Dynamics and infrared spectroscopy of the protonated water dimer.

Accurate infrared (IR) spectroscopy of protonated water clusters prepared in the gas phase has become possible in recent years,[1, 2, 3, 4, 5, 6] opening the door to a deeper understanding of the properties of aqueous systems and the hydrated proton, which are of main interest in central areas of chemistry and biology. Several computational studies have appeared in parallel, providing a necessary theoretical basis for the assignment and understanding of the different spectral features.[7, 8, 9, 6, 10] It has been recently demonstrated that the H5O + 2 motif, also referred to as Zundel cation, plays an important role in protonated water clusters of 6 or more water molecules and as a limiting structure, together with the Eigen cation (H9O + 4 ), of the hydrated