Orientational Relaxation Time Research Articles

Extended rotational diffusion and orientational relaxation of symmetric top molecules in a strong dc electric field: Second-rank orientational correlation functions

Second-rank orientational correlation functions (pertaining to Kerr effect relaxation and Raman scattering) are obtained using the extended rotational diffusion (J-diffusion) model of symmetric top polar molecules in a strong constant external field. It is shown that the shape of the molecule noticeably affects all second-rank correlation functions and relaxation times in the rare collision limit. In the opposite limit of frequent collisions, the quantities of interest are shown to be shape independent as a consequence of vanishingly small inertial effects. An interpolation formula for the orientation relaxation times in the intermediate regime between the rare and frequent collision limits is also given.

Liquid−Vapor Interfaces of Simple Electrolyte Solutions: Molecular Dynamics Results for Ions in Stockmayer Fluids

The equilibrium and dynamical properties of liquid-vapor interfaces of electrolyte solutions are investigated by employing a simple model where the ions are represented by charged Lennard-Jones particles and the dipolar solvent molecules are characterized by the so-called Stockmayer potential. The technique of molecular dynamics simulation is employed to calculate the density profiles and the orientational structure of the interfaces, the surface tension, the translational and rotational diffusion coefficients, and also the dipole orientational relaxation times of both interfacial and bulk molecules. It is found that the ions prefer to stay in the interior of the solutions and tend to avoid the surfaces. The dipole vectors of the interfacial solvent molecules tend to align parallel to the surfaces. The surface tension shows an increasing trend with increase of ion concentration, and this increase of the surface tension is found to be the net outcome of a decrease of the solvent-solvent contribution and an increase of the ion-solvent and ion-ion contributions. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. The solvent molecules at the interfaces rotate and translate in the parallel direction at a faster rate than that of bulk molecules. The perpendicular diffusion, however, occurs at a slower rate for the interfacial molecules. Also, on increase of ion concentration of the solutions, the dynamical properties of the interfaces are found to change differently from those of the bulk solutions. The equilibrium and dynamical results of the present interfacial solutions are also compared with the results of liquid-vapor interfaces of aqueous NaCI solutions that were reported in an earlier study (Chem. Phys. Lett. 2003, 373, 87).

OPTICAL KERR EFFECT IN CARBON DISULFIDE UNDER HIGH PRESSURE

The optical heterodyne spectroscopy of optical Kerr effect (OKE) in liquid carbon disulfide (CS2) was performed under various high pressures using a diamond anvil cell (DAC). The relaxation time of orientation was determined up to 1.20 GPa at 295 K using OKE signals. It increased from 1.6 ps at atmospheric pressure to 10.7ps at 1.20 GPa. Low-frequency light scattering (LFLS) spectra were measured in order to compare the intermolecular dynamical processes in liquid CS2 with those of crystalline CS2. A damped oscillation in OKE is found as a precursor of librational motion observed in crystalline state. This change shows that the intermolecular dynamical processes in the liquid vary gradually to that in crystal with increasing pressures.

Dynamics of water molecules at liquid–vapour interfaces of aqueous ionic solutions: effects of ion concentration

The dynamical properties of water molecules at the liquid–vapour interfaces of aqueous NaCl solutions are studied by means of molecular dynamics simulations. The diffusion coefficients and the orientational relaxation times of the interfacial molecules of pure water and also the effects of ion concentration on these interfacial dynamical properties of aqueous solutions are investigated and the results are compared with those of the corresponding bulk phases. The inhomogeneous density, anisotropic orientational profiles and the surface tension are also calculated in order to characterize the location, width and the thermodynamic aspects of the interfaces and to explore their effects on the dynamical properties.

Tumbling instability in a shearing nematic liquid crystal: Analysis of broadband dielectric results and theoretical treatment

The rotational diffusion coefficient D⊥, the rotational viscosity coefficients γi (i=1,2), and the orientational relaxation time τ001 are investigated. γi are calculated by a combination of an existing statistical–mechanical approach and of broadband experimental results of the complex permittivity for the polar liquid crystal 4-n-octyl-4′-cyanobiphenyl (8CB). The flow dynamic behavior of the nematic liquid crystal in a shear flow, as well as the transition from a flow alignment regime to a tumbling instability are also investigated in 8CB near and far away of a charged surface. The relaxation of the alignment angle θ(τ) to its equilibrium θeq in a layer up to ∼1.5 μm is calculated using the Ericksen–Leslie theory for the case of planar alignment of 8CB at a charged indium tin oxide-coated glass plate and for the temperature range in which 8CB exhibits a nematic phase. In accordance with earlier Couette flow experiments, our calculations show a tumbling flow in the bulk nematic phase. The nature of the hydrodynamic instability under shear flow, in the vicinity, e.g., at a few tens of mK from a nematic–smectic A phase transition (TNA) in the nematic phase, and far from TNA is also discussed for 8CB.

Dynamic and dielectric properties of liquid crystals

The structural properties, static e j and relaxation e j ( ω ) permittivities ( j = || , ┴ ), time correlation functions ( i = 0, 1) and , and orientational relaxation times ( i = 0, 1) of 4- n -pentyl-4'-cyano- biphenyl (5CB) molecules in the nematic phase are investigated in the framework of the statistical-mechanical theory and the molecular dynamics method. The permittivities e j are calculated within a statistical-mechanical approach with the inclusion of translational, orientational, and mixed correlations in the description of the anisotropic systems. The time correlation functions and and the orientational relaxation times of 5CB molecules are calculated using the molecular dynamics method for liquid-crystal systems simu- lated by realistic intramolecular and intermolecular atom-atom interactions. The results of calculations and the experimental data for 5CB are in good agreement. © 2003 MAIK "Nauka/Interperiodica". Φ i0 1 t () Φ 00 t () τ i0 1 t ()

Rotational reorientation dynamics of nile blue A and oxazine 720 in protic solvents

The temperature dependence of the orientational relaxation times of the cationic oxazine dyes nile blue A and oxazine 720 have been determined in water, methanol, ethanol and ethyleneglycol, by measuring the picosecond decays of the polarization dichroism of transient absorption signals. It has been shown that the observed orientational relaxation times are attributed to the ground state solutes. The equilibrium geometries of the two ground state cations, their dipole moments and their atomic charge distributions have been computed in accurate density functional calculations. The hydrodynamic and dielectric contributions to the orientational relaxation times have been estimated on the bases of generalized Stokes–Einstein–Debye theory and of point charge dipole approach, using the molecular parameters obtained in the quantum chemical calculations. The results indicate that the solvent shells around the solutes significantly enhance the orientational relaxation times. The temperature dependence of this effect is negligible in water and methanol, small, but observable in ethanol, and strong in ethyleneglycol.

Translational and rotational diffusion in stretched water

We perform molecular dynamics simulations using the extended simple point charge SPC/E water model in order to investigate the dynamical behavior of supercooled-stretched water. We focus on the behavior of the translational diffusion coefficient, the orientational relaxation time, and the local hydrogen bond network. Decreasing density of pressure along an isothermal path, there is a minimum in the diffusion coefficient and a maximum in the orientational relaxation time, suggesting an initial enhancement and subsequent breakdown of the tetrahedral structure and of the hydrogen bond network as the density decreases. The analysis of the tetrahedral structure of the nearest neighbors help to clarify the relationship between the local structural changes and the system dynamics. We also find that the product of diffusion coefficient and relaxation time is nearly constant, indicating that the anomalous behavior observed in the rotational and translational diffusion arise from the same microscopic mechanism.

In siturheo-x-ray investigation of flow-induced orientation in layered silicate–syndiotactic polypropylene nanocomposite melt

This article describes experimental results for both the rheology and flow-induced orientation of a series of intercalated syndiotactic polypropylene nanocomposites which were prepared by melt intercalation in the presence or absence of an i-PP/maleic anhydride copolymer. The nanocomposites showed typical rheological signatures of well-dispersed interacalated nanocomposites such as a low frequency plateau in dynamic moduli and an apparent yield transition from very high viscosity at low shear stresses to low viscosity above a yield stress. In situ x-ray diffraction (XRD) measurements during shear provided direct evidence of rheology-microstructure links in these materials. It was found that the clay tactoids could be easily oriented by shear and that a high degree of orientation can be achieved after the yield transition. Further, the rheo-XRD apparatus allowed measurements of the relaxation of orientation upon the cessation of flow. The orientation relaxation time matched the characteristic relaxation times estimated from independent rheological measurements well.

Large-scale computer simulation of local segmental dynamics in amorphous atactic polystyrene

Abstract Molecular dynamics computer simulations of amorphous atactic polystyrene have been performed for chains of up to 320 monomer units in a temperature range from 100 to 600 K and in a pressure range from 0.1 to 1000 MPa. The MD-determined values of the glass transition temperature T g are in a good agreement with experimental PVT data at different values of applied pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers as a function of time. The long-time asymptotic slope of these dependencies is close to 0.6 at T > T g , showing diffusive behaviour. The cage effect when local translational motions are essentially frozen in the glassy state has been studied. Local orientational mobility is studied with the help of Legendre polynomials of the first, P 1 ( t ), and second, P 2 ( t ), order for the bonds in the main chain and along the phenyl side groups. Temperature dependencies of the orientational relaxation times at different values of pressure are well described by the Vogel–Fulcher law.

Water-Proton Relaxation by a Noncovalent Albumin-Binding Gadolinium Chelate: An NMRD Study of a Potential Blood Pool Agent

Magnetic dipolar interactions between solvent water protons and paramagnetic solutes, such as chelated Gd3+ ions, provide an efficient means for increasing both the longitudinal (1/T1) and transverse (1/T2) relaxation rates of solvent protons. For small paramagnetic chelate complexes, interactions by direct water coordination to the metal ion (inner sphere) and diffusion in the outer sphere environment of the metal complexes are the two main contributors to relaxation enhancement. At magnetic resonance imaging (MRI) fields and physiologic conditions, these contributions are additive and generally comparable in magnitude, whereas for macromolecular complexes, inner sphere contributions tend to dominate relaxation. The inner sphere contribution to 1/T1 is given by: 1/T1pis =q[Gd3+]/55.5(T1M + τM). Here, q is the number of coordinated inner sphere water molecules, T1M is the longitudinal relaxation time of the coordinated water protons, and τM is their lifetime on the metal ion. For coordinated Gd3+ ions with q = 1, both T1M and τM are of the order of microseconds. 1/T1 is a complicated function of the strength of the interaction, the magnitude of the applied MRI field B0, and an overall correlation time, τC. τC is given by τC−1 = τR−1 + τS−1 + τM−1, the inverse of the sums of the reciprocals of the orientational relaxation time of the complex τR, the electronic relaxation time of the paramagnetic metal ion τS, and τM, respectively. For Gd3+ ions, τS can also be a function of B0, often characterizable by another correlation time τV. In any event, the enhancement is maximized for kinetically labile complexes, when exchange of the inner sphere water is rapid (but not too rapid), such that τC << τM << T1M ( 1 Outhred RK George EP A nuclear magnetic resonance study of hydrated systems using the frequency dependence of the relaxation processes. Biophys J. 1973; 13: 83-96 Abstract Full Text PDF PubMed Scopus (30) Google Scholar , 2 Koenig SH Brown III, RD Relaxation of solvent protons by paramagnetic ions: dependence on magnetic field and chemical environment—implications for NMR imaging. Magn Reson Med. 1984; 1: 478-495 Crossref PubMed Scopus (187) Google Scholar , 3 Bertini L Luchinat C NMR of paramagnetic molecules in biological systems. Benjamin/Cummins, Menlo Park, Calif1986 Google Scholar ).

Energy bottlenecks and the rotational dynamics of a water-like fluid

Water is modeled as a spherical top with a hard, convex tetrahedral surface. In its collisions, water transfers linear and angular momentum according to the usual two-body kinematics; however, rotational energy transfer is diminished from the hard-body value since portions of the surface of water are blocked due to pre-existing hydrogen bonds. Owing to the bottleneck in transfer of rotational energy, convective effects normally important for water at subcritical densities are conjectured to be significant at densities where water forms hydrogen bond networks. The present theory is compared to experimental and computer simulation studies of orientational and angular momentum relaxation times, and the agreement or lack thereof is analyzed.

Anomalous orientational relaxation of solute probes in binary mixtures

The orientation of a solute probe in a binary mixture often exhibits multiple relaxation times at the same solvent viscosity but different compositions [Beddard et al., Nature (London) 294, 145 (1981)]. In order to understand this interesting observation, we have carried out (NPT) molecular dynamics simulation study of rotation of prolate ellipsoids in binary mixtures. The simulations show that for a broad range of model parameters the experimental behavior can be reproduced. The plot of orientational relaxation time versus the solvent viscosity, the latter changed by varying the composition, shows a nonmonotonic viscosity dependence. The nature of the plot is found to depend on the system parameters, especially on the solute–solvent interactions. A mode coupling theoretical analysis of this complex dependence of the rotational relaxation time τR on the binary viscosity η is presented. The theory can qualitatively explain the origin of the multiple relaxation time at the same viscosity.

Orientational relaxation in polymer and dye solutions and its consequence for the laser threshold

We compare orientational relaxation phenomena of a light-emitting polymer and a, spectroscopically similar, conventional laser dye in solutions of comparable viscosity. The orientational relaxation time of the laser dye is found to be very much smaller than that of the polymer. This result is attributed to the large difference in shape between the two light-emitting materials. We demonstrate that these very different reorientation times have a major influence on the operation of a gain-switched laser based on these gain media, in particular on its threshold.

Molecular dynamics simulations of aqueous NaCl and KCl solutions: Effects of ion concentration on the single-particle, pair, and collective dynamical properties of ions and water molecules

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations, ranging from 0 M to 4.5 M, to investigate the effects of ion concentration on the single-particle, pair, and collective dynamical properties of aqueous electrolyte solutions. The SPC/E model is used for water and the ions are modeled as charged Lennard-Jones particles. The single-particle dynamics is investigated by calculating the self-diffusion coefficients of ions and water molecules and also the orientational relaxation times. The pair dynamics is studied by evaluating the ion–water residence and water–water hydrogen bond time correlation functions. The relaxation of relative velocity autocorrelation function and the cross velocity correlation function of two hydrogen bonded water molecules are also investigated at varying ion concentration. Finally, we explore the collective dynamical properties by calculating the frequency dependent dielectric function and conductivity. It is found that the self and relative diffusion coeffcients decrease and the orientational relaxation times increase with ion concentration. The residence times of water molecules near ions and also the structural relaxation time of water–water hydrogen bonds show an increasing trend as the ion concentration is increased. The dielectric relaxation time is found to decrease with ion concentration for the solutions investigated here. The static conductivity of concentrated solutions shows significant departure from the Nernst–Einstein behavior due to formation of ion pairs. With an increase of frequency, the conductivity first increases substantially and then decreases at very high frequency. The initial increase of conductivity is attributed to the disruption of ion pairs on application of high frequency electric fields.

Freezing behavior in porous glasses and MCM-41

We report experimental measurements of the melting and freezing behavior of fluids in nano-porous media. The experimental studies are for nitrobenzene in the silica based pores of controlled pore glass (CPG), Vycor and MCM-41. Dielectric relaxation spectroscopy was used to determine melting points and the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. It was found that the confined fluid freezes into a single crystalline structure for average pore diameters greater than 20 σ, where σ is the diameter of the fluid molecule. For average pore sizes between 20 and 15 σ, part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. For pore sizes smaller than 15 σ, even the partial crystallization did not occur.

Picosecond transient dichroism and birefringence spectroscopy on pheophorbide-a molecules in solution

Picosecond transient dichroism and birefringence on pheophorbide-a in solution is investigated using a pump-and-probe experiment. In this experiment the sample is pumped resonantly to the S0-S1 transition by linear polarized light and probed by a continuum polarized 45° with respect to the pump beam polarization plane. The photoselective excitation of the sample generates a transient anisotropy resulting in a rotation of the probe beam polarization plane due to light-induced dichroism and in the generation of elliptically polarized light due to light-induced birefringence. The polarization state of the probe beam after passing the sample is analysed by means of angle-resolved measurements carried out at different time delays. From these measurements (i) the light-induced changes of the absorptive index and (ii) the light-induced changes of the refractive index are obtained separately. The results are in good agreement with calculations using Kramers-Kronig transforms. Moreover, (iii) the anisotropy spectrum of the absorptive index is measured to obtain information about the orientation of the transition dipole moments of the excited-state transitions and (iv) the orientation relaxation time of the S1-state is determined from the decay time of the transient anisotropy.

Melting/freezing behavior of a fluid confined in porous glasses and MCM-41: Dielectric spectroscopy and molecular simulation

We report both experimental measurements and molecular simulations of the melting and freezing behavior of fluids in nanoporous media. The experimental studies are for nitrobenzene in the silica-based pores of controlled pore glass, Vycor, and MCM-41. Dielectric relaxation spectroscopy is used to determine melting points and the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. Monte Carlo simulations, together with a bond orientational order parameter method, are used to determine the melting point and fluid structure inside cylindrical pores modeled on silica. Qualitative comparison between experiment and simulation are made for the shift in the freezing temperatures and the structure of confined phases. From both the experiments and the simulations, it is found that the confined fluid freezes into a single crystalline structure for average pore diameters greater than 20σ, where σ is the diameter of the fluid molecule. For average pore sizes between 20σ and 15σ, part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. For pore sizes smaller than 15σ, even the partial crystallization did not occur. Our measurements and calculations show clear evidence of a novel intermediate “contact layer” phase lying between liquid and crystal; the contact layer is the confined molecular layer adjacent to the pore wall and experiences a deeper fluid–wall potential energy compared to the inner layers. We also find evidence of a liquid to “hexatic” transition in the quasi-two-dimensional contact layer at high temperatures.

流体のダイナミクス 高圧あるいは高圧・高温流体中のイオンおよび反応ダイナミクスの測定

In this article, we have dealt not only with the dynamic behavior of nitrate anion in aqueous zinc nitrate solution at high temperatures and a fixed pressure but also with photoreduction of benzophenone by N, N, -diethylaniline in supercritical carbon dioxide with time-resolved absorption techniques. In the former cases the perpendicular orientational relaxation time (τ) of the anion significantly decreases with increasing temperature u p to 340°C the values of τ were 1. 86 and 0. 25 ps at 20 and 340°C, respectively. In the latter the rate constant at 40°C decreased greatly with increasing pressure from 9. 8 to 17. 2 MPa, which is considered to be due to the local aggregation of carbon dioxide around benzophenone and/or N, N, -diethylaniline.

Raman spectra and microdynamics of the hydroxide-ion in molten NaOH and NaClNaOH mixtures

Abstract Molten mixtures of sodium hydroxide and sodium chloride were examined using Raman spectroscopy. The shift of the valence vibration band ν OH , the change of the orientation relaxation time τ 2r , and the variation of the effective inertia moment I eff. of the hydroxide-ion depending on temperature and chemical composition of the melts indicated the formation of ionic groups Na + OH − …Cl − Na + thanks to appearance of hydrogen bonds. The degree of associate formation was found to increase with growing concentration of halide-ions and reducing temperature.