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 ol. 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 A(1)(+/-) and B-1(+/-) symmetry and between states of A(2)(+/-) and B-2(+/-) symmetry to the permanent dipole moment of the E+/- states.