Measurement of the far-infrared vibration-rotation tunneling spectrum of the perdeuterated water tetramer is described. Precisely determined rotational constants and relative intensity measurements indicate a cyclic quasi-planar minimum energy structure, which is in agreement with recent ab initio calculations. The O-O separation deduced from the data indicates a rapid exponential convergence to the ordered bulk value with increasing cluster size. Observed quantum tunneling splittings are interpreted in terms of hydrogen bond rearrangements connecting two degenerate structures.

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Using tunable far infrared laser absorption spectroscopy, 12 vibration-rotation-tunneling (VRT) subbands, consisting of approximately 230 transitions have been measured and analyzed for an 82.6 cm -1 intermolecular vibration of the water dimer-d 4 • Each of the VRT subbands originate from K; = 0 and terminate in either K~ = 0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A' rotational constant of 122.9 GHz is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined.to exhibit large dependencies with both the K~ quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method (lAM) treatment, which has been developed to model the ground state tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excitation indicate that the high barrier approximation does not appear to be applicable in the excited state. The data are consistent with a motion possessing a' symmetry, and the vibration is assigned as the Vs acceptor bending coordinate. This assignment is in agreement with the vibrational symmetry determined from the data, the results of high level ab initio calculations, and preliminary data assigned to the analogous vibration in D 2 0-DOH. 6600