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

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 geome try 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 correcte d 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 a s functions of temperature using standard statistical methods. Based o n the calculated binding energies and the thermodynamic functions, the incremental changes in enthalpies and free energies, DELTAH(n) and DE LTAG(n), for the gas phase equilibria (H2O)n-1 M+ + H2O --> (H2O)nM+ f or M+ = NH4+ and NH3.+, are evaluated in comparison with the experimen tal data for (H2O)n ... H4N+, the present results suggest conformation s for the hydrated complexes observed in the experiments. The total fr ee energy change for filling the first hydration shell is significantl y more negative for NH3.+ than for NH4+.