Strong and weak halogen bonds (XBs) in discrete aggregates
involving
the same acceptor are addressed by experiments in solution and in
the solid state. Unsubstituted and perfluorinated iodobenzenes act
as halogen donors of tunable strength; in all cases, quinuclidine
represents the acceptor. NMR titrations reliably identify the strong
intermolecular interactions in solution, with experimental binding
energies of approx. 7 kJ/mol. Interaction of the σ hole at the
halogen donor iodine leads to a redshift in the symmetric C–I
stretching vibration; this shift reflects the interaction energy in
the halogen-bonded adducts and may be assessed by Raman spectroscopy
in condensed phase even for weak XBs. An experimental picture of the
electronic density for the XBs is achieved by high-resolution X-ray
diffraction on suitable crystals. Quantum theory of atoms in molecules
(QTAIM) analysis affords the electron densities and energy densities
in the bond critical points of the halogen bonds and confirms stronger
interaction for the shorter contacts. For the first time, the experimental
electron density shows a significant effect on the atomic volumes
and Bader charges of the quinuclidine N atoms, the halogen-bond acceptor:
strong and weak XBs are reflected in the nature of their acceptor
atom. Our experimental findings at the acceptor atom match the discussed
effects of halogen bonding and thus the proposed concepts in XB activated
organocatalysis.