Shielding Anisotropy Research Articles

Stiff, Thermally Stable and Highly Anisotropic Wood-Derived Carbon Composite Monoliths for Electromagnetic Interference Shielding

Electromagnetic interference (EMI) shielding materials for electronic devices in aviation and aerospace not only need lightweight and high shielding effectiveness, but also should withstand harsh environments. Traditional EMI shielding materials often show heavy weight, poor thermal stability, short lifetime, poor tolerance to chemicals, and are hard-to-manufacture. Searching for high-efficiency EMI shielding materials overcoming the above weaknesses is still a great challenge. Herein, inspired by the unique structure of natural wood, lightweight and highly anisotropic wood-derived carbon composite EMI shielding materials have been prepared which possess not only high EMI shielding performance and mechanical stable characteristics, but also possess thermally stable properties, outperforming those metals, conductive polymers, and their composites. The newly developed low-cost materials are promising for specific applications in aerospace electronic devices, especially regarding extreme temperatures.

Open-Type Magnetic Shields for Optical Fiber Coil Protection With Coaxial Foil Tubes

To decrease the considerable nonreciprocal phase drift caused by environmental magnetic fields in fiber-optic gyroscope (FOG), magnetic shields on optical fiber coil (OFC) are necessarily used in medium- and high-accuracy FOG protection. Cubic boxes and spherical shells without any gaps have been widely adopted for decades in shields, posing challenge for tradeoff between weight limitation of shields and its shielding demand yet. Here, an open-type magnetic shields (OMSs) with several coaxial permalloy foil tubes in few hundred micrometers thickness is proposed, overlaying on the winding process of fiber coil and finally combining with OFC as an integration module (OFC-OMS). The shielding effectiveness (SE) and longitudinal availability were adjustable by changing permalloy tubes’ thickness, intervals between coaxial foil tubes, and extended length in endcaps. The optimized result showed that an OMS set with six coaxial permalloy foil tubes achieved a longitudinal availability of 82.6% for SE above 20dB in both axial and radial magnetic field and further revealed anisotropic shielding effect in axial magnetic field remarkably resulting in a 40-dB longitudinal availability of 70.1%. Total mass of OMS reduced by around 75% comparing with the conventional magnetic shields for OFC, which showed immense potentials on reduction of FOG size and processing cost.

Nuclear magnetic and nuclear quadrupole resonance parameters of β-carboline derivatives calculated using density functional theory

A density functional theory (DFT) calculations using B3LYP/6-311++G(d,p) method were carried out to investigate the relative stability of the molecules of β-carboline derivatives such as harmaline, harmine, harmalol, harmane and norharmane. Calculated nuclear quadrupole resonance (NQR) parameters were used to determine the 14N nuclear quadrupole coupling constant χ, asymmetry parameter η and EFG tensor (q zz ). For better understanding of the electronic structure of β-carboline derivatives, natural bond orbital (NBO) analysis, isotropic and anisotropic NMR chemical shieldings were calculated for 14N nuclei using GIAO method for the optimized structures. The NBO analysis shows that pyrrole ring nitrogen (N9) atom has greater tendency than pyridine ring nitrogen (N2) atom to participate in resonance interactions and aromaticity development in the all of these structures. The NMR and NQR parameters were studied in order to find the correlations between electronic structure and the structural stability of the studied molecules.

NMR Spectral Parameters in Graphene, Graphite, and Related Materials: Ab Initio Calculations and Experimental Results

The shielding tensor in 13C nuclear magnetic resonance (NMR) offers important information about the structural aspects of carbon materials from a local point of view. Not only the symmetry of the carbon site but also the presence of local structural distortions can affect the values of the isotropic shielding constant, the shielding anisotropy, and the deviation from axial symmetry. In this report, the 13C shielding in a single graphene sheet was calculated using density functional theory (DFT) via the gauge-including projector augmented plane wave (GIPAW) method. After performing convergence tests involving changes of k sampling and supercell size, the calculations were extended to graphene-based systems, including graphene bilayer and stacked graphene sheets, finally leading to hexagonal graphite. The calculated results showed good agreement with experimental values obtained by 13C NMR measurements in different types of carbon materials, evidencing the power of the DFT calculations for predicting NMR pa...

Structure and Properties of 9,14,15,16,17,18,19,20-Octahydro-9,14[1′,4′]-benzenobenzo[b]triphenylene

The compound 9,14,15,16,17,18,19,20-octahydro-9,14[1′,4′]-benzenobenzo[b]triphenylene, C28H24, was prepared by hydrogenation of the 4πs+4πs photocycloadduct of dibenz[a,c]anthracene and 1,3-cyclohexadiene with Pt/C in ethyl acetate. The X-ray diffraction analysis shows that the compound crystallizes in the monoclinic space group P21/c with the geometric parameters of a = 11.0090(17) Å, b = 13.733(2) Å, c = 13.091(2) Å, and β = 109.583(13)°. In addition to several close intramolecular contacts involving hydrogens derived from the dibenzanthracene moiety, long interannular C–C single bonds of about 1.593 Å are present. These bonds are shorter by about 0.18 Å than the corresponding bonds in the unsaturated precursor, which can be attributed to reduced strain in the more saturated polycyclic ring system. Anisotropic shielding of the four endo-methylene hydrogens in the 1H NMR spectrum is larger for the two hydrogens lying above the phenanthrene unit, which resonate at 1.03 ppm, than those above the benzenoid ring, which resonate at 1.24 ppm. Theoretical calculations reproduce the geometry with good agreement.

Modifier cation effects on 29Si nuclear shielding anisotropies in silicate glasses

We have examined variations in the 29Si nuclear shielding tensor parameters of SiO4 tetrahedra in a series of seven alkali and alkaline earth silicate glass compositions, Cs2O·4.81 SiO2, Rb2O·3.96 SiO2, Rb2O·2.25 SiO2, K2O·4.48 SiO2, Na2O·4.74 SiO2, BaO·2.64 SiO2, and SrO·2.36 SiO2, using natural abundance 29Si two-dimensional magic-angle flipping (MAF) experiments. Our analyses of these 2D spectra reveal a linear dependence of the 29Si nuclear shielding anisotropy of Q(3) sites on the Si–non-bridging oxygen bond length, which in turn depends on the cation potential and coordination of modifier cations to the non-bridging oxygen. We also demonstrate how a combination of Cu2+ as a paramagnetic dopant combined with echo train acquisition can reduce the total experiment time of 29Si 2D NMR measurements by two orders of magnitude, enabling higher throughput 2D NMR studies of glass structure.

Exploring the Ruthenium-Ligands Bond and Their Relative Properties at Different Computational Methods

We report some experimental bond distances and computational models of six ruthenium bonds obtained from DFT to higher computational methods like MP2 and CCSD. The bonds distances, geometrical RMSD, and the thermodynamic properties of the models from different computational methods are similar. It is observed that optimization of molecules of many light atoms with different functional methods results in significant geometrical variation in the values and order of the computed properties. The values of the hyperpolarizabilities, HOMO, LUMO, and isotropic and anisotropic shielding are found to depend greatly on the type of the functional used and the geometrical variation rather than on the nature of basis set used. However, all the methods rated modelled Ru-S, Ru-Cl, and Ru-O bonds as having the highest hyperpolarizabilities values. The infrared spectra data obtained from the different computational methods are significantly different from each other except for MP2 and CCSD which are found to be very similar.

Chiral Recognition of Diketopiperazine Cyclo(Pro-Gly) and Propranolol Using (-)-Epigallocatechin-3-O-gallate.

In the (1)H-NMR spectrum of a solution containing an equimolecular amount of cyclo(L-Pro-Gly), cyclo(D-Pro-Gly) and (-)-epigallocatechin-3-O-gallate (EGCg) in a D2O, a difference in the chemical shift of (1)H-NMR signal for H7α, H7β,8α of the Pro residue was observed. Judging from the crystal structures of the 2 : 2 complexes of EGCg and cyclo(L-Pro-Gly), cyclo(D-Pro-Gly), the difference in the chemical shift resulted mainly from a magnetic anisotropic shielding effect by the ring current from the B ring of EGCg. Therefore, it was considered that chirality of cyclo(Pro-Gly) was recognized by EGCg in the D2O solution. Furthermore, in the (1)H-NMR spectrum of a solution containing an equimolecular amount of racemic propranolol ((R)- and (S)-propranolols) and EGCg in D2O, the (1)H-NMR signal for H2 of the naphthalene group was observed as two doublets, suggesting that the racemic propranolol formed diastereomers of complexes with EGCg; as a result, chirality of propranolol was recognized by EGCg in the D2O solution.

The C-, Si-, Ge-Doped (6,3) Chiral BNNTs: A Computational Study

Abstract Electronic structure properties including bond lengths, bond angles, dipole moments (μ), energies, band gaps, nuclear magnetic resonance (NMR) parameters of the isotropic and anisotropic chemical shielding parameters for the sites of various atoms were calculated using density functional theory for C-, Si-, and Ge-doped (6,3) chiral BNNTs. The calculations indicated that average bond lengths were as follows: Ge–N>Si–N>C–N and Ge–B>Si–B>C–B. The dipole moments for C-, Si-, and Ge-doped (6,3) chiral BNNTs structures show fairly large changes with respect to the pristine model.

Chemical Shielding Anisotropies for Chloroform Exchanging between a Free Site and a Complex with Cryptophane-D: A Cross-Correlated NMR Relaxation Study.

The case of (13)C-labeled chloroform exchanging between a free site in solution and the encaged site within the cryptophane-D cavity is investigated using the measurements of longitudinal cross-correlated relaxation rates, involving the interference of the dipole-dipole and chemical shielding anisotropy interactions. A compact theoretical expression is provided, along with an experimental protocol, based on INEPT (insensitive nuclei enhanced by polarization)-enhanced double-quantum-filtered inversion recovery measurements. The analysis of the build-up curves results in a set of cross-correlated relaxation rates for both the (13)C and (1)H spins at the two sites. It is demonstrated that the results can be given a consistent interpretation in terms of molecular-level properties, such as rotational correlation times, the Lipari-Szabo order parameter, and interaction strength constants. The analysis yields the bound-site carbon-13 chemical shielding anisotropy, ΔσC = -58 ± 8 ppm, in good agreement with most recent liquid-crystal measurements and the corresponding proton shielding anisotropy, ΔσH = 14 ± 2 ppm.

129Xe NMR Investigation of the Anisotropic Environment of a Thermotropic Nematic Liquid Crystal 4-Cyano-4′-Pentylbiphenyl

The anisotropic environment of the thermotropic nematic liquid crystal (LC), 4-cyano-4′-pentylbiphenyl (5CB), is probed by using 129Xe NMR spectroscopy. The temperature versus 129Xe nuclear shielding of the xenon gas dissolved in 5CB LC has clearly shown the nematic-isotropic phase transition. The observed linear and non-linear behavior of 129Xe chemical shifts in the isotropic and nematic phases, respectively, are explained by using the pair-wise additive model. Furthermore, the parameters such as 129Xe shielding constant (σ0), the anisotropy of the shielding tensor (σ0), the temperature dependence of the isotropic shielding (ϵ), and the shielding anisotropy (χ), are derived.

Chiral recognition of diketopiperazines cyclo(Pro-Gly) using (−)-epigallocatechin-3-O-gallate

Upon a formation of complexes of EGCg and cyclo(l-Pro-Gly), cyclo(d-Pro-Gly) in D2O, a chirality of cyclo(Pro-Gly) was recognized by difference of the chemical shift of 1H NMR signal for H7α, H7β,8α of the Pro residue. Judging from the crystal structures of the 2:2 complexes of EGCg and cyclo(l-Pro-Gly), cyclo(d-Pro-Gly), such a difference of the chemical shift might be due to magnetic anisotropic shielding effect by the ring current from the B ring of EGCg.

Interpretation of the longitudinal (13)C nuclear spin relaxation and chemical shift data for five bromoazaheterocycles supported by nonrelativistic and relativistic DFT calculations.

The longitudinal relaxation times of (13)C nuclei and NOE enhancement factors for 2-bromopyridine (1), 6-bromo-9-methylpurine (2), 3,5-dibromopyridine (3), 2,4-dibromopyrimidine (4), and 2,4,6-tribromopyrimidine (5) have been measured at 25 °C and B0 = 11.7 T. The most important contributions to the overall relaxation rates of nonbrominated carbons, i.e., the relaxation rates due to the (13)C-(1)H dipolar interactions and the shielding anisotropy mechanism, have been separated out. For 3 and 5, additionally, the T2,Q((14)N) values have been established from (14)N NMR line widths. All of these data have been used to determine rotational diffusion tensors for the investigated molecules. The measured saturation recovery curves of brominated carbons have been decomposed into two components to yield relaxation times, which after proper corrections provided parameters characterizing the scalar relaxation of the second kind for (13)C nuclei of (79)Br- and (81)Br-bonded carbons. These parameters and theoretically calculated quadrupole coupling constants for bromine nuclei have allowed the values of one-bond (13)C-(79)Br spin-spin coupling constants to be calculated. Independently, the coupling constants and magnetic shielding constants of the carbon nuclei have been calculated theoretically using the nonrelativistic and relativistic DFT methods F/6-311++G(2d,p)/PCM and so-ZORA/F/TZ2P/COSMO (F = BHandH or B3LYP), respectively. The agreement between the experimental and theoretical values of these parameters is remarkably dependent on the theoretical method used.

DFT investigation of the electronic structure of amphetamines: effect of substituent on O and N chemical shielding and electric field gradient tensors

To understand the relationship between the electronic structure of amphetamines and their biological activity, a computational investigation was carried out to characterise the 17O and 14N, electric field gradient (EFG) tensors and nuclear quadrupole coupling constants (NQCC) as well as 17O, 15N, 13C and 1H chemical shielding tensors of amphetamines. Calculation were performed with the B3LYP and PW91P86 density functional theory (DFT) methods with the 6-311++G(d,p) basis set. The anisotropic (Δσ) and isotropic (σiso) shielding are in good agreement with the available experimental data. The results indicate that the EFG and σ at oxygen are insensitive to substitution at the N and C10 positions in amphetamines. In contrast, the nitrogen EFG and σ are quite sensitive towards substituent changes. Furthermore, it was found that EFG tensors are more sensitive than σ tensor to the electron density distribution in amphetamines. Biological activities of amphetamines were found to depend upon the electron density distribution in the region of N atoms.

Solar Surface Anisotropy effect on the Magnetic Field

AbstractWithin the literature there are at least 15 references indicating that the horizontal magnetic flux does not exactly balance vertical flux in sunspots, leading to the surprising result that div$\vec{B}$would depart from zero. Intuitively, this has to be related to the stratification at the surface of the star, due to which horizontal and vertical typical lengths are different. This surface anisotropy results from gravity, but how does gravity influence the magnetic field? To answer this question, a scenario has been proposed in two recent publications, based on anisotropic Debye shielding. The presentation reported in this paper was devoted to investigate the possibility and causes of a non-zero div$\vec{B}$. A scaling law associated with the anisotropy is able to reestablish the nullity of div$\vec{B}$, which would lead to a renewed MHD in the solar photosphere layer. An eventual observation in the laboratory is also reported.

Temperature effects in first-principles solid state calculations of the chemical shielding tensor made simple.

We study the effects of atomic vibrations on the solid-state chemical shielding tensor using first principles density functional theory calculations. At the harmonic level, we use a Monte Carlo method and a perturbative expansion. The Monte Carlo method is accurate but computationally expensive, while the perturbative method is computationally more efficient, but approximate. We find excellent agreement between the two methods for both the isotropic shift and the shielding anisotropy. The effects of zero-point quantum mechanical nuclear motion are important up to relatively high temperatures: at 500 K they still represent about half of the overall vibrational contribution. We also investigate the effects of anharmonic vibrations, finding that their contribution to the zero-point correction to the chemical shielding tensor is small. We exemplify these ideas using magnesium oxide and the molecular crystals L-alanine and β-aspartyl-L-alanine. We therefore propose as the method of choice to incorporate the effects of temperature in solid state chemical shielding tensor calculations using the perturbative expansion within the harmonic approximation. This approach is accurate and requires a computational effort that is about an order of magnitude smaller than that of dynamical or Monte Carlo approaches, so these effects might be routinely accounted for.

Quinoxaline-walled π-stacking molecules: synthesis, conformation, and luminescence properties

A series of semi-rigid N-phenylsuccinimides I (11Xy), N-arylmethylsuccinimides II (18Xy), and N,N′-(1,4-arylenebis(methylene))disuccinimides III (19XyX) walled by quinoxaline (QX), dimethylquinoxaline (diMQX), or benzoquinoxaline (BQX) ring were synthesized. Intramolecular π–π interactions in solution were detected by NMR and fluorescence spectroscopy, in terms of reciprocal anisotropic shielding and exciplex formation, respectively. The X-ray crystallographic analysis revealed the preferential conformations in solid state, which were compatible with the results of conformational studies by NMR and fluorescence spectroscopy. The results collectively indicated that 18Xy and 19XyX are driven by intramolecular π–π interactions in the mode of π-stacking (face-to-face) or T-shaped (edge-to-face) configuration to preferentially adopt the folded- and the double folded-conformation, respectively, where the spaced aromatic rings are oriented syn to each other in close proximity.

Electronic and Structural Properties of Ga-Doped (4,4) armchair SiCNT as a p-Semiconductor

The properties of pristine and Ga-doped silicon carbide nanotubes (SiCNTs) in two models (GaSi and GaC) as a p-semiconductor were investigated by density functional theory (DFT) calculations. The investigated structures were optimized, and quantum molecular descriptors and isotropic (CSI) and anisotropic (CSA) chemical shielding parameters were calculated for the optimized structures. We extended the DFT calculation to predict the electronic and structural properties of Ga-doped silicon carbide nanotubes, which are important for the production of solid-state devices and other applications. According to the calculations, the GaSi model is a better p-semiconductor than the GaC model in the production of solid-state devices.

Singularities in the lineshape of a second-order perturbed quadrupolar nucleus and their use in data fitting

Even for large quadrupolar interactions, the powder spectrum of the central transition for a half-integral spin is relatively narrow, because it is unperturbed to first order. However, the second-order perturbation is still orientation dependent, so it generates a characteristic lineshape. This lineshape has both finite step discontinuities and singularities where the spectrum is infinite, in theory. The relative positions of these features are well-known and they play an important role in fitting experimental data. However, there has been relatively little discussion of how high the steps are, so we present explicit formulae for these heights. This gives a full characterization of the features in this lineshape which can lead to an analysis of the spectrum without the usual laborious powder average.The transition frequency, as a function of the orientation angles, shows critical points: maxima, minima and saddle points. The maxima and minima correspond to the step discontinuities and the saddle points generate the singularities. Near a maximum, the contours are ellipses, whose dimensions are determined by the second derivatives of the frequency with respect to the polar and azimuthal angles. The density of points is smooth as the contour levels move up and down, but then drops to zero when a maximum is passed, giving a step. The height of the step is determined by the Hessian matrix—the matrix of all partial second derivatives. The points near the poles and the saddle points require a more detailed analysis, but this can still be done analytically. The resulting formulae are then compared to numerical simulations of the lineshape.We expand this calculation to include a relatively simple case where there is chemical shielding anisotropy and use this to fit experimental 139La spectra of La2O3.

Electromagnetism in a strongly stratified plasma showing an unexpected effect of the Debye shielding

Abstract In the literature, we found 15 references showing, without exception, that the sunspot photospheric magnetic field vertical gradient is on the order of 3–4 G/km, with field strength decreasing with height, whereas the horizontal gradient is nine times weaker, on the order of 0.4–0.5 G/km. This is confirmed by our recent THEMIS observations. As a consequence, the vanishing of div B → is not realized, and the present paper is devoted to the investigation of this problem. We point out that the photosphere is a strongly stratified plasma, having different horizontal and vertical characteristic lengths of aspect ratio 1/9 as the different terms contributing to the observed div B → . The velocities are also anisotropic under the stratification effect. As a consequence, the Debye volume is a flattened sphere. We show that in this case div B → may mathematically depart from zero. Anisotropic shielding constitutes an alternative to the existence of monopoles for being responsible for non-zero div B → . We evaluate that the solar corona is conversely not a strongly stratified plasma, so that the conditions are very different there.