posted on 2008-01-18, 00:00authored byBenjamin W. Gung, Yan Zou, Zhigang Xu, Jay C. Amicangelo, Daniel G. Irwin, Shengqian Ma, Hong-Cai Zhou
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.