Search for “quadrupole-bound” anions. I

In a classical model, some anions exist due to the attraction between an electron and a molecule’s dipole moment. When the dipole moment is sufficiently large $(μcrit>2.5 D),$ an electron can be trapped. Can a sufficiently large quadrupole moment produce the same effect? To help answer this question, we can search for molecules with a large quadrupole moment and use predictive, ab initio, correlated quantum chemistry methods to assess whether an anion forms and, if it does, to discover its nature. For this purpose, coupled-cluster calculations are reported for the structure and properties of $KnClm$ and $KnClm−$ $(n,m=0–2).$ The $KCl2$ superhalogen was found to have an electron affinity of 4.2 eV and is stable towards dissociation by 26 kcal/mol. The $(KCl)2$ dimer has a rhombic ground state with a large electric quadrupole moment. Rhombic and linear configurations of the $(KCl)2−$ anion correspond to stationary states that are nearly degenerate in total energy. The rhombic anion has a single, weakly bound state that could be a “quadrupole-bound” state on the basis of a comparison of its characteristics with those of dipole-bound states. Linear $KClKCl−$ has seven excited states; four of them can be identified as dipole-bound states. KCl and $KCl2$ possess rather similar dipole moments and their anions have two excited dipole-bound states each.