TUNNELING DYNAMICS, SYMMETRY, AND FAR-INFRARED SPECTRUM OF THE ROTATING WATER TRIMER .1. HAMILTONIAN AND QUALITATIVE MODEL

Hamiltonian is derived for the rotating water trimer with three intern al motions-the rotations of the monomers about their hydrogen bonds. W e obtain an expression of the kinetic energy operator, which is a non- trivial extension of earlier heuristic forms used for the non-rotating trimer. The Coriolis coupling operator between the single-axis monome r angular momenta and the overall trimer rotation is given for the fir st time. To analyze the effects of the tunneling and Coriolis splittin gs on the energy levels of the trimer, we introduced a qualitative mod el for the pseudo-rotation and donor tunneling. By perturbation theory and application of the permutation-inversion groups G(6) and G(48) we obtain algebraic expressions for the splittings due to pseudo-rotatio n and donor tunneling, respectively. The pseudo-rotation does not prod uce any internal angular momentum and does not yield first order Corio lis splitting, but in second order the Coriolis coupling lifts various degeneracies and gives rise to observable J-dependent splittings. Don or tunneling splits every pseudo-rotation level into a quartet and tho se levels in this quartet that belong to the three-dimensional irreps of G(48) into doublets. For J>0 a rather complex pattern of larger (fo r the internal states with G(6) labels k=+/-1 and +/-2) and smaller (f or the levels with k=0 and k=3) splittings is obtained, especially for the substates with \K\=1 which are Coriolis coupled to the K=0 substa tes. The results of calculations in the companion paper, together with the model introduced in the present paper, will be used to interpret all the tunneling splittings observed in high-resolution spectra of (H 2O)(3) and (D2O)(3). (C) 1996 American Institute of Physics.