Volume 111, Issue 1, 01 July 1999
 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


Fifthorder nonlinear Raman processes in molecular liquids using quasicw noisy light. I. Theory
View Description Hide DescriptionFifthorder nonlinear Raman processes using broadband, incoherent light are treated for a multiply resonant, multicomponent mixture. In particular, the theoretical development of the direct and the sequential fifthorder analogs of coherent Raman scattering is presented. Of the complete formalism, only the dominant doubly Raman resonant hyperpolarizability contributions to the signal intensity are discussed in this article. Furthermore, application is made to simulate fifthorder signals from a variety of hypothetical molecular liquids. It is seen how the direct and the sequential processes can distinguish themselves in a mixture, in a neat liquid with more than one Raman coherence, and also whenever the Raman active modes are taken to be anharmonic. This theoretical treatment anticipates experimental results presented in the following paper.

Fifthorder nonlinear Raman processes in molecular liquids using quasicw noisy light. II. Experiment
View Description Hide DescriptionFifthorder analogs of coherent Raman scattering generated in a number of molecular liquids using broadband quasicw noisy light are presented. It is seen how the signal for the direct fifthorder process, which probes the dynamics of both a fundamental vibration and its overtone, is often contaminated by a sequential process, that is only capable of probing the vibrational dynamics of the fundamental. Although these two processes are virtually indistinguishable when a single Raman resonance is excited, we find that when a second Raman resonance is available within the experimental window governed by the bandwidth of the noisy light, new frequency components in the signal arise and the two competing fifthorder processes become distinguishable. These new frequency components, as well as their decay, are explained in terms of spectral filtering of the noisy light by the Raman resonances. This spectral filter analogy predicts which of the two competing processes dominates in an equimolar mixture of benzene and benzene and also in neat pyridine (each providing two available fundamentals). It is found by this new method that the sequential event is present and apparently fully dominates the direct process for all molecular liquids studied here.

Anomalous cryoprotective effectiveness of trehalose: Raman scattering evidences
View Description Hide DescriptionResults of Raman scatteringmeasurements performed on aqueous solutions of the homologous disaccharides (trehalose, maltose, and sucrose) are reported. To get some insight into the effects of disaccharides on the hydrogen bondnetwork of water, and to clarify the reasons that make trehalose the most effective in protecting organisms from dehydration and freezing, we investigate the intramolecular OH stretching mode. To carry out this study, two different approaches are employed: namely, a decomposition of the isotropic spectra into an “open” and a “closed” contribution, and a spectral stripping procedure to extract the “collective” contribution from the polarized spectra. Both procedures agree in suggesting that disaccharides promote, with a different strength, a destructuring effect on the tetrahedral Hbondnetwork of pure water. This result makes plausible the hypothesis that disaccharides obstruct the crystallization process reducing the amount of freezable water, namely destroying the network of water compatible with that of ice. What conclusively emerges from this Raman scattering study is that the greater bioprotective action of trehalose on biological structures is to be connected with its greater destructuring effect on the tetrahedral Hbondnetwork of water.

Nuclear magnetic resonance line shapes of methyllike quantum rotors in lowtemperature solids
View Description Hide DescriptionDissipative dynamics of a tunneling, methyllike rotor, whose spatial coordinate is weakly coupled to a thermal bath, are described using the reduced density matrix (RDM) approach. It is found that, owing to selection rules imposed on thermally induced transitions by the symmetrization postulate, there are two sorts of coherences between the rotor eigenstates that live long enough to be observed on the nuclear magnetic resonance(NMR) time scale. One comprises degenerate pairs of Kramers sublevels at sequential librational levels of the rotor. The other involves nearly degenerate pairs each of which engages one Kramers sublevel and the remaining sublevel, separated from the Kramers doublet by tunneling quantum. These are the coherences which are seen in the inelastic neutron scattering (INS) patterns of methyllike rotors. From the RDM equation of motion, augumented with spindependent terms relevant in the presence of an external magnetic field, the NMR line shape equation is derived. With no loss of information it can be formulated in terms of only the spin degrees of freedom. Its dissipative part includes two rate constants that describe damping of the longlived tunneling and Kramers coherences, respectively; coherent tunneling is represented in the Hamiltonian part by an apparent spinspin coupling. These rate constants are the widths of the inelastic and quasielastic lines, respectively, in the INS spectra of methyllike rotors; the apparent coupling constant is the shift of the inelastic line. This seems to be the first full exposition of the parallelism between INS and NMRimages of tunneling rotors. Rationalization of previous findings involving a rotor was achieved by use of a simple model of rotorbath couplings, combined with inferences from numerical simulations of NMR line shapes.

Solvation response of polar liquid mixtures: Waterdimethylsulfoxide
View Description Hide DescriptionThe solvation dynamics following the instantaneous creation of a positive or negative electronic charge in a previously neutral solute immersed in different waterdimethyl sulfoxide (DMSO) mixtures, spanning the entire composition range, is analyzed by molecular dynamics simulations. The solvation responses are strongly dependent on the sign of the solute charge, being considerably faster in the presence of cations for all mixtures considered. In terms of the composition dependence, the mixtures’ solvation response to the creation of the anion departs substantially from the pure solvents’, whereas for the cation, the mixtures’ responses are close to those exhibited by pure DMSO. In the case of anions, the mixture overall solvation time, defined as the time integral of the nonequilibrium response, can be as large as ten times the solvation time in pure DMSO, the slowest of the two cosolvents. The DMSO contribution to the mixtures’ solvation response may present an intriguing negative branch in the rotationaldiffusion regime which persists for times much longer than the time scales typically found in other polar liquids and mixtures. This negative portion is nearly cancelled by an equally longlasting positive contribution from water, resulting in a fastdecaying, total response curve which is typical of many polar liquid environments. This behavior is rationalized in terms of the time evolution of the first solvation shell around each type of solute.

Diffusion on fractals with interacting internal boundaries
View Description Hide DescriptionWe studied random walks interacting with the internal boundaries (borders of lacunas) of Sierpinski carpets (SC), which are infinitely ramified fractals with fractal dimensions between and The probability of steps along the borders is times the probability of steps in the bulk, where represents attraction and represents repulsion. The meansquare displacement and the mean fraction of adsorbed steps of Nstep walks were calculated using numerical simulations and extrapolations to the fractal limit considering the expected forms of finitesize corrections. The asymptotic fraction of adsorbed steps m is exactly calculated and confirms the accuracy of the simulations’ results. m varies continuously with u, and then there is no adsorptiondesorption transition. As thus we estimated the diffusion exponents and and the coefficients and in the attractive and in the repulsive cases, respectively, and the noninteracting exponent is universal and smaller than and decreases with the temperature. is universal and larger than because the attraction helps the diffusing particles to contour the obstacles of the disordered substrate. In a SC with decreases with the temperature T, for all T; in a SC with is maximum for which consequently provides a condition for fastest diffusion. We suggest the investigation of these features in real systems.

Fundamental measure theory for mixtures of parallel hard cubes. II. Phase behavior of the onecomponent fluid and of the binary mixture
View Description Hide DescriptionA previously developed fundamental measure functional [J. Chem. Phys. 107, 6379 (1997)] is used to study the phase behavior of a system of parallel hard cubes. The singlecomponent fluid exhibits a continuous transition to a solid with an anomalously large density of vacancies. The binary mixture has a demixing transition for edge–length ratios below 0.1. Freezing in this mixture reveals that at least the phase rich in large cubes always lies in the region where the uniform fluid is unstable, hence suggesting a fluid–solid phase separation. A method is developed to study very asymmetric binary mixtures by taking the limit of zero size ratio at fixed solvent fugacity. With this procedure the mixture is exactly mapped onto a onecomponent fluid of parallel adhesive hard cubes. At any density and solvent fugacity the large cubes are shown to collapse into a closepacked solid. Nevertheless the phase diagram contains a large metastability region with fluid and solid phases. Upon introduction of a slight polydispersity in the large cubes, the system shows the typical phase diagram of a fluid with an isostructural solid–solid transition (with the exception of a continuous freezing). Consequences about the phase behavior of binary mixtures of hard core particles are then drawn.

Temperature and density dependence of selfdiffusion in supercooled liquid
View Description Hide DescriptionThe selfdiffusion coefficient of liquid has been determined by computer simulation for seven densities (ρ) and eight temperatures, spanning more than two decades of in the supercooled and nearmelting normal liquid, and bracketing the isobar. SuperArrhenius behavior of vs an increase in slope with decreasing is found at 1 atm, but normal Arrhenius dependence holds along all seven different isochores, even at the highest density and lowest The superArrhenius behavior is a consequence of the variation in density at constant pressure. Physically meaningful activation energies, representative of the heights of the barriers to diffusion, depend upon ρ only, are smaller than the isobaric slope, and may be obtained by correcting it or from an Arrhenius plot at constant density. Barriers to diffusion are indeed higher at lower temperatures, but only due to the higher density. The importance of vs ρ as the “control variable” for diffusion is examined. Temperature and density play comparable roles near the melting states, and the relative importance of grows with supercooling. However that growth is due to the higher activation energy, itself controlled by density.

Thermodynamics and structural properties of the dipolar Yukawa fluid
View Description Hide DescriptionWe report computer simulations and a theoretical study of the thermodynamics and structure of a dipolar Yukawa system. A comparison between the analytical mean spherical approximation (MSA) solution, perturbation theory and Monte Carlo simulation data of pressure, internal energy and dielectric constant is given. In the perturbation theory, the MSA solution of hard core Yukawa fluid is used as a reference system. It was found that the MSA solution is reasonable only at lower dipole moments, while the perturbation theory gives good results at low and high values of dipole moment.Liquid–vapor coexistence data of dipolar Yukawa fluid are also obtained by Monte Carlo simulation and by both MSA and perturbation theory. It was found that at high dipole moments the liquid–vapor equilibrium disappears while chainlike structures appear in the low density fluid phase. The appearance of chainlike structures of dipolar Yukawa fluid is discussed in comparison with the Stockmayer fluid.
