March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Chapter 3. Bonding Weaker Than Covalent

3.B. π–π Interactions

Many theoretical and experimental studies clearly show the importance of π–π interactions,⁷⁷ which are fundamental to many supramolecular organization and recognition processes.⁷⁸ Perhaps the simplest prototype of aromatic π–π interactions is the benzene dimer.⁷⁹ Within dimeric aryl systems such as this, possible π–π interactions are the sandwich and T-shaped interactions shown. It has been shown that all substituted sandwich dimers bind more strongly than a benzene dimer, whereas the T-shaped configurations bind more or less favorably depending on the substituent.⁸⁰ Electrostatic, dispersion, induction, and exchange-repulsion contributions are all significant to the overall binding energies.⁸⁰

The π electrons of aromatic rings can interact with charged species, yielding strong cation–π interactions that are dominated by electrostatic and polarization effects.⁸¹ An interaction with CH units is also possible. For CH–π interactions in both alkyl- and aryl-based model systems, dispersion effects dominate the interaction, but the electrostatics term is also relevant for aryl CH–π interactions.⁸²

Detection of π–π interactions has largely relied on NMR based techniques (e.g., chemical shifts variations),⁸³ and Nuclear Overhauser Effect Spectroscopy (NOESY) or Rotating-Frame NOE Spectroscopy (ROESY).⁸⁴ Diffusion-ordered NMR spectroscopy (DOSY) has also been used to detect π–π stacked complexes.⁸⁵