Nuclear Magnetic Resonance Linewidth Research Articles

Nuclear magnetic resonance probe for supercritical water and aqueous solutions

A nuclear magnetic resonance (NMR) probe for high pressure, high temperature studies is presented. While applicable to many physical systems, the device is optimized for the study of the physics and chemistry of supercritical water and its solutions. The design is modular and is particularly simple, using readily available parts and materials. A new approach is presented for elimination of the magnetic field from the heater currents. The probe has been used to 600 °C and 400 bar. The rf performance is quite good; the NMR linewidth is about 0.1 ppm full width at half-height at any pressure and temperature.

NMR and studies of non-Fermi-liquid behaviour in disordered heavy-fermion systems

Evidence has emerged from nuclear magnetic resonance (NMR) and muon spin-rotation experiments that non-Fermi-liquid (NFL) behaviour in heavy-fermion alloys can in some cases be due to an inhomogeneous distribution of Kondo temperatures arising from disorder in the random alloy. Such `Kondo disorder' implies a broad distribution of the heavy-fermion spin polarization, which is reflected in the widths of NMR and lines. A simple model for the shape and width of the distribution of accounts for the temperature and field dependence of the bulk susceptibility in the NFL alloys , x = 1.0 and 1.5, and then agrees with NMR values of the width of the susceptibility distribution with no further adjustable parameters. Comparison of NMR and estimates of indicates that the susceptibility is disordered on an atomic length scale. In contrast, lines in are too narrow for Kondo disorder to account for NFL properties unless, as seems unlikely, the correlation length is long in this alloy. Similarly, small low-temperature NMR linewidths make it unlikely that Kondo disorder is the origin of NFL behaviour in .

13C and1H amorphous linewidths in semicrystalline poly(4-hydroxybutyrate) and poly(3-co-4-hydroxybutyrate) aboveTg by high resolution and solid-state NMR experiments

The high-resolution 13C and 1H nuclear magnetic resonance (NMR) linewidths of semi-crystalline poly(4-hydroxybutyrate), P4HB, and poly(3-hydroxybutyrate-co-4-hydroxybutyrate), (P3/4HB-18, 18% 4HB units) in the amorphous phase and in the melt are studied as a function of temperature and magnetic field strength. Measurements of the 13C spin-spin relaxation times under the same experimental conditions show that the natural line-width is a minor contributor to the line-broadening observed in the 13C spectra of the solid polymers. A variety of coherent averaging solid-state NMR methods are used to examine possible contributions from various line-broadening mechanisms. It is shown that magnetic susceptibility and chemical shift dispersion are the major factors for the broadening of the proton and carbon resonances of P4HB in the amorphous phase and the melt, respectively. Incomplete motional narrowing due to a slow motional mode restricted in amplitude by the presence of crystallites and/or chain constraints was found to be the major line-broadening factor for P3/4HB-18 in the amorphous phase. Correlations between crystalline morphology, physical and mechanical properties, and polymer chain dynamics are discussed, along with the way these factors affect the NMR linewidth data presented. © 1996 John Wiley & Sons, Inc.

Nuclear Magnetic Resonance (NMR) of Porous Silicon

AbstractPorous silicon (PS) was characterized by 1H, 19F and 29Si solid-state nuclear magnetic resonance (NMR). On freshly prepared samples, hydrogen contents were between 3 × 1014 and 3 × 1015 per cm2 of PS surface area, while fluorine concentrations were below the detection limit. Cross-polarization (CP) was used to selectively observe the 29Si near the hydrogen passivation. The features of the 29Si NMR spectra are assigned as (O)2(Si)Si-H (-50 ppm), (O)3Si-H (-84 ppm), (Si)3Si-H (-91 ppm), (Si)2Si-H2 (-102 ppm) and (O)4Si (-109 ppm). Changes resulting from low temperature annealing in air and an HF soak were observed by both NMR and infrared spectroscopy. The 29Si NMR line widths for PS fall between those for crystalline silicon and those for amorphous hydrogenated silicon films (a-Si:H), suggesting disorder near the PS surface is intermediate between these extremes. However, comparison of the isotropie chemical shift values shows that the bonding in the disordered regions of PS differs from that found in a-Si:H. In addition, the sharp 29Si NMR resonance observed in the bulk single crystal starting material can not be resolved in the spectra of PS. Thus, well-ordered silicon nanocrystallites in the PS are either several bond-lengths removed from hydrogen or comprise only a small fraction of the PS layer.

Characterization of Porous Silicon by Solid-State Nuclear Magnetic Resonance

Solid-state nuclear magnetic resonance (NMR) was used to characterize porous silicon (PS) surfaces. On freshly prepared samples, the range of hydrogen content measured by 1H NMR was equivalent to 0.5−3 monolayers, while fluorine concentrations were below the 19F NMR detection limit. The 1H nuclei were used to selectively cross-polarize (CP) 29Si near the hydrogen passivation. This method was used to study the passivation of an as-prepared, thick (116 μm), high surface area (893 m2/g), photoluminescent (700 nm) PS sample. CP followed by polarization inversion (CPPI) provided some spectral editing. Changes resulting from low-temperature annealing in air and an HF soak were followed by both NMR and infrared spectroscopy. The features of the 29Si NMR spectra are assigned as (O)2(Si)Si−H (−50 ppm), (O)3Si−H (−84 ppm), (Si)3Si−H (−91 ppm), (Si)2Si−H2 (−102 ppm), and (O)4Si (−109 ppm). These assignments are discussed in relationship to experimental measurements and correlations of 29Si NMR chemical shifts for other materials. The 29Si NMR line widths for PS fall between those for crystalline silicon and those for amorphous hydrogenated silicon (a-Si:H), suggesting that disorder near the PS surface is intermediate between these extremes. However, comparision of the isotropic chemical shift values shows that the bonding in the disordered regions of PS differs from that found in a-Si:H. In addition, the sharp 29Si NMR resonance observed in the bulk single crystal starting material cannot be resolved in the spectra of PS. Thus, well-ordered silicon nanocrystallites in the PS are several bond lengths removed from hydrogen or comprise only a small fraction of the PS layer.

Physicochemical characterization of a model digestive mixture by 2H NMR.

2H nuclear magnetic resonance (NMR) spectra were obtained at 30.87 MHz for 8% (w/v) aqueous dispersions of mixtures of bile salts (MBS), mixed intestinal lipids (MIL; myristic acid, monomyristoylglycerol, dimyristoylphosphatidylcholine = 5:1:1), and cholesterol, in which a single lipid component is selectively 2H-labeled. Using the observation that the time-averaged quadrupole splitting of a C2H3 group varies according to whether it exists in a micellar, multilamellar or solid phase, one-, two-, and three-phase regions in the equilibrium phase diagram have been identified. From the intensities of the singlets and powder patterns in the wide-line 2H NMR spectra, the relative amounts of these organized molecular assemblies were determined. With different C2H3-labeled components in samples of identical total composition, the chemical composition of each phase was calculated for one point (20 mol % cholesterol; 50 mol % MIL, and 30 mol % MBS) in a two-phase region of the phase diagram where the 2H NMR spectrum displayed both a sharp spectral component and a broad uniaxial powder pattern. X-ray diffraction measurements on this sample confirmed that the uniaxial powder pattern in the NMR spectra can be assigned to multilamellar vesicles. At this same point in the phase diagram with the 2H label on the alpha-methylene site of myristic acid, both narrow and broad (delta v = 37 kHz) spectral components were again observed. Relaxation time (T1 and T2) measurements of the sharp spectral component indicate that this peak arises from rapidly tumbling aggregates which, at a total lipid concentration of 8% (w/v), are micellar particles and not unilamellar vesicles. These experiments demonstrate the feasibility of structural investigations of model digestive mixtures by 2H NMR.

Multi-nuclear and multi-dimensional nuclear magnetic resonance investigation of silver iodide-silver phosphate fast ion conducting glasses

The results of a multi-nuclear nuclear magnetic resonance (NMR) study of (AgI) x (Ag 2O) y (P 2O 5) 1 − x − y glasses are reported. Using the two-dimensional variable-angle correlation spectroscopy experiment, the isotropic and anisotropic chemical shift interactions of phosphorus were determined as a function of silver iodide and silver oxide composition. From these measurements we determine the average conformation of the phosphate groups. In addition, the 109Ag NMR spectra were recorded, as a function of both composition and temperature. At high silver oxide concentrations, interaction between the silver and phosphate groups has been detected, but in glasses in the series the (AgPO 3) x (AgI) 1 − x 31P NMR is strikingly independent on the silver iodide composition. The temperature dependence of the silver NMR linewidths in these systems shows clearly the silver mobility, and at lower temperatures no evidence for multiple distinct silver binding sites was observed. The silver chemical shift is strongly dependent on both composition and temperature. The former effect is interpreted in terms of the influence on the chemical shift of binding to oxygen versus iodine.

Examination of the mixed-alkali effect in (Li,Na) disilicate glasses by nuclear magnetic resonance and conductivity measurements

Results from 29Si, 23Na and 7Li magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, 7Li NMR relaxation and electrical conductivity in a series of [Li (1 − x) · Na x ] 2O · 2SiO 2 (disilicate) glasses are used to investigate the mixed-alkali effect. From the 29Si NMR spectra there is relatively little change of the network with alkali composition. 23Na and 7Li NMR linewidths and shifts change continuously as a function of composition, indicating that the alkali ions are intimately and uniformly mixed rather than separated into lithium and sodium-rich domains. The activation energy from electrical conductivity shows a distinct maximum at the central composition ( x = 0.5), whereas the local activation energy for lithium motion determined from NMR shows only a smaller but monotonic increase as the lithium-content decreases.

Study of synthetic diamonds by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Four Ib-type synthetic diamond crystals were studied by dynamic nuclear polarization (DNP)-enhanced high resolution solid state13C nuclear magnetic resonance (NMR) spectroscopy. The home built DNP magic-angle-spinning (MAS) NMR spectrometer operates at a field strength of 1.9 T and the highest DNP enhancement factor of synthetic diamonds came near to 103. Comparing with Ib-type natural diamonds, the13C NMR linewidths of synthetic diamonds in static spectra are broader. The13C spin-lattice relaxation time and DNP polarization time of synthetic diamond are shorter than those of Ib-type natural diamond. From the hyperfine structure of the DNP enhancement curve, four kinds of nitrogen-centred free radicals could be identified in synthetic diamond.

129Xe Nuclear magnetic resonance study on a solid-state defect in HZSM-5 zeolite

HZSM-5 and NaZSM-5 zeolites of various Si Al ratios ranging from 20 to 130 have been characterized by 129Xe nuclear magnetic resonance (NMR) spectroscopy at 296 K, following evacuation at temperatures between 673 and 823 K. Increases in the aluminum content and evacuation temperature resulted in significant increases in the 129Xe NMR line width and chemical shift of xenon adsorbed on HZSM-5. Similarly, the electron spin resonance (ESR) signal at g=2.000±0.005 increased in intensity with increasing Al content or evacuation temperature. In the case of NaZSM-5, evacuation at high temperature did not affect the 129Xe NMR spectrum, and the ESR signal disappeared. These results are attributed to paramagnetic solid-state defects in HZSM-5 arising from the hydroxyl groups that bridge the Al and Si atoms in the zeolite framework.

Study of natural diamonds by dynamic nuclear polarization-enhanced 13C nuclear magnetic resonance spectroscopy

The results of a study of two types of natural-diamond crystals by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state 13C nuclear magnetic resonance (NMR) are reported. The home-built DNP magic-angle spinning (MAS) 13C NMR spectrometer operates at 54 GHz for electrons and 20.2 MHz for carbons. The power of the microwave source was about 30 W and the highest DNP enhancement factor came near to 10 3. It was shown that in the MAS spectra the 13C NMR linewidths of the I b-type diamond were broader than those of I aB3-type diamond. From the hyperfine structure of the DNP enhancement as a function of frequency, four kinds of nitrogen-centred and one kind of carbon-centred free radicals could be identified in the I b-type diamond. The hyperfine structures of the DNP enhancement curve that originated from the anisotropic hyperfine interaction between electron and nuclei could be partially averaged out by MAS. The 13C polarization time of DNP was rather long, i.e. 1500 s, and the spin—lattice relaxation time (without microwave irradiation) was about 300 s, which was somewhat shorter than anticipated. Discussions on these experimental results have been made in this report.

A 7Li NMR study of the superconducting spinel Li1+xTi2-xO4

The superconducting oxide spinel system Li1+xTi2-xO4 (0<or=x<or=1/3) has been studied in the temperature range 160-340 K by nuclear magnetic resonance (NMR) of the 7Li nucleus. Elucidation of the two different Li sites (tetrahedral and octahedral) for x>0 has been achieved via dilution of the 7Li nuclei with 6Li. NMR linewidths show a narrowing with increasing temperature for low x, indicating that the Li ions are becoming mobile around room temperature. Observed chemical shifts show an increase at the metal-insulator transition, demonstrating that the Li nuclei act as weakly coupled probes of the bulk paramagnetic susceptibility. Analysis of these shifts, together with nuclear spin-lattice relaxation times, has allowed the Knight shift due to the Li(2s) contact to be calculated. This reveals that there is approximately 7% Li 2s character in the unpaired-electron wave function at the Fermi surface in LiTi2O4.

Wide-line solid-state NMR of wood: Proton relaxation time measurements on cell walls biodegraded by white-rot and brown-rot fungi

Wood biodegradation is carried out mainly by white-rot and brown-rot fungi. These microorganisms secrete extracellular enzymes that are able to degrade the wood cell walls to carbon dioxide and water. Both scientific and commercial interests have generated a need to develop an in situ analytical test able to assess the level of cell wall biodegradation. In this paper wide-line solid state nuclear magnetic resonance (NMR) spectroscopy has been applied to this problem. An NMR method has been developed based on proton spin-lattice (T 1) and spin-spin (T 2) relaxation time measurements on wood samples subjected to fungal attack. Beech (Fagus sylvatica) and pine (Pinus sylvestris) wood degraded for 6 weeks by the white-rot fungus Coriolus versicolor and by the brown-rot fungus Coniophora puteana showed a significant increase in the NMR relaxation times as the biodegradation took place. T 1 showed increase up to 8.8-fold for white-rotted samples and 1.5-fold for brown-rotted samples. T 2 showed an increase of up to 2.1-fold for white-rotted samples and 1.7-fold for brown-rotted samples. Statistical analysis of the data (t-test) confirmed the significance of these variations. Such variations in relaxation times as a result of biodegradation are interpreted as being due to an increase in mobility of the cell wall's molecular components and of the water present in the cell walls, as a consequence of the oxidative breakdown by the action of fungal enzymes. In all samples white-rot fungi showed a more marked degradative power than brown-rot on both beech and pine wood. This finding offers the opportunity to develop a nondestructive test able to assess the level of degradation in wood samples.

Inhomogeneous spin-polarization anomaly in the Kondo alloy Y1−xUxPd3

Abstract Host 89 Y nuclear magnetic resonance line widths have been measured in the Kondo alloy Y 1− x U x Pd 3 , x = 0, 0.05, 0.1, and 0.2, from 1.5 to 300 K. An anomalous temperature dependence of the 89 Y line shape indicates that the U-induced host electronic spin polarization is modified on a distance scale orders of magnitude shorter that the Kondo coherence length. This strongly suggests a qualitative difference between the ground states of Y 1− x U x Pd 3 and more conventional Kondo alloys.

13C NMR investigation of CVD diamond: Correlation of NMR and Raman spectral linewidths

Six CVD diamond thin films were examined by magic angle spinning (MAS) 13C nuclear magnetic resonance (NMR), Raman, and electron spin resonance spectroscopy. The use of film samples cut to the diameter of the magic-angle spinning rotor provided ease of spinning and the opportunity to obtain good signal-to-noise spectra in 4 to 16 h. MAS NMR linewidths exhibit a linear correlation with Raman linewidths and reflect the optical quality of the material. Residual MAS NMR linewidths most likely arise from a combination of crystal defect sites and paramagnetic effects.

Lithium-7 NMR and ionic conductivity studies of gel electrolytes based on polyacrylonitrile

Composite gel electrolytes prepared from mixtures of poly(acrylonitrile) (PAN), ethylene carbonate (EC), propylene carbonate (PC), and LiClO[sub 4] or LiAsF[sub 6] have been investigated by complex impedance, differential scanning calorimetry (DSC), and [sup 7]Li nuclear magnetic resonance (NMR) spectroscopy. The ionic conductivity of a gel containing LiAsF[sub 6] reaches 10[sup [minus]2] S/cm at 60[degrees]C. Although the conductivity of the gels approach that found in EC/PC liquid electrolytes, NMR line width, and spin-lattice relaxation time (T[sub 1]) measurements indicate that even short-range ionic mobility is impeded by the presence of the PAN. As in the case of amorphous polyether-salt polymer electrolytes, the onset of [sup 7]Li motional line narrowing in the gels is strongly correlated with the DSC-determined glass transition temperature. 6 figs., 1 tab.

Evaluation of diamond films by nuclear magnetic resonance and Raman spectroscopy

The quality of chemically vapor deposited diamond films was assessed in terms of sp 2 sp 3 content as determined by solid-state nuclear magnetic resonance (NMR) and Raman spectroscopy. While the results of the two techniques are in qualitative agreement, only the NMR spectra yield quantitative values for the sp 2 sp 3 ratio. Only sp 3 carbon was observed in the NMR spectra of very high quality hot-filament, microwave plasma, and d.c. arc-jet chemically vapor deposited films. As expected, Raman spectroscopy is extremely sensitive to sp 2 bonded carbon, identifying small amounts below the detection limit of the NMR spectrometer. Comparison of the two techniques, however, indicates that Raman spectroscopy may be so sensitive to sp 2 bonded carbon that sp 3 bonded carbon in films containing as much as 90% sp 3 bonded material may remain undetected. NMR linewidths indicate that the sp 3 carbon in such material shows more disorder than that found in high-quality polycrystalline films.

Ionic conductivity and 7Li NMR Study of Poly(ethylene glycol) complexed with lithium salts

Complexes formed between lithium salts and low molecular weight (0.5 kg mol −1) poly(ethylene glycol) have been investigated by differential scanning calorimetry, 7Li nuclear magnetic resonance (NMR), and complex impedance measurements. 7Li NMR linewidths and hence ionic mobilities are strongly influenced by polymer segmental motion above the glass transition. Strong cation—anion interactions in LiBF 4 complexes are evidenced by substantial 7Li 19F dipolar coupling. Ionic conductivity results are similar to those reported for widely studied poly(ethylene oxide) complexes, in that broad conductivity vs salt concentration maxima are found. Finally, addition of up to 30 wt% of the inert ceramic powder γ-LiAlO 2, while improving the mechanical properties of the samples, is found to have a negligible effect on ionic conductivity.

Magnetic field at the surface of high-T C superconducting oxides by magnetic resonance

The magnetic field at the surface of high z. sbnd; T c superconducting oxide, YBa 2Cu 3O 7−x, was studied by nuclear magnetic resonance (NMR) in silicone oil coated on the surface. An analysis of the NMR linewidth gives information on the critical current density J c and equilibrium magnetization. The result was in good agreement with magnetization by SQUID magnetometer. Unusual temperature dependence of the J c was found in a sample with T c of 70 K.

The magnetic field at the surface of high-T c superconducting oxides by nuclear magnetic resonance : Critical current density and magnetic penetration depth

We report on the surface magnetic field of high- T c superconducting oxides powder, YBa 2Cu 3O 7− x by nuclear magnetic resonance (NMR). Anomalous broadening of NMR linewidth of probe material (silicone oil) coated on the surface was observed below the superconducting transition temperature. We interpreted this line broadening in terms of (1) equilibrium bulk magnetic moment, (2) inhomogeneous flux density distribution due to flux pinning and (3) vortex lattice structure. We tried to deduce contributions of flux pinning and vortex lattice to NMR linewidth, which where analyzed to obtain information on the critical current density and penetration depth.