Vibrations, tunneling, and transition dipole moments in the water dimer

The wave functions of the dimers (H2O)(2) and (D2O)(2) computed earlier [Gr oenenboom et ol. J. Chem. Phys. 2000, 113, 6702] are analyzed. Their tunnel ing and vibrational behavior is inspected and compared with earlier experim ental assignments of the tunneling and vibrational modes. The transition di pole moments between different vibration-rotation-tunneling states are comp uted, and suggestions are made for possible spectroscopic observations of n ew far-infrared lines. A group-theoretical analysis shows that only the sta tes of E+/- symmetry of the water dimer have a permanent dipole moment. A m odel is presented which relates the values of the parallel transition dipol e 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 per manent dipole moment of the E+/- states.