Hydration Energies and Reactivity of the Hypohalite Anions

Herein is a dissection of the energetic contributions to a correlation between the rates and driving forces for oxygen atom transfer from three inorganic peroxides to the halides. Experimental data are used to calculate conventional and absolute hydration enthalpies for OCl–, OBr–, and OI–. It is found that the hydration enthalpies are more exothermic for the OX– species in comparison to their X– congeners, and it is found that the hydration enthalpies are approximately constant on progressing from OCl– to OI–. Both of these trends are contrary to expectations based on simple models of ionic hydration. Similar trends are seen in the Gibbs energies of hydration. The strong decrease in E° from OCl– to OI– is seen to arise primarily from these differing trends in hydration energies rather than the gas-phase oxygen atom affinities of the halides. These effects show that the Marcus-like driving-force dependence for oxygen atom transfer from peroxides to the halides arises from the differing trends in hydration energies rather than in the intrinsic O–X– bond strengths.