CHARACTERIZATION OF A CAGE FORM OF THE WATER HEXAMER

WATER has been studied more extensively than any other liquid, Set its microscopic properties remain poorly understood. The difficulty in ob taining a rigorous molecular-scale description of water structure is l argely a consequence of the extended, dynamic hydrogen-bonded network that exists throughout the liquid(1). Studies of the structure and dyn amics of isolated small clusters of water molecules(2-6) provide a mea ns of quantifying the intermolecular forces and hydrogen-bond rearrang ements that occur in condensed phases. Experiments(2-7) and theory(8) strongly suggest that the water trimer, tetramer and pentamer have cyc lic minimum energy structures. Larger water clusters are expected(8) t o have three-dimensional geometries, with the hexamer representing the transition from cyclic to such three-dimensional structures. Here we report investigations by terahertz laser vibration-rotation tunnelling spectroscopy(3) of the structure of the water hexamer. A comparison o f our results with quantum Monte Carlo simulations of this Species sug gests that the most stable form of (H2O)(6) is indeed a cage-like stru cture, held together by eight hydrogen bonds (Fig. 1).