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+(H2O)(2) [and D+(D2O2] 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 H5O2+). 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.