Search for "quadrupole-bound" anions. I.

In a classical model, some anions exist due to the attraction between an el ectron and a molecule's dipole moment. When the dipole moment is sufficient ly large (mu(crit)>2.5 D), an electron can be trapped. Can a sufficiently l arge quadrupole moment produce the same effect? To help answer this questio n, we can search for molecules with a large quadrupole moment and use predi ctive, ab initio, correlated quantum chemistry methods to assess whether an anion forms and, if it does, to discover its nature. For this purpose, cou pled-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 elec tron 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 quadrup ole moment. Rhombic and linear configurations of the (KCl) 2 2 anion corres pond to stationary states that are nearly degenerate in total energy. The r hombic anion has a single, weakly bound state that could be a "quadrupole-b ound" state on the basis of a comparison of its characteristics with those of dipole-bound states. Linear KClKCl- has seven excited states; four of th em can be identified as dipole-bound states. KCl and KCl2 possess rather si milar dipole moments and their anions have two excited dipole-bound states each. (C) 1999 American Institute of Physics. [S0021-9606(99)31324-6].