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 (d10) water pentamer measured near 2.4 THz (81.2 cm-1) is presented. The vibrationally averaged rotational constants correspond rigorously to those of a quasiplanar (C5h) oblate top whereas the equilibrium cyclic structure is predicted to be slightly asymmetric due to puckering of the oxygen framework and uneven distribution of the free O−D bonds above and below the ring. The vibrational averaging which underlies the symmetric top behavior, the absence of a first-order Stark effect, as well as the origin of the observed intermolecular vibration, is consequently rationalized using a five-dimensional model of the pseudorotation, analogous to that established for the water trimer. Pseudorotation is induced by the nearly barrierless “flipping” of the monomers about their donor hydrogen bonds and by accompanying hydrogen bond network puckering motions. The observed vibration is tentatively assigned to the k (pseudorotational quantum number) = 5 (upper) ← 0 pseudorotational transition classified under the cyclic molecular symmetry group G10 (isomorphic to C5h); new transitions are also predicted from this model. The donor (bifurcation) tunneling responsible for the spectral splittings observed in each pseudorotational state in the water trimer does not produce observable splittings in the pentamer-d10, but is predicted to do so for the normal isotopic pentamer-h10. The experimentally deduced inter-oxygen separations for water clusters up to the pentamer (for which the vibrationally averaged result is RO-O = 2.76 Å) as a function of the cluster size exhibit exponential contraction toward the corresponding distance in ice Ih.