Charge Redistribution Effects Research Articles

Sodium and potassium salts of dichloroisocyanuric acid and their hydrates as antimicrobials agents studied by 35Cl-NQR spectroscopy and DFT calculations

The electronic structure of dichloroisocyanuric acid derivatives was analysed by 35Cl-NQR spectroscopy and DFT calculations. Here we concentrate our attention on three different factors: type of metallic substituent (sodium and potassium), temperature of the sample (liquid nitrogen and room) and degree of hydration (an amount of water molecules attached to analysed compounds). In particular, all the variations in 35Cl-NQR frequencies upon hydration of salts containing sodium and potassium ions are explained as a consequence of H-bonds formation and accompanied effects of charge redistribution. Our studies can be useful in searching for the derivatives of dichloroisocyanuric acid revealing higher antimicrobial activity.

A molecular dynamics modelling of cation hydrolysis effects

A model of a primitive cation M Z+ in aqueous solution is introduced in order to clarify the influence of ion charge on hydration structure of cations. A molecular dynamics study for this purpose was performed using a flexible non-constrained model of water molecules. It is shown that the increase of cation charge stabilized an octahedral arrangement of cation and significantly modified the intramolecular geometry of water molecules in hydration shell. For highly charged cations M 4+ and M 5+ the effect of deprotonation of some water molecules in hydration shell was observed. This phenomenon was treated as a cation hydrolysis. For correct description of this phenomenon the considered model was improved by introducing the effects of charge redistribution between hydrolysis products, which essentially modified and stabilized the hydrated–hydrolyzed structure of cation.

Harmonic Distortion in Switched Current Cells Due to Settling Error

An analysis of the harmonic distortion in switched-current cells produced by the non-linear settling error is presented. Two approaches for computing the harmonic components are addressed: discrete-time Fourier series and power series expansion. They are based on the large signal behavior of the SI cell. A compact and flexible expression is obtained with series expansion. An alternative expression is presented for cases where only the settling error magnitude is required. The effect of charge redistribution between the input and the sampling nodes is analyzed. It is shown that including that effect, the harmonic distortion is increased, and the DC gain of the SI integrator is reduced. An analysis of the total harmonic distortion when clock-feed-through and non-linear settling error are both taken into account is presented. It is demonstrated that minimum distortion is reached for a given capacitive value. For a SI cell designed with 0.8 μm standard CMOS technology working at a sampling frequency of 8 MHz, a minimum THD of −72 dB can be obtained with a Cgs a 2.4 pF.

A molecular dynamics study of uranyl hydration

Molecular dynamics calculations were carried out in order to investigate the hydration structure of uranyl in aqueous solution. The CF1 model of flexible water molecules is used. This model allows one to investigate a hydrolysis reaction for water molecules in the first uranyl hydration shell. Charge redistribution effects on hydrolysis products are also taken into account. We found five ligands in uranyl hydration shell, which is of bipyramidal pentacoordinated structure. The charge redistribution effects resulted in ligands of four water molecules and one hydroxyl, which was found closer to uranium than the other ligands.

Field penetration induced charge redistribution effects on the field emission properties of carbon nanotubes—a first-principle study

The effect of field penetration induced charge redistribution on the field emission properties of carbon nanotubes (CNTs) have been studied by the first-principle calculations. It is found that the carbon nanotube becomes polarized under external electric field leading to a charge redistribution. The resulting band bending induced by field penetration into the nanotube tip surface can further reduce the effective workfunction of the carbon nanotubes. The magnitude of the redistributed charge Δ Q is found to be nearly linear to the applied external field strength. In addition, we found that the capped (9, 0) zigzag nanotube demonstrates better field emission properties than the capped (5, 5) armchair nanotube due to the fact that the charge redistribution of π electrons along the zigzag-like tube axis is easier than for the armchair-like tube. The density of states (DOS) of the capped region of the nanotube is found to be enhanced with a value 30% higher than that of the sidewall part for the capped (5, 5) nanotube and 40% for the capped (9, 0) nanotube under an electric field of 0.33 V/Å. Such enhancements of the DOS at the carbon nanotube tip show that electrons near the Fermi level will emit more easily due to the change of the surface band structure resulting from the field penetration in a high field.

Effect of an external electric field on the charge transport parameters in organic molecular semiconductors

Internal reorganization energies and interchain transfer integrals are two key parameters governing the charge-transport properties of organic semiconducting materials. Here, in order to model some aspects of device operation in field-effect transistors based on conjugated oligomers, we investigate via semiempirical quantum-chemical calculations the way these two parameters are modified when a static electric field in the range 106–108 V/cm is applied along the long axis of pentacene and sexithienyl molecules. For the highest fields, a pronounced redistribution of the charges along the oligomer chains occurs, which is accompanied by significant geometric distortions. However, these charge redistribution effects are found not to impact significantly the transport parameters.

Vibrational properties and microstructure of reactively sputtered hydrogenated carbon nitrides

The present study is focused on the vibrational and structural characterization of a set of disordered hydrogenated carbon nitride (a-CN:H) thin films grown by reactive sputtering. A comparative analysis of the experimental results as achieved by Raman and infrared (IR) spectroscopies is made. The disorder-induced features of IR and Raman spectra are discussed as well in light of the current assessment on the vibrational properties of carbon-based materials. Some differences between a-CN:H and nonnitrogenated a-C:H materials are evidenced and attributed to the effects of charge redistribution and bond polarization due to the presence of nitrogen. In order to justify such a hypothesis, the dielectric constant, the dynamic effective charge, and the IR cross section determined by the charge transfer effects are calculated and found to be in agreement with the corresponding experimental values.

Chemical insights from high-resolution X-ray photoelectron spectroscopy and ab initio theory: propyne, trifluoropropyne, and ethynylsulfur pentafluoride.

High-resolution carbon 1s photoelectron spectroscopy of propyne (HC triple bond CCH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC triple bond CCF3) and ethynylsulfur pentafluoride (HC triple bond CSF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.

Dipole Moment Studies of n-butanol, i-butanol and t-butanol with Chlorobenzene Complexes

The dipole moments of H-bonded complexes of n-butanol, i-butanol and t-butanol with chlorobenzene in benzene were investigated from dielectric measurements at 455 kHz and at temperature 303.16 K. The dipole moment of the complex (μab), interaction dipole moment (Δμ) and induced polarization (Pab) for thermodynamically most favoured geometry of 1:1 complexes in these systems were determined. The results shows that the complexation is due to charge redistribution effect in all cases studied and the tendency of complex formation is maximum in n-butanol system.

Charging Dynamics of Self-Assembled InAs Quantum Dots Investigated by Wavelength Selective Optically Induced Charge Storage Measurements

We present results on spectrally resolved photo-resistance studies of optically induced charge storage effects in self-organized InAs quantum dots (QDs). The stored charge can be detected and erased electrically. Our results show that spectrally resolved optical QD charging provides information on carrier dynamics in the QDs. We find a difference in the charge redistribution effect within the inhomogeneous distribution of the QD ground state ensemble for stored electrons as compared to stored holes at a given temperature. For electron storage the spectrally resolved photo-resistance measurements show a shift of the stored charge ensemble with respect to the PL emission spectrum of the QD ensemble. For hole storage a symmetric broadening of the stored charge ensemble is found.

Charge redistribution effect on the properties of charge transfer complexes HnR?XY and HnR?X2 (X, Y = F, Cl, Br, I; R = O, S, N, P)

Systematic studies on structures, energies, charge transfer, dipole moments, and ionic character of a series of weakly bonded charge transfer (CT) complexes (D⋅AB, D = H2O, H2S, NH3, PH3, AB = F2, Cl2, Br2, I2, BrCl, IBr, ClF, ICl, BrF, IF) have been carried out by the hybrid Hartree–Fock density functional theory (HF-DFT) method, where those results are validated by available experimental and theoretical investigations. Employing the Hohenberg–Kohn theorem, the property of a multicomponent system is formulated with contributions from both component properties and the charge redistribution (CR) effect, which describes the electronic coupling between components. For any property of a multicomponent system, provided that the intercomponent coupling is weak enough, the first-order approximation can be applied, which yields a linear correlation of the component contribution to the CR effect. In fact, this kind of linear relationship can be evidenced by all the studied properties including the geometry, energy, charge transfer, dipole moment, and ionic character of all 40 complexes. This approximation quantitatively describes the relative contribution of the components to a given property, which shows the same tendency in a series of complexes. Based on the investigations of the CT effect on the intermolecular bond energy and the total dipole moment, it has been found that the principal bonding character of the title complexes was ascertained to be ionic with the exception of the F2 complexes, which agrees well with the calculated ionic character. The CT effect, though small in a quantitative aspect, is directly connected to various kinds of system properties. The effectiveness and consistency of the present type of calculations in multicomponent systems may allow their wider applications in the study of intermolecular interactions. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 327–338, 2001

Effect of low-temperature interphase charge transport at the Si/SiO2 interface on the photoresponse of silicon barrier structures

The effect of low-temperature electron charge redistribution at the Si/SiO2 interface between the interphase states and the conduction band of an n-Si crystal on the temperature behavior of conductance, photovoltage, and photocurrent in Si barrier structures with edge surface electron channels was studied in the temperature range of 77–300 K. The dynamics of the channel-current response to the voltage changes in the dark and under illumination can be explained qualitatively by dispersive hopping transport of holes in SiO2, which induces electron transfer to, and accumulation at, the Si surface near the barrier contact. The leveling off of the photovoltage at low temperatures and the nonmonotonic temperature dependence of the photocurrent are attributed to the nonmonotonically increasing, localized hole density at the Si/SiO2 interface and the free electron density at the Si surface with decreasing temperature, which reflects changes in the valence of oxygen complexes.

Superconducting Single Crystals of Tl2Ba2CaCu2O8 and YBa2Cu4O8: Crystal Structures in the Vicinity of Tc

The structures of superconducting Tl 2 Ba 2 CaCu 2 O 8 (T c ∼ 110 K) and YBa 2 Cu 4 O 8 (T c ∼70 K) were investigated by single-crystal X-ra y diffraction techniques in a wide temperature range. Some of the structure parameters show anomalous behaviour in the vicinity of the critical temperature. The most pronounced structural change is the shift of the apical O atoms in CuO 5 pyramids towards the CuO 2 planes, indicating charge redistribution near T c . This behaviour of the apical O atoms seems to be the common structural feature of hole-doped high-T c superconductors. Another effect of charge redistribution at T 1 concerns a very small decrease of in-plane Cu-O distances and/or O-O distances in CuO 5 pyramids. Both effects represent an averaged structure response to charge (hole) redistribution upon the phase transition to the superconducting state.

Raman-active phonons in Bi 2Sr 2Ca n−1 Cu nO 2 n+4 d ( n=1,2,3) – phonon assignment and charge redistribution effects

The phonon Raman spectra of Bi 2Sr 2Ca n−1 Cu n O 2 n+4 d ( n=1,2,3), high- T c superconductors have been investigated in a number of single-crystal and polycrystalline samples. From the polarization and doping dependence, and from a comparison with previous reports on other high- T c cuprates, we identify the A 1g and B 1g symmetry modes that are Raman allowed within the ideal body-centered-tetragonal unit cell. On the basis of the assignments proposed here, we discuss the influence of aliovalent substitutions on the phonon modes in Bi 2Sr 2− x La x CuO 6+ d and Bi 2Sr 2Ca 1− y Y y Cu 2O 8+ d . We interpret the observed changes in phonon frequencies and intensities as a combination of internal pressure and charge redistribution effects induced by doping.

A robust and physical BSIM3 non-quasi-static transient and AC small-signal model for circuit simulation

A new non-quasi-static (NQS) MOSFET model, which is applicable for both large-signal transient and small-signal ac analysis, has been developed. It employs a physical relaxation time approach to take care of the finite channel charging time to reach equilibrium and the effect of instantaneous channel charge re-distribution. The NQS model is formulated independently from the dc I-V and the charge-capacitor model, thus can be easily applied to any existing simulators. The model has been implemented in the newly released BSIM3 version 3, and comparison has been made among this model, common quasi-static (QS) SPICE models and PISCES two-dimensional (2-D) numerical device simulator. While predicting accurate NQS behavior, the time penalty for using the new model is only about 20-30% more than the common QS models. It is much less than the time required by other NQS models reported. Limitations and compromises between simplicity, efficiency and accuracy are also discussed.

Effect of charge redistribution through secondary phase formation on PTCR characteristics of n-BaTiO3 ceramics

Heterogeneous addition of alumina, up to 10 mol%, as a grain boundary modifier in $nBaTi0_3$ PTCR ceramics modifies the p-T characteristics whereby $T_{max}$ is shifted from 455 to 700 K, accompanied by the decrease in $p_{max}$. Whereas $B_2O_3$ shifts the $T_{max}$ to 410 K with little change in $p_{max}$. The grain boundary segregation of alumina leads to the formation of secondary phases, $BaAl_6TiO_{12 }$ and $Ba_3Al_{10}TiO_{20}$. The changing pattern of trap states is evident from the presence of $Ti^{4+}-O^--Al^{3+}$ type hole centers at the grain boundary layer regions, as identified by the EPR spectra. The charge redistribution among the inter-band gap defect states across the Curie temperature is substantiated by the EPR results.

The transformations of the EL6 deep level defect in n-GaAs: Is EL6 a DX-like center?

AbstractBased on the charge redistribution effect, as observed by the present authors, and the earlier reported large lattice relaxation and persistent photoconductivity phenomena associated with the EL6 defect seen in doped, undoped, semiinsulating(SI) and low temperature grown GaAs LT-GaAs), it is suggested that this defect be classified as a DX-center. A tentative unified atomic model is proposed for all the native defects EL2, EL3, EL5, and EL6 observed in GaAs.

Density functional calculations of the influence of substitution on singlet–triplet gaps in carbenes and vinylidenes

NDensity functional theory calculations of multiplet splittings are presented that agree closely with experimental measurements for six carbenes and six vinylidenes. The calculations are further analyzed to gauge the relative importance of different factors influencing the stabilities of the different spin and electronic states. In the carbene series, with halogen substituents, orbital rehybridization effects and charge redistribution effects are large. The magnitude of π-conjugation (back-bonding) is calculated to be only moderately larger (6–8 kcal mol−1) for singlets than for triplets based on natural bond orbital-derived conjugation energies. In the vinylidene series, substituion effects are primarily associated with through-space and through-bond inductive stabilization effects, especially hyperconjugation; π-conjugation effects are found to be small © 1997 John Wiley & Sons, Ltd.

Experimental and Theoretical Approach to Hydrogen-Bonded Diastereomeric Interactions in a Model Complex

Binding affinities of (R,R)-1,2-cyclohexanediamine (R) to (R,R)-1,2-cyclopentanediol (R5) and (S,S)-1,2-cyclopentanediol (S5) and to the corresponding cyclohexanediols (R6 and S6) have been measured in benzene and in CCl4 at 298 K by microcalorimetry, and unexpected differences between the diastereomeric complexes are observed. Long time scale (0.1 μs) molecular-dynamics simulations of the two smaller diastereomeric complexes, R/R5 and R/S5, in a simplified solvent model are reported. A direct free energy calculation gives results in good agreement with the experimental values measured in benzene for the first pair, but nearly identical results for the second pair, which is at variance with experiment. A systematic analysis of the dependence of simulation results on model parameters is performed, and no possibility is found to improve the enantioselectivity by parameter tuning. Other possible causes for discrepancies are specific solute−solvent or solvent−solvent interactions, electronic charge redistribution effects, or formation of clusters of more than two molecules. Owing to the long time scales reached, a well-converged picture of the dynamics is obtained, and the species present at equilibrium can be studied in detail. The average lifetime of the complex is found to be about 200 ps, whereas that of a hydrogen bond is only about 5 ps. Besides the unbound state, the dominant species observed in the simulations for both diastereomeric pairs are singly hydrogen-bonded complexes, with a clear preference for a O to N over the N to O hydrogen bond. Many other hydrogen-bonding patterns (bridged, double) are also observed in minor amounts.

Alkylation Products of a Calix[8]arene Trianion. Effect of Charge Redistribution in Intermediates.

The behavior of a well-defined calix[8]arene trianion under alkylation conditions was investigated by reacting the tetraethylammonium salt of p-tert-butylcalix[8]arene trianion with p-methylbenzyl bromide. Chromatography of the reaction mixture afforded the following derivatives in order of decreasing yields: 1,3,5,7-tetrakis-, 1,3-bis-, 1,3,5-tris-, 1,2,4-tris-, and mono(p-methylbenzyl)-p-tert-butylcalix[8]arene, besides trace amounts of 1,4- and 1,5-disubstituted compounds. Comparable results were obtained using p-tert-butylbenzyl bromide. The observed regioselectivity has been explained by assuming that after each alkylation step the left negative charge is redistributed, by exchange equilibria, over the remaining phenolic groups with preferential localization at those positions where contiguous H-bond stabilization is possible and the electrostatic repulsion is minimized. The occurrence of protonation/deprotonation equilibria allows the formation of 1,3,5,7-tetrasubstituted calix[8]arene.