Solvation Entropy Made Simple

The entropies of molecules in solution are routinely calculated using gas phase formulae. It is assumed that, because implicit solvation models are fitted to reproduce free energies, this is sufficient for modeling reactions in solution. However, this procedure exaggerates entropic effects in processes that change molecularity. Here, computationally efficient (i.e., having similar cost as gas phase entropy calculations) approximations for determining solvation entropy are proposed to address this issue.The $S_\omega$, $S_\epsilon$, and $S_{\epsilon\alpha}$ models are nonempirical and rely only on physical arguments and elementary properties of the medium (e.g., density and relative permittivity). For all three methods, average errors as compared to experiment are within chemical accuracy for 110 solvation entropies, 11 activation entropies in solution, and 32 vaporization enthalpies. The models also make predictions regarding microscopic and bulk properties of liquids which prove to be accurate. These results imply that $\Delta H_\text{sol}$ and $\Delta S_\text{sol}$ can be described separately and with less reliance on parametrization by a combination of the methods presented here with existing, reparametrized implicit solvation models.