Dipole Moment Constant Research Articles

Effect of an organic molecular coating on control over the conductance of carbon nanotube channel

It is shown that the coating of carbon nanotubes with molecules with a constant dipole moment changes the conductance of the tubes due to a variation in the structure of energy levels that participate in charge transport. The I–V characteristics of the investigated structures exhibit significant dependence of the channel conductance on the gate potential. The observed memory effect of conductance level can be explained by the rearrangement of polar groups and molecules as a whole in an electric field. The higher the dipole moment per unit length and the weaker the intermolecular interaction, the faster the rearrangement process is

Effect of molecular structure on fluid transport through carbon nanotubes

We perform molecular dynamics simulations to study the transport of geometrically modified water models through channels of carbon nanotube (CNT) membranes. We use two modifications to an existing water model (extended simple point charge SPC/E) as representative surrogates of molecular fluids: (1) bent model (model B) in which the HOH angle is varied while keeping the dipole moment constant by adjusting the OH bond length and (2) modified bent model (model MB) in which the HOH angle changes without any change in OH bond length thereby changing the dipole moment. Interestingly, we find that the fluid transport is a nonmonotonic function of the bond angle for both fluid models. This observed trend is not anticipated based on the fluid density as a function of the bond angle inside and outside of the nanotube channel. However, the average residence time of transmitted molecules through the channel provides an approximately inverse linear correlation with the observed flux, independent of the fluid model. Based on these correlations, we have developed an empirical design parameter connecting fluid transport through CNTs as a function of average occupancy (number of fluid molecules inside the nanotube) and the average residence time. Our results suggest that transport through carbon nanotubes can be sensitive to small changes in the structure of fluid molecules that can potentially be utilised for mixture separation.

Dielectric and structural properties of aqueous nonpolar solute mixtures

The dielectric properties and molecular structure of water mixtures with different nonpolar solutes (methane and noble gases) are studied using molecular dynamics. The water-water, water-solute, and solute-solute interactions are calculated using the combination of a polarizable potential [J. Li, Z. Zhou, and R. J. Sadus, J. Chem. Phys. 127, 154509 (2007)] for water plus the Lennard-Jones potential. The effect of solute size and concentration on the solubility of the system, hydrogen bonding, dielectric constant, and dipole moment are investigated over a temperature range of 278-750 K and solute percentage mole fractions up to 30%. Solute particles affect the structure of water, resulting in the compression of oxygen-oxygen and oxygen-hydrogen radial distribution functions. The influence of the solute extends both to relatively low concentrations and high temperatures. The coordination numbers of aqueous solutions of the nonpolar solutes appear to be proportional to the size of the solute particles. Our study shows the destructive influence of the nonpolar solute on both the tetrahedral water structure and hydrogen bond formation at solute concentrations greater than 30%. The presence of nonpolar particles typically decreases both the dielectric constant and dipole moment. The decrease of dielectric constant and water dipole moment is directly proportional to the solute concentration and temperature.

Surface tension and phase coexistence for fluids of molecules with extended dipoles

Molecular dynamics simulations of fluids of molecules with extended dipoles were performed, with increasing distance between point charges but with a constant dipole moment, to obtain thermodynamic properties. It was found that the effect of varying the dipole length on the dielectric constant in the liquid phase, the vapor-liquid equilibria, and the surface tension was negligible for dipolar lengths up to half the particle diameter. By comparing thermodynamic properties of the predictions of the extended dipole model with those for the Stockmayer fluid of point dipoles, it was found that extended dipoles are equivalent to point dipoles over a wide range of dipole lengths, and not only near the point dipole limit, when the separation length is very small compared with the mean distance between particles. Finally, phase equilibrium results of extended dipoles were compared to those obtained from the discrete perturbation theory for a Stockmayer potential.

Structure and polarization properties of water: Molecular dynamics with a nonadditive intermolecular potential

The temperature and density dependence of the structure and polarization properties of bulk water were systematically investigated using the ab initio MCYna potential [Li et al., J. Chem. Phys. 127, 154509 (2007)], which includes nonadditive contributions to intermolecular interactions. Molecular dynamics simulations were conducted for isochores of 1, 0.8, and 0.6 g/cm^{3} and temperatures from 278 to 750 K. Special attention was paid to the structural change of water in the range from the normal boiling point to supercritical temperatures. At temperatures below the normal boiling temperature, water exhibits a tetrahedral structure along the 0.8 and 0.6 g/cm^{3} isochores. A significant collapse of the hydrogen bonding network was observed at temperatures of 450, 550, and 650 K. The MCYna potential was able to successfully reproduce the experimental dielectric constant. The dielectric constant and average dipole moments decrease with increasing temperature and decreasing density due to weakened polarization. A comparison is also made with SPC-based models.

The effect of halogen substitution on the structure and electronic spectra of fluorone dyes

By means of the B3LYP density functional method with the use of the polarized continuum model PCM, we have performed quantum-chemical computations of the electronic absorption and fluorescence spectra of fluorone dianions: fluorescein, dibromofluorescein, eosin, erythrosine, and Rose Bengal in vacuum and methanol. We have revealed conformational features of the structure of fluorone dianions (charge redistribution, changes in the bond lengths and angles between bonds) second by the halogen substitution, the transition from the ground state to an excited state, and the change of the solvent (vacuum-methanol). Absorption and fluorescence wavelengths, constant dipole moments, transition dipole moments, and oscillator strengths have been calculated. We have showed that, upon halogenation of fluorones, the absorption spectra are redshifted and the Stokes shift decreases, which is qualitatively consistent with experimental results.

GROUP 12 ELEMENTS AND THEIR SMALL CLUSTERS: ELECTRIC DIPOLE POLARIZABILITY OFZn,CdANDHg,Zn2DIMER AND HIGHERZnnMICROCLUSTERS AND NEUTRAL, CATIONIC AND ANIONIC ZINC OXIDE MOLECULES (ZnO,ZnO+ANDZnO-)

This review is in general about group 12 elements and their small microclusters. In this part, after presenting an extensive literature survey of the electric dipole polarizability studies of the Zn , Cd and Hg atoms, we specifically target zinc-containing small clusters, beginning with the Zn2dimer, the Zn3trimer, higher Znnclusters and the neutral, cationic and anionic zinc oxide clusters: ZnO , ZnO+and ZnO-. We tabulated experimental and theoretical results for the spectroscopic constants (dissociation energy Deor D0, bond length re, fundamental frequency we, anharmonicity constant wexeand dipole moment μe) of the diatomic clusters and the first and second ionization potentials IP1and IP2and electron affinity EA of the species reviewed.

Reverse-High Performance Liquid Chromatography Mechanism Explained by Polarization of Stationary Phase

Original results and conclusions on Reverse Phase High Performance Liquid Chromatography (RPLC) mechanism are here presented. So far, none of the theoretical approaches applied to the RPLC mechanism can explain the retention and elution mechanisms of most of the analytes by RPLC, especially neutral organic compounds. Our experiences allowed us to state that RPLC retention mechanism most likely occurs through polarization of stationary phase (usually dielectric surfaces) submerged into solvents with huge dielectric constant and high dipole moment (i.e. water and/or acetonitrile) at high pressures as those applied in RPLC systems. The surface polarized interacts with polarizable target analytes (i.e. naphthalene, pyrene or benzo(ghi)perylene) in such a way that the retention depends on the medium polarizability of the target compound. The higher is the medium polarizability of the compound retained the higher is its retention time.

Enhanced Sensing of Nonpolar Volatile Organic Compounds by Silicon Nanowire Field Effect Transistors

Silicon nanowire field effect transistors (Si NW FETs) are emerging as powerful sensors for direct detection of biological and chemical species. However, the low sensitivity of the Si NW FET sensors toward nonpolar volatile organic compounds (VOCs) is problematic for many applications. In this study, we show that modifying Si NW FETs with a silane monolayer having a low fraction of Si-O-Si bonds between the adjacent molecules greatly enhances the sensitivity toward nonpolar VOCs. This can be explained in terms of an indirect sensor-VOC interaction, whereby the nonpolar VOC molecules induce conformational changes in the organic monolayer, affecting (i) the dielectric constant and/or effective dipole moment of the organic monolayer and/or (ii) the density of charged surface states at the SiO(2)/monolayer interface. In contrast, polar VOCs are sensed directly via VOC-induced changes in the Si NW charge carriers, most probably due to electrostatic interaction between the Si NW and polar VOCs. A semiempirical model for the VOC-induced conductivity changes in the Si NW FETs is presented and discussed.

Wavepacket and potential reconstruction by four-wave mixing spectroscopy: preliminary application to polyatomic molecules

We have recently shown how the excited-state wavepacket of a polyatomic molecule can be completely reconstructed from resonant coherent anti-Stokes Raman spectroscopy [Avisar and Tannor, Phys. Rev. Lett., 2011, 106, 170405]. The method assumes knowledge of the ground-state potential but not of any excited-state potential, however the latter can be computed once the excited-state wavepacket is known. The formulation applies to dissociative as well as bound excited potentials. We demonstrate the method on the Li2 molecule with its bound first excited-state as well as with a model dissociative excited state potential. Preliminary results are shown for a model two-dimensional molecular system. The calculations assume constant transition dipole moment (Condon approximation), delta-pulse excitation and a single excited-state potential, but we discuss the implications of removing these assumptions.

Variation in thickness of wetting water films in nonuniform electric field

We study the effect of an increase in the thickness of a water film at the inner surface of a glass capillary filled with a NaCl solution and with a disperse octane particle in the form of a column bounded by circular menisci under the action of an external dc electric field. Experimental data are obtained on the film thickness depending on the field characteristics. Analysis of experimental data shows the conformity of experimental data to early model ideas of authors. The projection of the constant dipole moment of a water molecule onto the field direction is estimated on the basis of these ideas.

A theory of elementary photophysical processes with the participation of excess electrons in polar liquids

The main concepts of the new theory of processes with the participation of excess electrons in polar liquids are considered. The theory takes into account that (1) polar liquids are electrostatically inhomogeneous (local potentials on molecules are different) and (2) a molecule can accept an electron for a short time to produce an anion in an unstable state with a certain energy and lifetime. A discrete model of a substance consisting of molecules with constant dipole moments is used. Excess electrons in a liquid are described by energy distribution density, and the behavior of electrons, by quantum mechanics equations. The experimental data on the photoionization of water and aqueous solutions of salts and the low threshold energy of photons (∼6.5 eV) at which solvated electrons appear in water are explained. The absorption spectra of water with excess electrons at the first and subsequent time moments after their photogeneration are reproduced theoretically. The dependence of the photoemission of solvated electrons from potassium-ammonia solutions on the energy of photons is interpreted. The continuous spectrum of spontaneous radiation of solvated electrons in liquid ammonia and water is calculated. The optical absorption spectra of solvated electrons in such polar liquids as water and ammonia are reproduced.

Electric deflection studies on lead clusters

The dielectric response to an inhomogeneous electric field has been investigated for Pb(N) clusters (N=7-38) within a molecular beam experiment. The experiments give clear evidence that lead clusters with 12, 14, and 18 atoms possess permanent dipole moments. For these cluster sizes, the permanent electric dipole moments strongly determine the response to the electric field, leading to a significantly increased apparent polarizability. An adiabatic polarization mechanism allows a semiquantitative explanation of the observed susceptibility anomalies. The beam profiles of most of the lead clusters with N not equal12, 14, and 18 also display a small broadening induced by the electric field, indicating permanent dipole moments of about (0.01-0.02) D/atom. Nearly constant dipole moments per atom for larger lead clusters (N>20) manifest in a linear increase in the polarizability per atom. Also, for lead clusters such as Pb(25), which do exhibit almost no measurable beam broadening, the polarizabilties are increased compared to the bulk value. This could be partially explained by the electronic structure of the lead clusters but might be also a consequence of quenched permanent dipole moments because for highly flexible clusters only an increased beam deflection, but no broadening, will be observed.

RbCs bands observation and interpretation

We calculated the approximate form of absorption bands of RbCs molecules in the 450–1000 nm wavelength range. The manifold of potential curves for the Hund’s coupling case a was used to predict the absorption spectra assuming constant transition dipole moments, which can serve as a first approximation for the comparison with experimental results and their interpretation. Any substantial departure of an observed spectrum from the predicted one would point to the need of more complex calculations comprising transition-dipole-moment functions and spin–orbit interaction. Comparison with RbCs absorption bands from hot vapor is discussed. The band at 717 nm is identified as the 11Σ+→11Π transition (B–X band) and the diffuse band at 563 nm as the 11Σ+→31Π transition.

Efficient coherent population transfer between theaΣ3and theXΣ1states ofK39Rb85

We present calculations for a coherent population transfer [stimulated Raman adiabatic passage (STIRAP)] between the two lowest electronic ground states of the KRb molecule. In contrast to previous work on STIRAP in molecular systems, position dependent transition dipole moments are included in the calculation to take the mixed singlet-triplet nature of the intermediate state into account. Additionally, we incorporate the kinetic energy term and position dependent electronic states of the molecule. Transfer efficiencies of up to 58% can be achieved for moderate laser pulses in the picosecond range. Calculations with constant transition dipole moments show a much higher transfer of 94% and thus overestimate the efficiency significantly.

Investigation of heat capacities of proteins by statistical mechanical methods

In this study additional heat capacity of the proteins in water dissociation have been investigated by statistical mechanical methods. For this purpose, taking electric field E and total dipole moment M as thermodynamical variables and starting with the first law of thermodynamics, an expression which reveals the thermodynamical relation between additional heat capacity in effective electric field Δ C E and additional heat capacity at the constant total dipole moment Δ C M , has been obtained. It is found that, difference between the heat capacities depends linearly on temperature. To establish the hydration effect during the folding and unfolding of the proteins, physical properties of the apolar dissociation have been used [G. Oylumluoglu, et al., Physica A 361 (2006) 255–262]. In the thermodynamical investigation of the protein system, in order to introduce the chemical potential μ (here it takes place of pH), one has to consider the system as a macro-canonical ensemble. In this study, the macro-canonical ensemble is obtained from the canonical ensemble. In this approach the proteins are taken in a heat bath, and also it is supposed that the system is in a particle reservoir. When this reservoir reaches to an equilibrium the number of particles take an average value. In this study, with the purpose of revealing the additional effect to the heat capacity, the partition functions of the proteins obtained in single protein molecule approach are taken. In this way, additional free energy has been related to heat capacities. Calculating the heat capacity Δ C E and taking the heat capacity at constant total dipole moment Δ C M from the experimental data, the outcomes of the performed calculations have been investigated for Myoglobin and other proteins.

Using DL_POLY to study the sensitivity of liquid structure to potential parameters

Two case studies are presented showing the local structure in liquids and how it responds to changes in the intermolecular potential. The idea is to use realistic and unrealistic potentials in order to determine the sensitivity of local liquid structure to potential parameters. The first case study concerns two families of modified water models. In the “hybrid” family, the hydrogen bond strength is reduced, but the geometry kept constant; in the second family, the molecular geometry is changed by reducing the bond angle, keeping a constant molecular dipole moment. The local structure is measured by radial distribution functions, three-dimensional probability distribution functions and three-body angular correlations. The second case study concerns the ionic liquid dimethylimidazolium chloride ([C1mim]Cl). The effect of reducing the hydrogen bonding potential of the cations while maintaining their charge is examined.

Pulsed transparency of anisotropic media with Stark level broadening

The propagation of few-cycle electromagnetic pulses through an anisotropic medium is considered. The effect of a constant dipole moment of the resonance transition on the formation of self-induced transparency pulses and their spectral composition is studied.

Noncommutative Dipole Field Theories

Assigning an intrinsic constant dipole moment to any field, we present a new kind of associative star product, the dipole star product, which was first introduced in [hep-th/0008030]. We develop the mathematics necessary to study the corresponding noncommutative dipole field theories. These theories are sensible non-local field theories with no IR/UV mixing. In addition we discuss that the Lorentz symmetry in these theories is ``softly'' broken and in some particular cases the CP (and even CPT) violation in these theories may become observable. We show that a non-trivial dipole extension of N=4, D=4 gauge theories can only be obtained if we break the SU(4) R (and hence super)-symmetry. Such noncommutative dipole extensions, which in the maximal supersymmetric cases are N=2 gauge theories with matter, can be embedded in string theory as the theories on D3-branes probing a smooth Taub-NUT space with three form fluxes turned on or alternatively by probing a space with R-symmetry twists. We show the equivalences between the two approaches and also discuss the M-theory realization.

Viscosity of a Dilute Suspension in a High‐Frequency Electromagnetic Field

The action of a high‐frequency electromagnetic field on a dilute suspension of spherical particles with a constant dipole moment is studied using statistical mechanics. An expression for effective viscosity is obtained. It is shown that the shear viscosity of the dilute suspension depends on the frequency, magnitude, and direction of the high‐frequency electromagnetic field. Depending on the frequency of the high‐frequency electromagnetic field, the rotation of the suspension particles is decelerated or accelerated, with the viscosity increasing or decreasing, respectively. It is shown that the acceleration of the suspension particles by a high‐frequency electromagnetic field “and, hence, the decrease in shear viscosity” has a resonant nature.