Quantitative Study of Interactions between Oxygen Lone Pair and Aromatic Rings: Substituent Effect and the Importance of Closeness of Contact

Current models describe aromatic rings as polar groups based on the fact that benzene and hexafluorobenzene are known to have large and permanent quadrupole moments. This report describes a quantitative
study of the interactions between oxygen lone pair and aromatic rings. We found that even electron-rich
aromatic rings and oxygen lone pairs exhibit attractive interactions. Free energies of interactions are
determined using the triptycene scaffold and the equilibrium constants were determined by low-temperature
1H NMR spectroscopy. An X-ray structure analysis for one of the model compounds confirms the close
proximity between the oxygen and the center of the aromatic ring. Theoretical calculations at the MP2/aug-cc-pVTZ level corroborate the experimental results. The origin of attractive interactions was explored
by using aromatic rings with a wide range of substituents. The interactions between an oxygen lone pair
and an aromatic ring are attractive at van der Waals' distance even with electron-donating substituents.
Electron-withdrawing groups increase the strength of the attractive interactions. The results from this
study can be only partly rationalized by using the current models of aromatic system. Electrostatic-based
models are consistent with the fact that stronger electron-withdrawing groups lead to stronger attractions,
but fail to predict or rationalize the fact that weak attractions even exist between electron-rich arenes and
oxygen lone pairs. The conclusion from this study is that aromatic rings cannot be treated as a simple
quadrupolar functional group at van der Waals' distance. Dispersion forces and local dipole should also
be considered.