Coherent electric field characterization of molecular chirality in the time domain

Intrinsic handedness encountered in molecular sciences plays an essential role in diverse physical, chemical and biological processes. Optical activity spectroscopy has enabled one to characterize such molecular handedness (chirality) and demonstrated its unique ability to provide stereo-specific structural insight into chiral molecular systems including biopolymers, chiral drugs, and superchiral materials. However, more extended applications including time-resolved studies have often been hindered by inherent limitations of conventional differential methods utilizing both left- and right-handed radiations. The latest methodological advance is heterodyned detection methods measuring wave interferences between signal and reference fields, which allowed direct characterizations of coherent chiroptical signals in a flash. With its ultimate sensitivity, the heterodyned chiroptical method promises to open new possibilities of transient electronic or vibrational optical activity measurements in the ultrafast time domain.     

Electron photodetachment in the presence of a static electric field via correlation functions: revisitation of a very schematic atomic model

The photodetachment cross section of an electron from a negative atomic ion in the presence of a strong, static, electric field has been evaluated as a function of the frequency of the electromagnetic radiation according to a very schematic model previously considered by other authors. Emphasis is placed on the quality of the results obtained by approximating the quantum mechanical propagator of a particle moving in the presence of electric field.     

Forces at the nuclei of a molecule in the presence of non-uniform electric field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electric field has been used to define electric multipole moment operators. Perturbation theory has been applied to evaluate the induced electronic moments and electric field at the nuclei in the presence of spatially non-uniform electric fields of high intensity. Multipole nuclear electric shielding tensors have been defined to describe the contributions arising from a non-homogeneous electric field. These quantities are useful to rationalize linear and non-linear responses of a molecule in the presence of intense external electric perturbations.     

Ionic mass transport through a homogeneous membrane in the presence of a uniform electric field

The time-dependent theoretical solution to ionic mass transport through a uniform membrane under the influence of a uniform electric field is derived. Equivalent lag times and steady-state fluxes are then determined. The enhancement ratio of ionic flow (at a given voltage) to passive flow is shown to be proportional to the voltage for large augmenting voltages, and to decrease exponentially for large retarding voltages. Positive enhancement factors can be as large as 120 for triply charged ions, with one volt imposed across a membrane. The lag times decrease with increasing voltage magnitudes, and can (for triply charged ions, with one volt imposed across a membrane) be as small as 1/20 of the passive lag time.     

Three-dimensional square water in the presence of an external electric field

In this work we study a tridimensional statistical model for the hydrogen-bond (HB) network formed in liquid water in the presence of an external electric field. This model is analogous to the so-called square water, whose ground state gives a good estimate for the residual entropy of the ice. In our case, each water molecule occupies one site of a cubic lattice, and no hole is allowed. The hydrogen atoms of water molecules are disposed at the lines connecting nearest-neighbor sites, in a way that each water can be found in 15 different states. We say that there is a hydrogen bond between two neighboring molecules when only one hydrogen is in the line connecting both molecules. Through Monte Carlo simulations with Metropolis and entropic sampling algorithms, and by exact calculations for small lattices, we determined the dependence of the number of molecules aligned to the field and the number of hydrogen bonds per molecule as a function of temperature and the intensity of the external field. The results for both approaches showed that, different of the two-dimensional case, there is no maximum in the number of HBs as a function of the electric field. However, we observed nonmonotonic behaviors as a function of the temperature of the quantities of interest. We also found the dependence of the entropy on the external electric field at very low temperatures. In this case, the entropy vanishes for the value of the external field for which the contributions to the total energy coming from the HBs and the field become the same.     

Dissociation of a diatomic molecule induced by discontinuous reversals of a static electric field

A diatomic molecule modeled by a Morse oscillator is shown to undergo dissociation if the external electric field, in which the molecule is placed, undergoes discontinuous reversals. A threshold frequency of reversal is found to exist. The classical behavior is compared with its quantum counterpart. The mechanism of dissociation is analyzed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Stark and field-born resonances of an open square well in a static external electric field

The resonance positions, widths (inverse lifetimes), and wave functions of a square-potential well in the presence of a static electric field are calculated by using the outgoing boundary conditions. Our study concentrates on the field-born states that, unlike the well-known Stark resonances, are not associated with the field-free bound states. The effect of a lower cutoff of the static field on the field-born resonance phenomena is studied. The feasibility of experiments, where the isolated long-lived and overlapping short-lived field-born resonances can be explored, is discussed.     

A note on chirality in NMR spectroscopy

Using simple symmetry arguments we give proofs of the derivations of the manifestation of chirality in the chemical shift and spin-spin coupling constant in nuclear magnetic resonance and relate our proofs to earlier discussions.     

On the microscopic origin of bending of graphene nanoribbons in the presence of a perpendicular electric field

Herein, we predict that graphene nanoribbons will be nonplanar under the influence of a critical perpendicular field. Our investigation demonstrates that the perpendicular field induces mixing of σ and π orbitals in graphene nanoribbons through the second order Stark effect which eventually modulates the electron-nuclear interaction strongly in favor of a bent structure.     

Quasi-elastic light scattering studies on T7 DNA in the presence of a sinusoidal electric field

The mobilities and lifetimes of electrons and holes as well as the triplet exciton lifetime have been measured in a range of samples originating from a long anthracene crystal grown from the vapour phase in the zone refining tube. The results have demonstrated the influence of chemical and physical imperfections on these parameters. The triplet lifetime has been found to be fairly insensitive to impurities, as opposed to the lifetime of electrons and holes. The measurements of the charge-carrier mobility as a function of temperature have revealed that hole transport in a defective anthracene crystal is controlled by structural traps 0.26–0.27 eV deep, whereas the electron mobility. contrary to theoretical predictions. is lattice controlled over the investigated temperature range. These localized states can be photochemically eliminated. which suggests that paired anthracene molecules (“incipient” dimers) are responsible for the observed trapping asymmetry. These observations, by analogy with dimeric species forming excimers while excited, have been interpreted in terms of new chemical species formed as a result of trapping at “incipient” dimer pairs.     

Drifting undulations in an achiral smectic C liquid crystal driven by a static electric field

We report a novel dc field-driven propagative instability associated with the thermally induced layer undulations of the smectic C phase in a phenyl benzoate. While the undulations are two-dimensional, the drift is observed only along the wave vector q parallel to the c director; undulations with orthogonal q and c remain stable. The drift, which is nonhysteretic, takes place in a hopping way between equilibrium positions; it has a well-defined threshold in a given region, but the threshold varies rather widely for different regions. The average propagation velocity increases linearly from zero with the control parameter epsilon until epsilon approximately 2 but tends to saturate thereafter. Significantly, the drift direction reverses on switching the field polarity. The mechanism responsible for the drift appears to involve a coupling between the transverse field gradients due to the conductivity anisotropy and the transverse component of the flexoelectric polarization.     

Electronic properties of bilayer and trilayer graphyne in the presence of electric field

The influence of electric field on the electronic properties of bilayer and trilayer graphyne has been studied using the density functional theory. We have investigated alpha graphyne due to its analogous to graphene. The bilayer and trilayer graphyne with different stacking style configurations have been considered. Our results indicate that the electronic properties of alpha graphyne are insensitive to the number of graphyne layer and configuration. The bilayer and trilayer graphyne are semimetal similar to the monolayer graphyne. It is found that applying a uniform electric field perpendicular to the graphyne sheet changes the electronic properties of AB-stacked bilayer and ABC-stacked trilayer graphyne so that they become semiconductor. The band gaps of the bilayer and trilayer graphyne with these configurations are enhanced by increasing strength of the electric field. Therefore, possibility of controlling the electronic properties of graphyne by applying electric field makes graphyne as a good candidate for next generation nanoelectronic devices.     

Cationic polymerizations in the presence of an electric field with polarity periodically reversed

A study has been made of the polymerizations of anethole, isobutylvinylether and cyclohexylvinylether by iodine in ethylene dichloride solution, at 25° under high vacuum in the presence of a constant or periodically reversed polarity d.c. field.

The influences of the reversal frequency, distance between the electrodes as well as mechanical stirring of the solution on the electric field effect (measured as R_PE/R_PO) have been investigated. Experiments were carried out to evaluate separately, in both compartments of the cell, the amounts of titratable iodine and polymer formed as well as the polymerization rate after the electric field application.

The field induced rate enhancement can be cancelled by suitable reversal frequencies and mechanical stirring, and depends on the distance between the electrodes.

The results confirm that the field effect is mainly due to concentration gradients of active species as a consequence of field induced electrolytic phenomena.     

Water between plates in the presence of an electric field in an open system

Molecular dynamics simulations of water at 298 K and 1 atm of pressure are used to investigate the electric-field dependence of the density and polarization density of water between two graphite-like plates of different sizes (9.8 x 9.2 and 17.7 x 17.2 A) in an open system for plate separations of 8.0, 9.5, and 16.4 A. The interactions with water were tuned to "hard-wall-like" and "normal" C-O hydrophobic potentials. Water between the larger plates at 16.4 A separation is layered but is metastable with respect to capillary evaporation at zero field (Bratko, D.; Curtis, R. A.; Blanch, H. W.; Prausnitz, J. M. J. Chem. Phys. 2001, 115, 3873). Applying a field decreases the density of the water between the plates, in apparent contradiction to thermodynamic and integral equation theories of bulk fluid electrostriction that ignore surface effects, rendering them inapplicable to finite-sized films of water between hydrophobic plates. This suggests that the free energy barrier for evaporation is lowered by the applied field. Water, between "hard-wall-like" plates at narrower separations of 9.5 A and less, shows a spontaneous but incomplete evaporation at zero field within the time scale of our simulation. Evaporation is further enhanced by an electric field. No such evaporation occurs, on these time scales, for the smaller plates with the "hard-wall-like" potential at a separation of 8.0 A at zero field, signaling a crossover in behavior as the plate dimension decreases, but the water density still diminishes with increasing field strength. These observations could have implications for the behavior of thin films of water between surfaces in real physical and biological systems.     

Molecular orientation via a dynamically induced pulse-train: wave packet dynamics of NaI in a static electric field

We regard the rovibrational wave packet dynamics of NaI in a static electric field after femtosecond excitation to its first electronically excited state. The following quasibound nuclear wave packet motion is accompanied by a bonding situation changing from covalent to ionic. At times when the charge separation is present, i.e., when the bond-length is large, a strong dipole moment exists and rotational excitation takes place. Upon bond contraction, the then covalently bound molecule does not experience the external field. This scenario repeats itself periodically. Thus, the vibrational dynamics causes a situation which is comparable to the interaction of the molecule with a train of pulses where the pulse separation is determined by the vibrational period.     

Semiclassical model for the absorption or emission of a photon by a charged particle in an intense static electric field

Quantum simulations made using Floquet methods show that a charged particle can exchange energy with an oscillating potential barrier in discrete quanta , where is the frequency of oscillation. However, this exchange is classically forbidden because no other mass is included in the model, so that energy and momentum could not both be conserved in the absorption or emission of a photon. We define a semiclassical mechanism for these inelastic processes in which a photon may be absorbed by a charged particle moving against an intense static electric field, or emitted when the particle moves with this field. In this model, the particle has an energy loss Q in photon absorption, and an energy gain Q in photon emission. Then the particle travels a short distance at constant momentum until the energy increment Q is made up by the interaction with the static electric field, after which the particle resumes classical motion with the initial energy plus or minus exactly one quantum. We use the energy–time uncertainty relation to determine the minimum value for the static electric field that is required for this process, and this value is typical of the experimental conditions for laser-assisted scanning tunneling microscopy and laser-assisted field emission where the exchange of quanta is found to occur.     

NMR evidence for planar chirality in natural porphyrins

The ¹H NMR spectra of several six-coordinate cobalt(III) porphyrins of general formula L₂Co(DPDME)CI, where DPDME=deuteroporphyrin dimethyl ester and L is an optically active ligand, have been measured at 250 or 400 MHz. In a few cases, two signals of equal intensity are observed for the L ligand protons. This magnetic non-equivalence, which never exceeds 0.04 ppm, is thought to arise from the planar chirality of the porphyrin ring, which makes a proton in the ligand L above the porphyrin ring and the corresponding proton below this ring diastereotopic.     

Optimization of heat transfer properties on ferrofluid flow over a stretching sheet in the presence of static magnetic field

Journal of Thermal Analysis and Calorimetry - The main emphasis of the present research is to investigate the composite effects of magnetization force and rotational viscosity on two-dimensional...