Structure and torsional dynamics of the water octamer from THz laser spectroscopy near 215 μm.

Clusters of eight water molecules play an important role in theoretical analysis of aqueousstructure and dynamics but have proven to be challenging experimental targets. Herewe report the high-resolution spectroscopic characterization of the water octamer.Terahertz (THz) vibration-rotation-tunneling (VRT) spectroscopy resolved 99 transitionswith 1 part per million precision in a narrow range near 46.5 wave numbers, which wereassigned to the h16octamer via detailed isotope dilution experiments. Fitting to a semi-rigid symmetric top model supports predictions of two coexisting cuboidal structures andprovides precise values for the changes in their rotational constants. Comparison withtheory and previous spectroscopic data provides a characterization of the two structuresand the observed torsional vibration and supports the prediction that the D2d symmetry structure is lower in energy than the S4 isomer