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 chemically distinct H-bond topologies of the cage structure were optimized for geometry and vibrationally analyzed with the PM3 semiempirical method. The structures, 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. With structures of the same number of SD-SA water molecules, stability is anticorrelated with the dipole moment. The global minimum-energy structure has been identified and is one of four of particularly stable cage structures related by free hydrogen flipping. The lowest energy structures may interconnect as a result of large-amplitude quantum mechanical motion. The global minimum cage structure is also found to be more stable than the lowest energy, topologically enumerated structures of the prism, book, chair, and boat frameworks.