TERAHERTZ LASER SPECTROSCOPY OF THE WATER PENTAMER - STRUCTURE AND HYDROGEN-BOND REARRANGEMENT DYNAMICS

The detailed analysis of a parallel vibration-rotation-tunneling (VRT) band of the isolated cyclic perdeuterated (d(10)) water pentamer meas ured near 2.4 THz (81.2 cm(-1)) is presented. The vibrationally averag ed rotational constants correspond rigorously to those of a quasiplana r (C-5h) Oblate top whereas the equilibrium cyclic structure is predic ted to be slightly asymmetric due to puckering of the oxygen framework and uneven distribution of the free O-D bonds above and below the rin g. The vibrational averaging which underlies the symmetric top behavio r, the absence of a first-order Stark effect, as well as the origin of the observed intermolecular vibration, is consequently rationalized u sing a five-dimensional model of the pseudorotation, analogous to that established for the water trimer. Pseudorotation is induced by the ne arly barrierless ''flipping'' of the monomers about their donor hydrog en bonds and by accompanying hydrogen bond network puckering motions. The observed vibration is tentatively assigned to the k (pseudorotatio nal quantum number) = 5 (upper) <-- 0 pseudorotational transition clas sified under the cyclic molecular symmetry group G(10) (isomorphic to C-5h); new transitions are also predicted from this model. The donor ( bifurcation) tunneling responsible for the spectral splittings observe d in each pseudorotational state in the water trimer does not produce observable splittings in the pentamer-d(10), but is predicted to do so for the normal isotopic pentamer-h(10). The experimentally deduced in ter-oxygen separations for water clusters up to the pentamer (for whic h the vibrationally averaged result is R0-0 = 2.76 Angstrom) as a func tion of the cluster size exhibit exponential contraction toward the co rresponding distance in ice Ih.