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Dynamical processes related to viscous flow in a supercooled arsenic selenide glass-forming liquid: Results from high-temperature 77Se NMR spectroscopy
摘要： Dynamical processes in supercooled AsSe9 liquid are studied using 1D and 2D 77Se NMR spectroscopic techniques. The results demonstrate the coexistence of two distinct dynamical processes: one is a slow process related to the chemical exchange between the various Se environments by bond scission/renewal and the other is the fast Se chain segmental motion, which leads to the rapid averaging of the chemical shift anisotropy. The timescales of these two processes become increasingly similar as the glass transition is approached from above. The slow process is found to be closely coupled to viscosity over the entire temperature range of investigation and its activation energy is consistent with the Se-Se/As bond energies. On the other hand, the activation energy of the fast process is significantly higher, which may be indicative of its cooperative nature. This process becomes coupled to viscosity only in the immediate vicinity of the glass transition.
关键词： Supercooled liquid,Relaxation,Chalcogenide,Viscous flow,Glass transition,NMR spectroscopy
更新于2025-09-11 14:15:04
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Radical induced cationic frontal twin polymerization of Si-spiro compound in combination with bisphenol-A-diglycidylether
摘要： The radical induced cationic frontal polymerization (RICFP) of the twin monomer with 2,2’-spirobi[4H-1,3,2-benzodioxasiline] combination (SPIRO) in bisphenol-A-diglycidylether (BADGE) has been developed to fabricate nanostructured hybrid material with domain sizes of 2–5 nm. In one reaction step, an interpenetrating network of phenolic resin, SiO2 and the epoxy resin is formed in a very short time period, initiated by UV light. That indicates that both polymerization reactions take place simultaneously. The influence of monomer composition on molecular structure was investigated by means of solid state NMR spectroscopies. The envisaged nanostructure of the resulting organic-inorganic hybrid materials is proven by transmission electron microscopy high-angle annular dark-field scanning (HAADF-STEM). The thermal properties of the hybrid materials are comparable to “state of the art” materials with the advantage that the silica quantity can be adjusted on demand.
关键词： Nanostructured hybrid material,HAADF-STEM,Solid state NMR spectroscopy,Radical induced cationic frontal polymerization,Twin polymerization
更新于2025-09-11 14:15:04
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[IEEE 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018) - Paris, France (2018.7.8-2018.7.13)] 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018) - Development of a Shunt for High Intensity DC and AC Currents with NMR Measurement of the Magnetic Field
摘要： This paper describes the development of a new design of a DC and AC shunt to high intensity current. The working principle is based on the measurement of the magnetic field using a nuclear magnetic ressonance or a NMR method. The traceability comes only from frequency and a geometric parameter. The preliminary results of a prototype are presented. Also, it is described a new non-inductive method to measure the magnetic AC component. The final device - a shunt to be used as a primary standard - is intended to operate in kA range.
关键词： Current measurement,high intensity current,NMR shunt,AC shunt,NMR measurement,current standards
更新于2025-09-10 09:29:36
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Investigation of luminescence mechanism of Nd3+-doped calcium aluminate glasses: Effect of glass-formers
摘要： The relationship between the luminescence properties and the glass structure of Nd3+-doped CaO-Al2O3-(B2O3, SiO2, or Ga2O3) glasses are investigated. The luminescence properties of CaO-Al2O3-B2O3 system are twice as much as that of the CaO-Al2O3-(SiO2 or Ga2O3) system. The glass structure and the coordination environment of the Nd3+ ions were investigated using nuclear magnetic resonance, Raman and electron paramagnetic resonance spectroscopy. All Al, Ga, and Si atoms are four coordinated. The B atoms have three- and four-coordinated boron units, however, most of them are three coordinated. The hyper?ne sublevel correlation spectra reveal that the Al and B nuclei are in the coordination sphere of Nd3+ ions for aluminoborate glasses; however, only 27Al signals could be observed for aluminosilicate and aluminogallate glasses. Owing to the increased number of AlO4-BO3 connections and 11B nuclei around the Nd3+ ions, B has a signi?cant e?ect on the luminescence properties of the Nd3+ ions. The increase in GaO4 or SiO4 does not change the three dimensional-tetrahedral network, and the coordination environment of Nd3+ ions varies slightly. Thus, the e?ect of Ga2O3 and SiO2 on the luminescence properties of the Nd3+ ions is not as great as that of B2O3.
关键词： Glass structure,Local environments of rare-earth ions,EPR,Luminescence properties,NMR
更新于2025-09-10 09:29:36
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Absorption and luminescence studies of Sm3+ ions activated in distinct phosphate glasses for reddish orange light applications
摘要： In the present work, we report the optical properties of Sm3+ ions doped glasses having chemical compositions (70-x) NH4H2PO4+15Na2O+15M2O + xSm2O3 (where x = 0.5 mol% and M = Li, Mg, Ca, Sr, Ba). Structural details are obtained from XRD, SEM, FTIR, Raman and solid-state 31P NMR spectra. Absorption (UV-NIR), emission and lifetime measurements have been performed and discussed using Judd-Ofelt theory. Various radiative parameters are calculated and discussed. Visible emissions from 4G5/2 excited state have been characterized and the experimental lifetimes of these levels have been compared with those obtained theoretically by using J-O approach. Quantum e?ciencies are reported. The CIE chromaticity co-ordinates of the present glass matrices con?rmed reddish-orange light emission. Hence the prepared samples can be treated as candidates for reddish orange light applications.
关键词： Photoluminescence,Phosphate glass,Absorption,FTIR,Emission cross-section,Solid state 31P NMR
更新于2025-09-10 09:29:36
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Spectroscopic Investigation (FT-IR, FT-Raman, NMR and UV-Vis),Conformational Stability, NBO and Thermodynamic Analysis of 1-(2-Methoxyphenyl) Piperazine and 1-(2-Chlorophenyl) Piperazine by DFT Approach
摘要： The Vibrational and electronic properties of phenyl substituted compounds 1-(2-methoxyphenyl)piperazine and 1-(2-chlorophenyl)piperazine have been investigated by FT-IR, FT-Raman, NMR and UV-Vis spectral measurements. Density functional theory (DFT) method, using B3LYP functional, with 6-311++G (d,p) basis set, has been performed for assigning vibrational frequencies of the title compounds, which also helps to derive useful information about the structure of the chosen compounds. A detailed interpretation of the Infrared and Raman spectra of the two molecules were reported based on potential energy distribution (PED). 13C and 1H NMR chemical shifts of the molecules were calculated using the gauge independent atomic orbital (GIAO) method. These studies satisfactorily agree the experimental data. Charge density distribution and site of chemical reactivity of the molecule have been studied by mapping electron density isosurface with molecular electrostatic potential (MEP). Stability of the molecules arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital (NBO) analysis. The compounds have similar HOMO (Highest Occupied Molecular Orbitals) - LUMO (Lowest Unoccupied Molecular Orbitals) gap due to similarity in their structures. The compounds show (π→ π*) transitions in the UV- Visible range.
关键词： NMR,1-(2-methoxyphenyl)piperazine,Natural bond orbitals,DFT,1-(2-chlorophenyl)piperazine,HOMO-LUMO
更新于2025-09-10 09:29:36
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Investigation of Structure and Dynamics in a Photochromic Molecular Crystal by NMR Crystallography
摘要： A photochromic anil, N-(3,5-di-t-butylsalicylidene)-4-amino-pyridine, has been studied by single-crystal X-ray diffraction, multinuclear magic-angle spinning NMR and first-principles density functional theory (DFT) calculations. Interpretation of the solid-state NMR data on the basis of calculated chemical shifts confirms the structure is primarily composed of molecules in the ground-state enol tautomer, while thermally-activated cis-keto and photoisomerised trans-keto states exist as low-level defects with populations that are too low to detect experimentally. Variable temperature 13C NMR data reveals evidence for solid-state dynamics which is found to be associated with fast rotational motion of t-butyl groups and 180° flips of the pyridine ring, contrasting the time-averaged structure obtained by X-ray diffraction. Comparison of calculated chemical shifts for the full crystal structure and an isolated molecule also reveals evidence for an intermolecular hydrogen-bond involving the pyridine ring and an adjacent imine carbon which facilitates the flipping motion. The DFT calculations also reveal that the molecular conformation in the crystal structure is very close to the energetic minimum for an isolated molecule, indicating that the ring dynamics arise as a result of considerable steric freedom of the pyridine ring and which also allows the molecule to adopt a favourable conformation for photochromism.
关键词： NMR crystallography,photochromic,anil,solid-state NMR,DFT calculations,molecular dynamics
更新于2025-09-09 09:28:46
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Silver polymer complexes as precursors of nanocomposites based on polymers of 1-vinyl-1,2,4-triazole
摘要： Bioactive silver-containing (Ag content of about 6.8-8.2 wt %) polymer complexes based on the synthetic homopolymer of 1-vinyl-1,2,4-triazole and its copolymers have been synthesized and characterized. Structural features of macromolecular complexes were studied by magnetic resonance techniques (EPR, NMR) and other analytical methods (FTIR, SEM, UV-vis, atomic absorption analysis, etc.). The spatial structure of the polymer chain fragment and silver ion/1-vinyl-1,2,4-triazole complexes were investigated by quantum-chemical calculations. The formation of coordinatively saturated polymer complexes of Ag(I) and highly oxidized forms of Ag(II) silver was analyzed. The sites of ions localization and reduced metallic silver in the structure of the polymer matrix were determined.
关键词： Complex,EPR,Polymer,Silver,1-Vinyl-1,2,4-triazole,NMR
更新于2025-09-09 09:28:46
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The effect of the length of terminal n-alkyl carboxylate chain on self-assembling and photosensitive properties of chiral lactic acid derivatives
摘要： A new series of photosensitive azo materials possessing a chiral alkyl lactate moiety and terminal n-alkyl carboxylate unit close to the azo group has been synthesised and studied. The length of the n-alkyl carboxylate chain has been systematically varied in order to establish the effect of the molecular structure on the self-assembling behaviour. Two series of materials possessing hexyl and dodecyl alkyl chains in the chiral part of the molecule have been studied. It has been shown that the length of both the alkyl chains strongly influences the mesomorphic behaviour, however, each chiral/achiral chain has different utility to tune the mesomorphic properties. With exception of the compound with the longest chains, all studied compounds exhibited the chiral tilted ferroelectric smectic C* phase. Based on the combination of terminal alkyl chains, chiral nematic, orthogonal smectic A*, and twist grain boundary smectic A* phases have been detected on cooling beyond the SmC* phase. The presence of the photosensitive functional N=N group in the molecular core allowed further tuning of the material properties by UV light illumination. The E-to-Z photoisomerization of the azo group and subsequent thermal back-isomerisation have been studied in solution by nuclear magnetic resonance and most importantly in the mesophases on bulk samples. We report on UV-induced isothermal switching from chiral smectic and nematic mesophases directly into the isotropic phase, respectively, and differences in the textures of mesophase upon restoration of the ordered liquid.
关键词： E-Z isomerisation,ferroelectric phase,photosensitive liquid crystals,azo group,chiral liquid crystals,NMR spectroscopy,optical switches
更新于2025-09-09 09:28:46
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Nuclear and Electron Magnetic Resonance Spectroscopies of Atomically Precise Gold Nanoclusters
摘要： Atomically precise gold nanoclusters display properties that are unseen in larger nanoparticles. When the number of gold atoms is sufficiently small, the clusters exhibit molecular properties. Their study requires extensive use of classic molecular physical chemistry and, thus, methods such as theory and vibrational spectroscopies, electrochemistry, density functional molecular dynamics calculations, and of course nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. NMR and EPR studies have been mostly carried out on the benchmark, stable molecules Au25(SR)18, Au38(SR)24, Au102(SR)44, and Au144(SR)60 (where SR = thiolate). In this Account, we showcase examples primarily taken from our previous and ongoing NMR and EPR studies, which we hope will trigger further interest in the use of these sensitive, though often underutilized, techniques. Indeed, 1D and 2D NMR spectra of pure, atomically precise clusters can be very detailed and informative. Molecular clusters are molecules and, thus, have discrete energy levels and undergo stepwise oxidation or reduction. The effect of the charge state on the chemical shifts and line shapes is a function of the ligand type (ligands differ due to specific bonds with different Au atom types) and the position of the chemical group along the ligand backbone: for groups near the Au core, they can be very dramatic. Ligand-protected gold clusters are hard?soft molecules where a hard metal core is surrounded by a dynamic molecular layer. The latter provides a nanoenvironment that interfaces the cluster core with the surrounding environment and can be permeated by molecules and ions. NMR spectroscopy is especially useful to assess its structure. For example, the data show that whereas long alkanethiolates form bundles, shorter chains exhibit more conformational freedom and are quite folded. NMR spectroscopy allows studying diastereotopic effects and provides information on possible hydrogen bonds of ligands with sulfur or surface gold atoms. EPR spectroscopy is a very precise technique to check and characterize the magnetic state of gold clusters or clusters doped with foreign-metal atoms. Electron nuclear double resonance (ENDOR) provides a powerful tool to assess the interaction of an unpaired electron with nuclei, as we showed for 197Au and 1H. It can be used as a sensitive probe of the spin-density distribution in nanoclusters: for example, it showed that the singly occupied molecular orbital may span outside the Au core by nearly 6 ?. Solid-state EPR spectroscopy has provided compelling evidence that the specific ligands and the crystallinity degree are very important factors in determining the interactions between clusters in the solid state. Depending on the condition, paramagnetic, superparamagnetic, ferromagnetic, or antiferromagnetic behavior can be observed. Time-resolved EPR was successfully tested to determine the efficiency of singlet-oxygen generation via sensitization of Au25 clusters. This Account thus demonstrates some of the remarkable insights that can be gained into the properties of atomically precise clusters through detailed NMR and EPR studies.
关键词： EPR,gold nanoclusters,molecular properties,atomically precise,NMR
更新于2025-09-09 09:28:46