Isotropic Peak Research Articles

Local electronic structure of LiMn 2O 4 probed by solid state 7Li-NMR

Crystalline LiMn 2O 4 materials were synthesized through a low temperature reflux (LTR) process. The LTR material exhibits an essential electrochemical behavior to present only one transition during the charge/discharge process and gives promises for the future application. The X-ray data hardly distinguishes either conventional high temperature (HT) or LTR material since both materials possess similar molecular structure. However, solid-state NMR results suggest substantial difference for their electronic structure. The 7Li-NMR of HT material exhibits the isotropic peak at 529 ppm and spreads for 160 kHz wide indicating the interaction between the lithium nucleus and the conducting electrons around the nucleus. The LTR material presents the main isotropic peak at 217 ppm but spreads for almost 400 kHz wide asymmetrically implying the existence of multiple electronic environments.

A Possible Cepheid‐like Luminosity Estimator for the Long Gamma‐Ray Bursts

We present a possible Cepheid-like luminosity estimator for the long gamma-ray bursts based on the variability of their light curves. To construct the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/Konus data for five bursts, Ulysses/GRB data for one burst, and NEAR/XGRS data for one burst. Spectroscopic redshifts, peak fluxes, and high-resolution light curves are available for 11 of these bursts; partial information is available for the remaining nine bursts. We find that the isotropic equivalent peak luminosities L of these bursts positively correlate with a rigorously constructed measure V of the variability of their light curves. We fit to these data a model that accommodates both intrinsic scatter (statistical variance) and extrinsic scatter (sample variance). We find that L ~ V. If one excludes GRB 980425 from the fit, on the grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not secure, the luminosity estimator spans ≈2.5 orders of magnitude in L, and the slope of the correlation between L and V is positive with a probability of 1 - (1.4 × 10-4) (3.8 σ). Although GRB 980425 is excluded from this fit, its L and V values are consistent with the fitted model, which suggests that GRB 980425 may well be associated with SN 1998bw and that GRB 980425 and the cosmological bursts may share a common physical origin. If one includes GRB 980425 in the fit, the luminosity estimator spans ≈6.3 orders of magnitude in L, and the slope of the correlation is positive with a probability of 1 - (9.3 × 10-7) (4.9 σ). In either case, the luminosity estimator yields best-estimate luminosities that are accurate to a factor of ≈4, or best-estimate luminosity distances that are accurate to a factor of ≈2. Regardless of whether GRB 980425 should be included in the fit, its light curve is unique in that it is much less variable than the other ≈17 light curves of bursts in our sample for which the signal-to-noise ratio is reasonably good.

Filtering of Spinning Sidebands in 1D MAS NMR Spectra

Spinning sidebands (SSBs) in the MAS NMR spectrum of a polycrystalline solid are related to the principal values of the chemical shift or quadrupole coupling tensors. At present, 2D methods are widely used to sort out the SSBs for each isotropic peak. Here a simple and efficient method for separating the SSBs in 1D MAS NMR spectra is described. It is based on finding the optimal spinning rate with a mathematical algorithm and subsequently treating the spectra with filtering functions.

X-ray diffraction peak broadening and lattice strain in LaNi5-based alloys

Abstract Peak broadening in X-ray powder diffraction (XRD) profiles of LaNi5-based alloys after hydriding and dehydriding processes was investigated in order to clarify the mechanism of formation of lattice strain in hydriding and dehydriding. The Rietveld method was used to evaluate the degree of peak broadening and to determine anisotropic peak broadening axis for LaNi5 and LaNi5−αMα (M: Mn, Fe, Cu, Al; α=0.25, 0.5) before hydriding, after activation and after 1000 hydriding–dehydriding cycles. All the alloys studied showed anisotropic broadening vectors of the same direction 〈110〉 after activation. The degree of the peak broadening, however, strongly depended on the substitution elements. Hydriding–dehydriding cycles did not influence the direction of the anisotropic peak broadening axis, while both anisotropic and isotropic peak broadening increased with number of cycles. It was found that the lattice strain analyzed from the peak broadening in X-ray diffraction profiles corresponded to dislocations with Burgers vectors 〈hk0〉 observed by transmission electron microscope.

Chloroform encapsulated in p-tert-butylcalix[4]arene: Structure and dynamics

A complex of p-tert-butylcalix[4]arene and chloroform has been prepared and characterized in the solid state using, for the first time for any calixarene complex, the Rietveld powder XRD method, as well as variable-temperature 2H and 13C solid-state NMR. The 2H NMR spectra are composed of a central isotropic peak, attributed to a small fraction of CDCl3 in the free motion regime inside the crystal lattice, and a quadrupolar powder pattern. 2H spin–lattice relaxation times lead to activation energies for the molecular motion of 4.9±0.5 and 6.0±0.6 kJ mol−1 for the two spectral components, respectively. The temperature dependence of the static 2H spectra shows a very unusual increase in the effective quadrupolar coupling constant with temperature, caused by the change in the geometry of the complex. We show that the complexation is caused by weak dispersion interactions. The 13C MAS NMR spectrum of the guest molecule changes upon prolonged heating, while thermal analysis confirms the presence of an exothermic transition attributed to an intermediate state before the choroform is released.

The Cancer Chemopreventive Agent Resveratrol Is Incorporated into Model Membranes and Inhibits Protein Kinase C α Activity

Resveratrol is a phytoalexin found in grapes and other foods that cancer chemopreventive and other biological activities have been attributed recently. We report that resveratrol is able to incorporate itself into model membranes in a location that is inaccessible to the fluorescence quencher, acrylamide. Differential scanning calorimetry revealed that resveratrol considerably affected the gel to liquid-crystalline phase transition of multilamellar vesicles made of phosphatidylcholine/phosphatidylserine and increased the temperature at which the fluid lamellar to HII inverted hexagonal transition took place in multilamellar vesicles made of 1,2-dielaidoyl-sn-phosphatidylethanolamine. Such a transition totally disappeared at 2.5 mM of resveratrol (resveratrol/lipid molar ratio of 2:1). This effect on 1,2-dielaidoyl-sn-phosphatidylethanolamine polymorphism was confirmed through 31P-NMR, which showed that an isotropic peak appeared at high temperature instead of the HII-characteristic peak of 42 mM of resveratrol (resveratrol/lipid molar ratio of 1.5:1). Finally, resveratrol inhibited PKCα when activated by phosphatidylcholine/phosphatidylserine vesicles with an IC50 of 30 μM, whereas when the enzyme was activated by Triton X-100 micelles the IC50 was 300 μM. These results indicate that the inhibition of PKCα by resveratrol can be mediated, at least partially, by membrane effects exerted near the lipid–water interface.

Orientational behavior of phospholipid membranes with mastoparan studied by 31P solid state NMR

Solid state 31P NMR spectroscopy was used to study the perturbing effect of the wasp venom peptide mastoparan (MP) on lipid bilayers composed of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG). The 31P chemical shift anisotropy of multilamellar vesicles decreased with increasing peptide concentration, indicating that MP interacts strongly and selectively with the charged DMPG head group. Macroscopically oriented MP-lipid samples between glass plates were studied by 31P NMR as a function of tilt angle. These spectra showed the coexistence of orientation-dependent lamellar signals as well as an isotropic peak, suggesting that MP can induce non-lamellar phases in DMPC/DMPG membranes.

31P and 27Al MAS NMR investigations of some lead phosphate glasses

The 31P MAS NMR spectra were measured and analyzed for the two phosphate glasses that contain distinct metals (i.e., In or Ba), different amounts of PbO, and the same content of P 2O 5 and Al 2O 3. The 31P MAS NMR spectra for the two glasses have a single isotropic peak at −3.77 or −4.76 ppm, indicating that orthophosphate species are the dominant P sites in the glasses studied. The 27Al MAS NMR spectrum measured for the lead–barium–aluminum phosphate glass shows that the aluminum ions present in the glass are in 4-, 5-, and 6-fold coordination with oxygen and that the Al(OP) 4 is the dominant moiety for the glass, suggesting the formation of orthophosphate species in the glass.

Structural study of a lead–barium–aluminum phosphate glass by MAS-NMR spectroscopy

The 31 P - and 27 Al -MAS-NMR spectra of a lead–barium–aluminum phosphate glass were measured and analyzed, and the short range structure of the glass are discussed. The 31 P -MAS-NMR spectrum of the glass studied has a single isotropic peak at −4.76 ppm, indicating that orthophosphate species are the dominant P-sites in the glass. The 27 Al -MAS-NMR spectrum of the glass shows that the aluminum ions present in the glass are in 4-, 5-, and 6- fold coordination with oxygen, and that the Al(OP) 4 is the dominant moiety for the glass, suggesting the formation of orthophosphate species in the glass.

Characterization of the benzene molecule adsorbed in USY zeolite using double quantum filtered NMR spectral analysis

The investigations of deuteron NMR relaxation including T 1, linewidths and double quantum filtered (DQF) spectral analyses are examined preliminary to probing the dynamics and exchange process of low-loading benzene molecules adsorbed into ultra-stable Y-type (USY) zeolite. DQF NMR is a sensitive method for the determination of the residual quadrupolar interaction resulting from the local order. The usual observed deuteron spectra of benzene- d 6 in USY retain the isotropic peak with no solid pattern over a broad temperature range. However, when observed in DQF spectra, an additional central line appears. It is evidence of the benzene in isotropic motion exchanging with the benzene situated at adsorption sites with non-vanishing quadrupolar interaction. We modified the cone model to describe the in-plane rotation around the hexad axis and the restricted wobbling motion of the benzene ring on the surface of the interaction site. The population profile of adsorbate thus determined provides useful knowledge of the site distribution in supercages of USY zeolite. Comparison with the result of NaY in terms of site distribution is also discussed briefly.

Injection locking of photonic paint

The narrow-linewidth laser emission from a scattering gain medium can be forced to oscillate in a narrower emission bandwidth with the introduction of a seed in much the same way as the injection-seeding technique used to produce single-mode oscillation from conventional lasers. The system exhibits complete spectral collapse at all wavelengths other than that of the seed and a large enhancement of the isotropic emission peak intensity. A theory explaining the emission characteristics and the minimum threshold seed energy required for locking is presented.

Unusual 31P Chemical Shielding Tensors in Terminal Phosphido Complexes Containing a Phosphorus−Metal Triple Bond

Phosphorus-31 NMR data for one-coordinate phosphorus compounds are extremely scarce in the literature.1,2 Until recently, the only stable compounds with a terminal phosphorus atom involved in a triple bond were phosphaalkynes, RCtP,3,4 and the isoelectronic iminophosphenium cations, [RNtP]+.5 The isotropic 31P chemical shift range for these compounds extends from 96 to -207 ppm. In general, phosphaalkynes and iminophosphenium ions are found to have their 31P chemical shifts at lower frequency (more shielded) compared to phosphaalkenes and iminophosphines, respectively. This trend is analogous to the relationship between 13C chemical shifts in alkynes and alkenes. Recently, the first terminal phosphido (P3-) complexes containing a phosphorus-metal triple bond were successfully synthesized.6,7 Surprisingly, 31P NMR signals in these complexes were found above 1000 ppm. Observation of such unusual 31P chemical shieldings suggests that phosphorusmetal triple-bonding schemes may differ substantially when compared to phosphorus-carbon and phosphorus-nitrogen triple-bonding schemes. Here we report the 31P chemical shielding tensors for the terminal phosphido (MtP) complexes Mo(P)[N(Bu)Ar]3 (1, Ar ) C6H5; 2, Ar ) 3,5-C6H3Me2) and M(P)(NN3) (NN3 ) [Me3SiNCH2CH2)3N]; 3, M ) Mo; 4, M ) W). The anisotropies are all in excess of 2000 ppm, which is by far the largest 31P chemical shielding anisotropy (CSA) ever observed. A typical solid-state 31P NMR spectrum8 of the terminal phosphido complexes obtained with magic-angle-spinning (MAS) is shown in Figure 1. In addition to the isotropic peak, a large number of rotational sidebands extend over a range of ca. 2000 ppm. From the rotational sideband intensities, it is possible to compute the principal components of the chemical shielding tensor.9,10 For complexes 1-4, orientations of the principal components of the 31P chemical shielding tensor are constrained by crystallographic symmetry. For example, complex 1 crystallizes in the space group I4h3d with its MotP bond lying along the crystallographic C3 axis.11 Therefore, the 31P chemical shielding tensor in 1 must be axially symmetric with the unique axis lying along the MotP direction. Similarly, complexes 2 and 4 crystallize in the space groups P63 and Pa3h, respectively; each complex has its MtP vector lying along a cystallographic C3 axis.6,7 The structure of 3, although not yet determined, is expected to be identical to 4. Our NMR results also confirmed that the 31P chemical shielding tensor in 3 is axially symmetric, suggesting the presence of an axis with C3 or greater symmetry.

Emulsion and Vesicle Formation of Retinol and Retinyl Palmitate with Egg Yolk Phosphatidylcholine

Spreading pressure measurements showed that retinyl palmitate (retinol palmitate, ReO-Pal) and egg yolk phosphatidylcholine (PC) were practically immiscible and separated into PC bilayers and ReO-Pal liquid. On the contrary, substantial amounts of retinol (ReOH) were incorporated into PC bilayers. The moral ratios of PC in the outer surface and the inside of sonicated particles of these lipid mixtures were determined on the basis of 1H-NMR measurements in the presence of a paramagnetic cation Pr3+. The higher ratio for the PC/ReO-Pal particles than 1.4 (the maximum value for vesicle structure) showed that part of the PC in the inner leallel of vesicle bilayers was moved to the outer lealled by addition of the neutral lipid, indicating formation of emulsion particles. Production of emulsion particles in the sonicated dispersion of PC/ReO-Pal mixtures was confirmed by electron microscopy (EM). Small angle X-ray scattering (SAXS) and EM measurements on (vortexed) hydrated lipid mixtures of PC/ReO-Pal demonstrated that excess PC, coexisting with emulsion particles, was organized into regular bilayer lamellas with a repeating distance of 64.8 nm. On the other hand, vesicles with irregularly stacked lamellar structures were observed in EM pictures of hydrated PC/ReOH mixtures. The irregular structures caused broad SAXS spectra with a repeating distance of about 70 nm and a sharp isotropic peak in 31 P-NMR spectra. Thus, esterification and hydrolysis of retinoids lead to remarkable changes in the structure organization with phospholipids.

Combined EXAFS and powder diffraction analysis

A method has been developed which allows the simultaneous refinement of X-ray powder diffraction data and one or more EXAFS spectra from the same sample using a single coordinate description of the structure. The positional parameters of the atoms are refined together with isotropic thermal factors, peak shape and amplitude parameters, and the EXAFS energy zeros for each absorption edge. Where correlations between shells can be calculated, as with Cu for which Debye theory can be used, EXAFS mean square displacements can be derived from the isotropic thermal factors, otherwise these must be introduced as separate variables. The program determines the point symmetry and radial coordinates of each site occupied by an atom for which EXAFS data are available, allowing a full multiple scattering calculation to be performed for each site. Mixed or partial occupancy of sites is permitted. The method allows us to determined accurately the position of oxygen and other light atoms in materials where the diffraction pattern is dominated by heavy atoms, and to determine the occupancy of sites where elements of similar scattering amplitude are involved. Results are particularly good where the EXAFS of several absorbing atoms are available.

Density and temperature study of the noncoincidence effect in liquid carbon disulfide

The noncoincidence of the anisotropic and isotropic peak frequencies in the allowed Raman v 1 vibrational mode of liquid CS 2 is studied over an extended density range (1.18 to 1.58 g cm 3 ) by varying pressure (0.001 to 5.0 kbar) and temperature (−55 to 80°C). The noncoincidence value is negative and the absolute value increases as a function of both increasing density, at constant temperature, and increasing temperature, at constant density. The noncoincidence of the anisotropic and isotropic peak frequencies is attributed to a quadrupole induced transition dipole coupling mechanism because changes in the noncoincidence value can be correlated with changes in the intensity of the forbidden v 1 infrared mode.

ESR characterisation of SO−3 and SO−4 radicals in X-irradiated kainite (KMgClSO4.3H2O)

The ESR spectra of the kainite (KMgClSO4.3H2O) crystal revealed an intense isotropic (g = 2.004) peak C attributed to the SO−3 radical and two pairs of lines (A1, A2) and (B1, B2) bearing intensity ratio 5:3. The intensity and linewidth variation of peak C suggested that the signal contains an unresolved shf structure. The power saturation studies on SO−3 indicate that its ESR line is homogeneously broadened and its line shape is Lorentzian. The spin—lattice and spin—spin relaxation times (T1 and T2) of SO−3 have been estimated to be 0.44 s and 656 μs, respectively. The analysis of the anisotropic pairs of lines show that they constitute two sets A and B and are due to two chemically inequivalent SO−4 radical species in the lattice. The ESR spectra of the polycrystalline samples recorded at 300 and 77 K confirm the isotropic behaviour of SO−3 and chemical inequivalence of two types of SO−4 radicals. The principal g-values of the SO−4 radical were evaluated to be: g1 = 2.007, g2 = 2.011, g3 = 2.014 for species A and g1 = 2.008, g2 = 2.012, g3 = 2.015 for species B. The low microsymmetry of the SO2−4 ion in the lattice seems to promote the radiation damage.

High‐field nuclear magnetic resonance of thallium in zeolites

AbstractThallium‐205 NMR is reported for several zeolite samples (A, X and Y; hydrated and dehydrated) with thallium(I) exchanged for sodium. Detection at high field (7.05 T) clarifies the spectrum appearance, and it is seen that chemical shift anisotropy (CSA) dominates some of the band shapes. The large CSAs dictate that the spectrum is recorded piecewise by a frequency‐stepped spin‐echo integral (FUSSI) method. Magic‐angle sample spinning has been observed to give only a very minor line‐narrowing effect. Threefold‐coordinated zeolite A Tl+ sites adjacent to six‐membered rings [Tl(1) and Tl(3)] are both assigned to a resonance which has large CSA (δiso 1183, δδ—1842), while the remaining zeolite A site [Tl(2)] gives an isotropic line shape (δiso 300). Dehydrated zeolite X gives an isotropic peak (δiso 854) which is assigned to the double six‐ring Tl(I), and a broader anisotropic resonance which is tentatively attributed to supercage Tl(III). Data for dehydrated zeolite Y indicate that this has a disordered cation distribution.

High-resolution solid state 13C NMR studies of ferrocene as a function of magic angle sample spinning frequency

High-resolution solid state 13C NMR spectra of polycrystalline ferrocene have been recorded, at room temperature, under conditions of magic angle sample spinning and high power 1H decoupling. It is shown that, contrary to naive expectations, the linewidth of the isotropic peak in the spectrum increases as the magic angle sample spinning frequency is increased. It is demonstrated, via experiments with variable decoupler field strength and experiments on ferrocene- d 10, that this effect results from the efficiency of the 1H decoupling being modulated by magic angle sample spinning. The fact that narrower linewidths are obtained at low magic angle sample spinning frequency for this material has important implications in the wider context of establishing optimum conditions for recording high-resolution 13C NMR spectra of solids, particularly when there is appreciable molecular motion in the solid.

On Wolff's law of trabecular architecture

Several studies suggest that the yield strain in cancellous bone may be uniformly distributed and isotropic. Yield strain was reported to be independent of textural anisotropy in bovine cancellous bone [Turner, J. biomech. Engng, 111, 1–5 (1989)] and it is plausible that yield strain is isotropic in human cancellous bone as well. In this paper, it is hypothesized that uniform, isotropic strain represents a goal of cancellous bone adaptation, i.e. cancellous bone alters its structure to maintain uniform, isotropic peak strains. Therefore, textural anisotropy must exactly cancel the anisotropy of the peak principal stresses imposed upon cancellous bone. When evaluating the relationships between mechanical properties of cancellous bone and trabecular architecture, it was found that over 90% of the variance of yield strength can be explained by one term— ϱ 2, H ̃ 3 (where ϱ is apparent density and H ̃ is the normalized anisotropy (fabric) constant). Furthermore, this single term explains 70–78% of the variance in Young's modulus of cancellous bone. Based upon these findings, it was postulated that fabric adaptation goes as H i H j =| σ ̂ i σ ̂ j | 1 3 , where H 1 and H j are fabric eigenvalues in the i- and the j-direction and σ ̂ i and σ ̂ j are peak principal stresses.

Isotropic reconstruction. A statistical method for correction of effects due to finite spinning speeds in MAS spectra

The purpose of this Communication is to demonstrate that other techniques for suppression or removal of sidebands from MAS NMR spectra that are similar to the qualitative method the eye uses when comparing spectra of a sample taken at different spinning speeds can be envisioned. We report a technique for sideband removal and isotropic intensity recovery which uses statistical methods to describe the behavior of each isotropic peak and its attendant sidebands