Rehm-Weller behaviour

Theoretical Discussions of Marcus- and Rehm-Weller Behaviour in... 

The considerable amount of experimental data concerned with the rates and free energies of e.t. processes can be fitted into two distinct trends, which are referred to commonly as Rehm-Weller behaviour and Marcus behaviour. The distinction is based on the absence or presence of the M.I.R. in the plots of Ink versus AG, and much of the research in the last few years has been concentrated on the theoretical basis of these two opposite behaviours [77],... 

It can be noticed that these processes are essentially charge recombinations, with a few examples of charge shifts [82], Photoinduced charge separations between neutral chromophores do not appear in this table, either because the range of free energies was not sufficient to reach the M.I.R., or because the M.I.R. was not observed this has carried on the trend already noted in the early 1980s, that photo-induced charge separations follow Rehm-Weller behaviour. 

The Mataga-Kakitani (M-K) theory is based on the rather general observation that e.t. processes which show the M.I.R. are mostly charge recombinations and charge shifts, whereas the photo-induced charge separations which start from neutral reactants follow Rehm-Weller behaviour. It is then suggested that the difference is due to the electric field which acts on the solvent in the field of ions or ion pairs, partial dielectric saturation of polar solvents would be reached, and this would restrict solvent motion. No such dielectric saturation effect would exist in the solvent shell of neutral reactants, so that solvent motion remains unhindered. 

Prasad, E. Gopidas, K.R. Photoinduced electron transfer in hydrogen bonded donor-acceptor systems. Study of the dependence of rate on free energy and simultaneous observation of the Marcus and Rehm-Weller behaviours. J. 30. Am. Chem. Soc. 2000, 122 (13). 3191-3196. 

Figure 16.13 Rehm-Weller behaviour in the bimolecular photoinduced electron transfer in acetonitrile. The experimental points arefromref. [29]. The rate constants of the reactions with AG < -50kJmol are diffusion controlled. ISM calculations employedX =/p = 3.8 X 10 kJ mol" A , l +l = 2.8 A, n = 1.41 and A = 160 or 273 kJ mol". The intrinsic barrier of TM-2 was fitted to reproduce the rates in the normal region with Z= 10" M" sec". ...

Experimental Data of Rehm-Weller-and Marcus Behaviour. . . 119... 

Weller behaviour, so that in intermolecular reactions the rate constant remains at the diffusion controlled limit in the most exergonie processes in apparent contradiction with the results of Marcus theory [84], On the other hand, observations of the M.I.R. have been made essentially in the case of thermal charge recombination reactions, with a few examples of charge shifts. Why should some e.t. systems follow the Marcus model, and others not This is the fundamental theoretical question which has been addressed to by different authors in recent years and several interesting models have been suggested to account for the different behaviours of the Rehm-Weller and Marcus systems. 

In the early 1980s, the classical Marcus theory was reanalyzed to consider the influence of quantum effects, notably electron tunneling [23]. Qualitatively, this gives the right direction, since it increases the observed rate constants when the thermally activated process becomes slow but it was concluded that it could not account for the quantitative discrepancies of observed Rehm-Weller type plots from Marcus behaviour. For this reason the hypothesis was retained that the... 

A concensus has developed in recent years that the interesting M-K theory may not provide the right approach towards a better understanding of the differences between e.t. systems which follow Rehm-Weller or Marcus behaviour. Another suggestion made recently by Rau therefore deserves attention [90]. This is still based on the general observation that intermolecular systems limited by... 

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