Terahertz Laser Spectroscopy of the Water Pentamer:  Structure and Hydrogen Bond Rearrangement Dynamics

Abstract

The detailed analysis of a parallel vibration−rotation-tunneling (VRT) band of the isolated cyclic perdeuterated (d 10) 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 (C 5 h ) 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 C 5 h ); 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-d 10, but is predicted to do so for the normal isotopic pentamer-h 10. The experimentally deduced inter-oxygen separations for water clusters up to the pentamer (for which the vibrationally averaged result is R O - O = 2.76 Å) as a function of the cluster size exhibit exponential contraction toward the corresponding distance in ice Ih.

Rearrangements and tunneling splittings of protonated water trimer

“…Qualitative estimates for the magnitude of the splittings suggested that the flip should lead to observable effects in both (H 2 O) 5 and (D 2 O) 5 , while bifurcation tunneling might produce splittings at the limit of current resolution [31]. The latest experimental results appear to be in agreement with these predictions [77]. …”

Terahertz Laser Vibration−Rotation Tunneling Spectroscopy and Dipole Moment of a Cage Form of the Water Hexamer

“…42,43 The size of a given water cluster responsible for an observed absorption feature was determined from an isotope mixture test described previously, 34 wherein the absorption line intensity was measured as a function of the H mole fraction in the H 2 O + D 2 O mixture employed for the production of water clusters. Careful intensity normalization 44 was performed by measuring the signals alternately between an isotope mixture and a pure reference sample. The logarithm of normalized intensities plotted against the H mole fraction ( Figure 2) established a cluster size of 5.99 ( 0.15 to 95% confidence, hence clearly establishing the hexamer as the spectral carrier.…”

Water Pentamer: Characterization of the Torsional-Puckering Manifold by Terahertz VRT Spectroscopy

“…The model of WW provided a reasonable first approximation to the torsional-puckering energy manifold; however, Liu et al 13 argued that the perturbative tunneling model of WW was insufficiently developed to fully characterize the hindered rotational motions of the water pentamer. Liu et al indicated two major shortcomings of the tunneling model of WW: (1) It only predicted a number of energy levels equal to the order of the MS group, nominally ten, since splittings beyond the ground state were not included.…”