Search for “quadrupole-bound” anions. I

Abstract

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.