HYDROGEN-BOND KINETICS IN LIQUID WATER

HYDROGEN bonds play a crucial role in the behaviour of water(1-4); the ir spatial patterns and fluctuations characterize the structure and dy namics of the liquid(5-7). The processes of breaking and making hydrog en bonds in tile condensed phase can be probed indirectly by a variety of experimental techniques(8), and more quantitative information can be obtained from computer simulations(9). In particular, simulations h ave revealed that on long timescales the relaxation behaviour of hydro gen bonds in liquid water exhibit non-exponential kinetics(7,10-13), s uggesting that bond making and breaking are not simple processes chara cterized by well defined rate constants. Here we show that these kinet ics can be understood in terms of an interplay between diffusion and h ydrogen-bond dynamics. In our model, which can be extended to other hy drogen-bonded liquids, diffusion governs whether a specific pair of wa ter molecules are near neighbours, and hydrogen bonds between such pai rs form and persist at random with average lifetimes determined by rat e constants for bond making and breaking.