Classical proton transfer barrier in flat-rhombic monolayer nanoconfined water

Ascertain the classical free energy barrier for proton transfer along the proton transfer coordinate ν in the flat-rhombic phase of monolayer water confined between graphene-like walls at 5 Å width under lateral pressure of 4 GPa and temperatures up to 300 K, given that classical simulations did not observe proton transfer events within nanosecond timescales.

Background

To quantify proton transfer, the authors compute free energy profiles along the proton transfer coordinate ν. In the flat-rhombic phase at 4 GPa, classical simulations showed no proton transfer events up to the hexatic transition near 300 K, preventing estimation of the barrier.

When nuclear quantum effects are included, finite barriers are observed even at 100 K, indicating that quantum motion enables proton transfer. However, the classical barrier remains unestimated due to the absence of transfer events, leaving a specific unresolved question about its magnitude under the studied conditions.