Ab initio theoretical study of dipole-bound anions of molecular complexes. Water trimer anion

We present results of ab initio calculations of our study of (H₂O)^-₃. The main conclusions of this work are as follows: the most stable cyclic structure of (H₂O)₃ has a dipole moment too small to form a dipole-bound state with an excess electron; the dipole-bound anion of the water trimer observed experimentally appears to be a hydrated complex of the water dimer anion, (H₂O)^-₂, by a single water molecule. The water trimer anion, (H₂O)^-₃, has an open shape. The calculated vertical electron detachment energy of this anion is predicted to be equal to 141 meV, which is in good agreement with the experimental value of Bowen and co-workers, equal to 142±7 meV. Although the open optimal geometry of the (H₂O)^-₃ anion obtained in the present calculations is an equilibrium structure, its energy is higher than the energy of the cyclic equilibrium structure of the neutral complex, indicating that the anion is a metastable system. Based on calculations, we predict significant differences in the IR vibrational spectra of (H₂O)₃ and (H₂O)^-₃, which may be used for identification of the two species.