Dynamics and Infrared Spectroscopy of the Protonated Water Dimer

The dynamics and infrared absorption spectrum of the protonated water dimer are reported by full quantum simulation. Strong couplings between the spectroscopically active proton-transfer motion and low-frequency, large-amplitude torsional modes are clearly identified and their role in the cluster dynamics is explained. These couplings are responsible for the characteristic doublet-peak around 1000 cm-1, which was not understood and subject of debate. This spectral feature is reproduced, assigned and explained.