Self-Exchange Electron Transfer Kinetics and Reduction Potentials for Anthraquinone Disulfonate

An electron transfer model for self-exchange reactions of 9,10-anthraquinone-2,6-disulfonate (AQDS) in aqueous solution has been formulated by using a combination of density functional theory (DFT) calculations and Marcus theory. One-electron self-exchange reactions are predicted to be fast (log k ≈ 6−9 M -1 s -1 ) but not diffusion limited. The internal component of the reorganization energy makes a large contribution to the total reorganization energy and cannot be neglected. Analysis and theoretical extensions of crystal structure data led to predicted precursor complex structures that, in the end, yielded theoretical electron transfer rates in good agreement with experimental ones. Electron transfer distances in solution are predicted to be in the 7−9 Å range. Calculated values of the electronic coupling matrix element indicate that the distinction between adiabatic and nonadiabatic electron transfer in this system likely occurs in this distance range as well. A set of reduction potentials was also produced by combining the density functional theory calculations with equilibrium expressions and the known acidity constants in the AQDS system.