Water clusters exposed

02 June 2012 - Chemical physics

Another one of those unsolved problems in chemistry solved! At least according to Pérez et al. who have news to report regarding water clusters (DOI). Simple water is not just a collection of water molecules: due to hydrogen bonding water molecules tend to cluster into larger assemblies but what do these assemblies exactly look like? The proposals vary from dimers to (H2O)100 networks but tetramers, pentamers and hexamers take prominent positions. Pérez has been looking at hexamers only and more specifically the hexamer isomers. The weapon of choice: rotational spectroscopy or more precisely a chirped-pulsed Fourier transform microwave (CP-FTMW) spectrometer. Rotational spectroscopy can only be applied to gas phase molecules which makes you wonder how it can be used in a typical bulk property. Experimental details are scarce but probably the method works by ejecting liquid water into a vacuum and then the clusters are analysed as they break up? A carrier gas is also involved. Somehow. The working temperature is not mentioned anywhere. Out comes a complex spectrum but based on computational modelling all peaks can be assigned to the heptamer and three hexamer isomers: a cage, a prism and a book with population ratio 1:1:0.25. Rest assured, there are complications: the ratio depends on the type of carrier gas and peaks multiply by using oxygen-18 enriched water.
The presence of a book geometry is counter-intuitive and instantly this blogs favorite. With 5 idle hydrogen atoms, this isomer should convert to the prism (three idle hydrogen atoms) with ease but does not. And what about a (H2O)6n ladderane?