A CRITICAL-ASSESSMENT OF CLASSICAL AND SEMICLASSICAL MODELS FOR ELECTRON-TRANSFER REACTIONS IN SOLUTION

The microscopic descriptions of electron transfer (ET) reactions given by Marcus theory (MT) and by the intersecting-state model (ISM) are c ompared with ab initio calculations on O-2/O-2(.-) and C6H6/C6H6.- mod el systems. The activated configuration calculated by ab initio method s agrees only with that of ISM. Solvent effects in self-exchange rates and in intervalence bands of mixed-valence symmetrical compounds. are shown to be smaller than predicted by dielectric continuum models. MT and ISM are used to calculate self-exchange and cross-reaction rates of transition-metal complexes and organic species. It is shown that a compensation of effects may take place in MT calculations. Absolute ET rate calculations using ISM are in good agreement with experiment whe n non-adiabatic factors are included in sigma(d) electron exchanges. It is shown that ISM reproduces different types of free-energy profile s (Rehm-Weller behaviour, Marcus inverted region, double inverted regi on) with only one adjustable parameter. The donor-acceptor distance de pendence, temperature dependence and free-energy dependence of intramo lecular ET reactions are calculated with two adjustable parameters. Th ese calculations are within an order of magnitude of the experimental rates.