Dipole-induced Dipole Research Articles

Polarizable Six-Point Water Models from Computational and Empirical Optimization

Tröster et al. (J. Phys. Chem B 2013, 117, 9486-9500) recently suggested a mixed computational and empirical approach to the optimization of polarizable molecular mechanics (PMM) water models. In the empirical part the parameters of Buckingham potentials are optimized by PMM molecular dynamics (MD) simulations. The computational part applies hybrid calculations, which combine the quantum mechanical description of a H2O molecule by density functional theory (DFT) with a PMM model of its liquid phase environment generated by MD. While the static dipole moments and polarizabilities of the PMM water models are fixed at the experimental gas phase values, the DFT/PMM calculations are employed to optimize the remaining electrostatic properties. These properties cover the width of a Gaussian inducible dipole positioned at the oxygen and the locations of massless negative charge points within the molecule (the positive charges are attached to the hydrogens). The authors considered the cases of one and two negative charges rendering the PMM four- and five-point models TL4P and TL5P. Here we extend their approach to three negative charges, thus suggesting the PMM six-point model TL6P. As compared to the predecessors and to other PMM models, which also exhibit partial charges at fixed positions, TL6P turned out to predict all studied properties of liquid water at p0 = 1 bar and T0 = 300 K with a remarkable accuracy. These properties cover, for instance, the diffusion constant, viscosity, isobaric heat capacity, isothermal compressibility, dielectric constant, density, and the isobaric thermal expansion coefficient. This success concurrently provides a microscopic physical explanation of corresponding shortcomings of previous models. It uniquely assigns the failures of previous models to substantial inaccuracies in the description of the higher electrostatic multipole moments of liquid phase water molecules. Resulting favorable properties concerning the transferability to other temperatures and conditions like the melting of ice are also discussed.

Calculations of the Electric Field in Solutions and Proteins with Polarizable Force Fields

A molecule in solution or a ligand bound to a protein experiences a complex range of electrostatic interactions from its environment. One way of describing these interactions in a collective fashion is to consider the total electric field they exert on the molecule of interest. A key advantage of this picture is that it provides a unifying language for comparing the relative importance of diverse specific interactions (e.g., hydrogen bonds) and nonspecific interactions (e.g., dipole-dipole and dipole-induced dipole), and it facilitates direct comparison to experimental observables (like vibrational Stark effects). We develop and demonstrate methods to calculate electric fields using molecular dynamics simulations with the AMOEBA polarizable force field, which enables us to describe the electrostatic characteristics of non-polar, polar, and hydrogen bonding environments in a consistent fashion. This consistency is not achievable with the more widely used continuum models or pair-wise additive force fields. The connection to experiment further enables us to test predictions from simulation and to benchmark the force fields employed. By offering a high-level description of electrostatics, polarizable force fields present a physically realistic picture of the organized environments of proteins and how functional properties emerge from them.

Quantitative Dielectric Measurements of Biomembranes and Oxides in Electrolyte Solutions at High Frequencies

A unique hallmark of atomic force microscopy is the ability to investigate matter with nanoscale spatial resolution in liquid environment. This has enabled access to materials properties, such as surface stiffness, electrochemical activity, piezoelectric response and surface charge/potential, in liquid with a tremendous impact in biology and electrochemistry. Yet, to date dielectric polarization, the fundamental property of materials to orient permanent or induced electric dipoles in response to an external electric field, has remained precluded to AFM-based techniques in liquid environment.Here we show that by operating Electrostatic Force Microscopy in liquid at high frequency (>1 MHz) with a heterodyne detection scheme dielectric material contrast can be obtained [1, 3]. Furthermore we demonstrate for the first time that the dielectric response of thin insulating films such as oxides and biomembranes can be imaged and quantified in ionic solutions using an AFM-based technique [2]. These results open a range of exciting possibilities. On the one hand, they will allow exploiting the unique capabilities of electrical SPM techniques to probe the local dielectric properties of nanoscale objects and materials in liquid environment in a similar way as it can be done in air. On the other hand, and probably more importantly, they will allow accessing polarization properties of systems that are stable only in ion-containing aqueous solutions and that for this reason to date have remained precluded. In particular, this is of fundamental interest in biology and electrochemistry, in which polarization properties at the nanoscale are crucial.The work was supported by the EU-FP7 (PEOPLE-2012-ITN-317116 NANOMICROWAVE) and ELECTROBACTERIA project.

Polarization Anisotropy of Multiple Localized Plasmon Resonance Modes in Noble Metal Nanocrescents

Multiple localized surface plasmon resonance (LSPR) modes induced in gold and silver nanocrescents exhibit anisotropic behavior with respect to polarization of incident light. Extinction spectroscopy with polarization control was used to characterize the optical properties of asymmetric nanocrescents with diameters ranging from 100 to 500 nm. The analysis shows that selective induction of distinct dipole and quadrupole LSPR modes can be accomplished through polarization of incident light. Finite-difference time-domain calculations of the LSPR responses of the nanocrescent corroborate the polarization-dependent optical behavior. Simulations reveal optical near field enhancements with unique spatial patterns associated with the individual resonance modes depending on the polarization and wavelength of incident light. Quantitative analysis of the anisotropic LSPR behavior supports the potential for controlling the near field through selective induction of specific resonance modes with particular near field d...

Fluorine mediated photocatalytic activity of BiPO4

The fluorine doped BiPO4 (F-BiPO4) photocatalyst was synthesized via an in situ fluorination method. The lattice oxygen of BiPO4 was substituted by fluorine, resulting in the increased polarizability and thus an increased induced dipole moment of BiPO4, which can increase the separation efficiency of electron-hole pairs. The photocatalytic activity of BiPO4 was enhanced by about 30% when the molar ratio of F/Bi was 0.03. Not only the discoloration rate of methylene blue (MB) but also the mineralization rates of MB and phenol were accelerated by F-doping. The enhancement of photocatalytic activity by F-doping could be attributed to the stronger adsorption ability of the substrate, the larger amount of active facet and higher separation efficiency of electron-hole pairs resulted from the increased polarizability. Overdoping would decrease the photocatalytic activity due to the increased recombination rate of electron-hole pairs at high defect concentration.

Investigation of attractive and repulsive interactions associated with ketones in supercritical CO2, based on Raman spectroscopy and theoretical calculations

Carbonyl compounds are solutes that are highly soluble in supercritical CO2 (scCO2). Their solubility governs the efficiency of chemical reactions, and is significantly increased by changing a chromophore. To effectively use scCO2 as solvent, it is crucial to understand the high solubility of carbonyl compounds, the solvation structure, and the solute-solvent intermolecular interactions. We report Raman spectroscopic data, for three prototypical ketones dissolved in scCO2, and four theoretical analyses. The vibrational Raman spectra of the C=O stretching modes of ketones (acetone, acetophenone, and benzophenone) were measured in scCO2 along the reduced temperature Tr = T∕Tc = 1.02 isotherm as a function of the reduced density ρr = ρ∕ρc in the range 0.05-1.5. The peak frequencies of the C=O stretching modes shifted toward lower energies as the fluid density increased. The density dependence was analyzed by using perturbed hard-sphere theory, and the shift was decomposed into attractive and repulsive energy components. The attractive energy between the ketones and CO2 was up to nine times higher than the repulsive energy, and its magnitude increased in the following order: acetone < acetophenone < benzophenone. The Mulliken charges of the three solutes and CO2 molecules obtained by using quantum chemistry calculations described the order of the magnitude of the attractive energy and optimized the relative configuration between each solute and CO2. According to theoretical calculations for the dispersion energy, the dipole-induced-dipole interaction energy, and the frequency shift due to their interactions, the experimentally determined attractive energy differences in the three solutes were attributed to the dispersion energies that depended on a chromophore attached to the carbonyl groups. It was found that the major intermolecular interaction with the attractive shift varied from dipole-induced dipole to dispersion depending on the chromophore in the ketones in scCO2. As the common conclusion for the Raman spectral measurements and the four theoretical calculations, solute polarizability, modified by the chromophore, was at the core of the solute-solvent interactions of the ketones in scCO2.

Polyamine ligand-mediated self-assembly of gold and silver nanoparticles into chainlike structures in aqueous solution: towards new nanostructured chemosensors.

Polyamine ligands are very versatile compounds due to their water solubility and flexibility. In the present work, we have exploited the binding ability of a polyamine molecular linker (L2−) bearing different functional groups, which favors the self-assembling of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) into 1D nanochains in aqueous solution. The chainlike assemblies of AuNPs and AgNPs were structurally stable for a long period of time, during which their characteristic optical properties remained unchanged. The mechanism of AuNPs and AgNPs chain assembly associated with the induction of electric dipole–dipole interactions arising from the partial ligand exchange of surface-adsorbed citrate ions by (L2−) was investigated. UV/Vis spectrophotometry and transmission electron microscopy (TEM) were used to determine timedependent structural changes associated with formation of the 1D nanoparticle structures. Finally, the sensing of Hg2+ in aqueous solution using AgNPs@(L)2− and AuNPs@(L)2− assemblies was also carried out in aqueous solution.

Polarizable Water Models from Mixed Computational and Empirical Optimization

Here we suggest a mixed computational and empirical approach serving to optimize the parameters of complex and polarizable molecular mechanics (PMM) models for complicated liquids. The computational part of the parameter optimization relies on hybrid calculations combining density functional theory (DFT) for a solute molecule with a PMM treatment of its solvent environment at well-defined thermodynamic conditions. As an application we have developed PMM models for water featuring ν = 3, 4, and 5 points of force action, a Gaussian inducible dipole and a Buckingham potential at the oxygen, the experimental liquid phase geometry, the experimental gas phase polarizability α(exp)(g) = 1.47 ų, and, for ν = 4 and 5, the gas phase value μ(exp)(g) = 1.855 D for the static dipole moment. The widths of the Gaussian dipoles and, for ν = 4 and 5, also the electrostatic geometries of these so-called TLνP models are derived from self-consistent DFT/PMM calculations, and the parameters of the Buckingham potentials (and the static TL3P dipole moment) are estimated from molecular dynamics (MD) simulations. The high quality of the resulting models is demonstrated for the observables targeted during optimization (potential energy per molecule, pressure, radial distribution functions) and a series of predicted properties (quadrupole moments, density at constant pressure, dielectric constant, diffusivity, viscosity, compressibility, heat capacity) at certain standard conditions. Remaining deficiencies and possible ways for their removal are discussed.

The Effect of the Molecular Size and Shape on the Volume Behavior of Binary Liquid Mixtures. Branched and Cyclic Alkanes in Heptane at 298.15 K

Excess molar volumes VE for 40 mixtures of heptane with a liquid alkane and apparent molar volumes in heptane for eight solid alkanes have been obtained at 298.15 K. They include five linear, 30 branched-chain, and 13 cyclic alkanes. Almost all systems exhibit negative VE values. For mixtures with open chain alkanes, VE increases from C5 to C7 and then decreases. A similar trend is shown by mixtures with cycloalkanes. VE values are compared with known HE data for mixtures with heptane and tetrachloromethane. Signs and trends of VE and HE are correlated with the free volume and interactional terms of the Flory theory. The partial molar volumes at infinite dilution in heptane, V°, have also been obtained and discussed together with literature data on other hydrocarbons and polar compounds. The calculated contributions to V° by CH3, CH2, CH and C groups are compared with previously determined contributions of polar groups. The lower contributions of the latter groups are explained with the volume contraction caused by dipole-induced dipole interaction. The volume effects associated with branching and cyclization have been evaluated and compared with the corresponding effects on solvation enthalpy. The branching effect, in the order of magnitude of few cm3·mol−1, and the larger negative values of cyclization volumes, down to −24 cm3·mol−1, are discussed in terms of packing and solute–solvent interactions, in analogy to polar organic solutes either in heptane and tetrachloromethane. A negative cyclization effect is also exhibited by the solvation enthalpies.

Coupling density functional theory to polarizable force fields for efficient and accurate Hamiltonian molecular dynamics simulations

Hybrid molecular dynamics (MD) simulations, in which the forces acting on the atoms are calculated by grid-based density functional theory (DFT) for a solute molecule and by a polarizable molecular mechanics (PMM) force field for a large solvent environment composed of several 10(3)-10(5) molecules, pose a challenge. A corresponding computational approach should guarantee energy conservation, exclude artificial distortions of the electron density at the interface between the DFT and PMM fragments, and should treat the long-range electrostatic interactions within the hybrid simulation system in a linearly scaling fashion. Here we describe a corresponding Hamiltonian DFT/(P)MM implementation, which accounts for inducible atomic dipoles of a PMM environment in a joint DFT/PMM self-consistency iteration. The long-range parts of the electrostatics are treated by hierarchically nested fast multipole expansions up to a maximum distance dictated by the minimum image convention of toroidal boundary conditions and, beyond that distance, by a reaction field approach such that the computation scales linearly with the number of PMM atoms. Short-range over-polarization artifacts are excluded by using Gaussian inducible dipoles throughout the system and Gaussian partial charges in the PMM region close to the DFT fragment. The Hamiltonian character, the stability, and efficiency of the implementation are investigated by hybrid DFT/PMM-MD simulations treating one molecule of the water dimer and of bulk water by DFT and the respective remainder by PMM.

Predictive models for estimating the vapor pressure of poly- and perfluorinated compounds at different temperatures

Poly- and perfluorinated compounds (PFCs) are a class of global environmental pollutants that are of concern regarding their environmental fate and adverse effects. However, data on the basic physicochemical property of PFCs are still limited. To fill part of the data gaps, temperature-dependent predictive models for vapor pressure of PFCs were developed based on previously reported experimental data. The applicability domain of the models was analyzed using the Williams plot and the influential points and the response outliers were identified. The statistical performance of the models was significantly improved by removing these influential points and response outliers. This procedure confirmed the importance of properly defining the applicability domain of the predictive models. It is shown that the main factors governing the vapor pressure of PFCs, are intermolecular dispersive interactions, hydrogen bonding, temperature, intermolecular dipole-induced dipole interactions and dipole–dipole interactions. Although the model obtained could be used to reliably predict the vapor pressures of certain PFCs at different temperatures, it is essential that the prediction must fall within the applicability domain of the model and the temperature range for reliable predictions.

Which density functional is close to CCSD accuracy to describe geometry and interaction energy of small noncovalent dimers? A benchmark study using Gaussian09

A benchmark study on all possible density functional theory (DFT) methods in Gaussian09 is done to locate functionals that agree well with CCSD/aug-cc-pVTZ geometry and Ave-CCSD(T)/(Q-T) interaction energy (Eint) for small non-covalently interacting molecular dimers in "dispersion-dominated" (class 1), "dipole-induced dipole" (class 2), and "dipole-dipole" (class 3) classes. A DFT method is recommended acceptable if the geometry showed close agreement to CCSD result (RMSD < 0.045) and Eint was within 80-120% accuracy. Among 382 tested functionals, 1-46% gave good geometry, 13-44% gave good Eint , while 1-33% satisfied geometry and energy criteria. Further screening to locate the best performing functionals for all the three classes was made by counting the acceptable values of energy and geometry given by each functionals. The meta-generalized gradient approximation (GGA) functional M06L was the best performer with total 14 hits; seven acceptable energies and seven acceptable geometries. This was the only functional "recommended" for at least two dimers in each class. The functionals M05, B2PLYPD, B971, mPW2PLYPD, PBEB95, and CAM-B3LYP gave 11 hits while PBEhB95, PW91B95, Wb97x, BRxVP86, BRxP86, HSE2PBE, HSEh1PBE, PBE1PBE, PBEh1PBE, and PW91TPSS gave 10 hits. Among these, M05, B971, mPW2PLYPD, Wb97x, and PW91TPSS were among the "recommended" list of at least one dimer from each class. Long-range correction (LC) of Hirao and coworkers to exchange-correlation functionals showed massive improvement in geometry and Eint . The best performing LC-functionals were LC-G96KCIS and LC-PKZBPKZB. Our results predict that M06L is the most trustworthy DFT method in Gaussian09 to study small non-covalently interacting systems.

Immunolocalization of Keratin‐Associated Beta‐Proteins (Beta‐Keratins) in Pad Lamellae of Geckos Suggest that Glycine–Cysteine‐Rich Proteins Contribute to Their Flexibility and Adhesiveness

The epidermis of digital pads in geckos comprises superficial microornamentation from the oberhautchen layer that form long setae allowing these lizards to climb vertical surfaces. The beta-layer is reduced in pad lamellae but persists up to the apical free margin. Setae are made of different proteins including keratin-associated beta-proteins, formerly indicated as beta-keratins. In order to identify specific setal proteins the present ultrastructural study on geckos pad lamellae analyzes the immunolocalization of three beta-proteins previously found in the epidermis and adhesive setae of the green anolis. A protein rich in glycine but poor in cysteine (HgG5-like) is absent or masked in gecko pad lamellae. Another protein rich in glycine and cysteine (HgGC3-like) is weakly present in setae, oberhautchen and beta-layer. A glycine and cysteine medium rich beta-protein (HgGC10-like) is present in the lower part of the beta-layer but is absent in the oberhautchen, setae, and mesos layer. The latter two proteins may form intermolecular bonds that contribute to the flexibility of the corneous material sustaining the setae. The pliable alpha-layer present beneath the thin beta-layer and in the hinge region of the pad lamellae also contains HgGC10-like proteins. Based on the possibility that some HgGC3-like or other cys-rich beta-proteins are charged in the setae it is suggested that their charges influence the mechanism of adhesion increasing the induction of dipoles on the substrate and enhancing attractive van der Waals forces.

Lateral induced dipole moment and polarizability of excitons in a ZnO single quantum disk

The lateral Stark shift of an exciton confined in a single ZnO quantum thin disk of radius R was calculated using a variational approach within the two bands effective mass approximation. It is shown that the exciton has a non negligible induced dipole moment when an external electric field is applied mainly for electron-hole separation below to the 3D excitonic Bohr radius. The behavior of the exciton lateral Stark shift proves the existence of an important correlation between the polarizability and the induced dipole moment.

Gate field induced ordered electric dipoles in a polymer dielectric for low-voltage operating organic thin-film transistors

The development of low voltage-operating organic thin-film transistors requires high-dielectric constant (high-K) materials for the device's gate dielectrics. The surface properties of these high-K materials must match those of organic semiconductors. A modification material coated on high-K dielectric is needed, and polyimide (PI) is a promising modifier to reduce the surface energy and the interface trap states (in the level of 1010 cm−2 eV−1) of the high-K dielectrics. In this study, surface characteristics of the dielectrics were identified and interface analyses at the dielectric/organic semiconductor interface were conducted through combined electrical force microscopy and impedance–admittance investigation. When the organic semiconductor pentacene was grown on the PI-modified dielectrics, the atomic force microscopy images and X-ray diffraction analyses showed larger grain size and higher crystallinity than those on native high-K dielectrics. Using polyimide-modified high-K materials as the gate dielectric, high performances (SS < 1 V per decade, μ above 0.1 cm2 V−1 s−1, and on/off ratio > 105) and low voltage-operating (<5 V) pentacene-based thin-film transistors were achieved. Although the gate field was decreased by inserting a PI layer, the effective gate field was compensated by an electric dipole-induced dipole field embedded in the PI layer. The mechanism of carrier accumulation at the PI/pentacene interface was also clearly described.

Two new azamacrocycle-based stationary phases for high-performance liquid chromatography: Preparation and comparative evaluation

In this paper, two new azamacrocycle-bonded stationary phases for high-performance liquid chromatography are described. The new phases were prepared by respectively coupling a 14-membered Curtis macrocycle and a 30-membered hexaazaannulene to γ-chloropropyltrimethoxylsilane-modified silica and characterized by elemental analysis and infrared spectroscopy. To understand the effects of the structures of the azamacrocyles and their functional groups upon the retention and separation, the chromatographic behaviors of the two stationary phases were compared by eluting alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs) and aromatic amines. The two new phases demonstrated unique selectivity and fine chromatographic performance, which were guaranteed by multiple interactions, including hydrophobic, hydrogen-bonding, π–π and dipole-induced dipole interactions between stationary phase and solutes. On the other hand, the difference between the structures of embedded azamacrocyles did lead to remarkable dissimilarity in the shape and planarity recognition of isomers, which was highlighted by the 30-membered hexaazaannulene-bonded phases’ superiority over 14-membered tetraazamacrocycle-bonded phases’ in discriminating specific PAHs. With a wide range of probes, the linear solvation energy relationship model was also applied to evaluate the chromatographic properties of the two stationary phases, further verifying the chromatographic results.

Multiproperty empirical isotropic interatomic potentials for CH4–inert gas mixtures

An approximate empirical isotropic interatomic potentials for CH4–inert gas mixtures are developed by simultaneously fitting the Exponential-Spline-Morse-Spline-van der Waals (ESMSV) potential form to viscosity, thermal conductivity, thermal diffusion factors, diffusion coefficient, interaction second pressure virial coefficient and scattering cross-section data. Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures for CH4–He and at T=87K for CH4–Ar are computed using theoretical values for overlap, octopole and hexadecapole mechanisms and interaction potential as input. Also, the quantum mechanical lineshapes of collision-induced light scattering (CILS) for the mixtures CH4–Ar and CH4–Xe at room temperature are calculated. The spectra of scattering consist essentially of an intense, purely translational component which includes scattering due to free pairs and bound dimers, and the other is due to the induced rotational scattering. These spectra have been interpreted by means of pair-polarizability terms, which arise from a long-range dipole-induced-dipole (DID) with small dispersion corrections and a short-range interaction mechanism involving higher-order dipole–quadrupole A and dipole–octopole E multipole polarizabilities. Good agreement between computed and experimental lineshapes of both absorption and scattering is obtained when the models of potential, interaction-induced dipole and polarizability components are used.

Two-dimensional structure of dipolar heterogeneous dumbbells

The influence of polarization on the two-dimensional structure of heterogeneous dumbbells is studied by Monte Carlo simulation. The dumbbell consists of two fused hard spheres whose centers are embedded with point dipoles and , respectively. The axis of the dumbbell can rotate freely in the x-y plane but and are always along the x-axis which is the polarization direction. When the dipolar strength μa is much larger than μb, the dumbbells are perpendicular to the polarization direction and form zigzag chains; when μa and μb are comparable, the dumbbells will form end-to-end chains if and are parallel, or side-by-side chains if and are antiparallel. All the chains are along the polarization direction and the detailed information about them is given through extracting the radial and angular distributions from the simulation results. Finally, we calculate the energies of four kinds of ideal chains to explain structural transition. Our simulation results can explain the very recent experiment by Nagao D. et al. (Langmuir, 28 (2012) 6546) which studied the assembly of dumbbells with inducible dipoles, where magnitude of the dipoles was frequency-dependent and by changing frequency, different assemblies could be obtained.

Predicted Ordered Assembly of Ethylene Molecules Induced by Polarized Off-Resonance Laser Pulses

We illustrate a new phenomenon in the dynamics of molecular ensembles subjected to moderately intense, far-off-resonance laser fields, namely, field-driven formation of perfectly ordered, defect-free assembly. Interestingly, both the arrangement of the constituting molecules within the individual assembly and the long-range order of the assembly with respect to one another are subject to control through choice of the field polarization. Relying on strong induced dipole-induced dipole interactions that are established in dense molecular media, the effect is expected to be general.

New insights into collision-induced rototranslational absorption and scattering spectra of gaseous methane at different temperatures

Quantum mechanical lineshapes of collision-induced absorption (CIA) and of collision-induced light scattering (CILS) at different temperatures for gaseous molecular methane are computed using as input theoretical values for induced dipole moments and pair-polarizability anisotropy. The quantum lineshapes of absorption are reproduced accurately by the extended Birnbaum Cohen (EBC) model with the parameters based on certain function of different coefficients for the octopole and hexadecapole mechanisms. For the scattering, the spectrum consists essentially of an intense, purely translational component which includes scattering due to free pairs and bound dimers and of another component due to the induced rotational scattering. This spectrum has been interpreted by means of pair-polarizability terms, which arise from a short range dipole-induced-dipole (DID) with small dispersion corrections and long range interaction mechanism involving higher-order dipole–quadrupole (A) and dipole–octopole (E) polarizabilities. Good agreement between computed and experimental lineshapes of both absorption and scattering is obtained with potential models constructed from thermophysical and transport properties.