Solvation dynamics dielectric relaxation

Loring R F, Van Y J and Mukamel S 1987 Time-resolved fluorescence and hole-burning line shapes of solvated molecules longitudinal dielectric relaxation and vibrational dynamics J. Chem. Phys. 87 5840-57... 

The observation of slow, confined water motion in AOT reverse micelles is also supported by measured dielectric relaxation of the water pool. Using terahertz time-domain spectroscopy, the dielectric properties of water in the reverse micelles have been investigated by Mittleman et al. [36]. They found that both the time scale and amplitude of the relaxation was smaller than those of bulk water. They attributed these results to the reduction of long-range collective motion due to the confinement of the water in the nanometer-sized micelles. These results suggested that free water motion in the reverse micelles are not equivalent to bulk solvation dynamics. 

To understand solvation dynamics, it is necessary to recall some aspects of dielectric relaxation in the framework of the simple continuum model, which treats the solvent as a uniform dielectric medium with exponential dielectric... 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

During the last two decades, studies on ion solvation and electrolyte solutions have made remarkable progress by the interplay of experiments and theories. Experimentally, X-ray and neutron diffraction methods and sophisticated EXAFS, IR, Raman, NMR and dielectric relaxation spectroscopies have been used successfully to obtain structural and/or dynamic information about ion-solvent and ion-ion interactions. Theoretically, microscopic or molecular approaches to the study of ion solvation and electrolyte solutions were made by Monte Carlo and molecular dynamics calculations/simulations, as well as by improved statistical mechanics treatments. Some topics that are essential to this book, are included in this chapter. For more details of recent progress, see Ref. [1]. 

Comparison of the evolution of the transient absorption in pure water with the indole solution demonstrates that the dynamics of the generated elections depends on the donor molecule. The initial evolution in pure water is similar to that in indole (350 50 fs), but shows an additional contribution on a timescale of 1 - 2 ps (Fig. 2b)). The formation of electrons stemming from indole is similar to the ionization and solvation process in pure water, but the time constant corresponding to the dielectric relaxation is missing. It indicates that the electron is not completely separated from the indole cation and the interaction with the parent molecule disables the dielectric relaxation that occurs in pure water on the timescale of 1 - 2 ps. 

An Evaluation of the Debye-Onsager Model. The simplest treatment for solvation dynamics is the Debye-Onsager model which we reviewed in Section II.A. It assumes that the solvent (i) is well modeled as a uniform dielectric continuum and (ii) has a single relaxation time (i.e., the solvent is a Debye solvent ) td (Eq. (18)). The model predicts that C(t) should be a single... 

Figure 3.21 MD simulation results for (a) wavevector-dependent dielectric relaxation and (b) solvation dynamics in acetonitrile at room temperature. The charge density TCF qq(k, t) is separated into single-molecule <3>qq(k, f) and pair q(/c, t) contributions. The results for

K. Ando and S. Kato, Dielectric relaxation dynamics of water and methanol solutions associated with the ionization of /V,/V-dimcltiylanilinc theoretical analyses, J. Chem. Phys., 95 (1991) 5966-82 D. K. Phelps, M. J. Weaver and B. M. Ladanyi, Solvent dynamic effects in electron transfer molecular dynamics simulations of reactions in methanol, Chem. Phys., 176 (1993) 575-88 M. S. Skaf and B. M. Ladanyi, Molecular dynamics simulation of solvation dynamics in methanol-water mixtures, J. Phys. Chem., 100 (1996) 18258-68 D. Aheme, V. Tran and B. J. Schwartz, Nonlinear, nonpolar solvation dynamics in water the roles of elec-trostriction and solvent translation in the breakdown of linear response, J. Phys. Chem. B, 104 (2000) 5382-94. 

B. Bagchi and A. Chandra, Polarization relaxation, dielectric-dispersion, and solvation dynamics in dense dipolar liquid, J. Chem. Phys., 90 (1989) 7338 -5 A. Chandra and B. Bagchi, Effects of molecular size in solvation dynamics, J. Phys. Chem., 94 (1990) 1874—6. 

Nandi N, Bhattacharyya K, Bagchi B. Dielectric relaxation and solvation dynamics of water in complex chemical and biological systems. Chem. Rev. 2000 100 2013-2045. 

The Interplay of Dielectric and Mechanical Relaxation in Solvation Dynamics... 

While the difference in the upwards and downwards solvent responses presented in Figure 3 is striking, this is not the first time that variations in solvation dynamics for the same solvent have been observed. Experimental studies have shown differences in solvation response for different probe molecules in the same solvent. This is a direct indication that probe molecules which have different excited state charge distributions and different mechanical interactions with the solvent produce differing relaxation dynamics. Computer simulations have also observed differing solvation dynamics for the forward and reverse transitions of the sudden appearance of charge, indicative of a solute-dependent solvent response. Moreover, theoretical work has shown that dielectric solvation dynamics is sensitive to the shape of a solute, and that solute size is intimately connected to viscoelastic relaxation. It is these effects which are manifest in the... 

Simulations of solvation dynamics following charge transfer at the water liquid/vapor interface[53,80] have shown that the solvent relaxation rate is quite close to that in bulk water, even though one might expect (based on the reduced interfacial dielectric constant and simple continuum model arguments) to have a significantly slower relaxation rate. The reason for this behavior is that the interface is deformed and the ion is able to keep its first solvation shell nearly intact. Since a major part of the solvation dynamics is due to the reorientation of first shell solvent dipoles, the rate relative to the bulk is not altered by much. 

The continuum dielectric theory used above is a linear response theory, as expressed by the linear relation between the perturbation T> and the response , Eq. (15.1b). Thus, our treatment of solvation dynamics was done within a linear response framework. Linear response theory of solvation dynamics may be cast in a general form that does not depend on the model used for the dielectric environment and can therefore be applied also in molecular (as opposed to continuum) level theories. Here we derive this general formalism. For simplicity we disregard the fast electronic response of the solvent and focus on the observed nuclear dielectric relaxation. 

The third part of this text focuses on several important dynamical processes in condensed phase molecular systems. These are vibrational relaxation (Chapter 13), Chemical reactions in the barrier controlled and diffusion controlled regimes (Chapter 14), solvation dynamics in dielectric environments (Chapter 15), electron transfer in bulk (Chapter 16), and interfacial (Chapter 17) systems and spectroscopy (Chapter 18). These subjects pertain to theoretical and experimental developments of the last half century some such as single molecule spectroscopy and molecular conduction—of the last decade. 

For most polar liquids, (e /Eo) < 1. As a result, the solvation time, Xg, is shorter than the dielectric relaxation time, Td- Since water is by far the most important solvent for biological systems, in the next section we will discuss some recent results on the solvation dynamics and dielectric relaxation of water. 

To explain the bimodal dielectric relaxation in aqueous protein solutions, Nandi and Bagchi proposed a similar dynamic exchange between the bound and the free water molecules [21]. The bound water molecules are those that are attached to the biomolecule by a strong hydrogen bond. Their rotation is coupled with that of the biomolecule. The water molecules, beyond the solvation shell of the proteins, behave as free water molecules. The free water molecules rotate freely and contribute to the dielectric relaxation process, whereas the rotation of the doubly hydrogen-bonded bound water molecules is coupled with that of the biomolecule and hence is much slower. The free and bound water molecules are in a process of constant dynamic exchange. The associated equilibrium constant, K, can be written as... 

In Section IV.A we will show that the substantially slower dielectric relaxation times of water in organized assemblies markedly slow down the solvation dynamics, in some cases by four orders of magnitude, compared to bulk water. 

As discussed in Section II.A, the solvation dynamics indicates the mobility of the solvent molecules in a medium. Among all solvents, study of relaxation properties of water in organized media is most important because the water molecules present in the confined environments control the structure, dynamics, and reactivity of biological systems. The results of the dielectric relaxation experiments, discussed in Section 11. A, suggest that the water molecules present in biological environments are substantially slower than those in ordinary water. We will now show that the solvation dynamics studies also indicate similar trends and reveal many other finer details. 

The role of vibrational relaxation and solvation dynamics can be probed most effectively by fluorescence experiments, which are both time- and frequency-resolved,66-68 as indicated at the end of Sec. V. We have recently developed a theory for fluorescence of polar molecules in polar solvents.68 The solvaion dynamics is related to the solvent dielectric function e(co) by introducing a solvation coordinate. When (ai) has a Lorentzian dependence on frequency (the Debye model), the broadening is described by the stochastic model [Eqs. (113)], where the parameters A and A may be related to molecular... 

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