Resulting Dipole Moment Research Articles

Spin torques and anomalous velocity in spin textures induced by fast electron injection from topological ferromagnets: The role of gauge fields

A new method for analyzing magnetization dynamics in spin textures under the influence of fast electron injection from topological ferromagnetic sources such as Dirac half metals has been proposed. These electrons, traveling at a velocity v with a non-negligible value of (where c is the speed of light), generate a non-equilibrium magnetization density in the spin-texture region, which is related to an electric dipole moment via relativistic interactions. When this resulting dipole moment interacts with gauge fields in the spin-texture region, an effective field is created that produces spin torques. These torques, like spin-orbit torques that occur when electrons are injected from a heavy metal into a ferromagnet, can display both damping-like and anti-damping-like properties. Finally, we demonstrate that such an interaction between the dipole moment and the gauge field introduces an anomalous velocity that can contribute to transverse electrical conductivity in the spin texture in a way comparable to the topological Hall effect.

Incorporating Polarization and Charge Transfer into a Point-Charge Model for Water Using Machine Learning

Rigid nonpolarizable water models with fixed point charges have been widely employed in molecular dynamics simulations due to their efficiency and reasonable accuracy for the potential energy surface. However, the dipole moment surface of water is not necessarily well-described by the same fixed charges, leading to failure in reproducing dipole-related properties. Here, we developed a machine-learning model trained against electronic structure data to assign point charges for water, and the resulting dipole moment surface significantly improved the predictions of the dielectric constant and the low-frequency IR spectrum of liquid water. Our analysis reveals that within our atom-centered point-charge description of the dipole moment surface, the intermolecular charge transfer is the major source of the peak intensity at 200 cm-1, whereas the intramolecular polarization controls the enhancement of the dielectric constant. The effects of exact Hartree-Fock exchange in the hybrid density functional on these properties are also discussed.

On Dipole Moments and Hydrogen Bond Identification in Water Clusters

It is demonstrated that the localized orbitals calculated for a water cluster have small delocalization tails along the hydrogen bonds, that are crucial in determining the resulting dipole moments of the system. (By cutting them, one gets much smaller dipole moments for the individual monomers-close to the values one obtains by using a Bader-type analysis.) This means that the individual water monomers can be delimited only in a quite fuzzy manner, and the electronic charge density in a given point cannot be assigned completely to that or another molecule. Thus, one arrives to the brink of breaking the concept of a water cluster consisting of individual molecules. The analysis of the tails of the localized orbitals can also be used to identify the pairs of water molecules actually forming hydrogen bonds.

Beyond CarbonK-Edge Harmonic Emission Using a Spatial and Temporal Synthesized Laser Field

We present numerical simulations of high-order harmonic generation in helium using a temporally synthesized and spatially nonhomogeneous strong laser field. The combination of temporal and spatial laser field synthesis results in a dramatic cutoff extension far beyond the usual semiclassical limit. Our predictions are based on the convergence of three complementary approaches: resolution of the three dimensional time dependent Schrödinger equation, time-frequency analysis of the resulting dipole moment, and classical trajectory extraction. A laser field synthesized both spatially and temporally has been proven capable of generating coherent extreme ultraviolet photons beyond the carbon K edge, an energy region of high interest as it can be used to initiate inner-shell dynamics and study time-resolved intramolecular attosecond spectroscopy.

On the influence of point defects on the structural and electronic properties of graphene-like sheets: a molecular simulation study

The influence of vacancies and substitutional defects on the structural and electronic properties of graphene, graphene oxide, hexagonal boron nitride, and boron nitride oxide two-dimensional molecular models was studied using density functional theory (DFT) at the level of local density approximation (LDA). Bond length, dipole moment, HOMO-LUMO energy gap, and binding energy were calculated for each system with and without point defects. The results obtained indicate that the formation of a point defect does not necessary lead to structural instability; nevertheless, surface distortions and reconstruction processes were observed, mainly when a vacancy-type defect is generated. For graphene, it was found that incorporation of a point defect results in a semiconductor-semimetal transition and also increases notably its polar character. As with graphene, the formation of a point defect in a hexagonal boron nitride sheet reduces its energy gap, although its influence on the resulting dipole moment is not as dramatic as in graphene. The influence of point defects on the structural and electronic properties of graphene oxide and boron nitride oxide sheets were found to be mediated by the chemisorbed species.

Application of Density Functional Theory To Study the Double Layer of an Electrolyte with an Explicit Dimer Model for the Solvent

Most theoretical studies of an electrical double layer, which is formed by an electrolyte in contact with a charged electrode, employ a primitive model in which the solvent is represented by a dielectric continuum. This implicit-solvent model is convenient because computations are comparatively simple. However, it suppresses oscillations in the density profiles of ionic species that result from the discreteness of the solvent molecules. Furthermore, the implicit-solvent model yields poor results for the capacitance. In comparison with experiment at fixed electrode charge density, it predicts a too small electrode potential, and the resultant capacitance is too large. This latter discrepancy can be compensated in part by postulating the existence of an often fictitious inner layer whose properties are parametrized to agree best with experiment. The use of an implicit solvent model and an inner layer helps in correlating experimental results but rests on a faulty microscopic picture. Unfortunately, explicit consideration of solvent molecules poses both theoretical and numerical difficulties and, as a result, studies using an explicit solvent model have been few and far between. In this study, we consider a simple nonprimitive or explicit solvent model in which each solvent molecule is represented by a dimer composed of touching positive and negative hard spheres, with a resulting dipole moment that is equal to that of a water molecule, and the ions are represented by charged hard spheres. The density profiles and charge-potential relationship of this model are examined using the classical density functional theory. We find that the introduction of an explicit solvent increases the electrode potential, at fixed electrode charge, without the need to postulate a parametrized inner layer. Because of the solvent polarity, the ion profiles become strong oscillatory and show local charge inversion near a highly charged electrode surface at all ion concentrations.

State-Selective Electron Transfer in an Unsymmetric Acceptor−Zn(II)porphyrin−Acceptor Triad: Toward a Controlled Directionality of Electron Transfer from the Porphyrin S2 and S1 States as a Basis for a Molecular Switch

A series of Zn(II) porphyrin (ZnP) compounds covalently linked to different electron acceptor units, naphthaleneimide (NI) and naphthalenediimide (NDI), are reported. The aim was to demonstrate a state-selective direction of electron transfer, where excitation to the lowest excited S(1) state of the porphyrin (Q-band excitation) would give electron transfer to the NDI unit, while excitation to the higher S(2) state (Soret-band excitation) would give electron transfer to the NI unit. This would constitute a basis for an opto-electronic switch in which the direction of electron transfer and the resulting dipole moment can be controlled by using light input of different color. Indeed, electron transfer from the S(1) state to NDI occurred in solvents of both high and low polarity, whereas no electron transfer to NDI was observed from the S(2) state. With NI as acceptor instead, very rapid (tau = 200-400 fs) electron transfer from the S(2) state occurred in all solvents. This was followed by an ultrafast (tau approximately 100 fs) recombination to populate the porphyrin S(1) state in nearly quantitative yield. The charge-separated state ZnP(+)NI(-) was spectroscopically observed, and evidence was obtained that recombination occurred from a vibrationally excited ("hot") ZnP(+)NI(-) state in the more polar solvents. In these solvents, the thermally relaxed ZnP(+)NI(-) state lies at lower energy than the S(1) state so that further charge separation occurred from S(1) to form ZnP(+)NI(-). This resulted in a highly unusual "ping-pong" sequence where the reaction went back and forth between locally excited ZnP states and charge-separated states: S(2) --> ZnP(+)NI(-)("hot") --> S(1) --> ZnP(+)NI(-) --> S(0). The electron transfer dynamics and its solvent dependence are discussed, as well as the function of the present molecules as molecular switches.

Electronic absorption spectra and nonlinear optical properties of CO2 molecular aggregates: A quantum chemical study

We have investigated the structural aspects of several carbon dioxide molecular aggregates and their spectroscopic and nonlinear optical properties within the quantum chemical theory framework. We find that, although the single carbon dioxide molecule prefers to be in a linear geometry, the puckering of angles occur in oligomers because of the intermolecular interactions. The resulting dipole moments reflect in the electronic excitation spectra of the molecular assemblies. The observation of significant nonlinear optical properties suggests the potential application of the dense carbon dioxide phases in opto-electronic devices.

Microsolvation of a partially bonded complex by non-polar and weakly polar microsolvents: a microwave and ab initio study of HCN–SO3–Ar and HCN–SO3–CO

Rotational spectra of HCN–SO3–Ar and HCN–SO3–CO, and several of their isotopically substituted derivatives, have been observed by Fourier transform microwave spectroscopy. Both complexes are symmetric tops with the Ar or CO weakly bound to the SO3 on the side opposite the HCN. The N–S and S–Ar distances in HCN–SO3–Ar are 2.5905(19) and 3.4465(50) Å, respectively. In HCN–SO3–CO, the N–S and S–C distances are 2.6563(14) and 3.0109(48) Å, respectively, representing a marked increase relative to the analogous distances in HCN–SO3 and SO3–CO. These results are in sharp contrast to previous work on HCN–HCN–SO3, in which the second HCN attaches to the HCN side of the HCN–SO3 and causes a large contraction of the partially formed N–S bond. Stark effect measurements have been performed and the resulting dipole moments are 4.230(10) D for HC15N–SO3–Ar and 3.678(11) D for HC15N–SO3–CO. These values are very near the differences between the dipole moments of the constituent dimers, suggesting that the dipole moments of the trimers are the vector sums of the dimer moments. Ab initio calculations for both complexes in the HCN–SO3–X and X–HCN–SO3 configurations are reported.

On the re-engineered TIP4P water models for the prediction of vapor–liquid equilibrium

We perform extensive Gibbs Ensemble Monte Carlo simulations to study the capability of some recently re-parameterizations of the original TIP4P model intended to predict accurately the vapor–liquid coexistence envelope of water, its critical point, and its temperature dependence for the vapor pressure and second virial coefficient, and complement this analysis with the characterization of some specific crystalline faces of ice. We also disclose some trends between the resulting dipole moment of the models and the Lennard–Jones parameters, the location of the negative charge, as well as the estimated critical temperature. Finally, we discuss the inability of these models to predict accurately and simultaneously the melting temperature and the temperature of maximum density.

Structural properties ofPbTe∕CdTeinterfaces from first principles

The structural and energetic properties of the $\mathrm{Pb}\mathrm{Te}∕\mathrm{Cd}\mathrm{Te}(100)$, (110), and (111) interfaces are investigated using an ab initio pseudopotential method within a slab approximation. A scheme is proposed to calculate interface energies for polar faces with a resulting dipole moment. The atomic relaxation gives rise to similar effects as those observed for free surfaces. At (110) interfaces the rebonding leads to pronounced additional lateral displacements. The $\mathrm{Pb}\mathrm{Te}∕\mathrm{Cd}\mathrm{Te}(111)$ interface is the most energetically stable interface studied. The interface energies are used to predict the equilibrium crystal shape of a PbTe quantum dot embedded in a CdTe host matrix by means of a Wulff construction.

Vibronic transition moments and line intensities for H 2O +

Ab initio calculations of the electronic dipole moment components for the A ̃ 2A 1 and X ̃ 2B 1 electronic states and the electronic transition moment for the A ̃ 2A 1 – X ̃ 2B 1 transition of H 2O + have been carried out. Parameterized analytical functions have been fitted through the computed ab initio data points, and the resulting dipole moment and transition moment surfaces have been used, along with potential energy surfaces derived from the ab initio results of Brommer et al. [J. Chem. Phys. 98 (1993) 5222], to simulate H 2O + spectra and to generate an extensive set of vibronic transition moments for the A ̃ – X ̃ and X ̃ – X ̃ band systems of H 2O +. The work is made with the dual purpose of facilitating further assignments of high-resolution spectra [J. Mol. Spectrosc. 219 (2003) 258] and of allowing cometary spectra of H 2O + to be simulated [Ap. J. 574 (2002) L183].

Prediction of molecular dipole moments from bond moments: testing of the method by DFT calculations on isolated molecules

The well-known principle of vector addition of bond moments, used as standard approximation when interpreting and predicting the experimental dipole moments, has been now checked on the calculated moments of isolated molecules with the accuracy not attainable by measurements in solution. Chloro derivatives of adamantane 1a–3a and 1,4-derivatives of bicyclo[2.2.2]octane 4–16 were used as model compounds: their dipole moments were calculated at the levels B3LYP/6-311+G(d,p) and B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p). In the case of two collinear dipoles, the resulting dipole moments were always slightly greater than the additive values but the additive approximation would be sufficient for practical purposes. The small difference can be explained by mutual induction of the two dipoles and can be roughly estimated by simple electrostatic calculation in spite of several uncertain parameters. The same applies to the adamantane derivatives 2a and 3a, in agreement with previous experiments in solution. In the case of the unsymmetrical CH2Cl groups in bicyclooctane derivatives 13–16, the additive values are always too great since the group moment does not lie exactly in the C–Cl bond. Nevertheless, the principle of vector addition can be used even in this case when an empirically determined group moment is used instead of a bond moment. Summarizing, the vector addition of bond moments is not exact theoretically but can be used rather broadly with a precision comparable to the accuracy of the experimental dipole moments estimated from solution measurements.

Application of the DVR method to the vibration-rotation spectrum of N 2O: derivation of the dipole moment derivatives in Radau coordinates

The rovibrational line intensities of the N 2O molecule in its ground electronic state have been used to determine its dipole moment derivatives using a fitting procedure. The wavefunctions calculation have been made using a Discrete Variable Representation method in Radau coordinates and an available potential energy surface. The resulting dipole moment derivatives are compared to previous ones obtained using normal coordinates approach.

Complete Structure of Gauche 1,1,2,2-Tetrafluoroethane Determined by Microwave Spectroscopy

The complete molecular structure of the high-energy gauche rotamer of 1,1,2,2-tetrafluoroethane (TFEA), Freon F134, has been determined using pulsed-molecular-beam Fourier transform microwave spectroscopy. Five different 13C- and 2H-substituted isotopomers, in addition to the normal species, were studied. Unlike the nonpolar anti rotamer, the gauche form has a permanent electric dipole moment, which permits a direct rotational analysis. The electric dipole moment was also determined using the Stark effect of two low-J transitions. The resulting dipole moment is μc = 8.186(7) × 10-30 C m [2.454(2) Debye] where the numbers in parentheses are the standard uncertainties (i.e., 1σ). The experimental results for the geometric parameters of TFEA are compared with two recent ab initio calculations.

Importance of Polarization in Simulations of Condensed Phase Energetic Materials

An embedded cluster model is used to estimate the molecular dipole moment of crystalline dimethylnitramine (DMNA). The electrostatic potential due to the crystal is included in the calculation via the SCREEP (surface charge representation of the electrostatic embedding potential) approach. The resulting dipole moment for DMNA in the crystalline environment is 6.69 D. This number is more than 40% greater than the gas-phase value and 15% greater than the estimated dipole moment in the liquid phase, thus providing evidence of a strong polarization effect in condensed phases of DMNA.

Solution structure of TsKapa, a charybdotoxin-like scorpion toxin from Tityus serrulatus with high affinity for apamin-sensitive Ca(2+)-activated K+ channels.

TsKapa (TsK), purified from the Buthidae Tityus serrulatus is a very high potent ligand for small-conductance apamin-sensitive calcium-activated potassium channels (SK). It is able to efficiently compete with apamin for binding on this channel (K0.5 = 0.3 nM) [Legros, C. et al., FEBS Lett. 390:81-84, 1996]. The solution structure of TsK has been determined by 2D-NMR techniques, which led to the full description of its 3D conformation: a short alpha helix from residues 14 to 20 and a three-stranded antiparallel beta sheet (residues 2-3, 27-29, and 32-34). The interaction of TsK with the SK potassium channel has been modeled according to the charge anisotropy of the ligand. The resulting dipole moment orientates TsK so that it presents toward the receptor, a surface, mainly basic, encompassing residues K18 and K19 on one side and R9 and Y8 on the other. Despite its three-dimensional structure that is related with scorpion toxins active on voltage-gated potassium channels such as charybdotoxin, the pharmacological activity and specificity of TsK is related with shorter scorpion toxins (i.e., possessing an only two-stranded beta sheet) such as scyllatoxin (also named leiurotoxin I) or P05.

Solution structure of TsKapa, a charybdotoxin‐like scorpion toxin from Tityus serrulatus with high affinity for apamin‐sensitive Ca2‐activated K channels

TsKapa (TsK), purified from the Buthidae Tityus serrulatus is a very high potent ligand for small-conductance apamin-sensitive calcium-activated potassium channels (SK). It is able to efficiently compete with apamin for binding on this channel (K0.5 = 0.3 nM) [Legros, C. et al., FEBS Lett. 390:81–84, 1996]. The solution structure of TsK has been determined by 2D-NMR techniques, which led to the full description of its 3D conformation: a short α helix from residues 14 to 20 and a three-stranded antiparallel β sheet (residues 2–3, 27–29, and 32–34). The interaction of TsK with the SK potassium channel has been modeled according to the charge anisotropy of the ligand. The resulting dipole moment orientates TsK so that it presents toward the receptor, a surface, mainly basic, encompassing residues K18 and K19 on one side and R9 and Y8 on the other. Despite its three-dimensional structure that is related with scorpion toxins active on voltage-gated potassium channels such as charybdotoxin, the pharmacological activity and specificity of TsK is related with shorter scorpion toxins (i.e., possessing an only two-stranded β sheet) such as scyllatoxin (also named leiurotoxin I) or P05. Proteins 29:359–369, 1997. © 1997 Wiley-Liss, Inc.

Structure and pyroelectric properties of vacuum deposited thin films of alkali metal stearates

Structure and pyroelectric properties of K, Na and Li stearate films, obtained by vacuum deposition, were studied. IR-spectroscopy, X-ray diffraction and electron microscopy reveal large difference in the molecular ordering and orientation in these films. The values of the pyroelectric coefficient γ are also substantially distinct. The largest value of γ equals to 1·10−6 Coul m−2 K−1 is observed for NaSt films. The manifestation of pyroelectricity in single-compound films is ascribed to the appearance of uncompensated resulting dipole moment in the films due to variation of molecular orientation and ordering with the film thickness.

Dipole binding: An experimental test for small cluster structure calculations

An experimental method for the discrimination between nearly degenerate isomers of size selected clusters of closed-shell polar molecules is presented. It is based upon electron attachment properties of dipole fields since a minimum value (≊2.5 D) of molecular dipoles is required for electron binding. When neutral clusters are created in different configurations only those with large enough resulting dipole moments bind electrons and give birth to stable anions. These dipole-bound anions have the geometries of their neutral parents which are here calculated within the framework of the exchange perturbation theory as developed by Claverie. Anions are created by Rydberg electron transfer to cold neutral clusters containing up to six molecules of acetonitrile, water, ammonia, or methanol. Structure and dipole moment calculations account well for the ‘‘magic’’ numbers observed in mass spectra distributions. Detailed comparisons between experimental data and calculated geometries lead to informations on the structure of several isomers of each studied neutral cluster.