Tunneling Splittings in the Water Hexamer Prisms Composed of Stacked Water Trimers.

We apply the modified WKB method to calculate tunneling splitting (TS) patterns in two hydrogen-bond isomers of the water hexamer, formed by the association of two water trimer rings stacked on top of each other with the same, clockwise-clockwise (CWCW), and the opposite, clockwise-counterclockwise (CWCCW), directionality of the in-ring hydrogen bonds. The TS patterns are composed of 78 (CWCW) and 90 (CWCCW) energy levels, which are interpreted in terms of the cluster rearrangements of flips, bifurcations and monomer rotations between equivalent symmetry-related minima of the cluster. In the vibrational ground state and the low-lying vibrationally excited states, we observe that two different monomer rotations, that both break two hydrogen bonds, explain the main features of the observable TS pattern in the CWCW form. In the CWCCW form, the dominant mechanism is a double flip, while several other mechanisms compete resulting in qualitatively different TS patterns in different vibrational states. The overall width of the TS pattern is larger in the more stable CWCCW form. Mild enhancements of the TS widths are observed in the fifth and sixth excited mode (numbered by increasing energy) in CWCW (19×) and CWCCW (4.8×) isomeric forms, respectively. We demonstrate that the present approach can be used to assign and interpret vibrational tunneling spectra of water clusters in terms of its distinct rearrangement mechaninsms when they compete for dominance in different low-lying vibrational states.