Characterization of a cage form of the water hexamer

Abstract

WATER has been studied more extensively than any other liquid, yet its microscopic properties remain poorly understood. The difficulty in obtaining a rigorous molecular-scale description of water structure is largely a consequence of the extended, dynamic hydrogen-bonded network that exists throughout the liquid¹. Studies of the structure and dynamics of isolated small clusters of water molecules^2–6 provide a means of quantifying the intermolecular forces and hydrogen-bond rearrangements that occur in condensed phases. Experiments^2–7 and theory⁸ strongly suggest that the water trimer, tetramer and pentamer have cyclic minimum energy structures. Larger water clusters are expected⁸ to 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 spectroscopy³ of the structure of the water hexamer. A comparison of our results with quantum Monte Carlo simulations of this species suggests that the most stable form of (H₂O)₆ is indeed a cage-like structure, held together by eight hydrogen bonds (Fig. 1).