Enumeration and evaluation of the water hexamer cage structure

A topological enumeration has identified all hydrogen bond arrangements of the (H2O)(6) cage, prism, book, chair. and boat frameworks. The 27 chemical ly distinct H-bond topologies of the cage structure were optimized for geom etry and vibrationally analyzed with the PM3 semiempirical method. The stru ctures, which differ only by the arrangement of the H-bonds, have minimized energies falling in a range of 10 kcal/mol and have dipole moments varying from 0 to 11 D. Stability of the structures is correlated with an increase in the number of single donor/single acceptor water (SD-SA) molecules. Wit h structures of the same number of SD-SA water molecules, stability is anti correlated with the dipole moment. The global minimum-energy structure has been identified and is one of four of particularly stable cage structures r elated by free hydrogen flipping. The lowest energy structures may intercon nect as a result of large-amplitude quantum mechanical motion. The global m inimum cage structure is also found to be more stable than the lowest energ y, topologically enumerated structures of the prism, book, chair, and boat frameworks.