Quantifying Uncertainties in Solvation Procedures for Modeling Aqueous Phase Reaction Mechanisms

Computational quantum chemistry provides fundamental chemical and
physical insights into solvated reaction mechanisms across many areas
of chemistry, especially in homogeneous and heterogeneous renewable
energy catalysis. Such reactions may depend on explicit interactions
with ions and solvent molecules that are nontrivial to characterize.
Rigorously modeling explicit solvent effects with molecular dynamics
usually brings steep computational costs while the performance of
continuum solvent models such as polarizable continuum model (PCM),
charge-asymmetric nonlocally determined local-electric (CANDLE), conductor-like
screening model for real solvents (COSMO-RS), and effective screening
medium method with the reference interaction site model (ESM-RISM)
are less well understood for reaction mechanisms. Here, we revisit
a fundamental aqueous hydride transfer reactioncarbon dioxide
(CO2) reduction by sodium borohydride (NaBH4)as a test case to evaluate how different solvent models
perform in aqueous phase charge migrations that would be relevant
to renewable energy catalysis mechanisms. For this system, quantum
mechanics/molecular mechanics (QM/MM) molecular dynamics simulations
almost exactly reproduced energy profiles from QM simulations, and
the Na+ counterion in the QM/MM simulations plays an insignificant
role over ensemble averaged trajectories that describe the reaction
pathway. However, solvent models used on static calculations gave
much more variability in data depending on whether the system was
modeled using explicit solvent shells and/or the counterion. We pinpoint
this variability due to unphysical descriptions of charge-separated
states in the gas phase (i.e., self-interaction errors), and we show
that using more accurate hybrid functionals and/or explicit solvent
shells lessens these errors. This work closes with recommended procedures
for treating solvation in future computational efforts in studying
renewable energy catalysis mechanisms.