CMIRS Solvation Model for Methanol: Parametrization, Testing, and Comparison with SMD, SM8, and COSMO-RS

The new continuum solvation model, composite method for implicit representation of solvent (CMIRS), proposed by Pomogaeva and Chipman and implemented in GAMESS was parametrized for methanol solvent, with the aim of using it for ionic reactions in solution. The model was tested for predicting single-ion solvation free energy, pK_a of acids and protonated bases, and the activation free-energy barriers of S_N2 and S_NAr reactions in methanol. A comparison was performed with other continuum models, such as SMD, SM8, and COSMO-RS. For a prediction of pK_a and free-energy barriers, the order of performance was CMIRS > COSMO-RS > SMD > SM8. In particular, the CMIRS model is much superior to the other continuum models for predicting pK_a of acids (without empirical corrections) and is able to evenly describe hard ions like methoxide and charge-dispersed ions like 2,4,6-trinitrophenol. On the basis of our results, we suggest that the field-extremum contribution, present in CMIRS, should be included in continuum solvation models, which can result in substantial improvement in the modeling of ionic reactions in solution.