Role of Explicit Hydration in Predicting the Aqueous Standard Reduction Potential of Sulfate Radical Anion by DFT and Insight into the Influence of pH on the Reduction Potential.

ABSTRACT

Sulfate radical anion (SO4•-) is a potent oxidant capable of destroying recalcitrant environmental contaminants such as perfluoroalkyl carboxylic acids. In addition, it is thought to participate in important atmospheric reactions. Its standard reduction potential (E°) is fundamental to its reactivity. Using theoretical methods to accurately predict the aqueous phase E° requires solvation with explicit water molecules. Herein, using density functional theory, we calculated the aqueous E° of SO4•- and evaluated sensitivity to explicit water count. The E° increased considerably with more waters until ca. 24 were included, after which change in E° was small. When a proton was added to these systems, the E° was similar regardless of the explicit water count and this value was similar to the E° for systems with a large number of explicit waters but no proton. This result agrees with literature evidence that the E° is pH independent. Natural Bond Orbital natural population analysis indicated that in the case of both SO42- and SO4•-, considerable charge was donated from the SO4 center to the explicit solvation shells.