π-Electron Densities in Molecules ΦCHX1X2 and Correlation with the Intensity of the 00 Band of the Secondary Transition

In molecules such as ΦX₁X₂X₃the intensity of the secondary transition of the benzene chromophore, strongly depends on long range interactions involving a σ·π coupling between orbitals belonging to the C_α-X bonds, and the πΦ system. Such a coupling distorts the symmetry of the π_Φ system(1–5). The secondary transition (towards 260 nm) which is forbidden when the symmetry is D_6h - as in the benzene molecule itself where only the low intensity progression A isobserved - becomes all the more allowed as the substituent is able to distort the πΦ cloud towards a C_2V symmetry. A new progression called B is observed. Its intensity is usually evaluated by the measure of its 00 band. The distorsion of symmetry arises because of the π donating or π withdrawing electronic effects of the substituent(1–7). These phenomena have been explained on the ground of a V shaped function(l) : ε₀₀ = f(σ), where σ is a parameter linked to the electronic effects of the substituents. When the whole substituent is a π donating one, its π donating effect is lowered by an increase of the electron attracting power of the groups X : the perturbation of the D_6h symmetry is lessened and ε₀₀ decreases. On the contrary, when the whole substituent is a π withdrawing one, an increase of the withdrawing power of the groups X increases the withdrawing power of the whole substituent, the distorsion of the D6h symmetry increases and ε₀₀ increases too. The minimum of the V shaped curve correspond to the species for which the π donating tendency of the aliphatic part of the substituent is quenched by the electron withdrawing tendency of the groups X. The D_6h symmetry of the πΦ cloud is restored. The transition is forbidden and the system A only is observed.     

How do concentration and dosage of the contrast agent affect the signal change in perfusion-weighted magnetic resonance imaging? A computer simulation

In this study we investigated the effect of varying both concentration and dosage of an intravenously administered MR contrast agent on the concentration-time curve in brain tissue. Aside from injection time and injection rate, our model considers the distribution of transit-time between injection site and brain but it is independent from pulse rate, heart volume or other circulation parameters. The width of the transit-time distribution and the concentration-time course in the brain were computed according to indicator dilution theory. We found that increasing the dosage of the administered contrast agent raises the maximum concentration of the agent in the brain, particularly if the injection time is short. Increasing the concentration of the agent (at fixed dosage and injection rate) also increases maximum concentration in the brain capillaries, particularly if the injection time is long. This increase, however, is less than that achieved by raising the dosage.     

Analysis of the 1S and 2S states of AQ and ΘQ with QCD sum rules

In this article,we study the ground states and the first radial excited states of the flavor antitriplet heavy baryon states AQ and ΘQ with the spin-parity JP =...     

Correlation of the magnetic and transport properties with polymorphic transitions in bismuth pyrostannate Bi2(Sn1 − x Cr x )2O7

Physics of the Solid State - The correlation of the electrical, magnetic, and structural properties of Bi2(Sn1 − x Cr x )2O7 solid solutions has been investigated for two compositions with x...     

Correlation of electrical conductivity with Mössbauer and Raman results in the alloy glass g-Ge1−xSnxSe20

The measured electrical conductivity of g-Ge_1−xSn_xSe₂ bulk alloy glasses is shown to be strongly correlated with the behavior of the Sn tetrahedral fraction and the Raman A₁ companion mode amplitude, both of which measure the density of intrinsic defects and which were previously found to be signatures of a molecular cluster network. The correlation is interpreted in terms of the variation with alloying of localized electronic states associated with the structure of this network.     

Special Features of the Correlation between the Uranium–Oxygen Interatomic Distances and the Frequencies of the Valence Vibrations of the UO2 2+ Group in Complex Compounds of Uranyl

We considered the factors due to which the dependences between the uranium–oxygen distances R ₀ and the frequencies of valence vibrations _as for complex compounds of uranyl are described by two independent expressions generally represented by equations of the type R ₀ = a + b_as ^–2/3 (b > 0) with the coefficients a and b differing in the regions R ₀ < 0.178 nm and R ₀ > 0.178 nm. It is shown that the reason is that there is a mutual effect of the ligands in the uranyl complex responsible for different partial contributions of the uranium orbitals and of the atoms of the first coordination sphere to the chemical bonding.     

Kinetics of Reactions of 1- and 2‑Naphthyl with Propyne and Allene

Bulletin of the Lebedev Physics Institute - Abstract—Quantum-mechanical ab initio calculations are applied to study the growth mechanism of polycyclic aromatic hydrocarbons via reactions of...     

Measurements of air and noble-gas broadening and shift coefficients of the methane R3 triplet of the 2ν3 band

By using a tunable diode laser spectrometer with one absorption White cell for low pressure and one photoacoustic cell for high pressure, line shape parameters of the R3 triplet of the 2ν₃ band of methane at 1.65 μm were measured. The absorption line was recorded by using the wavelength modulation spectroscopy technique with first harmonic detection. The broadening and shift coefficients were obtained by fitting the first harmonic absorption signal while varying the pressure of different perturbing gases: air and noble gases (helium, neon, argon, krypton and xenon). We present here the results for the R3 triplet. The observed shift and broadening coefficients behaviors are discussed. Received: 17 November 2000 / Revised version: 19 February 2001 / Published online: 27 April 2001     

Correlation between Quantum Entanglement and Quantum Coherence in the Case of XY Spin Chains with the Dzyaloshinskii–Moriya Interaction

Journal of Experimental and Theoretical Physics - Recently, there has been an increased interest in studying quantum entanglement and quantum coherence. Since both of these properties are...     

Analysis of the High-Resolution Infrared Spectrum of the PHD2 Molecule: ν1 and 2ν1 Bands

The results of investigation into the infrared spectra of the PHD₂ molecule including the ₁ fundamental band centered at 2324.005 cm^–1 (with a resolution of 4.2·10^–3 cm^–1) and the first 2₁ valence overtone centered at 4563.634 cm^–1 (with a resolution of 8.8·10^–3 cm^–1) are given in the present paper. Based on an analysis of the results obtained, 1340 and 1020 lines are referred to the ₁ and 2₁ bands, respectively. This data are used to calculate 316 and 248 vibrational-rotational energies of the (100000) and (200000) excited vibrational states, respectively. Since both bands can be considered as isolated, we take advantage of the Watson Hamiltonian (the reduction A in the I ^r representation) to describe their rotational structure. The calculated spectroscopic parameters of the examined states of the PHD₂ molecule correlate well with each other and with the corresponding parameters of the ground vibrational state.     

Use of fat suppression in R₂ relaxometry with MRI for the quantification of tissue iron overload in beta-thalassemic patients

Purpose

To assess the performance and results of R₂ relaxometry using a fat-suppressed (FS) multiecho sequence and compare these to conventional R₂ relaxometry in estimating tissue iron overload.

Materials and Methods

Relaxation rate values (R₂=1/T2) of the liver, spleen, pancreas and vertebral bone marrow (VBM) were estimated in 21 patients with β-thalassemia major, using a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence before (R₂) and after (R₂ FS) the application of chemically selective fat suppression.

Results

Hepatic and splenic R₂ FS values correlated with respective R₂ values (r=0.98 and r=0.96, P<.001), whereas correlations between R₂ FS and R₂ values for pancreas and VBM were not statistically significant. Bland–Altman plots show disagreement between R₂ and R₂ FS values, particularly for pancreas and VBM. Hepatic, pancreatic and VBM R₂ FS values correlated with serum ferritin (r=0.88, P<.001; r=0.51, P<.003; and r=0.75, P<.002, respectively). Hepatic R₂ FS values correlated with splenic R₂ FS (r=0.77, P<.03), pancreatic R₂ FS (r=0.61, P<.006) and VBM R₂ FS values (r=0.70, P<.001), whereas pancreatic R₂ FS values correlated also with VMB R₂ FS values. On the contrary, among the R₂ values of the above tissues, obtained without fat suppression, only hepatic R₂ values correlated with serum ferritin, whereas no correlation was documented between hepatic and pancreatic or VBM R₂ values. The application of fat suppression did not improve breathing or flow artifacts.

Conclusion

Application of fat suppression in the standard CPMG sequence improved the capability of MRI in noninvasive quantification of iron, particularly in lipid-rich tissues, such as vertebral bone marrow (VBM) and pancreas.     

Elasticity of the B2 phase and the effect of the B1–B2 phase transition on the elasticity of MgO

A study of the high-pressure anisotropy of MgO was conducted using first-principles calculations based on density functional theory within the generalized gradient approximations. The pressure dependence of the elastic stiffness coefficients and the anisotropy parameters, in both B1 and B2 phases, shows that for high-hydrostatic compression the easiest deformation is the shear along (100) plane and the the material's response to deformation and to shearing strains is quite the same. According to the calculations of the velocities of propagation of elastic waves, we deduced that MgO develop an elastic anisotropy, especially, in the B1 phase. We present the B2 phase elastic properties which are not already studied under high pressure.     

Correlation of light scattering and molecular dynamics in superionic Ba1−xLaxF2+x

Abstract

The squares of the Brillouin frequencies (Δω_B)² of the LA mode in the [100] direction related to the C₁₁ elastic constant show a linear decrease with temperature followed by significant deviations that occur above the corresponding transition temperatures (T_c), being at 850, 800, 970, 950 and 920 K for samples with x = 0.05, 0.1, 0.2, 0.3 and 0.5, respectively. The Raman linewidths show linear increases with temperature followed by rapid increases around the same temperatures at which the elastic constants start to show marked decreases. The complementary studies using Molecular Dynamics show that the diffusion coefficients start to increase markedly around the same temperatures observed experimentally.     

Propagation of light in （2＋1）-dimensional nonlinear optical media with spatially inhomogeneous nonlinearities

The （2＋1）-dimensional nonlinear SchrSdinger （NLS） equation with spatially inhomogeneous nonlinearities is investigated, which describes propagation of light in （2＋1）-dimensional nonlinear optical media with inhomogeneous nonlinearities. New types of optical modes and nonlinear effects in optical media are presented numerically. The results reveal that the regular split of beam can be obtained in （2＋1）-dimensional nonlinear optical media with inhomogeneous nonlinearities, by adjusting the guiding parameter. Furthermore, the stability of beam regular split is discussed numerically, and the results reveal that the beam regular split is stable to the finite initial perturbations.     

Structure and magnetism of the R2Fe14−xCoxB ferrimagnetic systems (R = Dy and Er)

R₂Fe_14−xCo_xB ferrimagnetic systems (R = Dy and Er) have been synthesized and studied by X-ray and magnetometry methods to determine the lattice parameters, Curie temperatures, saturation magnetizations, anisotropy fields and spin-reorientation temperatures. It has been established that the single phase materials, exhibiting a tetragonal crystal structure, can be formed in the Dy-based system only for x ⩽ 8, while in the Er-based system only for x ⩽ 5. An average increase of the Curie temperature of 58 K per one substituted Fe atom by Co is observed for both systems (x ⩽ 5). A characteristic maximum in composition dependence of the saturation magnetizations at 295 K is found for x ≃ 2, although this is less pronounced than in light rare earth-based Co-substituted systems. The composition dependencies of the anisotropy fields for the Dy-based system at 77 and 295 K show distinct maxima for low Co concentrations and a sharp decrease for higher Co content. The spin-reorientation temperature for the Er-based system is shifted towards higher temperatures by the Co substitution. A brief comparison between ferro- and ferrimagnetic R₂Fe_14−xCo_xB systems is included.     

Rapid luminescence saturation of the Mn2+ 3d shell in the Cd1−x MnxTe dilute magnetic semiconductor with a high manganese concentration

The dependence of manganese-ion intracenter-luminescence intensity on optical excitation level has been studied in the Cd_1−x Mn_xTe dilute magnetic semiconductor with 0.4<x<0.7. It is shown that the intracenter luminescence saturates due to effective nonlinear quenching already at low excitation levels. Mechanisms are proposed which can provide nonlinear quenching and offer a qualitative explanation for the temperature dependence of the luminescence saturation in samples with different manganese concentrations. Fiz. Tverd. Tela (St. Petersburg) 41, 49–53 (January 1999)