Solvation Free Energies of Ion Dissociations in Dichloromethane: En Route to Accurate Computations.

Calculating accurate free energies for solution-phase reactions is notoriously difficult. In our previous joint experimental and computational studies, we observed a striking failure of quantum mechanical calculations with popular implicit solvent models to even qualitatively reproduce the experimental trends of dissociation free energies of numerous proton-bound pyridine dimers in organic solvents [Pollice, R. . J. Am. Chem. Soc. 2017, 139(37), 13126-13140]; [Pollice, R. . Angew. Chem., Int. Ed. 2019, 58(40), 14281-14288]. In this article, we expand the computational study of the dissociation of proton-bound pyridine dimers in the gas phase and in dichloromethane (DCM). In an effort to determine the prerequisites for reproducing the experimental trends and magnitudes of the dissociation free energies (ΔGdiss) in solvent, we investigated the impact of accounting for the ensemble free energy, umbrella sampling, thermodynamic integration, and explicit solvation using semiempirical quantum mechanics and molecular mechanics. We estimated the effect of conformational free energy contributions with semiempirical quantum mechanics (SE). Molecular dynamics (MD) with explicit solvation and classical molecular mechanics (MM) was used as a method to treat not only the solute but also the solvent configurational entropy. We found that explicit solvation with MM is indeed capable of reproducing ΔGdiss in DCM for our test system within an acceptable error margin. We analyze and discuss the results and limitations of our approach for calculating the solvation free energy.