Strong orbital interactions of the Zundel cation H₅O₂⁺ with hydration shell water

We use O K-edge X-ray absorption spectroscopy in transmission to determine the electronic structure and hydrogen bond characteristics of the Zundel cation H₅O₂⁺ in solution. H₅O₂⁺ plays a key role in water-mediated proton transport in bulk water, hydrogen fuel cells and transmembrane proteins. For this, we elucidate spectroscopic signatures of O 1s core excitations to O-H σ* anti-bonding orbitals of hydrated protons. By a dedicated and previously established sample preparation procedure, we can systematically tune the hydration degree of the protons, starting from the Zundel cation solvated by acetonitrile, and exchange solvation layers with water, thus altering the hydrogen bond characteristics of H₅O₂⁺. The significant decrease of pre- and main-edge combined with a major increase of the post-edge transition cross sections point at strong interactions of the first hydration shell water molecules with the Zundel cation. With our flatjet system for x-ray absorption spectroscopy in transmission, now succesfully operating for the polar acetonitrile solvent, and in combination with previously obtained laboratory infrared spectroscopic data, we can establish a systematic structural approach to hydrated proton structures in solution.