Effects of Free Energy and Solvent on Rates of Intramolecular Electron Transfer in Organic Radical Anions.

Rates of intramolecular electron transfer from a 1,1'-biphenylyl radical anion to six different acceptors on an androstane scaffold are examined with the aid of a theory that was developed recently to include effects of vibrational relaxations and dephasing. The electronic-interaction matrix element and other parameters needed for the theory are obtained by quantum-mechanical/molecular-mechanical simulations of the reactions in five solvents ranging from iso-octane to methyltetrahydrofuran. Intramolecular vibrational modes that are coupled to electron transfer are resolved in simulations in iso-octane and cyclohexane. The energies and coupling factors for these modes allow extension of the theory to incorporate transitions to and from excited vibrational levels. The calculated rates of electron transfer agree well with experimental measurements from the literature, except for reactions in which excited electronic states of the products become important.