A dipole-bound dianion

The possibility of binding two electrons by the dipole potential of a molec ule was examined earlier by us using model potentials. That study suggested that large dipole moments mu=qR and large charge separation distances R (o r equivalently large charges q) would be required to achieve binding two el ectrons. For example, even with a charge q=1.5 a.u. which might be achieved using di- or tri-valent cations, a dipole moment exceeding 15.922 D is nee ded. The presence of inner-shell electrons even further increases the value of mu that is required because the dipole-bound electrons' orbital must be orthogonal to and excluded from such inner shells. In the present work, we discuss our efforts to find a real molecule that can actually bind two ele ctrons to a single dipole site. Numerical results are presented for the mon o- and dianions of a double 5-member carbon ring system substituted with a Ca atom and three superhalogen -PF5 groups. The dianion of this molecule is found to be geometrically stable and to have a vertical electron detachmen t energy of ca. 0.8 eV. Its two excess electrons occupy the same fully symm etric a(1) molecular orbital localized at the electropositive Ca end of the neutral system as is routinely observed in dipole-bound monoanions. Althou gh our final candidate is chemically unusual, it is hoped that our predicti ons about it will encourage others to search for more synthetically tractab le alternatives. (C) 2000 American Institute of Physics. [S0021-9606(00)309 15-1].