Induced Dipole Moment Research Articles

Polarizable continuum model solvation analysis of certain 5-substituted isoquinoline derivatives

The polarizable continuum model (PCM) analysis has been carried out using the B3LYP method with 6-311++G(d,p) basis set for 5-bromoisoquinoline (5-BIQ), 5-aminoisoquinoline (5-AIQ), 5-nitroisoquinoline (5-NIQ), 5-methylisoquinoline (5-MIQ), 5-chloroisoquinoline (5-CIQ) and 5-methoxyisoquinoline (5-MXIQ) in ten solvents with a wide range of dielectric constants. In this paper, we report electrostatic, dispersion and repulsive interaction components of Gibb's free energy of solvation along with cavitation energies for these six systems. The induced dipole moments of these six compounds in ten solvents are calculated for the three solutes. The interaction energies of the systems are discussed in terms of dielectric constant, index of refraction and surface tension of the solvents. The influence of substituent at 5-position of isoquinoline molecule on the solubility property is investigated.

Mechanisms of electromechanical coupling in strain based scanning probe microscopy

Electromechanical coupling is ubiquitous in nature and underpins the functionality of materials and systems as diverse as ferroelectric and multiferroic materials, electrochemical devices, and biological systems, and strain-based scanning probe microscopy (s-SPM) techniques have emerged as a powerful tool in characterizing and manipulating electromechanical coupling at the nanoscale. Uncovering underlying mechanisms of electromechanical coupling in these diverse materials and systems, however, is a difficult outstanding problem, and questions and confusions arise from recent experiment observations of electromechanical coupling and its apparent polarity switching in some unexpected materials. We propose a series of s-SPM experiments to identify different microscopic mechanisms underpinning electromechanical coupling and demonstrate their feasibility using three representative materials. By employing a combination of spectroscopic studies and different modes of s-SPM, we show that it is possible to distinguish electromechanical coupling arising from spontaneous polarization, induced dipole moment, and ionic Vegard strain, and this offers a clear guidance on using s-SPM to study a wide variety of functional materials and systems.

Infrared absorption of dense helium and its importance in the atmospheres of cool white dwarfs

Aims: Hydrogen deficient white dwarfs are characterized by very dense, fluid-like atmospheres of complex physics and chemistry that are still poorly understood. The incomplete description of these atmospheres by the models results in serious problems with the description of spectra of these stars and subsequent difficulties in derivation of their surface parameters. Here, we address the problem of infrared (IR) opacities in the atmospheres of cool white dwarfs by direct $ab$ $initio$ simulations of IR absorption of dense helium. Methods: We applied state-of-the-art density functional theory-based quantum molecular dynamics simulations to obtain the time evolution of the induced dipole moment. The IR absorption coefficients were obtained by the Fourier transform of the dipole moment time autocorrelation function. Results: We found that a dipole moment is induced due to three- and more-body simultaneous collisions between helium atoms in highly compressed helium. This results in a significant IR absorption that is directly proportional to $\rm \rho_{\rm He}^3$, where $\rho_{\rm He}$ is the density of helium. To our knowledge, this absorption mechanism has never been measured or computed before and is therefore not accounted for in the current atmosphere models. It should dominate the other collisionally induced absorptions (CIA), arising from $\rm H-He$ and $\rm H_2-He$ pair collisions, and therefore shape the IR spectra of helium-dominated and pure helium atmosphere cool white dwarfs for $\rm He/H>10^4$. Conclusions: Our work shows that there exists an unaccounted IR absorption mechanism arising from the multi-collisions between He atoms in the helium-rich atmospheres of cool white dwarfs, including pure helium atmospheres. This absorption may be responsible for a yet unexplained frequency dependence of near- and mid- IR spectra of helium-rich stars.

Substituent effect on supramolecular motifs in series of succinimide polycyclic keto derivatives – Spectroscopic, theoretical and crystallographic studies

The substituent effect on the supramolecular arrangement in a series of polycyclic monoimide keto derivatives crystals was studied. Single crystal X-ray diffraction and IR spectroscopic experiments were performed for seven related compounds, as well as the Hirshfeld surface analysis and quantum chemical calculations at HF and DFT levels in vacuo, in solution and for small clusters. The presence of CO group at the bridge of the main hydrocarbon skeleton implied the catemer motif of the NimideH⋯Oimide hydrogen bond in case of smaller substituents (H, MeO, EtO). For more voluminous groups (iBuO) or additional hydrogen bond acceptors (AcO, O) the steric hindrance increased and the imide⋯imide interactions were no longer present in the solid state. The NimideH⋯Oketo or NimideH⋯Oester hydrogen bonds were formed instead. The binding energy per one NH⋯O interaction calculated for supramolecular clusters at HF/6-31G(d,p) level was ca. 20kJmol−1, indicating moderate strength of this hydrogen bond. The solvation free energies and induced dipole moments were computed at B3LYP/6-311+G(d,p) level using the integral equation formalism model (IEF PCM) considering three solvents of various polarity: non-polar chloroform, polar aprotic dimethyl sulfoxide (DMSO) and polar protic water. The relations between the vibrational spectra and the crystal structure have been discussed. The following sequence of carbonyl stretching modes in IR spectra has been derived from quantum chemical calculations: (1) at the highest frequencies – the symmetric vibration of two imide CO bonds, (2) the vibrations of keto CO bonds attached directly to the polycyclic hydrocarbon skeleton, (3) the asymmetric vibration of two imide CO bonds, and (4) at the lowest frequencies – the vibration of ester CO group. The characteristic peaks observed in imide experimental IR spectra at about 3080cm−1 have been explained as overtone and combination bands of νCO stretching and γNH out-of-plane bending vibrations.

On the impact of intermolecular interactions between the quaternary ammonium ions on interlayer spacing of quat-intercalated montmorillonite: A molecular mechanics and ab-initio study

The work comprises the nature of intermolecular interactions in the organoclays modified with quaternary ammonium ions, the SOMASIF ME100 organofilized with N-octadecyltrimethylammonium ions and the commercially available Cloisite 15A. The potential energy scan was carried out for two large organoclay models with consistent-valence force field and a few others for comparison. Then, the two-body model of the interacting ions in their most and less preferable orientations was build for both systems and the more detailed, electrically neutral model of the two ions adsorbed on montmorillonite was constructed in order to investigate the non-additive interactions. The most of the work was devoted to study ab-initio calculations performed on the Unrestricted Hartree–Fock level of theory, but the electron correlation effects were also considered. The calculated interaction energy was partitioned into exchange, delocalization and electrostatic terms and the latter was distributed into its multipolar expansion. The specific intermolecular order occurring for the most and the least energetically preferred arrangements was discussed in detail with respect to the intermolecular interactions with the special emphasis on the structures underlying the occurrence of three distinctive phases in organoclay samples, for which the X-ray data was present in the literature. The basis-set dependency of the variational-perturbation scheme adopted to decompose the calculated Hartree–Fock interaction energies was examined with the use of cc-pVDZ and 6-31G basis sets. The comparison proved high reliability of the method used, even for the less extended 3-21G basis. The results revealed the presence of the relationship between the sum of two- and three-body delocalization energy terms and the inverse induced dipole moment, calculated for three body system as well as the independent functional relations between the second order electron correlation correction and Hartree–Fock delocalization and inverse Heitler–London exchange energy terms for the system of two N-octadecyltrimethylammonium cations. The equations proposed may help to predict the values of the electron correlation correction and the stabilizing delocalization energy term from the calculations at Hartree–Fock level of theory as well as from the measured module of the induced dipole moment for molecules.

Ion solvation in polymer blends and block copolymer melts: effects of chain length and connectivity on the reorganization of dipoles.

We studied the thermodynamic properties of ion solvation in polymer blends and block copolymer melts and developed a dipolar self-consistent field theory for polymer mixtures. Our theory accounts for the chain connectivity of polymerized monomers, the compressibility of the liquid mixtures under electrostriction, the permanent and induced dipole moments of monomers, and the resultant dielectric contrast among species. In our coarse-grained model, dipoles are attached to the monomers and allowed to rotate freely in response to electrostatic fields. We demonstrate that a strong electrostatic field near an ion reorganizes dipolar monomers, resulting in nonmonotonic changes in the volume fraction profile and the dielectric function of the polymers with respect to those of simple liquid mixtures. For the parameter sets used, the spatial variations near an ion can be in the range of 1 nm or larger, producing significant differences in the solvation energy among simple liquid mixtures, polymer blends, and block copolymers. The solvation energy of an ion depends substantially on the chain length in block copolymers; thus, our theory predicts the preferential solvation of ions arising from differences in chain length.

Theoretical and experimental examination of particle–particle interaction effects on induced dipole moments and dielectrophoretic responses of multiple particle chains

Dielectrophoresis (DEP), an electrokinetic phenomenon based on particle polarizations in nonuniform electric fields, is increasingly employed for particle and cell characterizations and manipulations in microdevices. However, particle number densities are rarely varied and particle-particle interactions are largely overlooked, but both affect particle's effective polarizations by changing the local electric field, which directly impacts particle assembly into chains. This work examines theoretical and experimental particle-particle interactions and dielectrophoretic responses in nonuniform electric fields, then presents individual and chain velocities of spherical polystyrene microparticles and red blood cells (RBCs) under DEP forces in a modified quadruple electrode microdevice. Velocities are independently compared between 1, 2, 3, and 4 polystyrene beads and RBCs assembled into chains aligned with the electric field. Simulations compared induced dipole moments for particles experiencing the same (single point) and changing (multiple points) electric fields. Experiments and simulations are compared by plotting DEP velocities versus applied signal frequency from 1 kHz to 80 MHz. Simulations indicate differences in the DEP force exerted on each particle according to chain position. Simulations and experiments show excellent qualitative agreement; chains with more particles experienced a decrease in the DEP response for both polystyrene beads and RBCs. These results advance understanding of the extent that induced dipole polarizations with multiple particle chains affect observed behaviors in electrokinetic cellular diagnostic systems.

Physical origins of weak H2 binding on carbon nanostructures: insight from ab initio studies of chemically functionalized graphene nanoribbons.

We have performed ab initio density functional theory calculations, incorporating London dispersion corrections, to study the absorption of molecular hydrogen on zigzag graphene nanoribbons whose edges have been functionalized by OH, NH2, COOH, NO2, or H2PO3. We find that hydrogen molecules always preferentially bind at or near the functionalized edge, and display induced dipole moments. Binding is generally enhanced by the presence of polar functional groups. The largest gains are observed for groups with oxygen lone pairs that can facilitate local charge reorganization, with the biggest single enhancement in adsorption energy found for "strong functionalization" by H2PO3 (115 meV/H2 versus 52 meV/H2 on bare graphene). We show that for binding on the "outer edge" near the functional group, the presence of the group can introduce appreciable contributions from Debye interactions and higher-order multipole electrostatic terms, in addition to the dominant London dispersion interactions. For those functional groups that contain the OH moiety, the adsorption energy is linearly proportional to the number of lone pairs on oxygen atoms. Mixed functionalization with two different functional groups on a graphene edge can also have a synergistic effect, particularly when electron-donating and electron-withdrawing groups are combined. For binding on the "inner edge" somewhat farther from the functional group, most of the binding again arises from London interactions; however, there is also significant charge redistribution in the π manifold, which directly reflects the electron donating or withdrawing capacity of the functional group. Our results offer insight into the specific origins of weak binding of gas molecules on graphene, and suggest that edge functionalization could perhaps be used in combination with other strategies to increase the uptake of hydrogen in graphene. They also have relevance for the storage of hydrogen in porous carbon materials, such as activated carbons.

Effective conductivity of composite with imperfect contact between elliptic fibers and matrix: Maxwell’s homogenization scheme

In the present paper, we use Maxwell homogenization scheme to calculate effective transverse conductivity of a fibrous composite with imperfect contact between the matrix and the parallel elliptic fibers. The developed theory extends the Hasselman–Johnson formula to the composites with elliptic fibers. The complete solution of the conductivity problem for single elliptic inclusion with imperfect interface provides the mathematical background of the method. In the Maxwell homogenization scheme, the effective inclusion is an ellipse with aspect ratio governed by the orientation distribution of elliptic fibers. The formula for effective conductivity is derived by equating the induced dipole moment of effective inclusion to the total dipole moment of individual inhomogeneities. In addition to the usual shape of inclusions, their volume content and phase conductivities, our model accounts also for the interface conductivity and orientation distribution of fibers. The obtained solution has been verified by comparison with the available in literature analytical solutions and numerical data for the particular cases. The reported numerical data validate the equivalent inhomogeneity shape choice. The interface conductivity and fiber orientation statistics are shown to be of primary importance for the effective conductivity of composite.

Tunable Fano resonance in hybrid graphene-metal gratings

Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphene's Fermi energy and the grating's geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.

Influence of ions on molecular vibrations and hydrogen bonds in methanol–water mixtures: MD simulation study

Solutions of NaCl, MgCl2 and CaCl2 in methanol–water mixtures have been investigated by the molecular dynamics simulations. Special attention has been paid to the OH stretching frequencies and hydrogen bonds of the water and methanol molecules in the subsystems; the coordination shells of the ions and the bulk solvent. The influence of the ions on the properties of the solvent molecules is limited to the first coordination shells and it depends on the ion charge density. Thus all cations affect the intramolecular bonds, valence angles, and the dipole moment and the Cl− ion influences only the length and vibrations of the nearest OH bond of the water molecules, but it does not affect the properties of the methanol molecules. Though the influence of the cations on the solvent molecules is strong, the interactions between the solvent molecules enhance the molecular polarization. In consequence the induced dipole moment and the OH frequencies of the water and methanol molecules in the cation shells depend also on the methanol mole fraction. The shift of the OH stretching frequency correlates linearly with the induced dipole moment, but it does not correlate with the strength of the hydrogen bonds.

Self-consistent electric field-induced dipole interaction of colloidal spheres, cubes, rods, and dumbbells

When calculating the interaction between electric field-induced dipoles, the dipole moments are often taken to be equal to their polarizability multiplied by the external electric field. However, this approach is not exact, since it does not take into account the fact that particles with a dipole moment affect the local electric field experienced by other particles. In this work, we employ the Coupled Dipole Method to calculate the electric-field-induced dipole pair interaction self-consistently: that is, we take into account many-body effects on the individual induced dipole moments. We calculate interactions of particles with nonvanishing dimensions by splitting them up into self-consistently inducible “chunks” of polarizable matter. For point dipoles, spheres, cubes, rods, and dumbbells, we discuss the differences and commonalities between our self-consistent approach and the aforementioned approach of pre-assigning dipole moments to either the point dipoles or, in the case of spatially extended particles, to the chunks making up the particle.

Ab initio calculations for the far infrared collision induced absorption by N2 gas

We present (far-infrared) Collision Induced Absorption (CIA) spectra calculations for pure gaseous N2 made for the first time, from first-principles. They were carried out using classical molecular dynamics simulations based on ab initio predictions of both the intermolecular potential and the induced-dipole moment. These calculations reproduce satisfactory well the experimental values (intensity and band profile) with agreement within 3% at 149 K. With respect to results obtained with only the long range (asymptotic) dipole moment (DM), including the short range overlap contribution improves the band intensity and profile at 149 K, but it deteriorates them at 296 K. The results show that the relative contribution of the short range DM to the band intensity is typically around 10%. We have also examined the sensitivity of the calculated CIA to the intermolecular potential anisotropy, providing a test of the so-called isotropic approximation used up to now in all N2 CIA calculations. As all these effects interfere simultaneously with quantitatively similar influences (around 10%), it is rather difficult to assert which one could explain remaining deviations with the experimental results. Furthermore, the rather large uncertainties and sometimes inconsistencies of the available measurements forbid any definitive conclusion, stressing the need for new experiments.

Improved light extraction with nano-particles offering directional radiation diagrams

We propose a unique approach for light extraction, using engineered nano-particles to efficiently decouple the light guided in transverse-magnetic guided modes into free-space radiation modes that leak out normally to the thin-film stacks. The underlying mechanism takes advantage of a small electric field variation at the nano-particle scale and induces a “polarization conversion,” which renders the induced dipole moment perpendicular to the polarization of the incident light. Our analysis is supported by 2D fully vectorial computational results. Potential applications for light emitting or photovoltaic devices are outlined.

Modulation of the work function of fullerenes C60 and C70 by alkaline earth metal adsorption: A theoretical study

The significant influence of alkaline earth metal (Be/Mg/Ca/Sr/Ba/Ra) adsorption on work functions of fullerenes C60 and C70 was investigated by first-principles calculations. The work functions of fullerenes C60 and C70 with Ca/Sr/Ba/Ra adatoms decrease linearly with the electronegativities of the alkaline earth metals. The work functions are also affected considerably by adsorption positions. The variations of the work functions depend on the changes of Fermi level (which is attributed to charge transfer) and the changes of vacuum levels (which is attributed to induced dipole moments). Moreover, the alkaline earth metal adsorption can also improve the electric conductivity of a fullerene mixture.

Effects of Iodine on the Relaxation Dynamics of a Photoexcited I–(H2O)4 Cluster

The Born-Oppenheimer molecular dynamics are used to examine the relaxation dynamics of the charge-transfer-to-solvent (CTTS) photoexcited electron in I(-)(H2O)4. The dynamics are initiated from the C1' cluster configuration, which contains a dangling water molecule. The iodine atom is found to exert a repulsive force on the photoexcited electron at the beginning but an attractive force at later times of the simulation. This dual repulsion-and-attraction role of the iodine atom is found to be dependent on the ratio of the iodine-electron distance to the radius of gyration of the excited electron, d/r. In the region of d/r < ∼0.8, the iodine exerts an exclusion-repulsion force on the excited electron. Conversely, for values of d/r > ∼1.0, the iodine can exert an attractive force on the excited electron due to the induced dipole moment of iodine. However, at large iodine-electron distances, the iodine-electron interaction becomes very weak, and as a result, this attractive force is expected to fade away. Due to the heavy mass of the iodine atom, the evolution of the iodine-electron distance is driven by the motion of solvent molecules and not iodine itself. The dangling water molecules and the dipolar field of water molecules are also important in the solvent dynamics. The influence of temperature on the iodine effects and the experimental implications of the findings are also discussed.

Optical Response of Atomic Gases to Ultrafast Pump-Probe Pulses

We present a computational study of the pump-probe response of a single atom to assess any microscopic anisotropy induced by strong, non-resonant optical fields. Using simulations of the Schrödinger equation for an atom exposed to a linearly polarized ultrafast pump pulse, we calculate the induced dipole moment along the probe polarization directions parallel and perpendicular to the pump polarization. Our simulations show birefringence ratios of approximately 0.7-0.9 on the timescale of tens of femtoseconds following the excitation pulse. In the regime studied, we conclude that excited bound states prepared by the pump are primarily responsible for the birefringence observed, but that the precise response reflects the probe pulse properties.

Electrophoretic mobility of weakly-charged (dipolar) hydrogels in water: Contribution of hydrogen-bonding in the solvent dipole layer

Counterintuitive observations by dynamic light-scattering experiments of negative electrophoretic mobility in uncharged, lightly charged, and later, densely-charged hydrogel nanoparticles are presented. A tentative theory, emphasizing the roles of electric field energy density and induced dipole moments in the dipolar and hydrogen-bonding solvent layer surrounding the particle, is introduced to explain and rationalize these observations. Addition of co-solvent glycine seems to produce a Kohlrausch boundary regulating effect which again illustrates the importance of the dipole layer and hydrogen bonds within it. Further alternative theories involving electric field gradients are discussed which may be relevant to other uncharged systems (such as gold nanoparticles). A contribution to the dipolar solvent-induced mobility is derived in Appendix A. A proposal for a new treatment of traditional (i.e. charged colloid particle) electrokinetic phenomena is given in a second Appendix (Appendix B).

High-order-harmonic generation of Ar atoms in intense ultrashort laser fields: An all-electron time-dependent density-functional approach including macroscopic propagation effects

We present an ab initio approach for the study of macroscopic high-order-harmonic generation (HHG) of rare-gas atoms (Ar) in intense ultrashort laser fields. The single-atom response is calculated by means of the self-interaction-free time-dependent density-functional theoretical (TDDFT) approach with proper long-range potential, and the macroscopic propagation effects are taken into account by solving Maxwell's equations using the induced dipole moments of the single-atom multiple electron shell. We perform all-electron nonperturbative investigation of macroscopic HHG of rare-gas atoms with the emphasis on the role of multiple valence electron subshells. In particular, we found that the valence electron subshell features a destructive interference distribution in multiphoton resonant HHG. In addition, our results show the Cooper minimum in macroscopic HHG from Ar atoms is located at 53 eV, in good agreement with the experimentally measured minimum position.

Molecules with an induced dipole moment in a stochastic electric field

The mean-field dynamics of a molecule with an induced dipole moment (e.g., a homonuclear diatomic molecule) in a deterministic and a stochastic (fluctuating) electric field is solved to obtain the decoherence properties of the system. The average (over fluctuations) electric dipole moment and average angular momentum as a function of time for a Gaussian white noise electric field are determined via perturbative and nonperturbative solutions in the fluctuating field. In the perturbative solution, the components of the average electric dipole moment and the average angular momentum along the deterministic electric field direction do not decay to zero, despite fluctuations in all three components of the electric field. This is in contrast to the decay of the average over fluctuations of a magnetic moment in a Gaussian white noise magnetic field. In the nonperturbative solution, the component of the average electric dipole moment and the average angular momentum in the deterministic electric field direction also decay to zero.