Quantitative Determination of Intermolecular Interactions with Fluorinated Aromatic Rings

✍ Scribed by Harry Adams; Jose-Luis Jimenez Blanco; Gianni Chessari; Christopher A. Hunter; Caroline M. R. Low; John M. Sanderson; Jeremy G. Vinter

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 273 KB
• Volume: 7
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/1521-3765(20010817)7:16<3494::aid-chem3494>3.0.co;2-2

✦ Synopsis

The chemical double mutant cycle approach has been used to investigate substituent effects on intermolecular interactions between aromatic rings and pentafluorophenyl p-systems. The complexes have been characterised using 1 H and 19 F NMR titrations, X-ray crystal structures of model compounds and molecular mechanics calculations. In the molecular zipper system used for these experiments, H-bonds and the geometries of the interacting surfaces favour the approach of the edge of the aromatic ring with the face of the pentafluorophenyl p-system. The interactions are generally repulsive and this repulsion increases with more electronwithdrawing substituents up to a limit of 2.2 kJ mol À1 , when the complex distorts to minimise the unfavourable interaction. Strongly electron-donating groups cause a change in the geometry of the aromatic interaction and attractive stacking interactions are found (À 1.6 kJ mol À1 for NMe 2 ). These results are generally consistent with an electrostatic model: the polarisation of the pentafluorophenyl ring leads to a partial positive charge located at the centre and this leads to repulsive interactions with the positive charges on the protons on the edge of the aromatic ring; when the aromatic ring has a high p-electron density there is a large electrostatic driving force in favour of the stacked geometry which places this p-electron density over the centre of the positive charge on the pentafluorophenyl group.