Vibrations, Tunneling, and Transition Dipole Moments in the Water Dimer

The wave functions of the dimers (H₂O)₂ and (D₂O)₂ computed earlier [Groenenboom et al. J. Chem. Phys. 2000, 113, 6702] are analyzed. Their tunneling and vibrational behavior is inspected and compared with earlier experimental assignments of the tunneling and vibrational modes. The transition dipole moments between different vibration−rotation−tunneling states are computed, and suggestions are made for possible spectroscopic observations of new far-infrared lines. A group-theoretical analysis shows that only the states of E^± symmetry of the water dimer have a permanent dipole moment. A model is presented which relates the values of the parallel transition dipole moments associated with tunneling between states of and symmetry and between states of and symmetry to the permanent dipole moment of the E^± states.