Specifically, protons diffuse by switching allegiance between neighboring oxygen atoms along hydrogen bonding proton wires, as imagined long ago by Grotthus. The transition state for separating hydronium and hydroxide ions occurs, in part, because the wire connecting the two ions breaks. In the absence of a nearby counter ion, the Grotthus diffusion process occurs on a picosecond time scale. On the other hand, the presence of a nearby counter ion produces a strongly biased electric field that acts to force the diffusion process on a much more rapid time scale, and most typically in the direction of recombination. Temporal fluctuations of the solvent, however, can oppose recombination and indeed lead to the initial separation of hydroxide and hydronium ions.
Transition states for autodissociation therefore involve the coincidence of two events.
First, there must be a solvent fluctuation in local potential energy propelling the proton away from a nascent hydroxide ion. Second, this fluctuation must be accompanied by a breaking of a hydrogen bond along the proton wire connecting the hydroxide and hydronium ions. Without this second event, recombination will occur on the typically sub-picosecond time scale of the solvent fluctuation. This two step procedure was pioneered by Charles Bennett and David Chandler. See, for example, C. H. Bennett, in Algorithms for Chemical Computations , ACS Symp.