TEMPERATURE EFFECTS ON INTRAMOLECULAR ELECTRON-TRANSFER KINETICS UNDER NORMAL, INVERTED, AND NEARLY OPTIMAL CONDITIONS

The temperature dependence of electron transfer over a wide range of d riving forces extending from far in the ''normal'' to deep into the '' inverted'' region is studied with the aid of a limited number of close ly related and fully rigid bridged donor-acceptor systems. The interpr etation of temperature-dependent electron-transfer kinetics is shown t o be complicated by the relatively large influence of the temperature dependence of the solvent dielectric properties. This problem becomes especially evident if the barrier is small, when it may lead to an ove rall nullification or even inversion of the temperature effect on the experimental rate. Upon correction for the temperature dependence of t he solvent properties, the rate of electron transfer is found to be in dependent of temperature in the inverted region, where nuclear tunneli ng becomes dominant, in contradiction to expectations based on the cla ssical Marcus treatment which, however, is shown to be capable of givi ng a qualitative description of the temperature dependence in the norm al region, even under close to ''optimal'' conditions.