Gradient Asymmetry Research Articles

Determination of the electric field gradient asymmetry from 2D nutation NQR spectra of 75As nuclei in oriented samples of As 2Se 3 semiconductor

The method of two-dimensional nutation nuclear quadrupole resonance in time domain is used to study the chalcogenide semiconductor As 2Se 3. In this system, the nuclear quadrupole resonance (NQR) resonance line width is as large as 10 MHz; therefore, the radiofrequency field produced by a pulsed NQR spectrometer can excite only a portion of the nuclear spins. The proposed method relies on polarizing the melted glass specimen in a strong magnetic field so that orientational disorder is partially removed. After hardening the sample is placed in a spectrometer r.f. coil in such a way that that the axes of sample polarization and r.f. coil coincide. We demonstrate the application of this method to determine η in glassy As 2Se 3.

Determination of the electric field gradient asymmetry from Berry’s phase in NQR of powder samples

An effect of Berry’s phase on the NQR spectrum of the rotating powder sample is described and applied for the determination of the electric field gradient asymmetry. The proposed method involves the analysis of the frequency singularities in the NQR powder patterns of the rotating samples. The Berry’s phases for the eigenstates, associated with an adiabatically changing quadrupole hamiltonian, are calculated for nuclei with a spin I = 3/2 and I = 1 as a function of the asymmetry parameter.

Two-Dimensional Nutation NQR Broad-Line Spectra in Oriented Samples

The NQR nutation method to determine the electric field gradient asymmetry parameter η in systems, where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins, is presented. In this situation, the recently developed spectroscopic methods are not applicable. Two-dimensional nutation NQR spectra of oriented powders are calculated and used to determine η at particular frequencies along a broad NQR line. The proposed method is useful for single crystals, oriented powders, glasses, and disordered systems even for small values of the asymmetry parameter. Therefore it can be used to evaluate fluctuations in η and the quadrupole coupling constant e2qQ due to inhomogeneities. We demonstrate the application of this method to oriented chalcogenide semiconducting glasses.

Layered Inclusion Compounds Containing Aniline and Polyaniline Studied by NQR and IR Spectroscopy

Temperature dependence of the 127I NQR frequencies in the intercalates of PbI2 with aniline were measured. Unusual temperature coefficients of frequency and the electric field gradient asymmetry parameters for these compounds were discussed, taking into account the structure peculiarities and the amplitude of iodine oscillations.

Surface enrichment of branched polymers in linear hosts: Effect of asymmetry in intersegmental interactions and density gradients

Variable density lattice treatment of surface enrichment of f-arm star-branched chains in star/linear polymer blends is compared with results of an analytical response theory proposed by Wu and Fredrickson [Macromolecules 29, 7919 (1996)]. We find that differences in treating the intersegmental interactions in the small interfacial region near a free surface lead to significant differences in the potentials by which polymer chain ends are attracted towards the surface. Consideration of an asymmetric relationship between segment potentials and density changes in polystyrene at 450 K and 0.1 MPa, for example, gives typically a threefold to fourfold enhancement in composition of star molecules at a vacuum interface. When contributions from gradients in density are included in the analysis even greater levels of surface enhancement (fivefold to sixfold increases) are observed. By appropriately estimating the attraction of chain ends and repulsion of branch points at a free surface, we show that concentration profiles of branched polymers predicted in the lattice model are consistent with results obtained in the analytical response theory.

Pure nuclear quadrupole resonance determination of the electric field gradient asymmetry for broad lines

We present an angle dependent nuclear quadrupole resonance (ADNQR) method to determine the electric field gradient asymmetry parameter η in systems where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins. In this situation, the recently developed spectroscopic methods are not applicable. ADNQR is useful for single crystals and oriented powders, and, for small η determines η 4. Therefore, it can be used to evaluate fluctuations in η due to inhomogeneities. We demonstrate the application of ADNQR experimentally to oriented superconducting YBa 2Cu 3O 7 powder.

Combined ab initio Computational and Solid-State17O MAS NMR Studies of Crystalline P2O5.

Ab initio calculations of the 1 7 O electrical field gradient (EFG) tensor quadrupolar coupling constant (C Q ) and asymmetry parameter (η Q ), along with the 1 7 O NMR isotropic chemical shift (δ i s o ) for the three crystalline polymorphs of P 2 O 5 are presented. These computational results are compared with experimental values for crystalline h-P 2 O 5 (hexagonal-form) obtained using a combination of solid-state 1 7 O magic angle spinning (MAS) NMR at three different magnetic field strengths (9.4, 14.1, and 19.6 T), two-dimensional (2D) multiple-quantum (MQ)MAS NMR, and 2D satellite-transition (ST)MAS NMR experiments. In addition ab initio studies of the model H 4 P 2 O 7 cluster allowed empirical correlations between the bridging oxygen EFG parameters and the P-O-P bond angle to be developed.

The nature and origin of taxonomic diversity gradients in marine bivalves

Abstract Bivalves have been fundamental in developing the understanding of large-scale biodiversity patterns in the marine realm. A new study based on 29 regional bivalve faunas indicates that both latitudinal and longitudinal gradients are not so regular in form as was once imagined. The northern hemisphere latitudinal gradient has a marked step, or inflection, at 20–30°N, and in the southern hemisphere Australia forms a distinct diversity hotspot. A longitudinal gradient runs between a large tropical high diversity focus in the southern China-Indonesia-Australia region and a somewhat smaller one in the Panamic-Caribbean region. It is unlikely that the marked asymmetry in both latitudinal and longitudinal gradients can be explained entirely by either the geometric patterns of species ranges in relation to geographical boundaries (the mid-domain effect) or the operation of contemporary, or equilibrium, factors. The role of history must be considered too, and in some cases this could involve a timescale of at least 60 Ma. For example, Australia has moved progressively northwards throughout the Cenozoic era and this seems to have led to the development of a unique assemblage of both tropical and temperate taxa around the continent. When Australia eventually collided with southeast Asia, some 15 Ma ago, three previously distinct tropical marine faunas were brought into close juxtaposition. However, something else seems to have been involved in order to generate a marked pantropical Neogene diversification event in bivalves and other marine invertebrate taxa. It is striking how this event coincided with a prolonged interval of global cooling and the suggestion has been made that glacioeustatic cycles may have promoted the formation of a tropical species diversity pump. In concert with regular and profound sea-level changes, new taxa were created in marginal tropical regions and then concentrated in certain core regions. These same glacioeustatic cycles may also have served to accentuate the latitudinal limits of many tropical taxa. As the knowledge of large-scale taxonomic diversity gradients in the marine realm has increased, so too has the realization that they are at least the partial products of major historical events.

Calculations of Multipulse Sequence in NQR of Spins [formula omitted

The general formalism of the interaction representation with respect to an operator which is its own inverse is developed and applied to pure NQR of spins I = 32. Under the assumption of no relaxation and no dipolar coupling, it is shown that the calculation of the response to pure NQR multipulse sequences can be performed with the same concepts used in high field NMR, such as coherence pathways. All the tools and mathematical expressions to predict the time evolution of the signal created by a pure NQR multipulse sequence are presented explicitly. It takes into account the off-resonance irradiation as well as the angular dependence of the excitation and detection for every value of the electric field gradient asymmetry parameter. Particular attention is devoted to the powder average, which is performed via a probability function derived analytically for the first time, leading to a drastic reduction of simulation times. The theory is illustrated by the study of the optimization and excitation bandwidths of one- to three-pulse sequences and compared to experimental results on Chloranil. We show that the three-pulse “stimulated echo” sequence gives a more uniform excitation profile than the traditional two-pulse echo sequence for powder samples. Thus, the “stimulated echo” sequence could be useful to cover a large spectrum when the experiment duration, or the signal to noise ratio, are not critical parameters. Analytical expressions for the nutation spectra obtained by one or two-pulse sequences are also derived for the first time.

Fronts Formed by Obliquely Reflecting Internal Waves at a Sloping Boundary

Abstract A characteristic of internal waves reflecting from sloping boundaries is that they form fronts that travel with the component of the phase speed of the waves up the boundary. The strength of the fronts is assessed by estimating the magnitude of nonlinear terms leading to the asymmetry of density gradients at the slope when waves travelling in a fluid of uniform buoyancy frequency are at nonnormal, or oblique, incidence to the slope. Strong nonlinearities, indicating fronts, are found for both supercritical (β > α) and subcritical (β < α) waves near critical slopes where the inclination of the boundary to the horizontal, α, matches that of the wave group velocity β. They are also found for subcritical waves when β is near sin−1[(sinα)/2]. Fronts become weaker as the angle at which the wave approaches the slope, the azimuth or incident angle, increases from zero (i.e., when waves are nonnormal), but not significantly so until this angle exceeds 30°.

Correlations between 29 Si, 17 O and 1 H NMR properties and local structures in silicates: an ab initio calculation

In order to gain insight into the correlations between 29Si, 17O and 1H NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)4 monomers (isolated as well as interacting), Si2O(OH)6 dimers (C2 symmetry) with the Si-O-Si angle fixed at 5° intervals from 120° to 180°, Si3O2(OH)8 linear trimers (C2 symmetry) with varying Si-O-Si angles, Si3O3(OH)6 three-membered rings (D3 and C1 symmetries), Si4O4(OH)8 four-membered ring (C4 symmetry) and Si8O12(OH)8 octamer (D4 symmetry). The calculated 29Si, 17O and 1H isotropic chemical shifts (δiSi, δiO and δiH) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated 17O quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated 17O QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between δiSi and the mean Si-O-Si angle for both Q1 and Q2, where Qn denotes Si with n other tetrahedral Si next-nearest neighbors. Both the δiO and the 17O electric field gradient asymmetry parameter, η of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the 17O QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable 17O NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the 17O QCC and the 2H QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H···O), and the δiH increase with the same parameter.

Ab initio calculations of of pyridine and its 2- and 3-chloroderivatives

Ab initio calculations of the C5H5N, 2-. and 3-ClC5H4N molecules by the RHF method in the valence split 6-31G* basis set with full optimization of the geometry have been carried out. The alternation of the charges on the atoms of the pyridine ring and of the populations of their px-orbitals is in agreement with the noninductive through -the-field interaction of the geminal atoms. The35Cl NQR frequencies and the electric field gradient asymmetry parameters at the35Cl nuclei in 2- and 3-CIC5H4N were estimated using the populations of the valentp-orbitals of the Cl atoms and their components. The35Cl NQR frequency for the first compound is lower than that for the second one, mainly due to the higher pσ-electron population of its Cl atom.

Two-dimensional nutation FT/MEM 35Cl nuclear quadrupole resonance spectroscopy and hydrogen bonding in 2,6-dichlorophenol

Two-dimensional (2D) nutation nuclear quadrupole resonance (NQR) spectroscopy of polycrystalline 2,6-dichlorophenol has been applied in order to study the effect of intramolecular hydrogen bonding on the electric field gradient (EFG) asymmetry parameter η. The superiority of the maximum entropy method (MEM) over the traditional fast Fourier transform (FFT) is demonstrated in analysis of 2D NQR data. The chlorine atom which is ortho to the hydroxyl group and involved in hydrogen bonding gave a value for η of 0.065 while the other chlorine gave a higher value of 0.140, independent of temperature in the range 77–284 K. These values are discussed in terms of hydrogen bonding and bond parameters.

Two-dimensional nutation FT/MEM 35Cl nuclear quadrupole resonance spectroscopy and hydrogen bonding in 2,6-dichlorophenol

Two-dimensional (2D) nutation nuclear quadrupole resonance (NQR) spectroscopy of polycrystalline 2,6-dichlorophenol has been applied in order to study the effect of intramolecular hydrogen bonding on the electric field gradient (EFG) asymmetry parameter η. The superiority of the maximum entropy method (MEM) over the traditional fast Fourier transform (FFT) is demonstrated in analysis of 2D NQR data. The chlorine atom which is ortho to the hydroxyl group and involved in hydrogen bonding gave a value for η of 0.065 while the other chlorine gave a higher value of 0.140, independent of temperature in the range 77–284 K. These values are discussed in terms of hydrogen bonding and bond parameters.

Hydrogen-bonding structural study of solid peptides and polypeptides containing a glycine residue by 17O NMR spectroscopy

Abstract Static 17 O NMR spectra of polyglycines I and II and glycylglycine peptides in the solid state were measured by the cross polarization technique. By computer simulations of these spectra, three NMR parameters (quadrupolar coupling constant ( e 2 qQ / h ), electric field gradient asymmetry parameter (η Q ), and chemical shift (δ)) were determined. From these results, it was found that as the hydrogen bond length decreased, the e 2 qQ / h value decreased. Furthermore, the principal values of the 17 O chemical shift tensor for both the peptides and polypeptides moved upfield with a decrease in the hydrogen bond length. From these experimental findings, it was clarified that 17 O NMR spectroscopy can provide a useful means of elucidating the hydrogen-bonding structure in solid peptides and polypeptides.

The effect of electric field gradient asymmetry on motionally averaged spin-1 powder patterns

The effect of an asymmetric electric field gradient tensor on anisotropic NMR spectra of spin-1 nuclei is investigated for fast molecular rotations and 2-site jump processes. When molecular rotations are fast, continuous, and complete, the peak-to-peak splitting of motionally averaged spin-1 NMR spectra can depend significantly on the inherent electric field gradient asymmetry, η, for η ≥ 0.05. Parameters describing low-symmetry molecular motion which are extracted from fitting experimental data depend strongly upon η. One implication of these phenomena is that even the modest electric field gradient asymmetry around deuterons in covalent bonds can contribute considerably to motionally averaged lineshapes, especially when molecular motions are not highly symmetric.

P,π-lnteraction in the Y—C [dbnd]C [rbond2] Group

The electric field gradient asymmetry parameters for 35 Cl nuclei in compounds of the series Cl 2 C CHOR, Cl 2 CHOCOR etc . do not corroborate a widespread representation of a p,π-interaction between the unshared electron pair of a Cl atom and the π-electron double bond system in these molecules.

Analysis of in-plane bit structure by magnetic force microscopy

This article uses a fully quantitative approach to describe the behavior of the minute tip of the magnetic force microscope (MFM) over longitudinally directed magnetic bits. Three possible magnetic structures are taken into consideration together with a realistic tip model. The magnetic and van der Waals interactions beween the tip and the sample are derived analytically to simulate a force gradient in MFM. The contours of the constant force gradient are presented and related to the known experimental data. The important role of the van der Waals force gradient in constant force gradient measurements is shown. The force gradient from a magnetic interaction is partly negative and partly positive and is balanced or increased by the (always positive) van der Waals force gradient. Force gradient asymmetry above the transition area is revealed when the MFM tip is tilted. By tilting the tip, the detection of two components of a stray field is accomplished. We found that the widths and heights of observed bumps of a force gradient depend on the width of the transition area and the deviation of the magnetization vector from the recording direction. These factors are especially important in determining what tolerance can be put on a read/write head’s performance. A lateral resolution of 90 nm could be achieved with our tip model.

51V nuclear magnetic resonance in polycrystalline ferroelastic BiVO 4

Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO 4 indicate that the 51V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using ∩ = A( T− T c ) B , B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.

The deuterium quadrupole coupling constants of di- and trihalomethanes

There have recently been published a number of papers reporting the deuterium quadrupole coupling constants (QCC) of small molecules dissolved in liquidcrystalline media (1-5). The results presented show, in many instances, wide variations in QCC values determined by different groups. These differences can be mainly attributed to the presence of different solvent-solute interactions in different liquid-crystal phases. Other causes of the range of values found arise from the difficulty in choosing appropriate bond lengths and making vibrational corrections to the dipolar couplings. Such uncertainties have led to some puzzling anomalies. For example, with the deuterohalomethanes, tribromomethane appears to have a considerably larger QCC than either trichloromethane or triiodomethane. These results have prompted us to determine the QCC values for deuterotrihalomethanes, C!HX3 (X = Cl, Br, or I), and dideuterodihalomethanes, C2H2X2, using solid-state deuterium quadrupole spin-echo techniques (6). Such measurements, at least for the di- and trihalomethanes, do not suffer so severely from uncertainties due to solvent effects, arising, for example, from the use of various liquid crystals containing aromatic and other more polar groups, or to uncertainties in bond lengths, vibrational averaging, and unknown electric field gradient asymmetry parameters. Spectra were obtained on a “home-built” spectrometer at 55.3 MHz using an 8.‘45 T, 3.0 in. bore Oxford Instruments (Qsney Mead, Oxford, U.K.) superconducting so’lenoid, a Nicolet (Madison, Wise.) 1180 computer and 1290 transient recorder, together with a variety of other digital and radiofrequency electronics. We also used a home-built solenoidal radiofrequency coil probe (sample volume -0.8 cm3), together with a quadrupole spin-echo sequence (6), for data acquisition. The 90” pulse width used was 3.3 ps. All samples (except [2H]triiodomethane) were obtained from Merck, Sharpe, and Dohme (Canada), and were used without further purification. [2H]Triiodomethane was prepared from [2H]trichloromethane and iodoethane, as reported in the literature (7). A typical experimental spectrum (displayed for the convenience of the reader as a “mirrored” on-resonance single-phase detected spectrum), together with its com