Nuclear Magnetic Resonance Linewidth Research Articles

Phase structure and mechanical properties of disentangled ultra-high molecular weight polyethylene/polyhedral oligomeric silsesquioxane nanocomposites in a solid state

Ultra-high molecular weight polyethylene (UHMWPE)/disilanolisobutyl polyhedral oligomeric silsesquioxane (POSS) nanocomposites in a disentangled state were synthesized by in situ ethylene polymerization. This is the first report of a detailed study on the phase structure and mechanical properties of disentangled UHMWPE/ POSS nanocomposites in a solid state. The phase structure and chain dynamics of UHMWPE nanocomposites were characterized using wide-line proton NMR and 13C cross-polarization/magic-angle spinning (CP/MAS) solid-state nuclear magnetic resonance (NMR). The results showed that a strong interaction between the POSS and nascent UHMWPE matrix occurred in a solid state, especially in the intermediate and monoclinic crystalline phases. This strong interaction between POSS particles and UHMWPE chains may result in improved melt recovery properties. Mechanical testing indicated that the lowest friction coefficients and highest breaking strength were achieved when 1 wt% loading of POSS was incorporated. The influence of the microstructure on lubrication and mechanical properties was also studied.

11B pulsed nuclear magnetic resonance study of HoB4

11B pulsed nuclear magnetic resonance (NMR) measurements were performed on a single crystal of HoB4 to investigate disorder-induced effects on the 4f spin structures and dynamics. The 11B NMR spectrum, shift, linewidth, and spin-lattice and spin–spin relaxation rates (1/T1 and 1/T2, respectively) were measured down to 3.5 K at a field of 8 T perpendicular to the c-axis. Above the Né el temperature, TN = 5.7 K, the 11B NMR linewidth is very large and the shift is also large and with negative sign. In addition, both are strongly temperature-dependent and the absolute shift value and linewidth increase at lower temperatures. This fact confirms that the hyperfine field at the boron sites originates mainly from the Ho 4f spins, as this is also the case for the susceptibility. Below TN, the 11B NMR spectrum displays a single, broad and featureless shape with an extremely large linewidth. This behavior is an unexpected result compared with typical NMR spectra in the magnetically ordered state for single crystal specimens, and is significantly different to that of other RB4 substances. Considering frustration and disorder effects on the static and dynamic NMR data, we conclude that this behavior originates from the magnetic frustration and quadrupole moment disorder effects on the NMR static data, since the Ho ions form a Shastry-Sutherland (SS)-type lattice in the ab plane. Above TN, both relaxation rates (1/T1 and 1/T2) are very large as a result of the rapid and random fluctuation of the Ho 4f moments, and they then increase toward the Né el transition. Below TN, both rates decrease significantly since the 4f spin fluctuations and dynamics are frozen as a result of the antiferromagnetic ordering.

Deformation Defects Supporting Quantum Readout of 29 Si Nuclear Spins in Si: P Deformed Crystals

High temperature (950 °C) plastic deformation of the 29Si enriched silicon crystals generates ˜10 15 cm−3 paramagnetic defects of the new type. The multiline spectrum of electron paramagnetic resonance (EPR) of the defects corresponds to S = 1 electronic spin of the clusters containing oxygen. Strong anisotropy and large linewidth of the EPR spectra (˜1 kOe) were observed. The Pake doublet split by spin-spin nuclear dipole interaction was analyzed in the nuclear magnetic resonance (NMR) spectra of the crystals. Contribution of the electron-nuclear interaction to the EPR linewidth is negligibly small, while the NMR linewidth is controlled by the dipole-dipole nuclear relaxation.

Slow reactant-water exchange and high catalytic performance of water-tolerant Lewis acids.

(31)P nuclear magnetic resonance (NMR) spectroscopic measurement with trimethylphosphine oxide (TMPO) was applied to evaluate the Lewis acid catalysis of various metal triflates in water. The original (31)P NMR chemical shift and line width of TMPO is changed by the direct interaction of TMPO molecules with the Lewis acid sites of metal triflates. [Sc(OTf)3] and [In(OTf)3] had larger changes in (31)P chemical shift and line width by formation of the Lewis acid-TMPO complex than other metal triflates. It originates from the strong interaction between the Lewis acid and TMPO, which results in higher stability of [Sc(OTf)3TMPO] and [In(OTf)3TMPO] complexes than other metal triflate-TMPO complexes. The catalytic activities of [Sc(OTf)3] and [In(OTf)3] for Lewis acid-catalyzed reactions with carbonyl compounds in water were far superior to the other metal triflates, which indicates that the high stability of metal triflate-carbonyl compound complexes cause high catalytic performance for these reactions. Density functional theory (DFT) calculation suggests that low LUMO levels of [Sc(OTf)3] and [In(OTf)3] would be responsible for the formation of stable coordination intermediate with nucleophilic reactant in water.

Strain and origin of inhomogeneous broadening probed by optically detected nuclear magnetic resonance in a (110) GaAs quantum well

We obtained strain and electric field gradient (EFG) in an $n$-GaAs/Al${}_{0.3}$Ga${}_{0.7}$As (110) quantum well (QW) by optically detected nuclear magnetic resonance (NMR). The dependence of the quadrupolar splitting on an angle between the QW plane and a static magnetic field provided the crystalline-orientation-dependent EFG and strain in the GaAs QW. We also explored the dependence of the NMR line widths on the direction of the external magnetic field with respect to the QW plane. It is likely that the nonuniform EFG as well as the hyperfine interaction governs the inhomogeneous broadening of NMR spectra.

Nuclear magnetic resonance at up to 10.1 GPa pressure detects an electronic topological transition in aluminum metal

High-sensitivity 27Al nuclear magnetic resonance (NMR) measurements of aluminum metal under hydrostatic pressure of up to 10.1 GPa reveal an unexpected negative curvature in the pressure dependence of the electronic density of states measured through shift and relaxation, which violates free electron behavior. A careful analysis of the Fermiology of aluminum shows that pressure induces an electronic topological transition (Lifshitz transition) that is responsible for the measured change in the density of states. The experiments also reveal a sudden increase in the NMR linewidth above 4.2 GPa from quadrupole interaction, which is not in agreement with the metal’s cubic symmetry.

31P nuclear magnetic resonance study of the proton-irradiated KTiOPO4

31P nuclear magnetic resonance (NMR) was employed to study the effects of proton irradiation on KTiOPO4 (KTP) in view of the previously studied paramagnetic impurity doping effects. High-resolution 31P NMR measurements showed significant increase in the isotropic chemical shifts of the two inequivalent phosphorus sites in the proton-irradiated KTP system, indicating decrease in the electron density around the phosphorous nuclei. The 31P NMR linewidths of the KTP system manifested anomalies associated with the superionic transition and with the polaron formation, which became much weaker after proton irradiation. Besides, the activation energy of the charge carriers increased significantly after proton irradiation.

Nuclear magnetic resonance as a probe of electronic states of Bi2Se3

We present magnetotransport and Bi-209 nuclear magnetic resonance (NMR) data on a series of single crystals of Bi2Se3, Bi2Te2Se and Cu_xBi2Se3 with varying carrier concentrations. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 ueV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies.

Charge density-dependent coil–globule transition of alkali metal polycarboxylates in aqueous organic solvent mixtures

The effects of polymer charge density on the counterion-specific and solvent-specific coil–globule transition of polycarboxylates were investigated for alkali metal salts of poly(styrene-alt-maleic acid) (PSaltMA) and poly(acrylic acid) (PAA) in aqueous organic solvent mixtures. The order of the transition region, namely, the counterion specificity for the transition in, e.g., aqueous dimethyl sulfoxide (DMSO), was the same for both polyelectrolytes, Na+ > K+ > Cs+ > Li+, while the discrepancy of the transition region between Na+ and Li+ systems was appreciably narrower for PSaltMA (approximately 20 vol%) than that of PAA (approximately 29 vol%). Such diminished counterion specificity for the former was ascribed to the nonuniform charge array. Namely, PSaltMA has two kinds of nearest charge arrays, one is the shorter spacing between the maleic acid carboxyl groups and the other is the longer one via one styrene group. Thus, the former may be favorable for binding of the smaller counterion (i.e., Li+) and the latter for the larger one (Cs+). Such a “size-fitting effect” for the counterion binding was in fact further confirmed with variously neutralized PAAs. For example, the counterion specificity in aqueous DMSO of PAA40 that was neutralized to 40 % was Cs+ > K+ > Na+ > Li+, showing that the largest counterion becomes most favorable in inducing the transition with increasing average charge spacing. In fact, the nuclear magnetic resonance line width measurement for 133Cs suggested that the counterion binding strength of the large counterion for PAA increases with decreasing charge density from 100 to 40 % neutralization.

On the Morphology of a Discotic Liquid Crystalline Charge Transfer Complex

Discotic liquid crystalline (DLC) charge transfer (CT) complexes, which combine visible light absorption with rapid charge transfer characteristics within the CT complex, can have a great potential for photovoltaic applications when they can be made to self-assemble in a bulk heterojunction arrangement with separate channels for electron and hole conduction. However, the morphology of some liquid crystalline CT complexes has been under debate for many years. In particular, the liquid crystalline CT complex built from the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF) and discotic molecules has been reported to have the TNF "sandwiched" either between the discotic molecules within the same column or between the columns within the aliphatic tails of the discotic molecules. We present a detailed structural study of the prototypic 1:1 mixture of the discotic 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6) and TNF. Nuclear magnetic resonance (NMR) line widths and cross-polarization rates are consistent with the picosecond time scale anisotropic thermal motions of the HAT6 and TNF molecules previously observed. By computational integration of Rietveld refinement analyses of neutron diffraction patterns with density experiments and short-range structural constraints from heteronuclear 2D NMR, we determine that the TNF molecules are vertically oriented between HAT6 columns. The data provide the insight that a morphology of separate hole conducting channels of HAT6 molecules can be realized in the liquid crystalline CT complex.

Dynamic processes and chemical composition of Lepidium sativum seeds determined by means of field-cycling NMR relaxometry and NMR spectroscopy

Proton nuclear magnetic resonance (NMR) techniques, such as field-cycling relaxometry, wide-line NMR spectroscopy, and magic angle spinning NMR spectroscopy, were applied to study the seeds of cress, Lepidium sativum. Field-cycling NMR relaxometry was used for the first time to investigate the properties of the whole molecular system of dry cress seeds. This method not only allowed the dynamics to be studied, but was also successful in the differentiation among the solid (i.e., carbohydrates, proteins, or fats forming a solid form of lipids) and liquid-like (oil compounds) components of the seeds. The (1)H NMR relaxation dispersion of oils was interpreted as a superposition of intramolecular and intermolecular contributions. The intramolecular part was described in terms of a Lorentzian spectral density function, whereas a log-Gaussian distribution of correlation times was applied for the intermolecular dipole-dipole contribution. The models applied led to very good agreement with the experimental data and demonstrate that the contribution of the intermolecular relaxation to the overall relaxation should not be disregarded, especially at low frequencies. A power-law frequency dependence of the proton relaxation dispersion was used for the interpretation of the solid components. From the analysis of the (1)H wide-line NMR spectra of the liquid-like component of hydrated cress seeds, we can conclude that the contribution of oil protons should always be taken into account when evaluating the spin-lattice relaxation times values or measuring the moisture and oil content. The application of (1)H magic angle spinning NMR significantly improves resolution in the liquid-like spectrum of seeds and allows the determination of the chemical composition of cress seeds.

Measurement of electron spin-lattice relaxation times in radical doped butanol samples at 1 K using the NEDOR method

The electron spin-lattice relaxation time (T1e) of TEMPO- and trityl-doped butanol samples at 2.5T and temperatures between 0.95K and 2.17K was studied by pulsed nuclear magnetic resonance (NMR) using the nuclear-electron double resonance (NEDOR) method. This method is based on the idea to measure the NMR lineshift produced by the local field of paramagnetic impurities, whose polarization can be manipulated. This is of technical advantage as measurements can be performed under conditions typically used for the dynamic nuclear polarization (DNP) process – in our case 2.5T and temperatures around 1K – where a direct measurement on the electronic spins would be far more complicated to perform.As T1e is a crucial parameter determining the overall efficiency of DNP, the effect of the radical type, its spin concentration, the temperature and the oxygen content on T1e has been investigated.For radical concentrations as used in DNP (several 1019spins/cm3) the relaxation rate (T1e−1) has shown a linear dependence on the paramagnetic electron concentration for both radicals investigated. Experiments with perdeuterated and ordinary butanol have given no indication for any influence of the host materials isotopes. The measured temperature dependence has shown an exponential characteristic. It is further observed that the oxygen content in the butanol samples has a considerable effect on the electron relaxation time and thus influences the nuclear relaxation time and polarization rate during the DNP.The experiments also show a variation in the NMR linewidth, leading to comparable time constants as determined by the lineshift. NEDOR measurements were also performed on irradiated, crystal grains of 6LiD. These samples exhibited a linewidth behavior similar to that of the cylindrically shaped butanol samples.

Incommensurate spin-density wave and a multiband superconducting phase in NaxFeAs revealed by nuclear magnetic resonance

We report a $^{23}$Na and $^{75}$As nuclear magnetic resonance (NMR) investigation of Na$_{x}$FeAs series ($x=1$, 0.9, 0.8) exhibiting a spin-density wave (SDW) order below $T_{\rm SDW}=45$, 50 and 43 K for $x=1$, 0.9, 0.8, respectively, and a bulk superconductivity below $T_c\approx 12$ K for x=0.9. Below $T_{\rm SDW}$, a spin-lattice relaxation reveals the presence of gapless particle-hole excitations in the whole $x$ range, meaning that a portion of the Fermi surface remains gapless. The superconducting fraction as deduced from the bulk susceptibility scales with this portion, while the SDW order parameter as deduced from the NMR linewidth scales inversely with it. The NMR lineshape can only be reproduced assuming an incommensurate (IC) SDW. These findings qualitatively correspond to the mean-field models of competing interband magnetism and intraband superconductivity, which lead to an IC SDW order coexisting with superconductivity in part of the phase diagram.

125Te NMR chemical shifts and tellurium coordination environments in crystals and glasses in the Ge–As–Sb–Te system

The local coordination environments of Te atoms have been investigated in crystalline and glassy binary and ternary tellurides in the system Ge–As–Sb–Te using 125Te solid-state wideline nuclear magnetic resonance (NMR) spectroscopy. The average 125Te NMR chemical shifts in these materials range from 300 to 1050, 90 to 700 and −2000 to −4100ppm for 2, 3 and 6-coordinated environments, respectively. Te atoms are predominantly 2-coordinated in binary Ge–Te, As–Te and ternary Ge–As–Te glasses. The 125Te NMR spectrum of the cubic Ge1Sb2Te4 phase with rock salt structure is consistent with a random distribution of Ge/Sb vacancies in the lattice. Besides the coordination number, the 125Te chemical shifts in these materials are also found to be sensitive to the chemical identity of the nearest neighbors. 125Te NMR spectroscopy shows significant future promise in its application as a technique complementary to diffraction and EXAFS in understanding the short-range structure of amorphous Ge–As–Sb tellurides.

In memory of Valentina Abramovna Ioffe (to the 100th anniversary of her birth)

The year 2010 marks the 100th anniversary of thebirth of Professor, Doctor of Physical and Mathematical Sciences, the participant of the Great PatrioticWar, Valentina Abramovna Ioffe, awarded manyorders and medals. Valentina Abramovna Ioffe cameto the Institute of Silicate Chemistry of the Academyof Sciences of the Soviet Union in 1951, immediatelyafter its foundation, and introduced a new directioninto research works performed at the institute, i.e., thestudy of electrical properties of glasses. More recently,she organized a group for the investigation of electricalproperties of dielectrics. In 1963, on the initiative ofthe Director of the Institute N.A. Toropov, the Laboratory of Electronic Processes was created, which,over the next 15 years, was headed by V.A. Ioffe.The first stage of investigations carried out in thelaboratory was concerned with the analysis of mechanisms of charge compensation in the lattice of transition and rareearth metal oxides, as well as with theinfluence of lattice defects on electrical conductivity.This period of the scientific activity of V.A. Ioffe wasdevoted to systematic investigations of dielectric lossesin silica glasses, ferrites, crystalline aluminosilicates,and rareearth aluminates at low temperatures. Iofferevealed a very high sensitivity of dielectric losses tothe degree of depolymerization of the structural glassnetwork. She paid much attention to the study of aluminates of some rareearth elements in which the permittivity reaches 600. More recently, these works wereawarded the medals of the AllUnion Exhibition ofAchievements of National Economy of the SovietUnion.The other side of the V.A. Ioffe activity consisted ininvestigating electrical properties of oxides of variablevalence elements and the mechanism of their conduction. These studies led to the development of the concept of the influence of structural defects on electricalproperties of complex oxides and to the necessity ofstudying the mechanism of defect formation. At thesame time, the range of investigation methodsemployed by V.A. Ioffe was extended by using opticaland magnetic resonance techniques, as well as methods for studying electrical conductivity, thermopower,and permittivity. Electron paramagnetic resonancespectrometers (RE1301, JEOL) and the widelinenuclear magnetic resonance spectrometer were usedover a wide range of temperatures (down to 77 K).Under the supervision of V.A. Ioffe, the procedure forgrowing highquality single crystals of complex oxidesof transition metals and rareearth ions by means ofspontaneous crystallization from own melt, crystallization from solutions in lowmelting salts, and gastransport reactions was implemented at the laboratory.The history of scientific investigations of the laboratory at the first stage of its development wasdescribed by Ioffe in [1].The second stage (since 1978) has been associatedwith the advent of a younger generation, who creatednew experimental and theoretical capabilities for investigations, i.e., the study in the range of liquidheliumtemperatures (down to 4 K) on the RE1306 spectrometer and the widespread use of computational methodsfor simulating physical properties of materials, whichmade it possible to investigate phase transitions and

Dynamics in the isotropic and nematic phases of bent-core liquid crystals: NMR perspectives

The dynamic properties of low-molecular-weight bent-core liquid crystals were revealed to be unusual with respect to the analogous rod-like mesogens. First pioneering investigations based on dynamic light scattering measurements were followed by intense and deep studies based on nuclear magnetic resonance (NMR) spectroscopy, mainly 2H NMR relaxation and line width analyses, 1H NMR relaxometry and diffusometry. These studies were of help in identifying the molecular motions responsible of the observed slow relaxation and the large 2H NMR spectral width. Viscosity and rheology measurements provided further elements confirming the slow dynamics of bent-core liquid crystals at a macroscopic level. All these experimental techniques indicate the presence of restricted dynamic motions as a distinctive feature of bent-core liquid crystals both in the isotropic and nematic phases. In this review, the case of several bent-core mesogens studied by different NMR methods is reported and the main results are discussed in view of the recent achievements concerning the tendency of bent-core molecules to form aggregates or temporary clusters both in the isotropic and nematic phases.

Comparison of the magnetic properties and the spin dynamics in heterometallic antiferromagnetic molecular rings

We present a comparison of the results obtained in the antiferromagnetic (AFM) homometallic ring ${\text{Cr}}_{8}$ with the results in three heterometallic rings with a Cr ion replaced by Cd, Ni, and Fe ions, i.e., ${\text{Cr}}_{7}\text{Cd}$, ${\text{Cr}}_{7}\text{Ni}$, and ${\text{Cr}}_{7}\text{Fe}$, respectively. The experimental results include magnetic susceptibility, $^{1}\text{H}$ nuclear-magnetic-resonance (NMR) spectra, spin-spin and spin-lattice relaxation rates data, collected in the temperature range $1.65<T<300\text{ }\text{K}$ at two applied magnetic fields. The data include both new results and previously published data. The static magnetic properties derived from susceptibility and $^{1}\text{H}$ NMR linewidth can be analyzed in a simple way in terms of the properties of the individual ions constituting the ring and their nearest-neighbor antiferromagnetic exchange coupling. The nuclear-spin-lattice relaxation rate at low temperature can be described phenomenologically by a model, which assumes a single-correlation time for the relaxation of the magnetization, as used previously for homometallic AFM rings. The correlation frequencies obtained from the fit of the data, increase by as much as two orders of magnitude in the ${\text{Cr}}_{7}\text{Ni}$ and ${\text{Cr}}_{7}\text{Fe}$ rings with respect to the homometallic ring ${\text{Cr}}_{8}$ and the diamagnetically substituted ${\text{Cr}}_{7}\text{Cd}$ ring. This result can be explained qualitatively in terms of a change in spin-phonon coupling due to the enhancement of crystal-field effects in the heterometallic rings. For a more quantitative analysis one should take into account the multi-Lorentzian behavior of the spin-spin correlation function for which detailed theoretical calculations are required. At temperatures higher than the magnetic exchange energy $J/{k}_{B}$, the mechanism for nuclear-spin-lattice relaxation changes since the fluctuations of the moments of the magnetic ions become weakly correlated. We find a fluctuation frequency much higher in the heterometallic rings as a result of the perturbation introduced by the substituted magnetic ion.

Solution NMR Investigation of the CD95/FADD Homotypic Death Domain Complex Suggests Lack of Engagement of the CD95 C Terminus

We have addressed complex formation between the death domain (DD) of the death receptor CD95 (Fas/APO-1) with the DD of immediate adaptor protein FADD using nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and size-exclusion chromatography with in-line light scattering. We find complexation to be independent of the C-terminal 12 residues of CD95 and insensitive to mutation of residues that engage in the high-order clustering of CD95-DD molecules in a recently reported crystal structure obtained at pH 4. Differential NMR linewidths indicate that the C-terminal region of the CD95 chains remains in a disordered state and (13)C-methyl TROSY data are consistent with a lack of high degree of symmetry for the complex. The overall molecular mass of the complex is inconsistent with that in the crystal structure, and the complex dissociates at pH 4. We discuss these findings using sequence analysis of CD95 orthologs and the effect of FADD mutations on the interaction with CD95.

Investigation of electrochemically hydrogenated carbon nanotubes

In the given work the results of investigation of hydrogen interaction with multiwall conic carbon nanotubes (CNT) during their electrolytic hydrogenation in a water solution of a sulphuric acid are presented. Hydrogen sorption by carbon nanotubes was studied by 1H nuclear magnetic resonance (NMR) spectroscopy. It has been found, that hydrogen chemisorption at dangling bonds of “bamboo”- type and “fish bone”-type carbon nanotubes from hydrocarbonic plasma occurs directly during their growth. The significant narrowing of the 1H NMR linewidth in the process of annealing and electrochemical hydrogenation of carbon nanotubes is revealed.

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.