Anionic water clusters with large vertical electron binding energies and their electronic spectra: (H₂O)₁₁^- and (H₂O)₁₄^-

The structures of four isomers of both (H₂O)₁₁^- and (H₂O)₁₄^- cluster anions have been determined by geometry optimization at the ROHF level. These structures exhibit vertical electron binding energies of the order of 0.4-0.6 eV and 0.8-1.0 eV for (H₂O)₁₁^- and (H₂O)₁₄^-, respectively, in satisfactory agreement with the maxima of the experimental photodetachment spectra. Three of the structures contain a H₃O⁺-OH^- ion pair; the others consist of nondissociated water molecules with maximally aligned dipole moments. The dipole moments of the corresponding neutral clusters are significant (12-18 Debye for (H₂O)₁₁, 18-24 Debye for (H₂O)₁₄). The excess electron in these cluster anions is a so-called surface electron, i.e., it resides outside the water hydrogen-bond network. The vertical excitation energies and oscillator strengths of these cluster anions have been calculated with the TDDFT method. All cluster anions exhibit a strong low-energy absorption band corresponding to the 3s → 3p transition of the excess electron. The peak of this absorption band is found between 0.55 and 0.75 eV for (H₂O)₁₁^- and between 0.77 and 0.95 eV for (H₂O)₁₄^-, in good agreement with the experimental estimates of 0.70 eV and 0.79 eV for (H₂O)₁₁^- and (H₂O)₁₄^-, respectively.