Critical
Evaluation of Implicit Solvent Models for
Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds
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Abstract
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