Relaxation time, solvent dynamic effect

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... 

A case of solvent-driven electronic relaxation has been observed [76] for [Re(Etpy)(CO)3(bpy)]+ in ionic liquids TRIR spectra have shown at early times a weak signal due to the II. state, in addition to much stronger bands of the 3MLCT state. Although no accurate kinetic data are available, the II. state converts to MI.CT with a rate that is commensurate with the solvent relaxation time. Fluorescence up-conversion provided an evidence [10] for population of an upper II. state in MeCN, which converts to CT with a much faster lifetime of 870 fs (Table 1). The solvent dynamic effect on the 3IL—>3CT internal conversion can be rationalized by different polarities of the II. and JCT states, Fig. 11. The solvent relaxation stabilizes the 3CT state relative to II., driving the conversion. 

HYNES - A comment. The results you mention on specific solvent effects in mixtures suggest that these systems would be interesting candidates to experimentally study solvent dynamical effects. All continuum theories for these effects say in one way or another that it is only the solvent dielectric relaxation time that matters. The specific solvation effects suggest to me that this picture would break down seriously for such systems, and reveal new and interesting dynamic solvent effects. 

This is no longer the case when (iii) motion along the reaction patir occurs on a time scale comparable to other relaxation times of the solute or the solvent, i.e. the system is partially non-relaxed. In this situation dynamic effects have to be taken into account explicitly, such as solvent-assisted intramolecular vibrational energy redistribution (IVR) in the solute, solvent-induced electronic surface hopping, dephasing, solute-solvent energy transfer, dynamic caging, rotational relaxation, or solvent dielectric and momentum relaxation. 

There is one important caveat to consider before one starts to interpret activation volumes in temis of changes of structure and solvation during the reaction the pressure dependence of the rate coefficient may also be caused by transport or dynamic effects, as solvent viscosity, diffiision coefficients and relaxation times may also change with pressure [2]. Examples will be given in subsequent sections. 

The description of the real process of dipole-orientational relaxation by one parameter xR is a first-order approximation which is far removed from reality even in studies with model solvents.(89) A set of relaxation times would exist in real systems. However, such an approximation is necessary since it allows rather simple models of relaxation to be developed and to be compared with the results of experiments. xR may be considered as a simple effective parameter characterizing the dynamic processes. 

So far, the discussion of concentrated electrolyte solutions has presumed that ionic relaxation is complete and so is a static correction. Dynamic electrolyte theories are still in their infancy and, in view of the rate of ionic relaxation compared with chemical reaction rates for dilute electrolytes (Sect. 1.6), such effects are probably not very important in concentrated electrolyte solutions containing reactants. The Debye— Falkenhagen [92] theory predicts a change in the relaxation time of electrolyte solutions with concentration, though experimental confirmation is scant [105]. At very high concentrations, small changes in the relaxation time ( 25%) of solvent relaxation can be identified (see also Lestrade et al. [106]). 

The study of 33S relaxation times and line widths has provided much information about dynamic behaviour and molecular association for sulpholane, sulphonic acids and sulphonate anions. It is interesting to observe that in all cases, 33S is an internal nucleus and is particularly sensitive to solvent and association effects, even though it is not directly exposed to these interactions. 

Dynamic solvent effect — is a phenomenon typical for adiabatic -> electron transfer and -> proton transfer reactions. This effect is responsible for a dependence of the reaction rate on solvent relaxation parameters. The initial search for a dynamic solvent effect (conventionally assumed to be a feature of reaction adiabatic-ity) consisted in checking the viscosity effect. However, this approach can lead to controversial conclusions because the viscosity cannot be varied without changing the -> permittivity, i.e. a dynamic solvent effect cannot be unambiguously separated from a -> static solvent effect [i]. Typically a slower solvent relaxation goes along with a higher permittivity, and the interplay of the two solvents effects can easily look as if either of them is weaker. The problems of theoretical treatment of the dynamic solvent effect of solvents having several relaxation times are considered in refs, [ii-iii]. 

Recent theoretical treatments, however, suggest instead that the dynamics of solvent reorganization can play an important and even dominant role in determining vn, at least when the inner-shell barrier is relatively small [43-45]. The effective value of vos can often be determined by the so-called longitudinal (or "constant charge ) solvent relaxation time, rL [43, 44]. This quantity is related to the experimental Debye relaxation time, rD, obtained from dielectric loss measurements using [43]... 

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... 

In the present study, the rate constants of the thermal-induced intramolecular ET reaction of pico-second order were first determined for mixed-valence biferrocene monocation (Fe(II),Fe(III)) in seven solvents at various temperatures by the NMR relaxation method. The procedure of the determination of the rate constants from the measured H spin-lattice relaxation times (Ti) is shortly mentioned and the effect of dynamical property of solvent on the ET rate is discussed. 

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