Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds
Quantum chemical implicit solvent models are used widely to estimate aqueous redox potentials. We compared the accuracy of several popular implicit solvent models (SM8, SMD, C-PCM, IEF-PCM, and COSMO-RS) for the prediction of aqueous single electron oxidation potentials of a diverse test set of neutral organic compounds for which accurate experimental oxidation potential and gas-phase ionization energy data are available. Using a thermodynamic cycle, we decomposed the free energy of oxidation into contributions arising from the gas-phase adiabatic ionization energy, the solvation free energy of the closed-shell neutral species, and the solvation free energy of the radical cation species. For aqueous oxidation potentials, implicit solvent models exhibited mean unsigned errors (MUEs) ranging from 0.27 to 0.50 V, depending on the model. The principal source of error was attributed to the computed solvation free energy of the oxidized radical cation. Based on these results, a recommended implicit solvation approach is the SMD model for the solvation free energy combined with CBS-QB3 for the gas-phase ionization energy. With this approach, the MUE in computed oxidation potentials was 0.27 V, and the MUE in solvation free energy of the charged open-shell species was 0.32 eV. This baseline assessment provides a compiled benchmark test set of vetted experimental data that may be used to judge newly developed solvation models for their ability to produce improved predictions for aqueous oxidation potentials and related properties.