Gas‐Phase Infrared Spectrum of the Protonated Water Dimer: Molecular Dynamics Simulation and Accuracy of the Potential Energy Surface

Ab initio molecular dynamics simulations show that the gas-phase infrared spectra of H5O2+ (blue line) and the H5O2+⋅Ar complex (black line) are very similar. The broad band between 800 and 1300 cm−1 is due to motions of the bridging proton in the OO direction coupled to torsion and wagging of the bridged water molecules. The bending motions of the proton perpendicular to the OO direction are not seen in the IR spectrum.