The homogeneous electron self-exchange reaction of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPPD/TMPPD˙+) shows the classical solvent dependence according to Marcus' theory. The rate constants are a linear function of γ=(1/n2–1/ε)(n= refractive index, ε= dielectric constant of the solvent). For 0.27 ⩽γ⩽ 0.53, log kex= const. ×γ. However, the reactions of tetracyanoquinonedimethane (TCNQ/TCNQ˙–) and tetracyanoethylene (TCNE/TCNE˙–) are dominated by the solvent dynamics, including the longitudinal relaxation time τL of the solvent. This is expressed by log (kexτLγ–1/2)= const. ×γ, for 0.05 ⩽γ⩽ 0.53. The role of solvent reorganization dynamics is discussed together with the decision equation of Ovchinnikova.
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Grampp et al. (1987) studied this question.