Prediction of Solvation Free Energies of Ionic Solutes in Neutral Solvents

The
prediction of solvation free energies is essential for a variety
of applications. Solvation free energies of neutral systems can be
predicted quite accurately. The accuracy of predictions for solvation
free energies of ionic solutes dissolved in neutral solvents, however,
has been reported to be worse by at least 1 order of magnitude. In
this study, the performance of three approaches for solvation free
energy prediction of several hundred ions dissolved in neutral solvents
is evaluated. The applied methods are COSMO-RS, cluster continuum
model (CCM) together with COSMO-RS, and COSMO-RS-ES. It is emphasized
that the reference data for model evaluation are subject to large
uncertainties stemming from the impossibility to measure the so-called
elusive absolute free energies of solvation of a single ion. Consequently,
such uncertainty must be considered during the evaluation of prediction
methods. Therefore, a straightforward approach to account for the
underlying uncertainty is applied here. Hereby, it is revealed that
the true performance of the method is better than what is often reported.
The average absolute deviation (AAD) of COSMO-RS is calculated to
be 2.3 kcal mol–1, while applying the CCM and COSMO-RS-ES
each results in AADs of 2.0 kcal mol–1. This accuracy
allows for qualitative assessment of solvation free energy-dependent
quantities, such as reaction rate constants.