Recent experimental
and theoretical work has debated whetherN-heterocyclic
carbenes (NHCs) are natively present in imidazolium-based
ionic liquids (ILs) such as 1-ethyl-3-methylimidazolium acetate ([EMIM+][OAc–]) at room temperature. Because NHCs
are powerful catalysts, determining their presence within imidazolium-based
ILs is important, but experimental characterization is difficult due
to the transient nature of the carbene species. Because the carbene
formation reaction involves acid–base neutralization of two
ions, ion solvation will largely dominate the reaction free energy
and thus must be considered in any quantum chemical investigation
of the reaction. To computationally study the NHC formation reaction,
we develop physics-based, neural network reactive force fields to
enable free energy calculations for the reaction in bulk [EMIM+][OAc–]. Our force field explicitly captures
the formation of NHC and acetic acid by deprotonation of a EMIM+molecule by acetate and in addition describes the dimerization
of acetic acid and acetate. Using umbrella sampling, we compute reaction
free energy profiles within the bulk IL and at the liquid/vapor interface
to understand the influence of the environment on ion solvation and
reaction free energies. Compared to reaction of the EMIM+/OAc–dimer in the gas phase, the bulk environment
destabilizes formation of the NHC as expected due to the large ion
solvation energies. Our simulations reveal a preference for the product
acetic acid to share its proton with an acetate in solution and at
the interface. We predict NHC content in bulk [EMIM+][OAc–] to be on the order of parts-per-million (ppm) levels,
with order-of-magnitude enhancement of NHC concentration at the liquid/vapor
interface. The interfacial enhancement of NHC content is due to both
poorer solvation of the ionic reactants and solvophobic stabilization
of the neutral NHC molecule at the liquid/vapor interface.