STRUCTURES, BINDING-ENERGIES, AND THERMODYNAMIC FUNCTIONS OF NH4+, NH3(CENTER-DOT+), AND THEIR H2O COMPLEXES

Ab initio calculations are performed for (H2O)n ... H4N+ and (H2O)n ... H3N.+ complexes for n = 0-5. For n = 0 and 1, the geometries of the complexes are optimized at the HF/6-31 + G* and MP2/6-31 + G* levels, and the energies are evaluated at the G2 level. For n = 2-5, the geometry optimizations and frequency calculations are carried out at the HF/6-31 + G* level, and the MP2/6-31 + G* energies are calculated at the HF optimized geometries. Basis set super-position errors are corrected by the Boys-Bernardi scheme at the HF/6-31 + G* level. The gas phase thermodynamic properties C(p)0, S0, and H-0 - H-0(0), are evaluated as functions of temperature using standard statistical methods. Based on the calculated binding energies and the thermodynamic functions, the incremental changes in enthalpies and free energies, DELTAH(n) and DELTAG(n), for the gas phase equilibria (H2O)n-1 M+ + H2O --> (H2O)nM+ for M+ = NH4+ and NH3.+, are evaluated in comparison with the experimental data for (H2O)n ... H4N+, the present results suggest conformations for the hydrated complexes observed in the experiments. The total free energy change for filling the first hydration shell is significantly more negative for NH3.+ than for NH4+.