Solvation structures of excess protons in aqueous surrounding are highly relevant to electric properties and for an understanding of proton transport in liquids and membranes. At ambient temperature, polar liquids undergo structural fluctuations on the femto- to picosecond time scale that have direct impact on proton solvation. By combining nonlinear two-dimensional infrared spectroscopy with ab-initio molecular dynamics simulations we investigate ultrafast proton dynamics in varying acetonitrile/water mixtures. The strategy of building the water environment step-by-step provides unique benchmark of aqueous solvation geometries via a spectroscopic comparison to excess protons selectively prepared with only a few water molecules in neat acetonitrile. We show that for the investigated mixing ratios protons are confined within a low barrier double-minimum potential, with large amplitude displacements driven by fluctuating electric fields. The results suggest that protons in water are predominantly solvated within dimeric water structures, i.e., within a H5O2+ Zundel motif. The observations are rationalized by in depth simulations of the excess proton dynamics where the proton is hosted within a varying number of water molecules. The simulations directly resemble the experimental conditions and show how the dimeric water solvation structure interacts with its closest water neighbor in a H7O3+ unit without persistent proton localization on a single water molecule.

Hydrated excess protons in acetonitrile/water mixtures: Solvation species and ultrafast proton motions

2019 Fall National Meeting

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