Volume 110, Issue 3, 15 January 1999
 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


Dynamical structure of water in alkali halide aqueous solutions
View Description Hide DescriptionThe lowfrequency Raman spectra of alkali halide AX (A=Na, K, Rb; X=Cl, Br) aqueous solutions with various temperatures and concentrations are investigated. We have found that at the supercooled temperature region the relaxation time of the KX and RbX aqueous solutions becomes shorter than that of pure water. Since in these solutions the viscosity decreases with increasing concentration, this result suggests that the relaxation time of the electrolytic solutions in the Raman region is strongly correlated with the viscosity. Moreover, through comparing the spectral profiles of the relaxation mode among AX aqueous solutions we demonstrate that the pattern of the concentration dependence of the relaxation mode parameters such as the activation energy changes between NaX and (KX, RbX) aqueous solutions.

A theoretical study of outersphere electron transfer reactions in electrolyte solutions
View Description Hide DescriptionA microscopic theory of outersphere electron transferreactions in electrolyte solutions is presented. Both static and dynamic effects of solvent and ion atmosphere on rates of electron transfer are calculated by employing molecular models. The donor–acceptor system is composed of two spheres and the electrolyte solution is composed of dipolar solvent molecules and ions which are treated at the same molecular level. A microscopic expression for the free energy of activation is derived by using density functional theory. The dynamic effects are calculated by using a molecular hydrodynamictheory which properly includes finite wave vector modes of relaxation of solvent and ion atmosphere. Explicit numerical results are presented for the activation free energy and the rate constant of electron transfer in solutions of varying ion concentration. It is found that ion atmosphere can make an important contribution to the activation free energy at finite ion concentration although the net increase in the activation energy is not very significant for the solutions studied in this work. This happens because, with increase of ion concentration, the ion atmosphere contribution to the total activation free energy increases, whereas the solvent contribution shows a decreasing trend. The solvent behaves as an effective less polar medium due to screening by ions and, therefore, its contribution to the activation free energy decreases as ion concentration is increased.

Phase behavior of the restricted primitive model and squarewell fluids from Monte Carlo simulations in the grand canonical ensemble
View Description Hide DescriptionCoexistence curves of squarewell fluids with variable interaction width and of the restricted primitive model for ionic solutions have been investigated by means of grand canonical Monte Carlo simulations aided by histogram reweighting and multicanonical sampling techniques. It is demonstrated that this approach results in efficient data collection. The shape of the coexistence curve of the squarewell fluid with short potential range is nearly cubic. In contrast, for a system with a longer potential range, the coexistence curve closely resembles a parabola, except near the critical point. The critical compressibility factor for the squarewell fluids increases with increasing range. The critical behavior of the restricted primitive model was found to be consistent with the Ising universality class. The critical temperature was obtained as and the critical density both in reduced units. The critical temperature estimate is consistent with the recent calculation of Caillol et al. [J. Chem. Phys. 107, 1565 (1997)] on a hypersphere, while the critical density is slightly lower. Other previous simulations have overestimated the critical temperature of this ionic fluid due to their failure to account for finitesize effects in the critical region. The critical compressibility factor for the ionic fluid was obtained as an order of magnitude lower than for nonionic fluids.

Numerical calculation of the rate of homogeneous gas–liquid nucleation in a LennardJones system
View Description Hide DescriptionWe report a computersimulation study of the absolute rate of homogeneous gas–liquid nucleation in a LennardJones system. The height of the barrier has been computed using umbrella sampling, whereas the kinetic prefactor is calculated using molecular dynamics simulations. The simulations show that the nucleation process is highly diffusive. We find that the kinetic prefactor is a factor of 10 larger than predicted by classical nucleationtheory.

Molecular radiative transport. III. Experimental intensity decays
View Description Hide DescriptionA critical experimental test of a previously developed theory of molecular radiative transport is described. It is concluded that the theory gives an accurate description of the effect of radiative transport on fluorescence observables. The numerical coefficients of the fluorescence decay are computed from a Monte Carlo integration procedure that mimics the photon trajectories inside a realistic sample cell, and is carried out only using known molecular and geometrical parameters. The predicted parameters are confronted with the experimental observables accessible in a typical singlephoton timing experiment, rhodamine 101 in ethanol being the system studied. The theoretical predictions quantitatively describe the effects of concentration and excitation and emission wavelengths experimentally observed in optical dense nondiffusing media for the two most common geometric arrangements: frontface and rightangle detection. It is shown that radiative transport leads to spatially heterogeneous fluorescence kinetics, as a direct consequence of the existence of a spatial distribution function of electronic excitation inside the sample cell. The agreement between theory and experimental results is good, with the average decay times predicted within ≃3% accuracy for frontface detection.

Reaction field effects on nitrogen shielding
View Description Hide DescriptionSolvation effects on nuclear magnetic shielding at nitrogen in the and molecules are studied using electronic structure calculations.Reaction field theory together with a dielectric continuum is invoked to describe solutesolvent electrostatic interactions, using a solute electronic isodensity contour to define the cavity surface. With the common approach that treats only surface polarizationeffects, it is found that the nitrogen shielding is very sensitive to the cavity size and usually provides results in reasonable agreement with experiment only for rather large cavity sizes corresponding to contour values of ∼0.000 25–0.0005 a.u. With a more complete treatment that also includes volume polarizationeffects arising from penetration of the solute charge density outside the cavity, the nitrogen shielding becomes much less sensitive to the cavity size and reasonable agreement with experiment can be obtained with contour values ∼0.001–0.003 a.u., which is more consistent with previous findings on the optimum cavity size for determination of free energies of solvation.

Deterministic identifiability of twostate excitedstate models with transients: Recovery of deactivation rate constants
View Description Hide DescriptionA deterministic identifiability analysis of kinetic models of twostate excitedstate processes in the presence of transient effects is performed to establish the conditions for the unique determination of the excitedstate species deactivation rate constants with no assumption regarding the kinetics of interconversion in the excited state. It is necessary that decays of at least one excitedstate species can be monitored separately to uniquely determine the excitedstate species deactivation rate constants. This conclusion holds for reversible and irreversible intermolecular as well as intramolecular excitedstate processes in the presence of transient effects. Sufficient conditions for the recovery of the deactivation rate constants are established for several cases of practical relevance.

Deterministic identifiability of photophysical kinetic models with transients via the method of similarity transformation
View Description Hide DescriptionThe method of similarity transformation is shown to be applicable to the problem of deterministic identifiability of photophysical kinetic models with transients. A twostate excitedstate model of irreversible intermolecular association with transients is demonstrated to be identifiable from two decay traces at two observation wavelengths and one coreactant concentration, when either only the species that associates with the coreactant is directly excited or the excited species spectra are separated such that each species can be monitored individually. When transients are absent, two decays at one coreactant concentration do not guarantee the recovery of the model parameters, which indicates that transient effects change identifiability criteria compared to the case with timeinvariant rate constants.

Phase behavior of ionic solutions: Comparison of the primitive and explicit solvent models
View Description Hide DescriptionGrand canonical Monte Carlo calculations are used to investigate the demixing transition in model ionic solutions where the solvent is explicitly included. Charged hard sphere ions in hard sphere, dipolar hard sphere and quadrupolar hard sphere solvents are considered and the results are compared with the primitive (continuum solvent) model. For all solvents considered, it is found that the demixing transition is in the same general region of the phase diagram and is roughly described by liquidvapor equilibrium in the primitive model. However, details such as the precise location of the critical point and the width of the unstable region depend upon the exact nature of the solvent.

Investigation of the shorttime photodissociation dynamics of trans1bromo2iodoethane in the A band absorption
View Description Hide DescriptionWe have obtained resonance Raman spectra and absolute Raman cross section measurements at five excitation wavelengths within the Aband absorption for 1bromo2iodoethane in cyclohexane solution. The resonance Raman spectra have most of their intensity in the fundamentals, overtones, and combination bands of six Franck–Condon active vibrational modes; the nominal CCI bend, C–I stretch, C–Br stretch, C–C stretch, wag with the Br atom attached to the group, and wag with the I atom attached to the group. The resonance Raman intensities and Aband absorption spectrum were simulated using a simple model and timedependent wave packet calculations. The simulation results and normal mode descriptions were used to find the shorttime photodissociation dynamics in terms of internal coordinate displacements. The Aband shorttime photodissociation dynamics for trans1bromo2iodoethane show that the C–I, C–Br, and C–C bonds as well as the CCI, CCBr, HCC, ICH, and BrCH angles have significant changes during the initial stages of the photodissociation reaction. This indicates the photodissociation reaction has a large degree of multidimensional character and suggests that the bromoethyl photofragment receives substantial internal excitation in so far as the shorttime photodissociation dynamics determines the energy partitioning. Comparison of our results for 1bromo2iodoethane with the Aband shorttime dynamics of iodoethane, 1chloro2iodoethane, and 1,2diiodoethane and the trends observed for their Aband absorption spectra suggest that both the C–I and C–Br bonds experience a noticeable amount of photoexcitation.

Dynamics and structure of solid hexafluoroethane
View Description Hide DescriptionNeutron Compton profiles were measured of atomic momentum distributions in solid hexafluoroethane at saturated vapor pressure from 20 to 170 K, and of the liquid phase at 180 K. There are conflicting reports in the literature, from different kinds of measurements, about the nature and extent of solid phases of Present neutron diffraction and thermal analysis shows that there is a clear crystallographic transformation near 103 K from a highertemperature bodycenteredcubic (bcc) phase which has considerable orientational disorder to a lowertemperature phase of different symmetry, which has relatively little disorder. The C and F atomic momentum profiles are found to have only small dependencies upon temperature and upon condensed phase. They are analyzed in terms of a dominant contribution from intramolecular vibrations plus contributions from translational and rotational motions of molecules. The nature of rotational contributions is of special interest, because of questions about the extent of static versus dynamic disorder in this kind of crystal. Brief comparisons to related solids such as are made.

High pressure studies of the Kramers turnover behavior for the excitedstate isomerization of 2alkenylanthracene in alkane
View Description Hide DescriptionThe isomerization rate of 2(2propenyl)anthracene (22PA) in the lowest excited singlet state was investigated in supercritical fluids (SCF); ethane and as well as in a series of liquid nalkanes at high pressures. Combining the present results with our preceding ones which had been examined in compressed liquid nalkane solvents [J. Phys. Chem. A 101, 2240 (1997)] we completed the curve of forward isomerizationrate constant against solventviscosity (η) over the entire friction range. We also discuss the dependence of on the inverse of solventselfdiffusion coefficient which is used as a measure of solvent collisional frequency. The Kramers turnover behavior of 22PA thus observed was compared with our previous result of 2vinylanthracene [J. Chem. Phys. 103, 5548 (1995)]. The values of both isomerization reactions around the turnover viscosity region never attains those which are predicted by transition state theory.

Shape and size of simple cations in aqueous solutions: A theoretical reexamination of the hydrated ion via computer simulations
View Description Hide DescriptionThe simplest representation of monoatomic cations in aqueous solutions by means of a sphere with a radius chosen on the basis of a welldefined property (that of the bare ion or its hydrate) is reexamined considering classical molecular dynamics simulations. Two charged sphere–water interaction potentials were employed to mimic the bare and hydrated cation in a sample of 512 water molecules. Shortrange interactions of trivalent cations were described by LennardJones potentials which were fitted from ab initio calculations. Five statistically independent runs of 150 ps for each of the trivalent spheres in water were carried out in the microcanonical ensemble. A comparison of structural and dynamical properties of these simple ion models in solution with those of a system containing the hydrate is made to get insight into the size and shape definition of simple ions in water, especially those that are highly charged. Advantages and shortcomings of using simple spherical approaches are discussed on the basis of reference calculations performed with a more rigorous hydrated ion model [J. Phys. Chem. B 102, 3272 (1998)]. The importance of nonspherical shape for the hydrate of highly charged ions is stressed and it is paradoxically shown that when spherical shape is retained, the big sphere representing the hydrate leads to results of ionic solution worse than those obtained with the small sphere. A lowcost method to generate hydrated ion–water interaction potentials taking into account the shape of the ionic aggregate is proposed.

Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane II. Density dependence in supercritical carbon dioxide
View Description Hide DescriptionWe have measured the isotropic Raman CH stretching spectrum of cyclohexane in supercritical at 49.7 °C and in liquid at room temperature over a range of densities from to where the critical number density for is 6.4 nm^{−3}. The axial and equatorial CH stretching bands in the spectrum shift to lower frequencies and broaden with increasing density. As was the case in an earlier study of cyclohexane in liquid solvents [G. J. Remar and R. A. MacPhail, J. Chem. Phys. 103, 4381 (1995)], the “perturbed hardfluid model” of BenAmotz and Herschbach provides a satisfyingly consistent description of the observed shifts in terms of competing contributions from repulsive and attractive solute–solvent forces along the CH bond. In particular, when the repulsive contribution to the shift is calculated according to the prescription developed in the liquid solution study, the attractive contribution is found to scale linearly with the density and with the polarizability derivative of the CH bond, as predicted by the model. The ratio of the equatorial to axial linewidths has a densityindependent value of 1.2, nearly the same value found for the liquid solutions and numerically equivalent to the ratio of polarizability derivatives for the CH bonds. This equivalence is consistent with Schweizer and Chandler’s theoretical result for the width of a band that is inhomogeneously broadened by attractive force fluctuations, but the density dependence is not; their result would predict a nonlinear density dependence with a maximum near whereas the observed linewidths show a nearly linear dependence on density. Neither the frequency shifts nor the linewidths show any clear evidence for a “local solvent density enhancement” that would be predicted for this mixture near the critical point. In the accompanying paper, Frankland and Maroncelli describe moleculardynamics simulations of cyclohexane in supercritical that reproduce the observed linewidths nearly quantitatively. They show convincing evidence that the linewidths are dominated by binary, collisional interactions between the hydrogen and the solvent, and they discuss the apparent absence of a density enhancement.

Moleculardynamics simulations of solvent effects on the C–H stretching Raman bands of cyclohexane in supercritical and liquid solvents
View Description Hide DescriptionMoleculardynamics simulations are used to elucidate the molecular basis for the solvent effects on the isolated C–H stretching bands observed in the Raman spectrum of cyclohexane The main focus is on modeling the density dependence of the spectrum in supercritical recently reported by Pan, McDonald, and MacPhail [J. Chem. Phys. 110, 1677 (1999)], but several liquidsolvents and have also been examined. The frequency shifts and line shapes of the Raman spectrum are simulated using a rigid solute and standard line shape theory in the limit of pure dephasing. Three models for the vibration–solvent coupling are considered. The simplest model, which is based on groundstate forces alone, provides a surprisingly good representation of the density dependence of the linewidths–line shapes but predicts the wrong sign for the gastosolution frequency shifts. This failure is due to the neglect of changes in bond polarizability upon vibrational excitation. Allowing for this polarizability difference via a semiempirical approach provides an accurate description of both the linewidths and frequency shifts with a physically reasonable vibrational difference potential. Interpretation of the instantaneous frequency shifts simulated with this model leads to the following general conclusions concerning the solvent effect on these spectra: (i) The relatively small gastosolution frequency shifts observed in experiment are the result of the near cancellation of much larger positive and negative contributions from repulsive and attractive interactions. (ii) Fluctuations in the instantaneous frequency are sufficiently fast (correlation times ∼100 fs) that the spectra are homogeneously broadened in all solvents examined. (iii) The dynamics of the solvent–solute interactions that determine the Raman line shapes are quite well described by an isolated binary collision (“IBC”) type picture. (iv) The simplicity of the dynamics, and the success of this IBC description, is due at least in part to the special, localized character of these isolated C–H stretching modes. (v) The linear density dependence of the linewidths observed in supercritical reflects the modest extent of local density augmentation in the cyclohexane– system.

Semiclassical simulations of multidimensional Raman echoes
View Description Hide DescriptionA hightemperature and a weaknonlinearity (lowtemperature) semiclassical expansion are developed for computing twodimensional vibrational Raman spectroscopies, and applied to an exactly solvable Brownianoscillator model. The origin of photon echoes is discussed using phasespacewavepackets. Impulsive and semiimpulsive echoes are shown to satisfy different phasematching conditions, and are generated in different directions.

Constrained BoltzmannGibbs measures and effective potential for glasses in hypernetted chain approximation and numerical simulations
View Description Hide DescriptionBy means of an effective potential associated with a constrained equilibrium measure and apt to study frozen systems, we investigate glassy freezing in simple liquids in the hypernetted chain (HNC) approximation. Unlike other classical approximations of liquid theory, freezing is naturally embedded in the HNC approximation. We give a detailed description of the freezing transition that is analogous to the one given in a large class of meanfield long range spin glass. We compare our findings with Monte Carlo simulations of the same system and conclude that many of the qualitative features of the transition are captured by the approximated theory.
