Excess Electrons Bound to Molecular Systems with a Vanishing Dipole but Large Molecular Quadrupole

Electron attachment properties of
covalent molecules and ion clusters
with vanishing dipole moments but large quadrupoles are studied with
coupled cluster ab initio methods. Selection of the molecules studied
is driven by two goals, finding a paradigm quadrupole-bound anion
and investigating whether there is a correlation between the magnitude
of the molecular quadrupole and the vertical attachment energy. Out
of all examined species, only the ion clusters and four of the covalent
molecules are found to support bound anions. The shapes and spatial
extents of the associated excess electron distributions are qualitatively
and quantitatively characterized, respectively. Two of the four covalent
systems are especially promising as paradigm systems because of advantageous
trade-offs regarding the number of isomers and conformers as well
as synthetic closeness to commercial sources. No correlation was found
between the vertical attachment energy and molecular quadrupole in
an analysis that included the newly identified bound anions, those
molecules, which were found not to support bound anions, and succinonitrile,
which had been studied before. Moreover, there is clearly no such
thing as a “critical quadrupole moment”. There are,
however, very strong electron correlation effects involved in the
binding of the excess electrons, and similar to succinonitrile, for
five out of six anions identified here, the molecular quadrupole of
the neutral itself is too weak to bind an excess electron, and electron
correlation in the form of dynamic polarization is required to do
so.