Autoionization in Liquid Water

The dissociation of a water molecule in liquid water is the fundamental event in acid-base chemistry, determining the pH of water. Because of the short time scales and microscopic length scales involved, the dynamics of this autoion-ization have not been directly probed by experiment. Here, the autoionization mechanism is revealed by sampling and analyzing ab initio molecular dynamics trajectories. We identify the rare ßuctuations in solvation energies that de-stabilize an oxygen-hydrogen bond. Through the transfer of protons along a hydrogen bond Òwire,Ó the nascent ions separate by three or more neighbors. If the hydrogen bond wire connecting the two ions is subsequently broken, a metastable charge-separated state is visited. The ions may then diffuse to large separations. If, however, the hydrogen bond wire remains unbroken, the ions recombine rapidly. Because of their concomitant large electric Þelds, the tran-sient ionic species produced in this case may provide an experimentally de-tectable signal of the dynamics we report. A randomly chosen, intact water molecul