Dynamics Of Molecular Liquids Research Articles

Using feedback for coherent control of quantum systems

A longstanding goal in chemical physics has been the control of atoms and molecules using coherent light fields. This paper provides a brief overview of the field and discusses experiments that use a programmable pulse shaper to control the quantum state of electronic wavepackets in Rydberg atoms and electronic and nuclear dynamics in molecular liquids. The shape of Rydberg wavepackets was controlled by using tailored ultrafast pulses to excite a beam of caesium atoms. The quantum state of these atoms was measured using holographic techniques borrowed from optics. The experiments with molecular liquids involved the construction of an automated learning machine. A genetic algorithm directed the choice of shaped pulses which interacted with the molecular system inside a learning control loop. Analysis of successful pulse shapes that were found by using the genetic algorithm yield insight into the systems being controlled.

A time dependent density functional method for the interaction site model

Abstract A new method is developed to study nonlinear dynamics of molecular liquids. It is a time dependent density functional method for the interaction site model. The nonlinear Langevin theory provides the formulation of the method. An approximation is introduced to calculate the reversible term.

Semischematic model for the center-of-mass dynamics in supercooled molecular liquids

We introduce a semi-schematic mode-coupling model to describe the slow dynamics in molecular liquids, retaining explicitly only the description of the center of mass degrees of freedom. Angular degrees of freedom are condensed in a q-vector independent coupling parameter. We compare the time and q-dependence of the density fluctuation correlators with numerical data from a 250 ns long molecular dynamics simulation. Notwithstanding the choice of a network-forming liquid as a model for comparing theory and simulation, the model describes the main static and dynamic features of the relaxation in a broad q-vector range.

Friction Coefficients and Correlation Times for Anisotropic Rotational Diffusion of Molecules in Liquids Obtained from Hydrodynamic Models and 13C Relaxation Data

A hydrodynamic model is presented to describe fully anisotropic rotational dynamics of asymmetric ellipsoids in liquids. The model by Gierer and Wirtz is extended to deal with anisotropic rotations and combined with the model by Youngren and Acrivos for slip boundary conditions. The combined model is applied to obtain the reorientational correlation times for the hydrocarbon 5,6-dimethyl-1,2,3,4-tetrahydro-1,4-methanonaphthalene in different solvents. The results from these calculations are compared to experimental values from 13C nuclear relaxation data. Calculated and experimental results agree very well, and the new combined model is believed to represent an important contribution for describing rotational molecular dynamics in liquids.

Subpicosecond two-dimensional Raman spectroscopy applying broadband nanosecond laser radiation

We report first experimental and theoretical results applying non-transform-limited nanosecond laser pulses for realizing a fifth order Raman effect in nitrobenzene. Similar to femtosecond experiments this experiment enables to distinguish principally the action of rapid and slow modulation interactions on the molecular dynamics in liquids. A fast decay time of 120 fs and a slow decay time of approximately 1.5 ps are deduced from the experiments. They are assigned to nuclear motions and the dephasing time of the ν = 1345 cm −1 vibration of nitrobenzene, respectively.

Classical description of activated conformational processes in molecular systems coupled to solvent degrees of freedom

Extended Fokker-Planck (FP) equations are generalized stochastic equations which describe the evolution of a set of coordinates, loosely referred to as solute, coupled with a relevant set of solvent variables. They are useful for the analysis of molecular dynamics in liquids, when a time-scale separation between probe motions and relaxation times of interactions with surroundings particles cannot be performed, because of persistence of slowly fluctuating components. In this article, we focus attention on a model system, made up of an angular coordinate and its conjugate momentum, submitted to a bistable potential, and coupled to a dissipative harmonic mode, to investigate the influence of polar solvents on reactive dynamics. The results are appropriate to describe dielectric effects, solvent-controlled conformational changes involving charge transfer which occur in photophysical processes, and the dynamic Stokes shifts observed in time-resolved fluorescence experiments. © 1996 John Wiley & Sons, Inc.

Motional diminishing of optical activity: a novel method for studying molecular dynamics in liquids and plastic crystals

Molecular dynamics calculations and optical spectroscopy measurements of weakly active infrared modes are reported. The results are qualitatively understood in terms of the “motional diminishing” of IR lines, a process analogous to the motional narrowing of a nuclear magnetic resonance (NMR) signal. In molecular solids or liquids where the appropriate intramolecular resonances are observable, motional diminishing can be used to study the fluctuations of the intermolecular interactions having time scales of 1 to 100 ps.

Water hydrogen-bond dynamics close to hydrophobic and hydrophilic groups

We discuss the analysis of molecular dynamics calculations where we simulate the kinetics of the breaking and forming of hydrogen bonds in distinctly different environments: liquid water and concentrated aqueous solution of DMSO. In our analysis, we consider reactive flux correlation functions computed for a variety of specific conditions and identify rate constants for bond making and breaking in terms of the liquid's molecular dynamics. According to the proposed mechanism of hydrogen-bond kinetics, hydrogen bonds most frequently break during a process of switching allegiance with a newly formed bond replacing the broken one. Simulations reveal bond dynamics in the mixture to be significantly slower than in pure water. This is interpreted in terms of reduced likelihood of fluctuations in the hydrogen-bond network, related to the presence of free hydrogen-bonding sites, which participate in the process of switching allegiances. Implications of our analysis for future experimental work are briefly discussed.

Molecular dynamics in liquids: spin-lattice relaxation of nitroxide spin labels.

Time domain pulsed saturation recovery and electron-electron double resonance spectroscopies were used to measure the spin-lattice relaxation rates of the electron and the nitrogen nucleus in nitroxide spin labels in liquids. The rotational correlation time range covered is from picoseconds to milliseconds. These rates are quantitatively explained by isotropic rotational Brownian dynamics, which modulate the interactions between the electron spin and the molecular angular momentum; the nitrogen and electron spins; and the solvent protons with both the electron and the nitrogen spins. This solves a 20-year-old problem that has limited scientific applications of nitroxides.

Intermolecular translational-rotational contribution to nuclear-spin relaxation in liquids.

The intermolecular contribution to spin relaxation by translational-rotational diffusion dynamics in molecular liquids is calculated for arbitrary relative time scales of the two stochastic processes. The resulting relaxation times are functions of variables that are proportional to ratios between rotational and translational diffusion constants, in substantial generalization of earlier theoretical approaches. In the first part the general treatment for symmetric-top molecules is given with a reduction to quadratures and a simpler version for the spherical case

Dynamics of molecular liquids: A comparison of different theories with application to wave vector dependent dielectric relaxation and ion solvation

Explicit expressions are given for the Fourier–Laplace transform of the van Hove function for fluids of particles interacting through orientation dependent pair potentials. The expressions are obtained from the Kerr approximation together with diffusion models for the self part of the van Hove function and apply at all frequencies and wavelengths. Both spherical (scalar diffusion constants) and nonspherical (tensorial diffusion constants) diffusors are considered and the theory is applied to k-dependent dielectric relaxation and ion solvation dynamics. The required equilibrium structure is obtained using the hypernetted chain (HNC) or reference hypernetted chain (RHNC) theories and Kerr/RHNC(HNC) results are given for fluids of dipolar hard spheres, dipolar hard ellipsoids, and water-like particles. Comparisons are made with earlier work based upon coupling equilibrium theories, such as the mean spherical approximation (MSA) for dipolar hard spheres with a dynamical equation of the Smoluchowski–Vlasov (SV) type. It is shown that for spherical diffusors with the self part of the van Hove function treated at the Fick–Debye level, the SV and Kerr equations are in fact equivalent. However, even for spherical diffusors, the results obtained can differ substantially depending upon the equilibrium theory and/or the molecular model employed. For nonspherical diffusors, the anisotropy of the rotational diffusion tensor can also be an important parameter influencing the k-dependent dielectric relaxation.

Molecular dynamics in liquids from femtosecond time-resolved impulsive stimulated scattering

Molecular motions in simple liquids are observed on the femtosecond time scale by means of impulsive simulated scattering. In carbon disulphide and benzene liquids, weakly oscillatory time-dependent responses are observed. These are interpreted in terms of molecular orientational motion which at short times is vibrational (i.e. librational) in character due to intermolecular interactions. In CS/sub 2/, the dephasing of the oscillations is primarily inhomogeneous. Temperature-dependent values of the configuration-averaged librational frequency and the extent of its inhomogeneity are determined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Intramolecular and intermolecular dynamics in molecular liquids through femtosecond time-resolved impulsive stimulated scattering

Recent femtosecond time-resolved impulsive stimulated scattering experimental results from molecular liquids are reviewed and new data are presented. Three main areas are discussed. First, the inertial molecular motions of simple liquids (CS 2 , benzene) are observed. The results can be interpreted in terms of intermolecular librations, which in some cases are clearly manifest in the form of oscillatory time-dependent responses. The temperature-dependent intermolecular librational frequency and dephasing dynamics in CS 2 liquid are determined. The data suggest that dephasing is predominantly inhomogeneous, and permit estimation of the extent of inhomogeneity in the librational frequency. Second, intramolecular vibrational oscillations are monitored directly in real time. The data are influenceed by both molecular vibrational and orientational dynamics. These experiments make possible time-resolved spectroscopy of vibrationally distorted molecules in well defined nonequilibrium configurations. Finally, the general occurrence of impulsive stimulated scat I. Etude des mouvements moleculaires inertiels de liquides simples (CS 2 , benzene); interpretation par des librations intermoleculaires qui, dans quelques cas, se manifestent par des reponses oscillatoires dependant du temps; determination de la variation temporelle de la frequence de libration et de la dynamique du dephasage pour CS 2 liquide, le dephasage etant principalement inhomogene; estimation du degre d'inhomogeneite du potentiel de libration. II. Etude des vibrations intramoleculaires en temps reel; influence des dynamiques moleculaires de vibrations et d'orientation; application a la spectroscopie resolue dans le temps de molecules vibrationnellement deformees dans des configurations bien definies hors d'equilibre. III. Discussion de l'apparition generalisee de la diffusion stimulee impulsionnelle chaque fois qu'une impulsion ultracourte traverse un milieu actif en Raman

Picosecond molecular dynamics in liquids via degenerate four-wave mixing

We report measurements of degenerate four-wave mixing in liquids to study the picosecond orientational dynamics of molecules and the transition of a probe molecule from many-particle to single-particle rotational behavior as a function of dilution. Data for CS 2 in pure and dilute alkane systems are presented, in which the molecular motion is characterized as diffusion, rather than impulsive, at times ≥ 0.9 ps, the limiting single-particle orientational correlation time in n-pentane at 300 K.

Electron-spin relaxation and molecular dynamics in liquids. I. Solvent dependence

Temperature-dependent ESR relaxation studies of PD-Tempone in a variety of solvents ranging from weakly interacting hydrocarbons to strongly interacting D2O are described. The empirical molar transition energy scale ET, which is a good measure of specific solvent polarity, is found to lead to a useful interpolation method for estimating the solvent dependence of the magnetic tensors required for accurate spin-relaxation studies. The temperature-dependent results are generally well-represented by a modified Stokes–Einstein type of η/T dependence for τR, the rotational correlation time. The ε parameter introduced by Freed and co-workers as an empirical correction to the nonsecular Debye spectral densities is found to correlate with ET, such that it is largest for the weakly interacting hydrocarbon solvents and approaches the Debye limit of ε = 1 for D2O solvent. This is discussed in terms of approximate fluctuating torque and slowly-relaxing local structure models, and the former appears more consistent with experiment. It is also pointed out that ε≳1 would result from the phenomenological feature of viscoelasticity.

A Pedestrian approach to the dynamics of molecular liquids

Abstract Some aspects of the difficulties encountered in describing irreversible phenomena by means of classical mechanics are considered.

Electron-spin relaxation and molecular dynamics in liquids. II. Density dependence

A pressure-dependent ESR relaxation study of the probe PD-tempone dissolved in toluene-d8 is described. Extensive results on rotational relaxation, in which τR(T,P) is varied over more than two orders of magnitude, are presented. These results are found to be inconsistent with a simple Stokes–Einstein type η/T behavior modified with a nonzero intercept. However, the data were successfully fit by the empirical form τR= cηβ(ρ−p̄)/T, where β is the isothermal compressibility, c is a constant, and p̄ is an (empirical) reference density whose inverse can be thought of as an ’’expanded volume’’. Thus, ρ−? is a measure of the strength of the anisotropic intermolecular interactions acting on the probe, while β−1 may be thought of as a measure of the total intermolecular interactions. These results for a solute of molecular size somewhat greater than that of the solvent molecules exhibit some differences when compared to a previous NMR study on neat toluene-d8. The ε parameter introduced by Freed and co-workers to fit their nonsecular spectral densities is found to be independent of temperature and pressure, and it is pointed out that this could be consistent with an intermolecular fluctuating torque model.

Proposals for a european collaborative project on the consistent evaluation of molecular dynamics in liquids

Proposals for a european collaborative project on the consistent evaluation of molecular dynamics in liquids

An electron paramagnetic resonance study of rotational and translational motion in solution

Electron paramagnetic resonance (E.P.R.) spectra of dilute and concentrated solutions of two nitroxide spin probes differing in size and shape have been examined in toluene and in the isotropic phase of the liquid crystal MBBA. The results are discussed in detail using the hydrodynamic theory of molecular relaxation in liquids. The feasibility of using electron spin relaxation data to test the Hynes-Kapral-Weinberg theory of molecular dynamics in liquids is also examined. It appears that electron spin relaxation results are useful for testing theories of reorientational relaxation in liquids; however, the translational diffusion constants obtained from E.P.R. linewidth studies are not accurate enough to test molecular theories of translational motion in liquids.

Power reflection spectroscopy near the brewster angle. Molecular dynamics of liquids

Power reflection spectroscopy has been investigated theoretically for potential use with dipolar liquids as a means of evaluating the molecular dynamics and interactions. Calculations have been made with and without a dynamic internal field correction. This does not affect in any way the major features of the spectra in σ and π polarisation at various values of ϕ, the incident angle. Near a Brewster angle (ϕB= tan–1Iµ1/2p, where Iµp is the high frequency permittivity), the reflectivity r develops a high frequency peak, typically in the mid-infrared. This brings the study of librational molecular dynamics within range of conventional spectrometers, provided that the problems of amplification and detection of the reflected rπ radiation can be overcome.