Dynamics Solvent

Zhong Y and McHale J L 1997 Resonance Raman study of solvent dynamics in electron transfer. II. Betaine-30 in... 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

Fleming G R and Cho M 1996 Chromophore-solvent dynamics Annu. Rev. Phys. Chem. 47 109-34... 

Pshenichnikov M S, Duppen K and Wiersma D A 1995 Time-resolved femtosecond photon echo probes bimodal solvent dynamics Phys. Rev. Lett. 74 674-7... 

The present approach has been applied to the experiment done by Nelsen et ah, [112], which is a measurement of the intramolecular electron transfer of 2,7-dinitronaphthalene in three kinds of solvents. Since the solvent dynamics effect is supposed to be unimportant in these cases, we can use the present theory within the effective ID model approach. The basic parameters are taken from the above reference except for the effective frequency. The results are shown in Fig. 26, which shows an excellent agreement with the experiment. The electronic couphng is quite strong and the perturbative treatment cannot work. The effective frequencies used are 1200, 950, and 800 cm for CH3CN, dimethylformamide (DMF), and PrCN [113]. 

Only if one takes into account the solvent dynamics, the situation changes. The electron transfer from the metal to the acceptor results in the transition from the initial free energy surface to the final surface and subsequent relaxation of the solvent polarization to the final equilibrium value Pqj,. This brings the energy level (now occupied) to its equilibrium position e red far below the Fermi level, where it remains occupied independent of the position of the acceptor with respect to the electrode surface. 

A. Malevanets and R. Kapral, Mesoscopic model for solvent dynamics, J. Chem. Phys. 110, 8605 (1999). 

Maroncelli M, Macinnis J, Flaming FR (1989) Polar solvent dynamics and electron-transfer reactions. Science 4899 1674-1681... 

Esr spectroscopy has also been used to study pure solvent dynamics in electron self-exchange reactions (Grampp et al., 1990a Grampp and Jaenicke, 1984a,b). When the systems are not linked by a spacer (i.e. TCNQ- /TCNQ (TCNQ = tetracyanoquinodimethane), the homogeneous bimolecular rate constants /chom are given by (10), with fcA the association constant and kET... 

Finally, we note that we have mostly limited attention so far to the self-consistent reaction field limit of dynamical solvent polarization, which is the only one that has been generally implemented (see next Section). Nevertheless, there are problems where the solute-solvent dynamical correlation must be considered, and we will address that topic in Section 5. 

The TST rate constant for electronically adiabatic ET reactions is the well-known Marcus rate constant kjjj [27-29], In the language of this chapter, solvent dynamical effects can alter the actual rate from this limit due to the friction influence. The corresponding GH equations for kct = / kfj are strictly analogous... 

R to P is slow, even when the isoenergetic conditions in the solvent allow the ET via the Franck-Condon principle. The TST rate for this case contains in its prefactor an electronic transmission coefficient Kd, which is proportional to the square of the small electronic coupling [28], But as first described by Zusman [32], if the solvation dynamics are sufficiently slow, the passage up to (and down from [33]) the nonadiabatic curve intersection can influence the rate. This has to do with solvent dynamics in the solvent wells (this is opposed to the barrier top description given above). We say no more about this here [8,11,32-36]. 

We now turn to the electronically adiabatic ET reaction problem (cf. Sec. 2.2). There has been a spate oftheoretical papers [8,11 28,33,35,36,50] dealing with the possible role of solvent dynamics in causing departures from the standard Marcus TST rate theory [27,28] (although many of these deal with nonadiabatic reactions). The ET reaction considered is a simplified symmetric model, A1 2 A1/2 A1/2 A1/2, in a model solvent similar to CH3C1. The technical and computational... 

Actually, all of the above results are in contradiction to the currently conventional view [32-35] that solvent dynamical effects for electronically adiabatic ET reactions are determined by solvent dynamics in the R and P wells, and not the barrier top region. This misses the correct picture, even for fairly cusped barrier. Instead, it is the solvent dynamics occurring near the barrier top, and the associated time dependent friction, that are the crucial aspects. It could however be thought possible that, for cusped barrier adiabatic ET reactions in much more slowly relaxing solvents, the well dynamics could begin to play a significant role. However, MD simulations have now been carried out for the same ET solute in a solvent where the... 

Experimental probing of the detailed solute-solvent dynamical mechanisms can be... 

Rasaiah, J. and Zhu, J. (1994) Solvent dynamics and electron transfer reactions,in Gauduel, Y. and Rossky, P. J.(eds.), Ultrafast reaction dynamics and solvent effects, AIP Press, New York,pp.421-434. 

Phelps, D. K., Weaver, M. J. and Ladanyi, B. M. Solvent dynamic effects in electron transfer molecular dynamics simulations of reactions in methanol, Chem. Phys., 176 (1993), 575-588... 

In recent years, there have been numerous studies examining the dynamics of proton transfer within the context of recently developed theoretical models. Reactions in the gas phase, in the solution phase, and in matrices have been examined [59-72]. Few of these studies, however, have addressed the issue of how the rate of proton transfer correlates with the thermodynamic driving force, which is an important correlation for discerning the validity of the various theoretical models. However, there have been two series of investigations by Kelley and co-workers [70, 71], and by Pines et al. [65, 66] that have sought to elucidate the role of solvent dynamics on the rate of proton transfer. 

M. Van Damme, J. Hofkens, F. G De Schryver, T. G. Ryan, W. Rettig, and A. M. Klock, Solvent dynamics and twisted intramolecular charge transfer in 4-cyano-4 -butyloxybiphenyl (4COB), Tetrahedron 45, 4693-4706 (1989). 

VIII. SOLVENT DYNAMIC EFFECTS ON ET REACTIONS AT ELECTRODES... 

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. 

The role of solvent dynamics in the electron transfer reaction at a Pt electrode was discussed utilizing the theory of Zusman. In this work, ° the solvent-dependent rate constant for the electron transfer reaction at an electrode was given as... 

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