Anisotropic rotational diffusion and transient nonlinear responses of rigid macromolecules in a strong external electric field

Nonlinear transient responses of polar polarizable particles (macromolecules) diluted in a nonpolar solvent to a sudden change in magnitude of a strong external dc field are evaluated using the anisotropic noninertial rotational diffusion model. The relaxation functions and relaxation times appropriate to the transient dynamic Kerr effect and nonlinear dielectric relaxation are calculated by solving the infinite hierarchy of differential-recurrence equations for statistical moments (ensemble averages of the Wigner D functions). The calculations involve matrix continued fractions, which ultimately yield the exact solution of the infinite hierarchy of differential-recurrence relations for the first- and second-order transient responses.     

Local hydrogen bonding dynamics and collective reorganization in water: ultrafast infrared spectroscopy of HOD/D(2)O

We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.     

Dielectric relaxation studies of binary mixtures of N-methylacetamide and N-methylformamide in benzene solutions using microwave absorption data

Using standard microwave X-band technique and by following Gopala Krishna's single frequency (9.90?GHz) concentration variational method, the dielectric relaxation times (τ) and the dipole moments (μ) of dilute solution of N-methylacetamide (NMA), N-methylformamide (NMF) and NMA?+?NMF binary mixtures in benzene solutions have been calculated at different temperatures. The energy parameters for the dielectric relaxation process for NMA?+?NMF binary mixture containing 30?mol% NMF have been calculated at 25, 30, 35 and 40°C and compared with the corresponding viscosity parameters. A good agreement between the free energy of activation from these two sets of values shows that the dielectric relaxation process like the viscous flow process can be treated as the rate process. From relaxation time behavior of NMA and NMF binary mixture in benzene solution, solute–solute types of the molecular association has been proposed.     

Molecular dynamics models and thermodynamic characteristics of hydrogen bonds in 1,2-ethanediol

A correlation between the lifetimes of hydrogen bonds and the thermodynamic characteristics of their formation and breaking, and the experimental relaxation times of dielectric spectra and the energy characteristics of relaxation processes, is observed via molecular dynamics (MD) simulation of the rearranging of the network structure of 1,2-ethanediol. The MD torsional frequency of the transition of gauche conformer tGg′ at 224.1 cm_?1 and the experimental frequency of the band maximum of torsional vibrations at 230 cm_?1 in the infrared spectrum correlate with the oscillation frequency of molecules at 240 cm_?1 inside clusters in the Dissado–Hill (DH) model. The MD and DH models indicate a predominantly parallel alignment of the electric dipole moments of conformers tGg′ in the three-dimensional network of hydrogen bonds of the liquid 1,2-ethanediol phase.     

Reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in water

The reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in deuterated water were studied by 1H NMR longitudinal relaxation rates. It is shown that the relaxation is purely dipolar in the non-extreme narrowing regime. The distance between the geminal protons could be determined from the NMR data, giving good agreement with the values generally used in correlation time calculations. The correlation times show an Arrhenius behaviour in good agreement with previously reported data from 13C measurements for a similar uncomplexed calixarene. The Arrhenius energies of activation are identical for the uncomplexed and the complexed calixarenes, suggesting a reorientational motion strongly dependent on the structure of the water cage around the complex. This is also in agreement with a complexation of the La(III) cation in the second sphere of solvation of the sulfonate groups, as shown by molecular dynamics simulations.     

Dielectric relaxation in OH—O bond complexes

Abstract

The dielectric absorption of the mixtures of Phenol with Acetone and Ethyl methyl ketone having different concentration ratios (3:1, 2:1, 1:1, 1:2 and 1:3) has been determined at microwave frequency 9.8 GHz, at 33°C. The dielectric data have been analyzed, to yield relaxation times and dipole moments, using Higasi method and Higasi, Koga and Nakamura method. All the mixtures yielded high values of the distribution parameter indicating the flexible nature of the OH-O type hydrogen bonding. The high values of the relaxation times and dipole moments give confirmative results for the association of the phenol with these electron donar molecules in all mixtures investigated.     

Dynamics of glass-forming liquids. VIII. Dielectric signature of probe rotation and bulk dynamics in branched alkanes

We have measured the dielectric relaxation of several glass forming branched alkanes with very low dielectric loss in the frequency range 50 Hz-20 kHz. The molecular liquids of this study are 3-methylpentane, 3-methylheptane, 4-methylheptane, 2,3-dimethylpentane, and 2,4,6-trimethylheptane. All liquids display asymmetric loss peaks typical of supercooled liquids and slow beta relaxations of similar amplitudes. As an unusual feature, deliberate doping with 2-ethyl-1-hexanol, 5-methyl-2-hexanol, 2-methyl-1-butanol, 1-propanol, or 2-methyltetrahydrofuran at the 1 wt % level generates additional relaxation peaks at frequencies below those of the alpha relaxation. The relaxation times of these sub-alpha-peaks increase systematically with the size of the dopant molecules. Because these features are spectrally separate from the bulk dynamics, the rotational behavior and effective dipole moments of the probes can be studied in detail. For the alcohol guest molecules, the large relative rotational time scales and small effective dipole moments are indicative of hydrogen bonded clusters instead of individual molecules.     

Rheodielectric spectroscopy on heterogeneous polymer systems —electrorheological fluids

The technique of rheodielectric spectroscopy is used to investigate the dielectric behaviour of electrorheological fluids (ERF) as a function of external electrical DC-field and/or shear rate. Commercial ERF's consisting of mesoscopic polyurethane particles in a silicone oil matrix were studied. The particles contain a salt which leads to strong dipole moments via the Maxwell-Wagner-Polarization (MWP) if subjected to an external electrical field. In an electrical field the dipoles and, concomittantly, the particles motion become correlated leading to the formation of solid-like structures and significant changes in the viscosity. We demonstrate that dielectric spectroscopy is capable of monitoring the field and shear rate effects in terms of relaxation strength and relaxation time of the MWP. In electrical or shear fields dipole-dipole correlations increase the MWP's relaxation strength, so that we are able to observe structure formation with dielectric spectroscopy, especially the time resolved response of the ERF to changes in the electrical field or the shear rate.     

Thermally activated escape rate for the Brownian motion of a fixed axis rotator in an asymmetrical double-well potential for all values of the dissipation

The Kramers theory of the escape rate of a Brownian particle from a potential well as extended by Mel'nikov and Meshkov is used to evaluate the relaxation times and the dynamic susceptibility for the rotational Brownian motion of fixed axis rotators in an asymmetric double-well potential. An expression for the escape rate valid for all values of the dissipation including the very low damping (VLD), very high damping (VHD), and crossover regimes is derived. It is shown that this expression provides a good asymptotic estimate of the inverse of the smallest nonvanishing eigenvalue lambda(1) of the underlying Fokker-Planck operator calculated by using the matrix-continued fraction method. For low barriers, where the Mel'nikov and Meshkov approach is not applicable, analytic equations for the correlation time tau( parallel) of the longitudinal dipole correlation function in the VLD and VHD limits are derived and a simple extrapolating equation valid for all values of the damping is proposed.     

Dielectric relaxation behavior of aqueous solutions of carbobetaines with varying intercharge distances

The dielectric relaxation behavior of aqueous triethylammonioalkanoate (carbobetaine: Et(3)nCB) solutions was examined as a function of frequency from 1.00 x 10(6) to 2.00 x 10(10) Hz (6.28 x 10(6) to 1.26 x 10(11) rad s(-1) in angular frequency); number of intercharge methylene groups, n = 1, 3, 4, 5, and 10; and solute concentration, c. Two major relaxation modes, fast and slow, were found in all solutions examined. Et(3)nCB systems with n = 5 and 10 possessed another, medium, relaxation mode with relaxation time tau(Dh) at high c values (above the contact concentration of solutes) in addition to the fast and slow modes. The fast mode with a relaxation time, tau(w), of approximately 10 ps was attributed to the rotational motion of bulk water molecules. The slow mode with a relaxation time, tau, of 0.08-1 ns, depending on the n value, was attributed to the overall rotational motion of each carbobetaine in aqueous solution. The concentration normalized relaxation strength, Deltaepsilonc(-1), and tau value of the slow relaxation mode increased with increasing n. These findings were quantitatively explained on the basis of changes in the intercharge distance resulting in increased size and dipole moment of the carbobetaines. Above the contact concentration, collisions between solute molecules likely hindered their rotational motions, leading to an increase in tau. The middle relaxation mode found in longer Et(3)nCBs (n = 5 and 10) with a relaxation time, tau(Dh), of approximately 0.2 ns, more than 20 times as long as that of bulk water molecules, tau(w), was attributed to the dehydration of water molecules tightly bound to all Et(3)nCBs examined (including those with n < 5). This mode was not observed in the solutions of Et(3)nCBs with n < 5, since the tau values corresponding to overall rotation were close to or shorter than the tau(Dh) values.     

Structure and dynamics of the aqueous liquid-vapor interface: a comprehensive particle-based simulation study

This research addresses a comprehensive particle-based simulation study of the structural, dynamic, and electronic properties of the liquid-vapor interface of water utilizing both ab initio (based on density functional theory) and empirical (fixed charge and polarizable) models. Numerous properties such as interfacial width, hydrogen bond populations, dipole moments, and correlation times will be characterized with identical schemes to draw useful conclusions on the strengths and weakness of the proposed models for interfacial water. Our findings indicate that all models considered in this study yield similar results for the radial distribution functions, hydrogen bond populations, and orientational relaxation times. Significant differences in the models appear when examining both the dipole moments and surface relaxation near the aqueous liquid-vapor interface. Here, the ab initio interaction potential predicts a significant decrease in the molecular dipole moment and expansion in the oxygen-oxygen distance as one approaches the interface in accordance with recent experiments. All classical polarizable interaction potentials show a less dramatic drop in the molecular dipole moment, and all empirical interaction potentials studied yield an oxygen-oxygen contraction as the interface is approached.     

Study of molecular interactions in the polar binary mixtures of N-methyl aniline and alcohols,using excess dielectric and thermodynamic parameters

Molecular interactions between the polar systems N-methyl aniline and alcohols (propan-1-ol/propan-2-ol) for various mole fractions at different temperatures are studied by determining the dielectric permittivity using LF impedance analyzer, Microwave bench and Abbe’s refractometer in radio, microwave and optic frequency regions respectively. The dipole moment, excess dipole moment, excess Helmholtz energy, excess permittivity, excess inverse relaxation time and excess thermodynamic values are calculated using experimental results. The optimized geometry, harmonic vibrational wave numbers and dipole moments of pure and equimolar binary mixtures have been calculated theoretically from the ab initio Hartree–Fock (HF) and Density Functional Theory (DFT – B3LYP) methods with 6-31+G¹ and 6-311+G⁷ basis sets using Spartan 08 modelling software. Conformational analysis of the formation of hydrogen bond in the equimolar binary mixture systems of N-methyl aniline and alcohols (propan-1-ol/propan-2-ol) is supported by experimental FT-IR spectra. The calculated wave numbers and dipole moments agree well with the experimental values. Further, the correlations among the parameters are discussed in detail.     

Effect of temperature and pressure on the structure, dynamics, and hydrogen bond properties of liquid N-methylacetamide: a molecular dynamics study

The structure and dynamical properties of liquid N-methylacetamides (NMA) are calculated at five different temperatures and at four different pressures using classical molecular dynamics simulations. Our results are analyzed in terms of pressure-induced changes in structural properties by investigating the radial distribution functions of different atoms in NMA molecule. It is found that the first peak and also the second peak of C-O and N-H are well defined even at higher temperature and pressure. It is also observed that the number of hydrogen bonds increase with application of pressure at a given temperature. On the other hand, the calculated hydrogen bond energy (E(HB)) shows that the stability of hydrogen bond decreases with increasing of pressure and temperature. Various dynamical properties associated with translational and rotational motion of neat NMA are calculated and the self-diffusion coefficient of NMA is found to be in excellent agreement with the experiment and the behavior is non-Arrhenius at low temperatures with application of pressures. The single particle orientational relaxation time for dipole vector and N-C vector are also calculated and it is found that the orientational relaxation time follows Arrhenius behavior with a variation of temperature and pressure.     

Influence of the anisotropic polarizability on the anomalous dielectric relaxation spectra

The nonlinear dielectric response due to the application of a strong dc bias electric field superimposed on a weak ac electric field is considered in the context of the anomalous diffusion (subdiffusion). A perturbation procedure is used to derive analytical expressions for the first three harmonic components of the electric polarization of an assembly of both polar and anisotropically polarizable symmetric-top molecules. To accomplish that, an infinite hierarchy of multiterm (21) differential-recurrence equations of noninteger order for the moments is established and solved for the stationary regime. The results so obtained are illustrated in the form of Argand diagrams and three-dimensional relaxation spectra for the complex nonlinear dielectric increment extracted from the first harmonic component of the electric susceptibility. These plots show the role and importance played by the fractional exponent and the parameter P measuring the influence of the dipole moment over the permanent one.     

Viscoelastic properties of dilute polymer solutions: The effect of varying the concentration

Dynamic viscoelastic behavior was investigated for solutions of polystyrene in tricresyl phosphate, a good solvent, at concentrations, c, less than the coil‐overlapping concentration, c*. At the infinite dilution limit, the behavior was in accord with the theory of Doi and Edwards involving the excluded volume potential and hydrodynamic interaction (HDI). Thus, the viscoelastic functions were completely derived from the intrinsic viscosity–molecular weight relation. At finite c, the complex modulus was represented by the sum of two terms. One was a Rouse–Zimm (RZ) term conveniently represented by the Zimm theory with an arbitrarily chosen value of the HDI parameter. The other was a term with a single relaxation time, longer than the longest RZ relaxation time, and with a high‐frequency modulus proportional to the square of c [the long‐time (LT) term]. The behavior of the RZ term indicated the stronger screening of HDI with increasing c. Using the experimental c dependence of the longest RZ relaxation time to get the relevant parameter, we compared the RZ viscoelastic function with the Muthukumar–Freed theory. The agreement was good at low concentrations, c < c*. The contribution of the LT term, which was not included in the theory, was quite significant at low frequencies; about 60% of the Huggins coefficient was attributable to this term. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 211–217, 2001     

Primary and secondary relaxations in bis-5-hydroxypentylphthalate

Broadband dielectric spectroscopy was used to study the relaxation dynamics in bis-5-hydroxypentylphthalate (BHPP) under both isobaric and isothermal conditions. The relaxation dynamics exhibit complex behavior, arising from hydrogen bonding in the BHPP. At ambient pressure above the glass transition temperature T(g), the dielectric spectrum shows a broad structural relaxation peak with a prominent excess wing toward higher frequencies. As temperature is decreased below T(g), the excess wing transforms into two distinct peaks, both having Arrhenius behavior with activation energies equal to 58.8 and 32.6 kJmol for slower (beta) and faster (gamma) processes, respectively. Furthermore, the relaxation times for the beta process increase with increasing pressure, whereas the faster gamma relaxation is practically insensitive to pressure changes. Analysis of the properties of these secondary relaxations suggests that the beta peak can be identified as an intermolecular Johari-Goldstein (JG) process. However, its separation in frequency from the alpha relaxation, and both its activation energy and activation volume, differ substantially from values calculated from the breadth of the structural relaxation peak. Thus, the dynamics of BHPP appear to be an exception to the usual correlation between the respective properties of the structural and the JG secondary relaxations.     

Linear viscoelastic properties of transient networks formed by associating polymers with multiple stickers

We have developed a single-chain theory that describes dynamics of associating polymer chains carrying multiple associative groups (or stickers) in the transient network formed by themselves and studied linear viscoelastic properties of this network. It is shown that if the average number N of stickers associated with the network junction per chain is large, the terminal relaxation time τ(A) that is proportional to τ(X)N(2) appears. The time τ(X) is the interval during which an associated sticker goes back to its equilibrium position by one or more dissociation steps. In this lower frequency regime ω<1/τ(X), the moduli are well described in terms of the Rouse model with the longest relaxation time τ(A). The large value of N is realized for chains carrying many stickers whose rate of association with the network junction is much larger than the dissociation rate. This associative Rouse behavior stems from the association/dissociation processes of stickers and is different from the ordinary Rouse behavior in the higher frequency regime, which is originated from the thermal segmental motion between stickers. If N is not large, the dynamic shear moduli are well described in terms of the Maxwell model characterized by a single relaxation time τ(X) in the moderate and lower frequency regimes. Thus, the transition occurs in the viscoelastic relaxation behavior from the Maxwell-type to the Rouse-type in ω<1/τ(X) as N increases. All these results are obtained under the affine deformation assumption for junction points. We also studied the effect of the junction fluctuations from the affine motion on the plateau modulus by introducing the virtual spring for bound stickers. It is shown that the plateau modulus is not affected by the junction fluctuations.     

Determination of the nonlinear dielectric increment in the Cole-Davidson model

The problem of the nonlinear dielectric relaxation of complex liquids is tackled in the context of the Cole-Davidson [J. Chem. Phys. 19, 1484 (1951)] model. By using an appropriate time derivative of noninteger order, an infinite hierarchy of differential-recurrence relations for the moments (expectation values of the Legendre polynomials) is obtained. The solution is established for the stationary regime of an ensemble of polar and symmetric-top molecules acted on by a strong dc bias electric field superimposed on a weak ac electric field. The results for the first three nonlinear harmonic components of the electric susceptibility are analytically established and illustrated with the help of Argand diagrams for the nonlinear dielectric increment and three-dimensional dispersion and absorption spectra for the second and the third harmonic components as a function of the anomalous exponent beta相似文献