In polar solvents the energies of the initial and final states change with time due to the fluctuation of the polarization of the solvent, and electron transfer takes place when they coincide. In nonpolar solvents the fluctuation of the polarization is essentially absent. In this case, how is the energy coincidence of the initial and final states brought about? In nonpolar solvents the energy E_IP(r) of the ion pair changes significantly with donor-acceptor distance r because the Coulomb interaction is not screened by the solvent, while the energy E_NP(r) of the neutral pair changes little with r. In nonpolar solvents the energy coincidence of the initial and final states is brought about by the change of donor-acceptor distance.
From the condition of energy coincidence the distance R at which electron transfer takes place is calculated as a function of the energy change ΔE_∞ of reaction, which represents the energy difference between the ion pair and the neutral pair at infinity. The electron transfer rate is expressed in terms of the distribution of acceptors around a donor, Landau-Zener transition probability at R, and the distribution of the relative velocity of donor and acceptor. We have calculated the electron transfer rate as a function of ΔE_∞ in the kinetic control case in which the diffusion of donor and acceptor is much faster compared with electron transfer at R. The effects of intramolecular vibration and diffusion of donors and acceptors are investigated. Comparison with experiment is also made.