The importance of thermodynamic quantities for the determination of the unknown conformation: Ab initio studies of K+(H2O)(3)

The structures, the energetics, and the spectra of K+(H2O)(3) have been stu died at HF and MP2 levels with the basis set of triple-zeta plus two sets o f polarization functions (TZ2P) fur water molecules. Two structures conside red are 3+0 (D-3), and 2+1 (C-2v). The 2+1 (C-2v) has two hydrogen bonds be tween the primary hydration and the secondary hydration shell water molecul es. They have similar binding energy and enthalpy. The most stable conforma tion of K+(H2O)(3) is entropy driven as shown in Na+(H2O)(5) and in Na+(H2O )(6) cases. The 3+0 (D-3) conformation is the most stable at 298 K and at 1 atm, based on Gibbs free energy changes (Delta G(r)). The thermal contribu tions to the enthalpy and the Gibbs free energy are corrected for the low f requency modes. The corrected Delta G(r) is in good agreement with the expe rimental value. Vibrational frequencies of two conformations are revealed a s their characteristics.