Reorientational relaxation and rotational–translational coupling in water clusters in a d.c. external electric field

Molecular dynamics simulations have been carried out for small water clusters (N=16, 32, 64) in a d.c. electric field at T=200 K. It was shown that for relatively weak fields, there was a significant decrease of reorientational and structural relaxation times for all cluster sizes examined. Regarding the molecular reorientational motions, in the strong field regime, a decoupling of tumbling and spinning librations was observed. Reorientational relaxation times of the dipole and vector joining the two hydrogen atoms were found to follow different relaxation laws, with the former decreasing and the latter increasing with electric field increase. These trends were qualitatively explained by invoking the Debye model with field-dependent friction for dipole librations and the symmetric double-well for spinning rotations on a plane perpendicular to the field axis. Finally, the interdependence of the reorientation on the translational modes of the cluster was indicated, with the translationally slow molecules being rotationally slow as well and vice versa.     

Highly noninstantaneous solitons in liquid-core photonic crystal fibers

The nonlinear propagation of pulses in liquid-filled photonic crystal fibers is considered. Because of the slow reorientational nonlinearity of some molecular liquids, the nonlinear modes propagating inside such structures can be approximated, for pulse durations much shorter than the molecular relaxation time, by temporally highly nonlocal solitons, analytical solutions of a linear Schr?dinger equation. The physical relevance of these novel solitons is discussed.     

Translational and rotational mode coupling in disordered ferroelectric KTa1−xNbxO3 studied by Raman spectroscopy

The coupling of translational modes to the reorientational motion is an essential property of systems with internal orientational degrees of freedom. Due to their high complexity most of those systems (molecular crystals, glasses, etc.) present a major puzzle for scientists. In this paper we analyze the Raman scattering of a relatively simple ferroelectric system, KTa_1−xNb_xO₃, which may serve as a model for more complicated cases. We show the presence of a strong coupling between translational and reorientational motion in the crystal. Our data suggest that this coupling is the main reason for the depolarized component of the second-order Raman spectra and that it is also responsible for the frequency decrease (softening) of the transverse acoustic mode down to the third of three transitions, below which reorientational motion is no longer allowed.     

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers. Received 27 September 2001 and Received in final form 18 January 2002     

A simulation study of vibrational relaxation of I− 3 in liquids

The temperature dependence of the vibrational relaxation of a flexible model of triiodide in a Lennard-Jones solvent (xenon) has been studied using equilibrium molecular dynamics simulations. The internal dynamics of the ion is calculated from a previously published semi-empirical valence bond model with a limited number of basis states. Vibrational decorrelation rates of the symmetric and antisymmetric stretching modes were found from the time correlation functions of the normal coordinate velocities and the vibrational energy relaxation rates from the time correlation functions of the kinetic energy in each mode. The vibrational dephasing rates and the energy relaxation rates decrease slowly as the temperature is lowered and do not show a discontinuity when the fluid solidifies, although the reorientational diffusion rates change rapidly at low temperatures. In order to interpret the results, perturbation theory expressions for the relaxation rates were evaluated for simulations of a rigid model of the ion and found to agree well with the direct observations. These showed that, unusually, both the solvent force and its derivative, the solvent potential curvature, contribute to the dephasing of the symmetric mode. The relevant fluctuation correlation times are very short, which may explain the insensitivity of the vibrational relaxation to the state of the solvent.     

Derivation of extremely slow dynamics of protein motion based on entropy invariance

It is a mysterious fact that protein systems often show an extremely slow dynamics of their molecular motions with time scales much longer than nanosecond order, although their characteristic frequencies obtained by the normal mode analysis fall in much shorter temporal regions. This Letter provides a heuristic account for why and how such extremely slow modes of protein motions naturally emerge from fast molecular modes on the basis of an idea of entropy invariance in the principal component analysis.     

Solution dynamics of the natural bioactive molecule capsaicin: a relaxation study

Nuclear magnetic resonance spectroscopy is a straightforward technique for studying molecular dynamics that range in timescale from picosecond (motions faster than molecular reorientation) to those that occur in real-time. This approach is important to highlight the behavior of bioactive molecules in solution, and to acquire information about action mechanisms and potential pharmacological effects. Proton and carbon-13 spin–lattice relaxation experiments were performed to calculate the reorientational correlation time for protonated carbons. Capsaicin showed complex dynamical properties and the results revealed two regions with different dynamical properties: the aliphatic region with fast reorientation motions and the aromatic region with slow motions.     

Reorientational motion in binary mixtures: molecular dynamics simulations

Molecular reorientational motion has been studied in a dilute solution of linear ‘tracer’ molecules in a solvent that exhibits liquid, plastic and crystalline phases. Molecular dynamics simulations have been used to extract reorientational correlation functions for both solvent and solute species as functions of temperature in all phases. The transition from the liquid to the plastic phase (upon cooling) results in less hindered tracer rotation, as evidenced by the more rapid decay of orientational correlation. These surprising dynamics are interpreted in terms of structural changes at freezing that lead to a less confining local tracer environment. The findings support a recent experimental result obtained from polarized Raman scattering on a solution of CS₂ tracers in a cyclohexane host.     

Structure and dynamics of liquid formamide at high pressure and high temperature: comparison of X-ray diffraction and molecular dynamics results

New molecular dynamics simulations with optimised potentials for liquid simulation are presented for liquid formamide at high pressures and high temperatures. The structural results are compared to those found by X-ray diffraction measurements, and the H-bonding structure is analysed in detail. While it is ambiguous from purely experimental data, the simulation results support the idea that pressure increase can enhance ring dimer formation at the expense of linear chain structure, but increasing temperature results in an opposite effect. Dynamical results (reorientational correlation times and lifetimes of hydrogen bonds) are in agreement with these findings.     

Evidence of the presence of opticlike collective modes in a liquid from neutron scattering experiments

Inelastic neutron scattering data from liquid DF close to the melting point show, in addition to spectra comprising quasielastic and heavily damped acoustic motions, an intense, nondispersive band centered at about 27 meV along with a broader higher energy feature. Observation of the former band provides the first direct verification of the existence within the liquid state of collective opticlike excitations as predicted by molecular dynamics simulations. The latter corresponds to mainly reorientational motions assigned from mode eigenvector analysis carried out by computer simulations.     

NMR investigations of polymer dynamics in a partially filled porous matrix

The interior surface of well-defined porous alumina membranes (Anopore) of 20 nm and 200 nm pore diameter, respectively, was coated with polymer layers generated from solution by the solvent evaporation method. Deposits of poly(dimethyl siloxane) (PDMS) with nominal thicknesses ranging from 0.15 to 4.5 nm --corresponding to submonolayer to multilayer films--were investigated, and were compared to poly(butadiene) (PB) as an example for non-wetting polymers. Molecular weights below and above the critical value were studied since the bulk dynamics of such polymers are known to be qualitatively different. First results of NMR relaxation dispersion experiments on these systems are presented, supplemented by transverse relaxation times and double-quantum measurements obtained from high-field NMR. A systematic decrease of relaxation times at low fields with decreasing polymer amount is found for PDMS, but molecules retain a high degree of mobility irrespective of molecular weight. The relaxation dispersion results are supported by T2 data and 1H residual dipolar coupling (RDC) constants, and are discussed in terms of molecular order and reorientational dynamics.     

Vibrational energy transfer in a protein molecule

Mode coupling in a protein molecule was studied by a molecular dynamics simulation of the intramolecular vibrational energy transfer in myoglobin at near zero temperature. It was found that the vibrational energy is transferred from a given normal mode to a very few number of selective normal modes. These modes are selected by the relation between their frequencies, like Fermi resonance, governed by the third order mode coupling term. It was also confirmed that the coupling coefficients had high correlation with how much the coupled modes geometrically overlapped with each other.     

1H nuclear magnetic resonance study of mixed-valent dodecylamine-intercalated vanadium oxide nanosheets

Alkyl-chain dynamics in the mixed-valent dodecylamine-intercalated vanadium oxide nanosheets (C12VONS) was investigated by means of 400-MHz ¹H nuclear magnetic resonance. Two C12VONS samples with distinct alkylammonium-to-alkylamine ratios showed distinct alkyl-chain dynamics, the alkylammonium chains being apparently more disordered than the alkylamine chains. The high-temperature spin–lattice relaxation is dominated by the conformational chain defects and the reorientational motions of the alkyl chains, respectively, in the alkylammonium-abundant system and in the alkylamine-abundant system. The reorientational and conformational changes of the alkyl chains appear to be uncorrelated with the magnetic transitions.     

Evidence for the weak steric hindrance scenario in the supercooled-state reorientational dynamics

We use molecular-dynamics computer simulations to study the translational and reorientational dynamics of a glass-forming liquid of dumbbells. For sufficiently elongated molecules the standard strong steric hindrance scenario for the rotational dynamics is found. However, for small elongations we find a different scenario--the weak steric hindrance scenario--caused by a new type of glass transition in which the orientational dynamics of the molecule's axis undergoes a dynamical transition with a continuous increase of the nonergodicity parameter. These results are in agreement with the theoretical predictions by the mode-coupling theory for the glass transition.     

Two-dimensional exchange nuclear quadrupole resonance spectroscopy of molecular crystals

A theoretical treatment of the 2D exchange NQR pulse sequence is presented and applied to the quantitative study of exchange processes in molecular crystals. It takes into account the off-resonance irradiation, which critically influences the spin dynamics. The response of a system of spins I = 3/2 in zero applied field, experiencing electric quadrupole couplings, to the three-pulse sequence is analysed. All the tools and mathematical expressions to predict the time evolution of the signal created by a pure NQR multipulse sequence are presented explicitly. The mixing dynamics by exchange and the expected cross-peak intensities as a function of the frequency offset have been derived. The theory is illustrated by a study of the optimization procedure, which is of crucial importance for the detection of the cross- and diagonalpeaks in a 2D exchange spectrum. The systems investigated here were hexachloroethane and tetrachloroethylene, which show threefold and twofold reorientational jumps about the carbon-carbon axis, respectively.     

The rate memory of a polymer close toT gas elucidated by reduced 4-D NMR echo experiments

We analyze the reorientational dynamics of a polymer close to the glass transition. Via comparison of two-time and four-time correlation functions, obtained from multidimensional nuclear magnetic resonance¹³C echo experiments, we are particularly sensitive to the fluctuations within the heterogeneous distribution of mobilities. From variation of the experimental parameters we obtain a detailed picture of the interplay between jump and diffusion dynamics, including the different types of motional heterogeneities and the geometry of the system.     

Single-particle and collective dynamics of protein hydration water: a molecular dynamics study

We present an analysis based on molecular dynamics simulations of water single particle and collective density fluctuations in a protein crystal at 150 and 300 K. For the collective dynamics, the calculations predict the existence of two sound modes. The first one around 35 meV is highly dispersive and the second one around 9 meV is weakly dispersive in the k range studied here (0.5相似文献   

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT ₁ process and double quantum filtered (DQF) spectral analyses.²H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.     

Study of vibrational relaxation and molecular reorientation in liquids: a Raman spectroscopic study

The vibrational relaxation and molecular reorientational processes in liquids have been studied in some carbonyl‐containing molecules using the acetonitrile (ACN) solvent. The vibrational and reorientational processes have been studied in correspondence with correlation times. The screening effect due to dielectric has been studied and the Onsager reaction field model has been tested. The study shows that repulsive types of intermolecular forces play an important role in complex systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Chain length effect on dynamical structure of poly (vinyl pyrrolidone)-polar solvent mixtures in dilute solution of dioxane studied by microwave dielectric relaxation measurement

Dielectric relaxation study of the binary mixtures of poly(vinyl pyrrolidone) (PVP) (Mw=24000, 40000 and 360000 g mol⁻¹) with ethyl alcohol (EA) and poly(ethylene glycol)s (PEGs) (Mw=200 and 400 g mol⁻¹) in dilute solutions of dioxane were carried out at 10.1 GHz and 35°C. The relaxation time of PVP-EA mixtures was interpreted by the consideration of a wait-and-switch model in the local structure of self-associated ethyl alcohol molecules and also the PVP chain length as a geometric constraint for the reorientational motion of ethyl alcohol molecules. The formation of complexes and effect of PVP chain length on the molecular dynamics, chain flexibility and stretching of PEG molecules in PVP-PEG mixtures were explored from the comparative values of dielectric relaxation time. Further, relaxation time values in dioxane and benzene solvent confirm the viscosity independent molecular dynamics in PVP-EA mixtures but the values vary significantly with the non-polar solvent environment.