Ab initio study on molecular and thermodynamic properties of water: a theoretical prediction of pK(w) over a wide range of temperature and density

The ionic product of water (pK(w)) has been calculated in a wide range of t emperature (0-600 degrees C) and density (0.6-1.4 g/cm(3)) by means of ab i nitio electronic structure theory combined with the extended reference inte raction site model in statistical mechanics for molecular liquids (RISM-SCF /MCSCF). We consider the autoionization process (H2O + H2O reversible arrow H3O+ + OH-) by regarding H2O, H3O+, and OH- as "solute" molecules in an aq ueous solution and evaluate molecular geometries, electronic structure, sol vation structure, and free energy components of these species as functions of thermodynamical conditions. The results for pK(w) obtained from the theo ry have shown a monotonical decrease with increasing density at all the tem peratures investigated, in good accord with the experimental observation. T he behavior is determined essentially by the difference in solvation free e nergies, Delta mu(H3O+) + Delta mu(OH-) - 2 Delta mu(H2O), associated with the reaction. The Delta mu(OH-) shows the density dependence, which is enti rely different from that of the other species and which gives rise to the o bserved behavior for Delta log K-w. It is shown through analyses of the ele ctronic structure of the "solutes" that the distinct density dependence of Delta mu(OH-) has the origin in its rather "soft" electronic cloud interact ing with solvent polarization.