Abstract: The conductor-like polarizable continuum model(CPCM) combined with UAKS cavity was applied to calculating the solvation free energy of 11 ammonium ions in aqueous solution at HF/6-31+G* and B3LYP/6-31+G* levels. The computational values were consistent with the experimental values, the mean errors and standard deviations were 0.17, 12.04 and 0.96, 10.96 kJ/mol, respectively. Using the calculated gas-phase proton-transfer free energies of the ammonium ions at the B3LYP/6-31+G* level of theory, the absolute pK_a values in aqueous solution were obtained and agreed well with the experimental values, the relevant mean errors and standard deviations were only 0.05, 1.50 and 0.45, 1.40 pK_a units, respectively. In order to validate the practicability of CPCM-UAKS model, the absolute pK_a value of noradrenalin was also calculated. The result shows that CPCM-UAKS method can be used to calculate the absolute pK_a values of ammonium ions compounds in aqueous solution accurately.

Key words: CPCM-UAKS model, Solvation free energy, pK_a, Noradrenalin