IR Spectroscopy of Protonation in Benzene-Water Nanoclusters: Hydronium, Zundel, and Eigen at a Hydrophobic Interface

The structure of ions in water at a hydrophobic interface influences important processes throughout chemistry and biology. However, experiments to measure these structures are limited by the distribution of configurations present and the inability to selectively probe the interfacial region. Here, protonated nanoclusters containing benzene and water are produced in the gas phase, size-selected, and investigated with infrared laser spectroscopy. Proton stretch, free OH, and hydrogen-bonding vibrations uniquely define protonation sites and hydrogen-bonding networks. The structures consist of protonated water clusters binding to the hydrophobic interface of neutral benzene via one or more pi-hydrogen bonds. Comparison to the spectra of isolated hydronium, zundel, or eigen ions reveals the inductive effects and local ordering induced by the interface. The structures and interactions revealed here represent key features expected for aqueous hydrophobic interfaces.