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

Understanding nuclear quantum effects (NQEs) in aqueous systems remains a challenge in computational chemistry. Prior research has shown that NQEs influence hydrogen-bonding and proton transfer in water. However, the extent of these effects in solutions containing strong bases like NaOH is less clear. Established knowledge includes the role of quantum fluctuations in proton dynamics but lacks detailed insights into their impact on solvation structures and transport properties. This gap motivated the use of advanced simulation techniques to explore NQEs in NaOH solutions. The need for high-dimensional neural network potentials arose from the limitations of classical models in capturing quantum behavior. No prior work had resolved the interplay between NQEs and solvation dynamics in such systems. Existing studies often focus on pure water or dilute solutions, leaving room for investigation into concentrated systems. This paper's contribution lies in addressing these unresolved questions through comprehensive simulations.

This study aims to investigate the impact of nuclear quantum effects (NQEs) on the properties of aqueous NaOH solutions. The specific problem involves understanding how quantum fluctuations affect solvation structures, hydrogen bonding, and proton transfer dynamics. The motivation stems from the need to bridge the gap between classical and quantum mechanical models in solution chemistry. The researchers propose to use molecular dynamics simulations to capture these effects accurately. By employing a high-dimensional neural network potential, the study seeks to model quantum behavior without excessive computational cost. The focus is on properties like radial distribution functions and power spectra, which are sensitive to NQEs. The goal is to quantify how these effects alter proton transfer rates and diffusion coefficients. This approach allows for a detailed analysis of NQEs across the solubility range of NaOH.

The study employs ring-polymer molecular dynamics simulations to model nuclear quantum effects in NaOH solutions. These simulations use a high-dimensional neural network potential trained on dispersion-corrected density functional theory reference calculations. The simulations capture both classical and quantum nuclear fluctuations. The neural network potential allows for reactive modeling of hydrogen bonding and proton transfer. The researchers analyze radial distribution functions to assess solvation structures around Na+ cations. Power spectra are computed to evaluate vibrational modes of water and hydroxide. Proton transfer barriers and rates are calculated to determine the influence of NQEs on proton dynamics. The simulations cover the full room-temperature solubility range of NaOH to ensure comprehensive results.

The simulations reveal that nuclear quantum effects (NQEs) have minimal impact on the solvation structure around Na+ ions. However, NQEs slightly strengthen hydrogen bonds between water molecules and hydroxide. The power spectra show a reduction in peak positions for HOH bending and OH stretching modes by approximately 50 and 100 cm-1, respectively. These shifts indicate a lowering of vibrational frequencies due to quantum fluctuations. Proton transfer barriers are significantly reduced by NQEs, leading to increased proton transfer rates. This effect is most pronounced in OH- diffusion coefficients, which rise due to faster proton movement. At high NaOH concentrations, the mean residence time of molecules in the first hydration shell around Na+ decreases. These findings suggest that quantum effects enhance proton mobility and alter solvation dynamics in concentrated solutions.

The authors state that nuclear quantum effects (NQEs) have a small but measurable influence on the properties of aqueous NaOH solutions. They propose that NQEs slightly strengthen hydrogen bonds between water and hydroxide ions. The reduction in vibrational peak positions indicates a shift in molecular motion due to quantum fluctuations. The researchers suggest that NQEs lower proton transfer barriers, increasing the rate of proton transfer. This effect is linked to higher diffusion coefficients for OH- ions at room temperature. The decrease in mean residence time around Na+ at high concentrations supports the idea that quantum effects alter solvation dynamics. These findings align with the hypothesis that quantum nuclear fluctuations influence proton mobility in aqueous systems. The study concludes that NQEs should be considered in modeling proton transfer and solvation in concentrated solutions.