title: The water hexamer: three-body interactions, structures, energetics, and OH-stretch spectroscopy at finite temperature.
abstract: Using a newly developed and recently parameterized classical empirical simulation model for water that involves explicit three-body interactions, we determine the eleven most stable isomers of the water hexamer. We find that the lowest energy isomer is one of the cage structures, in agreement with far-IR and microwave experiments. The energy ordering for the binding energies is cage > glove > book > bag > chair > boat > chaise, and energies relative to the cage are in good agreement with CCSD(T) calculations. The three-body contributions to the cage, book, and chair are also in reasonable agreement with CCSD(T) results. The energy of each isomer results from a delicate balance involving the number of hydrogen bonds, the strain of these hydrogen bonds, and cooperative and anti-cooperative three-body interactions, whose contribution we can understand simply from the form of the three-body interactions in the simulation model. Oxygen-oxygen distances in the cage and book isomers are in good agreement with microwave experiments. Hydrogen-bond distances depend on both donor and acceptor, which can again be understood from the three-body model. Fully anharmonic OH-stretch spectra are calculated for these low-energy structures, and compared with shifted harmonic results from ab initio and density functional theory calculations. Replica-exchange molecular dynamics simulations were performed from 40 to 194 K, which show that the cage isomer has the lowest free energy from 0 to 70 K, and the book isomer has the lowest free energy from 70 to 194 K. OH-stretch spectra were calculated between 40 and 194 K, and results at 40, 63, and 79 K were compared to recent experiments, leading to re-assignment of the peaks in the experimental spectra. We calculate local OH-stretch cumulative spectral densities for different donor-acceptor types and compare to analogous results for liquid water.
authors.affiliation: Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Tainter
authors.forename: J L
authors.initials: JL
authors.lastname: Skinner
mesh_terms: D006860:Hydrogen Bonding; D006878:Hydroxides / Q000737:chemistry*; D008872:Microwaves; D015394:Molecular Structure; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D006878:Hydroxides; D014867:Water; C031356:hydroxide ion