The hydration state of HO−(aq)

The HO−(aq) ion participates in myriad aqueous phase chemical processes of biological and chemical interest. A molecularly valid description of its hydration state, currently poorly understood, is a natural prerequisite to modeling chemical transformations involving HO−(aq). Here it is shown that the statistical mechanical quasi-chemical theory of solutions predicts that HO · [H2O]3− is the dominant inner shell coordination structure for HO−(aq) under standard conditions. Experimental observations and other theoretical calculations are adduced to support this conclusion. Hydration free energies of neutral combinations of simple cations with HO−(aq) are evaluated and agree well with experimental values.