Srabani Roy

Ultrafast solvation dynamics in water: Isotope effects and comparison with experimental results

A detailed theoretical study of solvation dynamics in water is presented. The motivation of the present study comes from the recent experimental observation that the dynamics of solvation of an ion in water is ultrafast and the solvation time correlation function decays with a time constant of about 55 fs. The slower decay in the long time can be described by a sum of two exponentials with time constants equal to 126 and 880 fs. The molecular theory (developed earlier) predicts a time constant equal to 52 fs for the initial Gaussian decay and time constants equal to 134 and 886 fs for the two exponential components at the long time. This nearly perfect agreement is obtained by using the most detailed dynamical information available in the literature. The present study emphasizes the importance of the intermolecular vibrational band originating from the O...O stretching mode of the O–H...O units in the initial dynamics and raises several interesting questions regarding the nature of the decay of this mode....

Solvation dynamics in liquid water. A novel interplay between librational and diffusive modes

A microscopic calculation of the solvation dynamics of an ion in liquid water is presented. The calculated solvation time correlation function shows an ultrafast Gaussian decay which carries about 70%–90% of the strength followed by a biexponential decay with time constants equal to 250 fs and 1 ps. These results are in excellent agreement with the computer simulations of Maroncelli and Fleming and also with the experimental findings of Barbara and Jarzeba. In addition, we find that both the rotational librations and the intermolecular translational vibrational modes of water contribute significantly to the initial Gaussian decay.

Ultrafast underdamped solvation: Agreement between computer simulation and various theories of solvation dynamics

A theoretical analysis of the three currently popular microscopic theories of solvation dynamics, namely, the dynamic mean spherical approximation (DMSA), the molecular hydrodynamic theory (MHT), and the memory function theory (MFT) is carried out. It is shown that in the underdamped limit of momentum relaxation, all three theories lead to nearly identical results when the translational motions of both the solute ion and the solvent molecules are neglected. In this limit, the theoretical prediction is in almost perfect agreement with the computer simulation results of solvation dynamics in the model Stockmayer liquid. However, the situation changes significantly in the presence of the translational motion of the solvent molecules. In this case, DMSA breaks down but the other two theories correctly predict the acceleration of solvation in agreement with the simulation results. We find that the translational motion of a light solute ion can play an important role in its own solvation. None of the existing theories describe this aspect. A generalization of the extended hydrodynamic theory is presented which, for the first time, includes the contribution of solute motion towards its own solvation dynamics. The extended theory gives excellent agreement with the simulations where solute motion is allowed. It is further shown that in the absence of translation, the memory function theory of Fried and Mukamel can be recovered from the hydrodynamic equations if the wave vector dependent dissipative kernel in the hydrodynamic description is replaced by its long wavelength value. We suggest a convenient memory kernel which is superior to the limiting forms used in earlier descriptions. We also present an alternate, quite general, statistical mechanical expression for the time dependent solvation energy of an ion. This expression has remarkable similarity with that for the translational dielectric friction on a moving ion.

Molecular theory of ultrafast solvation in liquid acetonitrile

A microscopic calculation of solvation dynamics of an immobile solute ion in liquid acetonitrile is presented. The dynamical information necessary for this calculation is obtained from the Kerr and dielectric relaxation of the neat liquid. The calculated solvation time correlation function is in excellent agreement with both the experimental and the simulated results.

Time dependent solution of generalized Zusman model of outersphere electron transfer reactions: Applications to various experimental situations

The Zusman model of the environmental effects on the outersphere electron transfer reaction has been widely used to study solvent effects on various important electron transfer reactions. We present here a generalized treatment of the Zusman model using a powerful Green’s function technique. This generalization enables us to obtain the time dependent solution of the model for various complicated situations often encountered in experiments. In addition, the present formulation allows for a unified description of the barrierless and the high barrier reactions for both the nonadiabatic and the weakly adiabatic limits of electron transfer reactions. A merit of the present description is that one need not assume an initial equilibrium population of the reactants and therefore, this method is particularly suitable for the treatment of photoelectron transfer reactions. The following four model situations have been studied. (a) Ground state, symmetric, and asymmetric electron transfer reactions. The reactant surf...

Microscopic theory of ion solvation dynamics in liquid methanol

A microscopic study of the solvation dynamics of a rigid ion in liquid methanol is presented. The theory is in good agreement with the available computer simulation results. We further find that the collective and single particle dissipative kernels (that is, the memory functions) show rather similar dynamics at ultrashort times.

Solvation dynamics, energy distribution and trapping of a light solute ion

Abstract In the theoretical treatments of the dynamics of solvation of a newly created ion in a dipolar solvent, the self-motion of the solute is usually ignored. Recently, it has been shown that for a light ion the translational motion of the ion can significantly enhance its own rate of solvation. Therefore, solvation itself may not be the rate determining step in the equilibration. Instead, the rate determining step is the search of the low energy configuration which serves to localize the light ion. In this article a microscopic calculation of the probability distribution of the interaction energy of the nascent charge with the dipolar solvent molecules is presented in order to address this problem of solute trapping. It is found that to a good approximation, this distribution is Gaussian and the second moment of this distribution is exactly equal to the half of its own solvation energy. It is shown that this is in excellent agreement with the simulation results that are available for the model Brownian dipolar lattice and for liquid acetonitrile. If the distortion of the solvent by the ion is negligible then the same relation gives the energy distribution for the solvated ion, with the average centered at the final equilibrium solvation energy. These results are expected to be useful in understanding various chemical processes in dipolar liquids. Another interesting outcome of the present study is a simple dynamic argument that supports Onsagers “inverse snow-ball” conjecture of solvation of a light ion . A simple derivation of the semi-phenomenological relation between the solvation time correlation function and the single particle orientation, reported recently by Maroncelli et al. (J. Phys. Chem. 97 (1993) 13), is also presented.

Adiabatic and nonadiabatic outersphere electron transfer reactions in methanol: Effects of the ultrafast solvent polarization modes

Recent studies have demonstrated that the solvation dynamics in common dipolar liquids like water and acetonitrile is dominated by an initial ultrafast Gaussian component which seems to account for about 60%–70% of the total energy relaxation. Methanol, on the other hand, exhibits a rather different behavior with a much smaller amplitude of the initial Gaussian component and the relaxation is primarily caused by a much slower exponential decay. In the present study, we have investigated the role of these solvent modes on both adiabatic and nonadiabatic outersphere electron transfer reactions in methanol. It is found that the rate of the adiabatic barrier crossing is greatly enhanced due to the ultrafast solvation. For nonadiabatic reactions, the relative importance of the solvent dynamic modes increases enormously compared to the situation when only the slow, overdamped modes are included. Another important conclusion is that because of the dominance of the inertial modes, the rate of electron transfer re...

Effects of solvent polarization relaxation on nonadiabatic outersphere electron transfer reactions in ultrafast dipolar solvents

Since the important work of Efrima and Bixon [J. Chem. Phys. 70, 3531 (1979)], it is believed that solvent polarization relaxation is usually too slow (compared to the rate of electron transfer) or the amplitude of energy fluctuation too large to have any noticeable effect on the dynamics of the nonadiabatic (NA) electron transfer reactions. On the other hand, recent studies have demonstrated that solvent polarization relaxation in several common dipolar liquids can proceed at a rate much faster than that anticipated in the earlier studies. This calls for a re‐examination of the role of solvent dynamics on NA electron transfer reactions in these ultrafast solvents. In this paper, the results of such studies are presented for NA reactions in water and acetonitrile. It is found that because of ultrafast solvation, many NA reactions may lie in the dynamic region where the solvent effects are just beginning to be important. The present study further reveals the following new results. (i) In the case of high b...

Dielectric relaxation in dipolar solid rotator phases

Abstract Dielectric relaxation in orientationally disordered dipolar solids often exhibits exotic features, such as a strong Cole-Cole relaxation for simple molecular solids. However, there does not seem to exist a detailed molecular theory of such phenomena. In this article, a molecular hydrodynamic theory of dielectric relaxation in solid rotator phases, such as plastic crystals, is presented.