THEORETICAL-STUDY FOR AUTOIONIZATION OF LIQUID WATER - TEMPERATURE-DEPENDENCE OF THE IONIC PRODUCT (PK(W))

The temperature dependence of the ionic product of water (pK(w)) is in vestigated theoretically by means of ab initio electronic structure th eory combined with the extended reference interaction site method in s tatistical mechanics of molecular liquids (RISM-SCF/MCSCF method). The chemical equilibrium H2O + H2O reversible arrow H3O+ + OH- is studied , in which water molecules? hydronium ions, and hydroxide ions are reg arded as ''solute'' molecules in aqueous solution. Molecular geometrie s, electronic structures, pair correlation functions, and free energy components of those species as well as their temperature dependence ar e calculated. It is shown that the hydroxide anion is polarized more e asily by surrounding solvent compared to the other species. The solven t-induced electronic structure relaxes toward that in the gas phase as temperature increases. The hydroxide anion exhibits the largest tempe rature dependence in the electronic structure as well as in solvation structure. It is found that changes in the solvation free energies dri ve the chemical equilibrium toward the left-hand side (association) as temperature increases, while energies associated with solvent-induced reorganization of electronic structure make the opposite contribution . The temperature dependence of pK(w). is dominated by the latter cont ribution, which gives rise to good agreement with the experimental res ults. It is suggested that the observed temperature dependence of pK(w ) is related to the great sensitivity of the electronic structure of O H- on the solvent effect.