  There are views prevalent in the noncovalent chemistry literature that i) the
O atom in molecules cannot form a chalcogen bond, and ii) if formed, this bond
is very weak. We have shown in this study that these views are not necessarily
true since the attractive energy between the oxygen atom of some molecules and
several electron rich anionic bases examined in a series of 34 ion-molecule
complexes varied from the weak (ca -2.30 kcal/mol) to the ultrastrong (-90.10
kcal/mol). The [MP2/aug-cc-pVTZ] binding energies for several of these
complexes were found to be comparable to or significantly larger than that of
the well-known hydrogen bond complex [FH...F]- (roughly -40 kcal/mol). The
nature of the intermolecular interactions was examined using the quantum theory
of atoms in molecules, second order natural bond orbital and symmetric adaptive
perturbation theory energy decomposition analyses. It was found that many of
these interactions comprise mixed bonding character (ionic and covalent),
especially manifest in the moderate to strongly bound complexes. All these can
be explained by a bonding to an anti-bonding orbital type donor acceptor charge
transfer delocalization. This study, therefore, demonstrates that the
covalently bound oxygen atom in molecules can have a significant ability to act
as an unusually strong chalcogen bond donor.
