Effects on Solvent Dynamics

There are both macroscopic effects of ions on the dynamics of electrolyte solutions relative to pure water and microscopic effects on the dynamics of the water molecules themselves. The former include the effects of ions on the fluidity of the solution as a whole, which is the reciprocal of its viscosity, and on the self-diffusion of the water. The latter pertain to the movements of individual water molecules their mutual orientations, the rate of breaking or making of hydrogen bonds, etc. Such effects have been measured experimentally, as discussed in Sects. 3.1.1-3.1.5, and are complemented by computer simulations in Sect. 3.1.6. 

Some questions arise from experimental facts pertaining to the relative viscosity, of certain dilute aqueous solutions, where r] is the dynamic viscosity of the solution and ) w that of water at the same temperature. Jones and Stauffer (1936) reported that for 0.09966 Mofcesium iodide at 25 = 0.98910, i.e., it is 1. 

Laurence and Wolfenden (1934) reported that for 0.09393 M of lithium acetate at 25°Cr]/r] = 1.0393g, i.e., it is 1. Essentially complete ionic dissociation of these solute electrolytes takes place in such dilute solutions. The question then arises why do the cesium and iodide ions depress the relative viscosity of the solution whereas lithium and acetate ions enhance it  

The terms structure making and structure breaking are attributed to Gurney (1953), but Cox and Wolfenden (1934) were the first to mention the notion of water structure in the connection of the viscosities. Furthermore, Frank and Evans (1945) have already used the term structure breaking (but not -making ) with regard to effects of the alkali metal and halide ions, except Li+ and F , on the partial molar entropies of dilute aqueous solutions. The Jones-Dole -coefficient, Eq. (2.35), is the quantitative measure of this effect, and this equation may be recast in the form  

The At, coefficients can be calculated from the conductivity of the salts, Eq. (2.36), but are generally obtained as the intercepts in plots of ((i / ) — 1)/ce vs. the square root of the electrolyte concentration, ce, and the B, values are the limiting slopes of such plots. The B, thus pertain to infinite dilution, and are, therefore, additive in the values for the constituent ions. 

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The list of experimentally accessible properties of colloid solutions is the same as the list of accessible properties of polymer solutions. There are measurements of single-particle diffusion, mutual diffusion and associated relaxation spectra, rotational diffusion (though determined by optical means, not dielectric relaxation), viscosity, and viscoelastic properties (though the number of viscoelastic studies of colloidal fluids is quite limited). One certainly could study sedimentation in or electrophoresis through nondilute colloidal fluids, but such measurements do not appear to have been made. Colloidal particles are rigid, so internal motions within a particle are not hkely to be significant the surface area of colloids, even in a concentrated suspension, is quite small relative to the surface area of an equal weight of dissolved random-coil chains, so it seems unlikely that colloidal particles have the major effect on solvent dynamics that is obtained by dissolved polymer molecules. 

Park N S and Waldeck D FI 1989 Implications for multidimensional effects on isomerization dynamics photoisomerization study of 4,4 -dimethylstilbene in / -alkane solvents J. Chem. Phys. 91 943-52... 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

Samanta, A. A., and S. K. Gosh. 1995. Density functional approach to the solvent effects on the dynamics of nonadiabatic electron transfer reactions. J. Chem. Phys. 102, 3172. 

However, a very limited number of studies focused on the effect of solvent dynamics on electron transfer reactions at electrodes.Smith and Hynes" introduced the effect of electronic friction (arising from the interaction between the excited electron hole pairs in the metal electrode) and solvent friction (arising from the solvent dynamic [relaxation] effect) in the electron transfer rate at metallic electrodes. The consideration of electron-hole pair excitation in the metal without illumination by light seems unrealistic. 

Femtosecond spectroscopic investigations in the spectral range 400-880 ran have permitted to discriminate specific OH effects on the dynamics of short lived UV excited CTTS states and transient near-IR (HO e )H20 pairs. The complex nature of ultrafast prehydration elementary redox reactions with nascent OH radical (strong acid) must be contemplated in the framework of ion-pairs dynamics, ion-solvent correlation function, short-range ordering water molecules, solvent screening or anisotropic electric field effects and short-time vibronic couplings. 

The force controls the remarkably persistent coherence in products, a feature that was unexpected, especially in view of the fact that all trajectory calculations are normally averaged (by Monte Carlo methods) without such coherences. Only recently has theory addressed this point and emphasized the importance of the transverse force, that is, the degree of anharmonicity perpendicular to the reaction coordinate. The same type of coherence along the reaction coordinate, first observed in 1987 by our group, was found for reactions in solutions, in clusters, and in solids, offering a new opportunity for examining solvent effects on reaction dynamics in the transition-state region. 

The use of 19F NMR for a variable temperature (VT) NMR study of fluorinated taxoids is obviously advantageous over the use of H NMR because of the wide dispersion of the l9F chemical shifts that allows fast dynamic processes to be frozen out. Accordingly, F2-paclitaxel 65 and F-docetaxel 66 were selected as probes for the study of the solution structures and dynamic behavior of paclitaxel and docetaxel, respectively, in protic and aprotic solvent systems.77 The inactive 2, 10-diacetyldocetaxel (73) was also prepared to investigate the role of the 2 -hydroxyl moiety in the conformational dynamics.89 While molecular modeling and NMR analyses (at room temperature) of 73 indicate that there is no significant conformational changes as compared to paclitaxel, the 19F NMR VT study clearly indicates that this modification exerts marked effects on the dynamic behavior of the molecule.77... 

In an investigation of the role of water in enzymic catalysis. Brooks and Karplus (1989) chose lysozyme for their study. Stochastic boundary molecular dynamics methodology was applied, with which it was possible to focus on a small part of the overall system (i.e., the active site, substrate, and surrounding solvent). It was shown that both structure and dynamics are affected by solvent. These effects are mediated through solvation of polar residues, as well as stabilization of like-charged ion pairs. Conversely, the effects of the protein on solvent dynamics and... 

C. Effect of solvent dynamics on electron transfer and Sumi-Marcits two-dimensional model... 

The nonlinear Smoluchowski-Vlasov equation is calculated to investigate nonlinear effects on solvation dynamics. While a linear response has been assumed for free energy in equilibrium solvent, the equation includes dynamical nonlinear terms. The solvent density function is expanded in terms of spherical harmonics for orientation of solvent molecules, and then only terms for =0 and 1, and m=0 are taken. The calculated results agree qualitatively with that obtained by many molecular dynamics simulations. In the long-term region, solvent relaxation for a change from a neutral solute to a charged one is slower than that obtained by the linearized equation. Further, in the model, the nonlinear terms lessen effects of acceleration by the translational diffusion on solvent relaxation. 

Forty years after Kramers seminal paper on the effect of solvent dynamics on chemical reaction rates (Kramers, 1940), Zusman (1980) was the first to consider the effect of solvent dynamics on ET reactions, and later treatments have been provided by Friedman and Newton (1982), Calef and Wolynes (1983a, 1983b), Sumi and Marcus (1986), Marcus and Sumi (1986), Onuchic et al. (1986), Rips and Jortner (1987), Jortner and Bixon (1987) and Bixon and Jortner (1993). The response of a solvent to a change in local electric field can be characterised by a relaxation time, r. For a polar solvent, % is the longitudinal or constant charge solvent dielectric relaxation time given by, where is the usual constant field dielectric relaxation time... 

In view of the simplifying assumptions which form the basis for TST, its success in many practical situations may come as a surprise. Bear in mind, however, that transition state theory accounts quantitatively for the most important factor affecting the rate—the activation energy. Dynamical theories which account for deviations from TST often deal with effects which are orders of magnitude smaller than that determined by the activation barrier. Environmental effects on the dynamics of chemical reactions in solution are therefore often masked by solvent effect on the activation free energy, IF (xso), with lF(x) given by Eq. (14.25). 

Our focus so far was on unimolecular reactions and on solvent effects on the dynamics of barrier crossing. Another important manifestation of the interaction between the reaction system and the surrounding condensed phase comes into play in bimolecular reactions where the process by which the reactants approach each other needs to be considered. We can focus on this aspect of the process by considering bimolecular reactions characterized by the absence of an activation barrier, or by a barrier small relative to ke T. In this case the stage in which reactants approach each other becomes the rate determining step of the overall process. 

It is important to realize that not only does the solvent environment modify the equilibrium properties and the dynamics of the chemical process, it often changes the nature of the process and therefore the questions we ask about it. The principal object in a bimolecular gas phase reaction is the collision process between the molecules involved. In studying such processes we focus on the relation between the final states of the products and the initial states of the reactants, averaging over the latter when needed. Questions of interest include energy flow between different degrees of freedom, mode selectivity, and yields of different channels. Such questions could be asked also in condensed phase reactions, however, in most circumstances the associated observable cannot be directly monitored. Instead questions concerning the effect of solvent dynamics on the reaction process and the inter-relations between reaction dynamics and solvation, diffusion and heat transport become central. 

Stochastic dynamics has been found to be particularly useful for introducing simplified descriptions of the internal motions of complex systems. When applied to small systems (e.g., a peptide or an amino acid sidechain) it is possible to do simulations that extend into the microsecond range, where many important phenomena occur. Simulation studies using this method have been carried out, for example, to explore solvent effects on the dynamics of internal soft degrees of freedom in small biopolymers, e.g., the dynamics of dihedral angle rotations in the alanine dipeptide (see Chapt. IX.B.l). 

C. L. Brooks 111 and M. Karplus, /. Mol. Biol., 208, 159 (1989). Solvent Effects on Protein Motion and Protein Effects on Solvent Motion. Dynamics of the Active Site Region of... 

An example of pressure effects on the dynamic properties of liquids is given in the study by Hasha et al. of conformational isomerization in liquid cyclohexane. In contrast to classical transition-state theory, stochastic models predict that for such reactions the transmission coefficient, k, should depend on the collision frequency between the solvent and the solute molecules, which is a measure of the coupling of the reaction coordinate with the... 

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