Nonempiric calculation (MP2/UHF/4-31++G**) shows the presence of inherent H3O and OH fragments in small water cluster cations. According to the arrangement of these fragments, the structures of cations are divided into two groups:  either OH fragment acts exclusively as a proton acceptor in all its hydrogen bonds, or it is directly bonded to H3O and acts also as a proton donor in the H-bond with a water molecule. At the external effect of about 0.4 eV, the former cations can dissociate into free or quasifree OH radical and a protonated water cluster of the corresponding size. An extrapolation of the adiabatic ionization potentials of water clusters to an infinite cluster size provides the value of 8.5 eV close to the experimental photoelectric threshold of amorphous ice. When the adiabatic hydration of the electron knocked out is taken into account, the energy of 6.8 eV should be sufficient for the ionization of an ice specimen.

ABSTRACT:Nonempirical direct molecular dynamics of neutral and cationic water clusters involving four to six molecules is studied to shed light on the character of the ionization of water clusters. Potential energy and forces are calculated in the second-order of the Møller-Plesset perturbation theory with a 6 -31ϩϩG(1d, 1p) basis set. Upon vertical ionization of a water cluster, H 3 O ϩ and OH fragments appear. The process needs no less than 40 fs, and in some cases, up to 100 or 150 fs. The resulting configuration of the cluster (neglecting its generally larger volume) is close to that obtained in the geometry optimization run. Vibrational excitation of a neutral cluster with a total energy about half its adiabatic ionization potential may promote the appearance of structures similar to that of the stable cation at certain moments in time. In particular, a configuration in which only one water molecule separates H 3 O ϩ and OH Ϫ fragments forms much faster than upon vertical electron detachment from a stable neutral cluster, in nearly 18 fs. Such structure reorganization is noticeably impeded by the rotational-vibrational excitation of the whole cluster, especially when hydrogen bonds are substantially distorted. The effect of distorting motions becomes very pronounced after 20 fs.

Water cluster anions (H2O) n - with n = 2−8 are considered as a gradual approach to the model of a hydrated electron. In the structures of cluster anions optimized at the unrestricted Hartree−Fock level, the excess electron density is always localized around protons of the hydrogen atoms uninvolved in hydrogen bonds. Yet, two types of structures are distinguished. In the first type, the excess electron is localized in an individual though deformed cluster. The second type embraces the face-to-face anionic species, in which two confronted clusters that are not joined via a usual hydrogen bond localize the electron. The vertical energies of the electron detachment from the anions estimated in the second order of the Møller−Plesset perturbation theory are positive already for the trimer anion consisting of the confronted monomer and dimer. A transformation of the plain chainlike neutral octamer into the most stable face-to-face anion upon adding an electron shows that eight molecules already can form a stable solvation shell of the excess electron. Varying the exponent of the diffuse s functions centered on hydrogen nuclei showed that there is probably an optimum exponent of about 0.020 for the description of the hydrated electron. Two types of excess electron localization are distinguished, namely, surface and interface.

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