Signal Quantum-level simulation resolves how protons move through water

Summary

An international research team simulated the motion of a hydrated proton in water at a full quantum-mechanical level of detail. The nature of the aqueous proton has traditionally been described through two limiting structural motifs, the Zundel and Eigen cations, but experimental infrared spectra of the solvated proton reveal a far more dynamic character, evidenced by distinct intensity modulations within the characteristic continuum absorption band. Recent ultrafast two-dimensional infrared spectroscopy has further suggested that solvation-induced structural distortions around the proton bridging two water molecules critically shape this infrared response. To clarify the role of this asymmetry, the team ran full-dimensional quantum dynamics simulations of an extended complex encompassing both Zundel and Eigen motifs, consisting of a proton surrounded by six water molecules. Systematically removing water molecules from the second solvation shell gradually shifted the system away from a perfectly symmetric Zundel-like structure toward Eigen-like spectral features. The work sheds light not only on the acidity of water but also on processes such as energy storage in batteries and signaling within living cells.