Recently, 13C–13C double quantum (DQ) Nuclear Magnetic Resonance (NMR) spectroscopy has been successfully applied to investigate chain-folding (CF) structure and packing structure of 13C enriched polymers after solution and melt crystallization. We review recent NMR studies for (i) packing structure, (ii) chain trajectory, (iii) conformation of the folded chains, (iv) nucleation mechanisms, (v) deformation mechanism, and (vi) molecular dynamics of semicrystalline polymers.
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Boyd, M., I. Therrien y Richard J. Pazur. "CHARACTERIZATION AND CORRELATION OF THE NETWORK CHAIN DENSITY TO THE PROPERTIES OF FLUOROELASTOMER RUBBER". Rubber Chemistry and Technology 93, n.º 2 (1 de abril de 2020): 274–85. http://dx.doi.org/10.5254/rct.20.80367.
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ABSTRACT The concentrations of triallyl isocyanurate (TAIC) in a peroxide-curable fluoroelastomer terpolymer containing 67 wt% of fluorine were varied to generate compounds of differing crosslink densities. Experimental analysis was undertaken using rheometry, hardness, stress–strain (Mooney–Rivlin), equilibrium solvent swell, and low-field nuclear magnetic resonance (NMR) using the double quantum (DQ) technique. Increasing the TAIC concentration caused a systematic rise in rheometry elastic torque, hardness, and tensile strength, whereas both elongation at break and swelling levels decreased. These results are concurrent with an enhanced overall level of crosslinking, which was confirmed by the steady increase of the Mooney–Rivlin C1 values. DQ NMR analysis using hydrogen and fluorine probes and subsequent application of fast Tikhonov regularization to the corrected intensity data were particularly useful in discerning the inhomogeneous nature of the compound morphology. The spatial distribution of the crosslink density suggests that the compound consists of small, highly crosslinked/entangled polymerized TAIC domains embedded within the elastic crosslinked matrix. A concentration of 3 phr of TAIC is optimal according to compression set testing.
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Fišera, Lubor, Igor Goljer y Libuše Jarošková. "Reaction of tetrahydrofuroisoxazoles with molybdenum hexacarbonyl. A new route to preparation of 3-substituted tetrahydro- and dihydrofuran derivatives". Collection of Czechoslovak Chemical Communications 53, n.º 8 (1988): 1753–60. http://dx.doi.org/10.1135/cccc19881753.
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3-(4-X-Phenyl)-3a,4,6,6a-tetrahydrofuro[3,4-d]isoxazoles Ia-If (X = H, CH3, CH3O, Cl, F, C6H5) react with molybdenum hexacarbonyl to give not only the expected cis-3-aroyl-4-hydroxytetrahydrofuran III, but also its trans isomer IV and 3-aroyl-2,5-dihydrofuran II. This paper concerns a new preparation of 3-aroyl-2,5-dihydrofuran derivatives starting from 2,5-dihydrofuran via 1,3-dipolar cycloaddition of nitrile oxides, cleavage with molybdenum hexacarbonyl and dehydratation with p-toluenesulfonic acid. The structure of cis-(III) and trans-(IV) derivatives was deduced from both the γ-effect in the 13C NMR and the DQ COSY experiment in the 1H NMR spectra.
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Peng, Wansu, Chengdong Feng, Jiawen Hou, Rongchun Zhang, Pingchuan Sun, Yun Gao y Xiaoliang Wang. "Probing the Dynamic Structural Evolution of End-Functionalized Polybutadiene/Organo-Clay Nanocomposite Gels before and after Yielding by Nonlinear Rheology and 1H Double-Quantum NMR". Polymers 14, n.º 8 (8 de abril de 2022): 1518. http://dx.doi.org/10.3390/polym14081518.
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Understanding the structural evolution process after the yielding of networks in polymer nanocomposites can provide significant insights into the design and fabrication of high-performance nanocomposites. In this work, using hydroxyl-terminated 1,4-polybutadiene (HTPB)/organo-clay nanocomposite gel as a model, we explored the yielding and recovery process of a polymer network. Linear rheology results revealed the formation of a nanocomposite gel with a house-of-cards structure due to the fully exfoliated 6 to 8 wt% organo-clays. Within this range, nonlinear rheologic experiments were introduced to yield the gel network, and the corresponding recovery processes were monitored. It was found that the main driving force of network reconstruction was the polymer–clay interaction, and the rotation of clay sheets played an important role in arousing stress overshoots. By proton double-quantum (1H DQ) NMR spectroscopy, residual dipolar coupling and its distribution contributed by HTPB segments anchored on clay sheets were extracted to unveil the physical network information. During the yielding process of a house-of-cards network, e.g., 8 wt% organo-clay, nearly one-fourth of physical cross-linking was broken. Based on the rheology and 1H DQ NMR results, a tentative model was proposed to illustrate the yielding and recovery of the network in HTPB/organo-clay nanocomposite gel.
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Rashid, Haroon, Yury Golitsyn, Muhammad Humayun Bilal, Karsten Mäder, Detlef Reichert y Jörg Kressler. "Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol)". Gels 7, n.º 1 (20 de febrero de 2021): 22. http://dx.doi.org/10.3390/gels7010022.
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Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.
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Fiorentino, Antonio, Brigida D'Abrosca, Palma Oriano, Annunziata Golino, Angela Natale y Pietro Monaco. "Structural Elucidation of a New Aromatic Metabolite from Melilotus Neapolitana and its Potential Allelopathic Effect on Wild Species". Natural Product Communications 2, n.º 2 (febrero de 2007): 1934578X0700200. http://dx.doi.org/10.1177/1934578x0700200210.
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From the ethereal extract of Melilotus neapolitana Ten., an herbaceous plant living in the Mediterranean macchia, a new aromatic metabolite was isolated. The structure 1-(2,4-dihydroxyphenyl)2-(4-hydroxy-2-methoxyphenyl)1,3-propandiol, was elucidated on the basis of its mass spectrum and 1D and 2D NMR spectroscopic data (DQ-COSY, TOCSY, HSQC, HMBC). The relative configuration was determined from NOESY/ROESY experiments and computational calculations. The compound was tested for its potential allelopathic effect on two wild plant species living in the same habitat as M. neapolitana.
Tesis sobre el tema "DQ-NMR"
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Le, Hel Cindy. "Propriétés et morphologies des thermoplastiques vulcanisés (TPV)". Electronic Thesis or Diss., Lyon, 2021. http://www.theses.fr/2021LYSE1239.
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Les thermoplastiques vulcanisés (TPV) sont des mélanges de polymères présentant à la fois des propriétés d'élasticité et de mise en œuvre et en forme. L'objectif de cette thèse est d'étudier les TPV à base d’un système PP/EPDM malgré la complexité de leurs formulations et de comprendre les mécanismes gouvernant la propriété de recouvrance élastique. La première partie de ce travail s'est concentrée sur l'influence de la chimie de réticulation sur les mécanismes de relaxation aux temps longs des réseaux élastomères réticulés. Il a été constaté que la réticulation radicalaire à partir de la décomposition thermique d’un peroxyde menait à une meilleure recouvrance élastique qu’avec la réticulation avec une résine phénolique. Cependant, la chimie de la réticulation n'est pas le paramètre de premier ordre qui influence cette propriété en ce qui concerne les TPV. En effet, elle dépend surtout de la formulation et du ratio thermoplastique/élastomère, ainsi, leurs influences sur les morphologies finales et sur la déformation rémanente en compression ont été étudiées dans une seconde partie. De plus, l'influence des charges inorganiques a également été étudiée dans les systèmes élastomères avec l'ajout de noir de carbone ou de silice ou dans les systèmes TPV avec l'ajout de noir de carbone. Il a été démontré que l'ajout de noir de carbone dans les systèmes réticulés au peroxyde est bénéfique et améliore les propriétés mécaniques : déformation rémanente après compression et comportement en traction (module d’Young, allongement à la rupture). En effet, la densité de réticulation et la compatibilité entre les polymères ont été améliorées. Enfin, une méthode RMN à bas champ a été utilisée pour l’étude de ces systèmes, notamment avec l'utilisation de la séquence Double Quanta (DQ) qui donne accès aux variations des mobilités moléculaires dans les matériaux avec la mesure du couplage dipolaire résiduelThermoplastics vulcanizates (TPVs) are polymer blends with interesting elasticity and processability properties. The objective of this thesis was to study PP/EPDM TPV despite the complexity of their formulations and to understand what influences the elastic recovery property. The first part of this work was focused on the influence of crosslinking chemistry and its statistics on the long-time relaxation mechanisms of crosslinked elastomeric networks. It was found that radical peroxide crosslinking provides better recovery elasticity than phenolic resin one. However, crosslinking chemistry has been seen to not be the first-order parameter that influences this property for TPV. It depends mostly on the formulation and the thermoplastic/elastomer ratio, their influence on the final morphology and compression set have been then studied in a second part. Moreover, the influence of inorganic fillers was also studied in elastomeric systems with the addition of CB or silica or in TPV systems with the addition of CB. It has been seen that the addition of CB in a system crosslinked with peroxides is beneficial for the improvement in mechanical properties: compression set or tensile test. Indeed, the crosslinking density and the compatibility between polymers were improved. Finally, a low-field NMR method has been applied to these systems, in particular with the use of the Double Quanta (DQ) sequence which gives access to the variations of molecular mobilities in the materials with the measurement of a residual dipolar coupling
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Baishya, Bikash. "Spectral Simplification In Scalar And Dipolar Coupled Spins Using Multiple Quantum NMR : Developments Of Novel Methodologies". Thesis, 2008. https://etd.iisc.ac.in/handle/2005/793.
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Spin selective MQ-SQ correlation has been demonstrated by either selective pulses in homo-nuclear spin systems in isotropic and weakly orienting chiral media or by nonselective pulses in hetero-nuclear spin systems in strongly aligned media. As a consequence of the spin selective correlation, the coherence transfer pathway from MQ to SQ is spin state selective. This two dimensional approach enables the utilization of the passive couplings (remote couplings) to break a complex one dimensional spectrum into many sub spectra. Each sub spectrum contains fewer transitions and hence fewer couplings (active couplings). The role of the passive couplings is to displace the sub spectra and measurement of the displacements taking into account their relative tilt provides the magnitude of the passive couplings along with relative signs. Further possibility of a spin state selective MQ-SQ resolved experiment to determine very small remote couplings otherwise buried within linewidth in one dimensional spectrum has been demonstrated. The resolution of the multiple quantum spectrum in indirect dimension has also been exploited to separate the sub spectra. The technique renders the analysis of complex spectrum in isotropic system much simpler. The potentialities of the technique have also been demonstrated for discrimination of optical enantiomers and derivation of the residual dipolar couplings from very complicated spectrum. The second order spectrum in strongly aligned media restrict selective excitation, however in hetero-nuclear spin system the nonselective pulses on protons do not interact with the hetero-nuclear spins. Thus the weakly coupled part of a strongly coupled spectrum has been exploited for simplifying the second order spectrum and thereby its analysis. Thus several methodologies derived from spin selective correlation has been demonstrated.
Enantiopure spectrum has been recorded from a mixture of R and S enantiomers by a novel pulse scheme called Double Quantum Selective Refocusing Experiment. The dipolar coupled methyl protons in weakly orienting media are utilized. The selective excitation of double quantum coherence reduces the three spin system into a two spin system and remote couplings are refocused which otherwise leads to broadening. The sum of passive couplings being different for the enantiomers resolution in the DQ dimension is enhanced and thereby their discrimination.
Finally several decoupling schemes has been compared in the indirect dimension of HSQC experiment to resolve 13C satellite spectra otherwise buried within line width for increased confidence in determining hetero-nuclear framework information.
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Baishya, Bikash. "Spectral Simplification In Scalar And Dipolar Coupled Spins Using Multiple Quantum NMR : Developments Of Novel Methodologies". Thesis, 2008. http://hdl.handle.net/2005/793.
Texto completoResumen
Spin selective MQ-SQ correlation has been demonstrated by either selective pulses in homo-nuclear spin systems in isotropic and weakly orienting chiral media or by nonselective pulses in hetero-nuclear spin systems in strongly aligned media. As a consequence of the spin selective correlation, the coherence transfer pathway from MQ to SQ is spin state selective. This two dimensional approach enables the utilization of the passive couplings (remote couplings) to break a complex one dimensional spectrum into many sub spectra. Each sub spectrum contains fewer transitions and hence fewer couplings (active couplings). The role of the passive couplings is to displace the sub spectra and measurement of the displacements taking into account their relative tilt provides the magnitude of the passive couplings along with relative signs. Further possibility of a spin state selective MQ-SQ resolved experiment to determine very small remote couplings otherwise buried within linewidth in one dimensional spectrum has been demonstrated. The resolution of the multiple quantum spectrum in indirect dimension has also been exploited to separate the sub spectra. The technique renders the analysis of complex spectrum in isotropic system much simpler. The potentialities of the technique have also been demonstrated for discrimination of optical enantiomers and derivation of the residual dipolar couplings from very complicated spectrum. The second order spectrum in strongly aligned media restrict selective excitation, however in hetero-nuclear spin system the nonselective pulses on protons do not interact with the hetero-nuclear spins. Thus the weakly coupled part of a strongly coupled spectrum has been exploited for simplifying the second order spectrum and thereby its analysis. Thus several methodologies derived from spin selective correlation has been demonstrated.
Enantiopure spectrum has been recorded from a mixture of R and S enantiomers by a novel pulse scheme called Double Quantum Selective Refocusing Experiment. The dipolar coupled methyl protons in weakly orienting media are utilized. The selective excitation of double quantum coherence reduces the three spin system into a two spin system and remote couplings are refocused which otherwise leads to broadening. The sum of passive couplings being different for the enantiomers resolution in the DQ dimension is enhanced and thereby their discrimination.
Finally several decoupling schemes has been compared in the indirect dimension of HSQC experiment to resolve 13C satellite spectra otherwise buried within line width for increased confidence in determining hetero-nuclear framework information.
Capítulos de libros sobre el tema "DQ-NMR"
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Hughes, Leslie P., Patrick M. J. Szell, Helen Blade y Steven P. Brown. "NMR Crystallography in Pharmaceutical Development". En Magnetic Resonance and its Applications in Drug Formulation and Delivery, 179–212. Royal Society of Chemistry, 2024. http://dx.doi.org/10.1039/9781788019996-00179.
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NMR crystallography is the combined use of experimental solid-state nuclear magnetic resonance (NMR) with density-functional theory (DFT) calculation of NMR parameters for a structure, as obtained, for example, by complementary diffraction or crystal structure prediction (CSP) approaches. We give an overview of how NMR crystallography can be applied to active pharmaceutical ingredients (APIs) and their formulations, including considering polymorphism, solvates and hydrates, salt and co-crystal formation, and amorphous dispersions. Specifically, the use of the gauge-including projector augmented wave (GIPAW) method, as implemented, for instance, in CASTEP or Quantum Espresso, is widely employed to calculate NMR chemical shifts for nuclei such as 1H, 13C, 14/15N, 19F, and 35Cl, as well as quadrupolar parameters for spin I ≥ 1 nuclei such as 14N and 35Cl, complementing experimental data obtained using magic-angle spinning (MAS). We describe the application of key MAS NMR experiments such as cross-polarisation (CP) MAS, notably for polymorph fingerprinting and determination of the number of distinct molecules in the asymmetric unit cell (Z′), and 1H-based two-dimensional experiments including heteronuclear correlation and double-quantum (DQ) MAS. Experiments probing internuclear dipolar couplings provide structural insight via identifying specific atomic proximities and determining specific distances and characterise dynamic processes via quantitative measurement of dipolar couplings.