Hemibonding between Water Cation and Water

The hemibonding interaction in the water dimer cation is studied using coupled cluster electronic structure methods. The hemibonded dimer cation geometry is a local minimum structure characterized by the two participating monomers having both a very short separation and a near parallel relative orientation. It is shown that the vertically ionized dimer at its optimum neutral geometry can convert to the hemibonded dimer cation structure with essentially no energetic hindrance. Direct conversion to the hemibonded structure is therefore an energetically facile alternative to the minimum energy path that connects the vertically ionized neutral water dimer to the global minimum proton-transferred structure. A substantial barrier must be surmounted to convert the hemibonded dimer cation to the proton-transferred structure. The optical absorption spectrum of the hemibonded dimer cation is characterized by three excited near-UV states, two of which have very large oscillator strengths. Relative resonance Raman intensities are estimated for the hemibonded dimer cation vibrational modes, finding the intermolecular stretching mode to be the most strongly enhanced when in near resonance with each of the near-UV excited states, and the anharmonicity and overtones of this mode are estimated. These results provide guidance for the possible observation of hemibonded cations in irradiated liquid water.