Reorientation Of Molecules Research Articles

Effect of Electric Field on Condensed-Phase Molecular Systems. I. Dipolar Polarization of Amorphous Solid Acetone

We investigated the dipolar reorientation of acetone molecules in amorphous solids under the influence of externally applied electric fields in the range of (0–4.3) × 108 V·m–1. The electric field ...

Dynamic surface tension of С10ЕО8 at the aqueous solution/hexane vapor interface as measured by bubble pressure tensiometry

Abstract The dynamic surface tension of aqueous solutions of C10EO8 at the interfaces with air and saturated hexane vapor was measured by the maximum bubble pressure tensiometer BPA for adsorption times t ≥ 0.01 s. It can be shown that the dynamic surface pressure of C10EO8 solutions in a saturated hexane atmosphere increases at short adsorption times with increasing surfactant concentration. The theory of diffusion controlled adsorption of two surfactants from the water and gas phases, respectively, was used for data analysis. The obtained experimental results agree with a model based on a reorientation of the C10EO8 molecules, assuming a strong intermolecular interaction between hexane and C10EO8 molecules in the surface layer. Alternatively, an additional enhancement of C10EO8 adsorption activity can be assumed due to the presence of hexane molecules at the interface.

Liquid crystal on subwavelength metal gratings

Optical and electrooptical properties of a system consisting of subwavelength metal gratings and nematic liquid crystal layer are studied. Aluminium gratings that also act as interdigitated electrodes are produced by focused ion beam lithography. It is found that a liquid crystal layer strongly influences both the resonance and light polarization properties characteristic of the gratings. Enhanced transmittance is observed not only for the TM-polarized light in the near infrared spectral range but also for the TE-polarized light in the visible range. Although the electrodes are separated by nanosized slits, and the electric field is strongly localized near the surface, a pronounced electrooptical effect is registered. The effect is explained in terms of local reorientation of liquid crystal molecules at the grating surface and propagation of the orientational deformation from the surface into the bulk of the liquid crystal layer.

Resonance local phonon mode and electron spin-lattice relaxation of formate-type free radicals studied by electron spin echo in Cd(HCOO)2·2H2O crystal

The results of X-band electron spin resonance (ESR) and electron spin echo (ESE) measurements for free radicals generated in Cd(HCOO)2·2H2O single crystal are presented. From ESR spectra analysis the radicals were identified as after x-ray irradiation and as HOCO after γ-ray irradiation. The room temperature g-factors are: g|| = 1.9969 and g⊥ = 2.0024 for and g1 = 2.0087, g2 = 2.0029 and g3 = 1.9960 for HOCO. Axial g-tensor symmetry for is due to fast reorientation of the radical molecule around the g||-axis. Assignment of HOCO is confirmed by hyperfine splitting (Amax = 0.4 mT) from a single distant proton. Spin lattice relaxation rate was determined from ESE measurements in temperature range 4–250 K. Both radicals relax via local resonance mode lying within acoustic phonon branch. The existing theories of electron spin-lattice relaxation via local resonance mode are critically reviewed and compared with experimental data. A new approximation is proposed giving local mode energy = 56 cm−1 for and = 44 cm−1 for the HOCO-radical.

An Electrically Controlled Nematic Liquid Crystal Core Waveguide With a Low Switching Threshold

We demonstrate an electrically-controlled liquid crystal (LC) core waveguide, using 4-cyano-4′-pentylbiphenyl (5CB) nematic liquid crystal, fabricated on a glass substrate. A negative photoresist, AZ15nXT layer was coated on the glass substrate to realize a channel waveguide of core thickness 4.8 μm. The LC core waveguide exhibits strong differential attenuation for propagation of the TE- (horizontal) and TM-like (vertical) polarizations of light. The experimental results show that the output power suddenly drops down by several dB at an applied voltage of 1 V, for a waveguide of length 10 mm, due to the re-orientation of the LC molecules. The waveguide can be used as an optical switch, as well as an optical attenuator.

Major reorientation of tRNA substrates defines specificity of dihydrouridine synthases

The reduction of specific uridines to dihydrouridine is one of the most common modifications in tRNA. Increased levels of the dihydrouridine modification are associated with cancer. Dihydrouridine synthases (Dus) from different subfamilies selectively reduce distinct uridines, located at spatially unique positions of folded tRNA, into dihydrouridine. Because the catalytic center of all Dus enzymes is conserved, it is unclear how the same protein fold can be reprogrammed to ensure that nucleotides exposed at spatially distinct faces of tRNA can be accommodated in the same active site. We show that the Escherichia coli DusC is specific toward U16 of tRNA. Unexpectedly, crystal structures of DusC complexes with tRNA(Phe) and tRNA(Trp) show that Dus subfamilies that selectively modify U16 or U20 in tRNA adopt identical folds but bind their respective tRNA substrates in an almost reverse orientation that differs by a 160° rotation. The tRNA docking orientation appears to be guided by subfamily-specific clusters of amino acids ("binding signatures") together with differences in the shape of the positively charged tRNA-binding surfaces. tRNA orientations are further constrained by positional differences between the C-terminal "recognition" domains. The exquisite substrate specificity of Dus enzymes is therefore controlled by a relatively simple mechanism involving major reorientation of the whole tRNA molecule. Such reprogramming of the enzymatic specificity appears to be a unique evolutionary solution for altering tRNA recognition by the same protein fold.

Molecular motion, dielectric response, and phase transition of charge-transfer crystals: acquired dynamic and dielectric properties of polar molecules in crystals.

Molecules in crystals often suffer from severe limitations on their dynamic processes, especially on those involving large structural changes. Crystalline compounds, therefore, usually fail to realize their potential as dielectric materials even when they have large dipole moments. To enable polar molecules to undergo dynamic processes and to provide their crystals with dielectric properties, weakly bound charge-transfer (CT) complex crystals have been exploited as a molecular architecture where the constituent polar molecules have some freedom of dynamic processes, which contribute to the dielectric properties of the crystals. Several CT crystals of polar tetrabromophthalic anhydride (TBPA) molecules were prepared using TBPA as an electron acceptor and aromatic hydrocarbons, such as coronene and perylene, as electron donors. The crystal structures and dielectric properties of the CT crystals as well as the single-component crystal of TBPA were investigated at various temperatures. Molecular reorientation of TBPA molecules did not occur in the single-component crystal, and the crystal did not show a dielectric response due to orientational polarization. We have found that the CT crystal formation provides a simple and versatile method to develop molecular dielectrics, revealing that the molecular dynamics of the TBPA molecules and the dielectric property of their crystals were greatly changed in CT crystals. The TBPA molecules underwent rapid in-plane reorientations in their CT crystals, which exhibited marked dielectric responses arising from the molecular motion. An order-disorder phase transition was observed for one of the CT crystals, which resulted in an abrupt change in the dielectric constant at the transition temperature.

Simultaneous measurements of molecular forces and electro-optical properties of a confined 5CB liquid crystal film using a surface forces apparatus.

Using a surface forces apparatus (SFA), we studied the forces associated with the reorientation of molecules of a common nematic thermotropic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB), confined between two conducting (silver) surfaces and its optical behavior under the influence of electric fields with varying magnitudes and field directions. A transient attractive force was observed due to partial reorientations of the liquid crystal molecules and the flow of free ions, in addition to a stronger constant capacitance attraction between the silver surfaces. At the same time, the optical properties of the liquid crystals were observed perpendicular to the silver surfaces. Observations of shifts and fluctuations of the extraordinary wave of the (multiple beam) interference fringes measure the refractive index of the director component parallel to the surface, which is sensitive to tilt motion (or reorientation) of the liquid crystal molecules that provided details of the anisotropic orientations of the molecules and domains. Any lateral differential refractive index change is easily observed by optical microscopy. The optical microscope imaging showed that the changes in the optical properties are due to convective flow at domain boundaries of the liquid crystal molecules (and possible free ions) between the two charged surfaces. At low electric fields, propagation of domain boundaries was observed, while at higher electric fields, hexagonal patterns of flowing molecules were observed. The interplay of the force measurements and optical observations reveal a complex dynamic behavior of liquid crystals subjected to varying electric fields in confined spaces.

Raising efficiency of organic solar cells with electrotropic additives

Incorporation of electrotropic additives with large molecular dipole moments into the bulk heterojunction layer of organic photovoltaic devices followed by electric field poling led to an increase of power conversion efficiency up to 7.97% from 7.17% for devices that did not utilize the additives and from 5.18% for devices with additives prior to poling. The improvement is due to more efficient extraction of photogenerated charge carriers, resulting in higher short circuit current density and fill factor. The observed effects are proposed to arise from a re-orientation of additive molecules in the external electric field, i.e., electrotropism, leading to a macroscopic alignment of their dipole moments. This leads to an increased built-in electrostatic potential difference in the device active layer post-poling. The dependence of device performance on the polarity of poling bias and reversibility of the effect are demonstrated, further supporting the proposed mechanism.

A Model for the Complex Dielectric Constant of Supercooled Liquid Water at Microwave Frequencies

Measurements of the complex dielectric constant, or relative permittivity, spectrum of liquid water are modeled for frequencies up to 1 THz and temperatures up to 60 °C by a combination of two components: a Debye function, which characterizes the reorientation of the water molecules under the influence of an electric field, and a distributed-resonance function, which is associated with vibrations of intermolecular hydrogen bonds. This model provides a good representation of cloud measurements below 200 GHz at temperatures as low as -25 °C.

Fixed-bed adsorption of toluene on high silica zeolites: experiments and mathematical modelling using LDF approximation and a multisite model

The adsorption of toluene (TOL) as a target volatile organic compound has been studied experimentally and modelled on various hydrophobic zeolites: Faujasite (FAU), ZSM-5 (Z) and Mordenite (MOR). The influence of the nature of the compensating cation (H+ or Na+) has also been investigated for ZSM-5 zeolite, which is known to possess three kinds of adsorption sites (sinusoidal channels, straight channels and intersections). Type I isotherms observed on FAU, Na-Z and MOR fitted well with the Langmuir model. A deviation from a type I isotherm was observed for H-Z, because of the structure of this zeolite. The Successive Langmuir Model was more successful to fit the ‘bump’ of the experimental curve than the Double Langmuir. Classical shapes were found for MOR, FAU and Na-Z breakthrough curves that were fitted with good accuracy using the Linear Driving Force (LDF) approximation. In the case of H-Z, a change of profile was observed during the dynamic adsorption and the differences seen between the Na-Z and H-Z behaviours were explained by the strong interactions between Na+ and adsorbed TOL at the intersection sites. The Na+ cations prevented reorientation of TOL molecules at the intersection and thereby avoided the filling of the sinusoidal channel segments. Thus, a specific model was developed for fitting the breakthrough curve of H-Z. The model developed took into account these two types of adsorption sites with the overall uptake for each site being given by an LDF approximation.

An update on the mechanism of the graphene–NO reaction

Cognizant of the key experimental facts from studies of carbonaceous solids ranging from soot to graphite, we performed a quantum chemistry study of the interaction of NO monomer or dimer with one or more zigzag sites. Thermodynamic and kinetic results were used to examine two alternative mechanisms proposed in the literature, and to compare them with the graphene–O2 reaction mechanism. The chemisorption stoichiometry similarities are striking; but the differences, especially regarding the intermediate role of N2O, have important practical implications. Monomer chemisorption on an isolated site is a dead-end and temporarily inhibiting process, similar to that of formation of a stable C–O surface complex in the graphene–O2 reaction. When two sites are available, successive monomer adsorption eventually leads to N2O formation subsequent to parallel reorientation of the first NO molecule. If three contiguous sites are available, N2 and CO are the principal products. Chemisorption of the dimer provides a straightforward path to N2 and CO2 when one site is available and to N2 and CO when two sites are available. The formation of N2O is also feasible in this case, both during adsorption and desorption; in the adsorption phase it is very sensitive to the details of the electron pairing processes.

Hydrogen bond dynamics in primary alcohols: a femtosecond infrared study.

Hydrogen-bonded liquids are excellent solvents, in part due to the highly dynamic character of the directional interaction associated with the hydrogen bond. Here we study the vibrational and reorientational dynamics of deuterated hydroxyl groups in various primary alcohols using polarization-resolved femtosecond infrared spectroscopy. We show that the relaxation of the OD stretch vibration is similar for ethanol and its higher homologues (∼0.9 ps), while it is appreciably faster for methanol (∼0.75 ps). The fast relaxation for methanol is attributed to strong coupling of the OD stretch vibration to the overtone of the CH3 rocking mode. Subsequent to excited state relaxation, the dissipation of the excess energy leads to structural relaxation of the alcohol liquid structure. We show that this relaxation of the H-bonded network depends on the alkyl chain length. We find that the anisotropy of the excitation decays by both thermal diffusion from excited OD groups to nonexcited molecules and reorientational motion. The reorientation is described well by a model employing two relaxation times that increase linearly with increasing alcohol size. The short reorientation time is assigned to the partial reorientation of molecules within the alcohol cluster, while the long reorientation times can be attributed to breaking and reforming of hydrogen bonds.

Pi(4,5)P2 Lipid Binding Induces a Reorientation of FGF2 Molecules Near Membrane Surface to Facilitate the Unconventional Oligomerization-Dependent Secretion Process as Revealed by a Combined FTIR/NMR/X-Ray Study

Fibroblast growth factor 2 (FGF2) secreted by an unconventional mechanism has been suggested through a membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-dependent process by involving FGF2 oligomerization and lipidic membrane pore formation (Steringer et l., J. Biol. Chem, 2012, 287, 27659), but the mechanism of lipid-induced oligomerization remains elusive. Herein, we demonstrate by a combined lipid monolayer/FTIR method that PI(4,5)P2 binding to FGF2 induce a reorientation of FGF2 molecule near membrane surface to allow the exposure of hydrophobic surface responsible for FGF2 dimer formation. By using the structural information of FGF2/ PI(4,5)P2 complex obtained by NMR and FGF2 dimeric contact surface obtained by X-ray, we further illustrate a molecular mechanism responsible for lipid-induced FGF2 oligomerization process. The results not only provide a structural basis for FGF2 oligomerization near membrane surface, but also suggest a novel PI(4,5)P2 - induced protruding, rather than insertion, process as a crucial event to trigger the proper orientation of FGF2 molecule for its oligomerizations.

Molecular reordering processes on ice (0001) surfaces from long timescale simulations.

We report results of long timescale adaptive kinetic Monte Carlo simulations aimed at identifying possible molecular reordering processes on both proton-disordered and ordered (Fletcher) basal plane (0001) surfaces of hexagonal ice. The simulations are based on a force field for flexible molecules and span a time interval of up to 50 μs at a temperature of 100 K, which represents a lower bound to the temperature range of earth's atmosphere. Additional calculations using both density functional theory and an ab initio based polarizable potential function are performed to test and refine the force field predictions. Several distinct processes are found to occur readily even at this low temperature, including concerted reorientation (flipping) of neighboring surface molecules, which changes the pattern of dangling H-atoms, and the formation of interstitial defects by the downwards motion of upper-bilayer molecules. On the proton-disordered surface, one major surface roughening process is observed that significantly disrupts the crystalline structure. Despite much longer simulation time, such roughening processes are not observed on the highly ordered Fletcher surface which is energetically more stable because of smaller repulsive interaction between neighboring dangling H-atoms. However, a more localized process takes place on the Fletcher surface involving a surface molecule transiently leaving its lattice site. The flipping process provides a facile pathway of increasing proton-order and stabilizing the surface, supporting a predominantly Fletcher-like ordering of low-temperature ice surfaces. Our simulations also show that eventual proton-disordered patches on the surface may induce significant local reconstructions. Further, a subset of the molecules on the Fletcher surface are susceptible to forming interstitial defects which might provide active sites for various chemical reactions in the atmosphere.

Vibrations and reorientations of NH3 molecules in [Mn(NH3)6](ClO4)2 studied by infrared spectroscopy and theoretical (DFT) calculations

The vibrational and reorientational motions of NH3 ligands and ClO4− anions were investigated by Fourier transform middle-infrared spectroscopy (FT-IR) in the high- and low-temperature phases of [Mn(NH3)6](ClO4)2. The temperature dependencies of full width at half maximum (FWHM) of the infrared bands at: 591 and 3385cm−1, associated with: ρr(NH3) and νas(N–H) modes, respectively, indicate that there exist fast (correlation times τR≈10−12–10−13s) reorientational motions of NH3 ligands, with a mean values of activation energies: 7.8 and 4.5kJmol−1, in the phase I and II, respectively. These reorientational motions of NH3 ligands are only slightly disturbed in the phase transition region and do not significantly contribute to the phase transition mechanism. Fourier transform far-infrared and middle-infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at TCc=137.6K (on cooling), which suggested lowering of the crystal structure symmetry. Infrared spectra of [Mn(NH3)6](ClO4)2 were recorded and interpreted by comparison with respective theoretical spectra calculated using DFT method (B3LYP functional, LANL2DZ ECP basis set (on Mn atom) and 6-311+G(d,p) basis set (on H, N, Cl, O atoms) for the isolated equilibrium two models (Model 1 – separate isolated [Mn(NH3)6]2+ cation and ClO4− anion and Model 2 – [Mn(NH3)6(ClO4)2] complex system). Calculated optical spectra show a good agreement with the experimental infrared spectra (FT-FIR and FT-MIR) for the both models.

Polarizability anisotropy relaxation in nanoconfinement: molecular simulation study of water in cylindrical silica pores.

We report the results of a molecular simulation study of polarizability anisotropy relaxation for water confined in approximately cylindrical silica pores, with diameters in the range from 20 to 40 Å. In our calculations, we use a polarizability model that includes molecular and interaction-induced components. In agreement with optical Kerr effect experimental data, we find strong confinement effects on the relaxation rate of water polarizability anisotropy. Given that water molecular polarizability anisotropy is small, much of the intensity of the polarizability anisotropy response comes from the interaction-induced component. However, we find that, at longer times, the relaxation properties of this component strongly resemble those of collective reorientation, the mechanism by which the molecular polarizability anisotropy relaxes. We also find that the relevant collective orientational relaxation differs considerably from single molecule reorientation and that this difference varies with the extent of confinement. Our investigation of the effects of axial-radial pore anisotropy indicates that these effects play a minor role in water polarizability anisotropy relaxation in this pore diameter range.

Is surface layering of aqueous alkali halides determined by ion pairing in the bulk solution?

This contribution aims to elucidate the connection between ion-ion-solvent interactions in the bulk of aqueous electrolyte solutions and the properties of their liquid-air interface. In particular, we were interested in the conditions under which ion pairs form at the surface and whether this is linked to ion pairing in the bulk. For this reason different combinations of hard (Cl(-), Li(+)) and soft ions (I(-), Cs(+)) were investigated. Ion hydration and possible ion association in the bulk was probed with dielectric relaxation spectroscopy. This technique monitors the cooperative reorientation of the dipolar solvent molecules and detects all ion-pair species possibly present in the solution. At the interface, the formation of contact ion pairs was investigated by infrared-visible-sum frequency spectroscopy (SFG). This nonlinear optical technique possesses an inherent surface specificity and can be used for the characterization of interfacial water. The intensity of the SFG-active vibrational stretching modes depends on the number of oriented water molecules. The electric field at the surface of a charged aqueous interface aligns the water dipoles, which in turn increases the SFG response. Hence, the enhancement of the oscillator strengths of the water vibrational modes can be used to draw some conclusions on the strengths and geometrical extension of the electric field. The formation of ion pairs at the interface reduces the intensity of the band associated with hydrogen-bonded water. The underlying theory is presented. The combined data show that there are no contact ion pairs in the bulk of the fluid and--at best--only small amounts of solvent shared ion pairs. On the other hand, the combination of hard/hard or soft/soft ions leads to the formation of ion pairs at the liquid-air interface.

Spatial and electrical switching of defect modes in a photonic bandgap device with a polymer-dispersed liquid crystal defect layer.

This paper investigates the spectral properties of a one-dimensional photonic crystal (PC) containing an inhomogeneous polymer- dispersed liquid crystal (PDLC) as a defect layer. Experimental results indicate that the voltage-induced reorientation of LC molecules between the light-scattering and transparent states in the PDLC enables the electrical tuning of the transmittance of defect-mode peaks in the spectrum of the PC/PDLC cell. Specifically, owing to the unique configuration of the spatial distribution of LC droplet sizes in the defect layer, a concept concerning the spatial switching in the wavelength of defect modes is proposed. As a result, the PC/PDLC hybrid cell is suggested as a potential element for realizing an electrically tunable and spatially switchable photonic bandgap device, which is polarizer-free and requires no alignment layers in the fabrication process.

Water confinement in faujasite cages: a deuteron NMR investigation in a wide temperature range. 2. Spectra and relaxation at high temperature.

Deuteron NMR spectra and spin-lattice relaxation were measured for D2O confined in NaX, NaY, and DY faujasites with various loadings at temperatures ranging from 200 to 310 K with the aim to study molecular mobility of confined water. Hysteresis of spin-lattice relaxation was observed for both DY and NaY(2.4) samples at 500% loading (280 water molecules per unit cell) in a heating-cooling cycle between 264.5 and 277.7 K. The hysteresis is most likely reflecting formation and decomposition of water clusters at different temperature. Spin-lattice relaxation rates obtained from the experiment are consistent with a picture of the fast magnetization exchange between two dynamically different deuteron populations. The observed relaxation behavior as a function of temperature and loading is most likely an effect of interplay between translational and rotational diffusion. Translational diffusion of water molecules is found to be related to the strength of the electrostatic interaction of water oxygen atoms to faujasite sodium cations, whereas water molecule reorientations seem to depend on the strength of hydrogen bonding to faujasite oxygen atoms and the strength of hydrogen bonds between water molecules, at outer and inner positions in water clusters, respectively.