Relaxation time, solvent dynamic effect electron transfer

The effect of solvent in an electron transfer is larger than simply through its energetic contribution to A. There is evidence that the dynamics of solvent reorganization, often represented in terms of a solvent longitudinal relaxation time, tl, contribute to the preexponential factor in (3.6.2) (47, 62-65), e.g., Since tl is roughly proportional... 

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. 

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]. 

Photoinduced electron transfer is a subject characterised, particularly at the present time, by papers with a strongly theoretical content. Solvent relaxation and electron back transfer following photoinduced electron transfer in an ensemble of randomly distributed donors and acceptors, germinate recombination and spatial diffusion a comparison of theoretical models for forward and back electron transfer, rate of translational modes on dynamic solvent effects, forward and reverse transfer in nonadiabatic systems, and a theory of photoinduced twisting dynamics in polar solvents has been applied to the archetypal dimethylaminobenzonitrile in propanol at low temperatures have all been subjects of very detailed study. The last system cited provides an extended model for dual fluorescence in which the effect of the time dependence of the solvent response is taken into account. The mechanism photochemical initiation of reactions involving electron transfer, with particular reference to biological systems, has been discussed by Cusanovich. ... 

If the molecule has dynamic motions on the timescale of the EPR experiment, this motion will lead to relaxation effects on the EPR line. Depending on the timescale and size of these motions, these effects may be observable directly in the cw-EPR spectrum or indirectly by pulsed EPR measurements of the relaxation times. In many cases, different dynamics may simultaneously contribute to the relaxation behavior of the electron spin system, as, for example, vibrational and rotational motion, conformational dynamics, phonon coupling to the frozen solvent, and nuclear spin dynamics. In these cases, it will be difficult to obtain specific information from these relaxation measurements. On the other hand, it is possible to highlight a specific time-scale window by the selection of pulse sequences and microwave frequencies that can lead, in favourable cases, to a direct relation between measured relaxation times and interesting molecular dynamics at the paramagnetic site. In these cases, very interesting molecular dynamical aspects of electron-transfer, catalytic, or photo-reactions, unobservable by other structural methods, can be studied directly by pulse-EPR techniques. 

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