Nuclear Quantum Effects in Sodium Hydroxide Solutions from Neural Network Molecular Dynamics Simulations

Nuclear quantum effects
(NQEs) cause the nuclei of light elements
like hydrogen to delocalize, affecting numerous properties of water
and aqueous solutions, such as hydrogen-bonding and proton transfer
barriers. Here, we address the prototypical case of aqueous NaOH solutions
by investigating the effects of quantum nuclear fluctuations on radial
distribution functions, hydrogen-bonding geometries, power spectra,
proton transfer barriers, proton transfer rates, water self-exchange
rates around the Na+ cations, and diffusion coefficients,
for the full room-temperature solubility range. These properties were
calculated from classical and ring-polymer molecular dynamics simulations
employing a reactive high-dimensional neural network potential based
on dispersion-corrected density functional theory reference calculations.
We find that NQEs have a very small impact on the solvation structure
around Na+, slightly strengthen the water–water
and water–hydroxide hydrogen bonds, and lower the peak positions
in the power spectra for the HOH bending and OH stretching modes by
about 50 and 100 cm–1, respectively. Moreover, NQEs
significantly lower the proton transfer barriers, thus increasing
the proton transfer rates, resulting in an increase of the diffusion
coefficient in particular of OH–, as well as a decrease
of the mean residence time of molecules in the first hydration shell
around Na+ at high NaOH concentrations.