title: Quantitative study of interactions between oxygen lone pair and aromatic rings: substituent effect and the importance of closeness of contact.
abstract: 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.
authors: Department of Chemistry & Biochemistry, Miami University, Oxford, Ohio 45056, USA. gungbw@muohio.edu
authors: Benjamin W
authors: BW
authors: Gung
authors: Yan
authors: Y
authors: Zou
authors: Zhigang
authors: Z
authors: Xu
authors: Jay C
authors: JC
authors: Amicangelo
authors: Daniel G
authors: DG
authors: Irwin
authors: Shengqian
authors: S
authors: Ma
authors: Hong-Cai
authors: HC
authors: Zhou
mesh_terms: D001555:Benzene Derivatives / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004583:Electrons; D057927:Hydrophobic and Hydrophilic Interactions; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D001555:Benzene Derivatives; D010100:Oxygen