Structures of Cage, Prism, and Book Isomers of Water Hexamer from Broadband Rotational Spectroscopy

Cristóbal Pérez, University of Virginia - Main Campus
Matt T. Muckle, University of Virginia - Main Campus
Daniel P. Zaleski, University of Virginia - Main Campus
Nathan A. Seifert, University of Virginia - Main Campus
Berhane Temelso, Bucknell University
George C. Shields, Bucknell University
Zbigniew Kisiel, Polish Academy of Sciences
Brooks H. Pate, University of Virginia - Main Campus


Theory predicts the water hexamer to be the smallest water cluster with a three-dimensional hydrogen-bonding network as its minimum energy structure. There are several possible low-energy isomers, and calculations with different methods and basis sets assign them different relative stabilities. Previous experimental work has provided evidence for the cage, book, and cyclic isomers, but no experiment has identified multiple coexisting structures. Here, we report that broadband rotational spectroscopy in a pulsed supersonic expansion unambiguously identifies all three isomers; we determined their oxygen framework structures by means of oxygen-18–substituted water (H218O). Relative isomer populations at different expansion conditions establish that the cage isomer is the minimum energy structure. Rotational spectra consistent with predicted heptamer and nonamer structures have also been identified.