Isomer-Selective Detection of Hydrogen-Bond Vibrations in the Protonated Water Hexamer

• DOI: 10.1021/ja401359t
• Publication Date: 2013-05-12T20:36:17Z
• Authors: Nadja Heine; Matias R. Fagiani; Mariana Rossi; Torsten Wende; Giel Berden; Volker Blum; Knut R. Asmis
• Keywords: FP7; EC; Molecular Structure and Dynamics; Water; Hydrogen Bonding; Stereoisomerism; Research Infrastructures; Vibration; 01 natural sciences; 0103 physical sciences; Quantum Theory; Protons; European Commission; SP4-Capacities

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).