Abstract:
Halogen and anion-n bondings represent unexpected types of intermolecular
interactions which attracted the attention of chemists over the last two decades.
Halogen bonding is an attraction of electron-rich species to halogen atoms. It is similar
to the well-known hydrogen bonding, and the differences between hydrogen and
halogen bonds have been hard to distinguish. Using UV-Vis and 1HNMR measurements
in solutions and X-ray crystallography in the solid-state, we were able to determine
when each bond was likely to form and the different characteristics that each bond
showed. In particular, it was found that the formation of halogen bonds leads to the
appearance of new and intense peaks in the absorption bands in the UV-Vis spectra
while hydrogen bonds would merely result in some shift of the less intense absorption
bands or reagents. Also, halogen bonds and anion-TT interactions are most commonly
explained as an attraction of electron-rich (negative) species to the areas of positive
charge (referred to as cr- and n-holes) which sometimes exists on the surfaces of
halogens and n-molecules. However, in the current work we demonstrated and clarified
exceptions to this model. Specifically, X-ray structural analysis showed that
arrangements of many anion-n and halogen-bonded reactants deviate substantially
from the location of the most positive or negative potentials on their surfaces. Instead,
they follow locations of the frontier orbitals of interacting species. This suggests that
frontier-orbital interactions represent the major factor determining the structures of these
intermolecular complexes.
