Tunneling splittings in the energetically low-lying structural isomers of the water hexamer: the prism, the cage and the book

Abstract

Tunneling splitting (TS) patterns of the energetically low-lying structural isomers of the water hexamer are calculated using the modified WKB (Wentzel–Kramers–Brillouin) method in full dimensionality. TSs in the water hexamer prism are determined for a number of excited low-frequency vibrational modes. Internal rotation of a double-donor water monomer is identified as the mechanism that potentially plays a role in the appearance of the TS pattern in vibrationally excited states in addition to the mechanisms that shape the TS pattern in the ground state. The ground-state TSs of the water hexamer cage were found to form a doublet of doublets. The finer splitting is two orders of magnitude smaller due to a stark difference in the barrier heights for bifurcations of the water monomers at the two opposite vertices of the cage. We also give the first estimates of the ground-state TSs in the water hexamer book structure. The TS pattern is again a doublet of doublets, with the wider doublet of similar size to that in the cage and the narrower doublets an order of magnitude larger than that in the cage. The case study of the cage and the book represents the first realistic application of instanton theories to systems with symmetrically inequivalent wells.