Cage Match

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Science  07 Sep 2012:
Vol. 337, Issue 6099, pp. 1150
DOI: 10.1126/science.337.6099.1150-a

What does water look like? Many think of waves and ripples, and various shades of color, reflected from whatever materials contain and constrain the liquid, disrupting an otherwise perfect clarity. Chemists want to know about the structure—how individual H2O molecules orient relative to one another and thereby lend water its range of remarkable properties. In this context, the hexamer (six molecules) has special significance: It's the smallest, and therefore most easily analyzed, three-dimensional assembly. Yet even this simplified sub-structure has been the subject of decades-long debate regarding which of three possible arrangements is the most stable. Only very recently were all three successfully observed in the same experiment (see Pérez et al., Reports, 18 May 2012, p. 897). This gave the so-called cage a slight edge over the prism, though results depended on the inert carrier gas used for spectral characterization. Wang et al. have now performed high-level theoretical calculations that give the prism the very slightest edge but suggest that the cage has nearly the same energy at the lowest temperature and soon overtakes the prism upon warming, on account of its greater entropy. Eventually, a book shape with the greatest entropy is predicted to overtake them both, closing the book, perhaps, on the whole question.

J. Am. Chem. Soc. 134, 11116 (2012).

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