MNDO-PM3 INVESTIGATION OF MOLECULE AND ION HYDRATION .1. GEOMETRY ANDTHERMODYNAMICS OF H(H2O)(N)(+) WATER CLUSTERS

Quantum-chemical calculations of the energy, electronic structure and thermodynamics of H(H2O)(n)(+) hydrated clusters have ben performed by the MNDO-PM3 semiempirical method. It was determined that when n is s mall H(H2O)(n)(+) ions have the hydrated H3O+ geometry. When the numbe r of water molecules increases to five each cluster can be both hydrat ed H3O+ and H5O2+ structures. However, the hydration of the H3O+ ion i s energetically preferable. The calculated values of stepwise hydratio n entropy are in fair correspondence with experiment. Nevertheless, th e entropy analysis does not allow distinction to be made between hydra ted H3O+ and H5O2+ ions because both isometric types have close Delta S-n-1,n(o) values and close translation, rotation and vibration compon ents.