Structure and energy of the positively ionized water clusters

Geometry optimization of small (H2O)n+ clusters (n ≤ 4) at the UHF/4–31 + + G** level indicates that the cations consist of two fragments: the OH radical and the H2n−1 O+n−1 ion. The latter can be considered as a thermodynamically stable combination of a distorted H3O+ ion and (n−2) H2O molecules. The H bond between the fragments becomes weaker with increasing cluster size. Extrapolation of the adiabatic ionization potentials calculated for the (H2O)n oligomers (n ≤ 4) at the MP2 level to an infinite cluster size provides the value of approximately 8.7 eV, which can be presumably necessary for the ionization of liquid water in a vacuum.