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Abstract

The properties of hydrogen ions in aqueous solution are governed by the ability of water to incorporate ions in a dynamical hydrogen bond network, characterized by a structural variability that has complicated the development of a consistent molecular level description of H⁺(aq). Isolated protonated water clusters, H⁺(H₂O)_n, serve as finite model systems for H⁺(aq), which are amenable to highly sensitive and selective gas phase spectroscopic techniques. Here, we isolate and assign the infrared (IR) signatures of the Zundel‐type and Eigen‐type isomers of H⁺(H₂O)6, the smallest protonated water cluster for which both of these characteristic binding motifs coexist, down into the terahertz spectral region. We use isomer‐selective double‐resonance population labeling spectroscopy on messenger‐tagged H⁺(H₂O)₆∙H₂ complexes from 260 to 3900 cm^-1. Ab initio molecular dynamics calculations qualitatively recover the IR spectra of the two isomers and allow attributing the increased width of IR bands associated with H‐bonded moieties to anharmonicities rather than excited state lifetime broadening. Characteristic hydrogen‐bond stretching bands are observed below 400 cm^-1.