Abstract: The solvation free energies of NH₂^-, NH₃, and NH₄⁺ in aqueous solution were calculated by means of a mixed discrete-continuum model, in which the solute and a limited number of solvent molecules were treated quantum chemically and the remaining solvent was simulated by the polarizable continuum model(PCM) method. The numbers of water molecules hydrogen-bonding directly with NH₂^-, NH₃, and NH₄⁺ are predicted to be 2, 4, and 4 respectively at the HF level. By comparing with the results obtained with the PCM method, our results show that because specific solute-solvent interactions are described at the quantum level and long-range interactions are introduced through a continuum model, the discrete-continuum model could simulate the solvation effects more accurately than the continuum model, especially in case where the solvent and the solute are strongly bound. The predicted solvation free energies for NH₂^- and NH₄⁺ are in good agreement with available experimental data. Due to the explicit consideration of specific solute-solvent interactions, the electron correlation effect is significant in the discrete-continuum model. Furthermore, from the gas phase to the solution, the large changes of geometries of hydrated solutes caused by the continuum field suggest that the geometry reoptimization in the solvent is necessary. In the discrete-continuum model, the geometries of hydrated ions closer to the real case would lead more accurate solvation free energies.

Key words: NH₂^-, NH₃, NH₄⁺, Discrete-continuum model, Solvation free energy