THEORETICAL-STUDIES OF INTRAMOLECULAR ELECTRON-TRANSFER REACTIONS - DISTANCE AND FREE-ENERGY DEPENDENCES

Electron tunneling through a square potential energy barrier is used t o calculate the distance-dependent factors of electron transfer (ET) p rocesses in metal-monolayer-metal junctions, donors and accepters disp ersed in rigid organic glasses, intramolecular ET in rigid donor-bridg e-acceptor species in solution and redox centers attached to electrode s through adsorbed monolayers. This tunneling model of distance-depend ent non-adiabatic factors is incorporated in the intersecting state mo del (ISM). The result is a simple semiclassical theory which is used t o calculate the rates of non-adiabatic ET reactions. When the electron is originally located in a pi molecular orbital of the donor and the reaction free energy is no lower than approximately -50 kJ mel(-1), n o adjustable parameters are necessary to calculate the intramolecular ET rates from a donor, through a rigid bridge, to an acceptor. Such ca lculated rates are within an order of magnitude of the experimental va lues. The model can also account for the ET rates of more exothermic r eactions provided that the value of an empirical parameter, which is c onstant for structurally related reactants and solvents of similar pol arity, is estimated. The physical meaning of this parameter is related to the dynamics of the reactions. The profiles of the distance and fr ee energy dependences of photoinduced ET rates are closely reproduced. The occurrence of distance-dependent non-adiabatic factors in intermo lecular sigma-d ETs is rationalized.