Gas-Phase Infrared Spectrum of the Protonated Water Dimer

The protonated water dimer is a prototypical system for the study of proton transfer in aqueous solution. We report infrared photodissociation spectra of cooled H + (H 2 O) 2 [and D + (D 2 O 2 ] ions, measured between 620 and 1900 wave numbers (cm −1 ). The experiment directly probes the shared proton region of the potential energy surface and reveals three strong bands below 1600 cm −1 and one at 1740 cm −1 (for H 5 O 2 + ). From a comparison to multidimensional quantum calculations, the three lower energy bands were assigned to stretching and bending fundamentals involving the O⋯H + ⋯O moiety, and the highest energy band was assigned to a terminal water bend. These results highlight the importance of intermode coupling in shared proton systems.