Small charged water clusters: Anions

Water cluster anions (H2O)(n)(-) with n = 2-8 are considered as a gradual a pproach to the model of a hydrated electron. In the structures of cluster a nions optimized at the unrestricted Hartree-Fock level, the excess electron density is always localized around protons of the hydrogen atoms uninvolve d in hydrogen bonds. Yet, two types of structures are distinguished. In the first type, the excess electron is localized in an individual though defor med 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 fr om the anions estimated in the second order of the Moller-Plesset perturbat ion theory are positive already for the trimer anion consisting of the conf ronted monomer and dimer. A transformation of the plain chainlike neutral o ctamer into the most stable face-to-face anion upon adding an electron show s that eight molecules already can form a stable solvation shell of the exc ess electron. Varying the exponent of the diffuse s functions centered on h ydrogen nuclei showed that there is probably an optimum exponent of about 0 .020 for the description of the hydrated electron. Two types of excess elec tron localization are distinguished, namely, surface and interface.