Environmental effects on vibrational proton dynamics in H5O2+: DFT study on crystalline H5O2+ClO4-.

The structure as well as IR and inelastic neutron scattering (INS) spectra of H5O2+ in crystalline H5O2+ClO4- were simulated using Car-Parrinello molecular dynamics with the BLYP functional. The potential of the O...H+...O fragment is very shallow. The Pnma structure, assumed in the X-ray study to be the most suitable choice, is a saddle point on the potential energy surface, while the P2(1)2(1)2(1) minimum structure is only 20 cm(-1) lower in energy. The computed INS and IR spectra enable us to achieve a complete assignment of the observed spectra. The broad band between 1000 and 1400 cm(-1) is due to the asymmetric stretch and one of the bending vibrations of the O...H+...O fragment, while the band between 1600 and 1800 cm(-1) is due to the bending vibration of the water molecules and the second bending of the O...H+...O fragment. Comparison with the vibrational spectra of isolated H5O2+, obtained using Born-Oppenheimer molecular dynamics simulation, reveals environmental effects on vibrational proton dynamics in strong H-bonded species. The most pronounced changes are found for the O...H+...O bending modes because the two bending coordinates become distinctly different for the structure that the H5O2+ ion assumes in the crystal.