Mechanism of hydroxide mobility

A suggested mechanism for hydroxide mobility in water identifies the fare 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. (C) 2000 Elsevier Science B.V. All rights reserved.