STRUCTURE AND ENERGY OF THE POSITIVELY IONIZED WATER CLUSTERS

Geometry optimization of small (H2O)(n)(+) clusters (n less than or eq ual to 4) at the UHF/4-31 + + G* level indicates that the cations con sist of two fragments: the OH radical and the H2n-1On-1+ ion. The latt er can be considered as a thermodynamically stable combination of a di storted H3O+ ion and (n - 2) H2O molecules. The H bond between the fra gments becomes weaker with increasing cluster size. Extrapolation of t he adiabatic ionization potentials calculated for the (H2O)(n) oligome rs (n less than or equal to 4) at the MP2 level to an infinite cluster size provides the value of approximately 8.7 eV, which can be presuma bly necessary for the ionization of liquid water in a vacuum. (C) 1997 John Wiley & Sons, Inc.