Mechanism of hydroxide mobility

A suggested mechanism for hydroxide mobility in water identifies the rate limiting step as a cleavage of a second shell hydrogen bond which converts a H₇O₄^- ion (triply coordinated hydroxide) to (HOHOH)^- (deprotonated water dimer). Proton transfer is enabled by an additional O-O bond contraction, not required in H₅O₂⁺. This explains why the activation energy for hydroxide mobility is larger than that of proton mobility by about 0.5 kcal/mol. The transfer cycle is terminated by hydrogen-bond formation to the other oxygen center. Available experimental data, and most of the computational results, can be rationalized in the framework of the above model.