Explicitly Unraveling the Roles of Counterions, Solvent Molecules, and Electron Correlation in Solution Phase Reaction Pathways

Studies utilizing continuum solvation methods can sometimes omit critically important solute-solvent interactions, while explicitly sampling full solution phase mechanisms accurately with Born-Oppenheimer molecular dynamics (BOMD) is computationally costly. In this work, we benchmark components for an alternative IRCMax-like procedure for refined analyses of electronic energies along reaction pathways. The procedure involves obtaining molecular clusters from nudged elastic band calculations for use in mixed explicit-continuum models. The reaction energetics from these models correspond well to energetics obtained from explicit models using periodic boundary conditions, and the clusters obtained are more amenable to treatments with high levels of quantum chemistry theory. We demonstrate this approach using CO2 reduction by NaBH4 and NaBH3OH in aqueous solution as test cases. We show that the local solvation environment containing explicit solvent molecules and a counterion within the entire first solvation shell significantly influences reaction energies. For the hydride transfers reported herein, the level of quantum chemistry theory used beyond that treated by standard GGA exchange correlation functionals normally plays a less significant role.