π-π interaction

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π-π interactions are forces that occur between π systems of molecules . Non- covalent interactions between π systems, especially aromatic rings, are still a topical research area in chemistry . On the one hand, this is due to the fact that π-π interactions, in addition to anion-π, cation-π and XH interactions, are important for many chemical and biochemical phenomena as well as for materials science. On the other hand, the research interest is based on the fact that the nature of the interaction is not yet fully understood. In fact, there is not even a consensus on whether there are any special π-π interactions.

The basis for the description of π-π interactions was provided by the model developed by Hunter and Sanders in 1990, which treats each atom like a local quadrupole consisting of the positively charged nucleus and two negative charges above and below the molecular level (which are represented by the π -Electrons are formed). According to this model, the interaction of the quadrupoles of the molecules involved determines the preferred geometry and the effect of the substituent on it.

These and analogous models based on them have proven to be very useful for the rationalization and prediction of experimentally encountered arrangements. The π systems are therefore preferably arranged in a T-shape or parallel to one another. In a parallel arrangement, charge-identical systems stack with an offset axis. In the case of complementary quadrupole moments ( benzene / hexafluorobenzene ), the binding partners stack up without any offset. The bond strength is typically given as up to 50 kJ · mol −1

However, the aforementioned models have several drawbacks and there are times when they fail. As highly accurate calculation methods such as CCSD (T) or SAPT became applicable to systems of a relevant size, a start was made to investigate the bonding situation in a theoretically sound manner. It was found that the prototypical sandwich and parallel shifted configurations of the benzene dimer are primarily bound by dispersion interactions, and that the decrease in exchange repulsion is responsible for the higher stability of the shifted isomer. Various studies on substitution effects produced results that contradicted the predictions of the Hunter-Sanders model. Wheeler and Houk suggested that the substituent effects do not act through a change in the electron density, but almost exclusively through direct interactions between the substituent and the neighboring vertex of the neighboring molecule. Thus, the Hunter-Sanders model is currently being questioned in the literature.

Individual evidence

  1. ↑ a b c d e Steven E. Wheeler, Jacob WG Bloom: Toward a More Complete Understanding of Noncovalent Interactions Involving Aromatic Rings . In: Journal of Physical Chemistry A . tape 118 , no. 32 , July 16, 2014, ISSN  1089-5639 , p. 6133-6147 , doi : 10.1021 / jp504415p .
  2. ↑ Christof Walter: Excitonic States and Optoelectronic Properties of Organic Semiconductors - A Quantum-Chemical Study Focusing on Merocyanines and Perylene-Based Dyes Including the Influence of the Environment. Dissertation. S107-109 ( uni-wuerzburg.de ).
  3. ↑ Stefan Grimme: Do Special Noncovalent π – π Stacking Interactions Really Exist? In: Angewandte Chemie International Edition . tape 47 , no. 18 , April 21, 2008, ISSN  1433-7851 , p. 3430-3434 , doi : 10.1002 / anie.200705157 .
  4. ↑ a b Chelsea R. Martinez, Brent L. Iverson: Rethinking the term “pi-stacking” . In: Chemical Science . tape 3 , no. 7 , 2012, ISSN  2041-6520 , p. 2191 , doi : 10.1039 / c2sc20045g .
  5. ↑ Christopher A. Hunter, Jeremy KM Sanders: The nature of .pi .-. Pi. interactions . In: Journal of the American Chemical Society . tape 112 , no. July 14 , 1990, ISSN  0002-7863 , pp. 5525-5534 , doi : 10.1021 / ja00170a016 .
  6. ↑ Franco Cozzi, Mauro Cinquini, Rita Annunziata, Tammy Dwyer, Jay S. Siegel: Polar / .pi. interactions between stacked aryls in 1,8-diarylnaphthalenes . In: Journal of the American Chemical Society . tape 114 , no. July 14 , 1992, ISSN  0002-7863 , pp. 5729-5733 , doi : 10.1021 / ja00040a036 .
  7. ^ TU Kaiserslautern: Stefan Kubik: π-π interaction
  8. ↑ Mutasem Omar Sinnokrot, C. David Sherrill: High-Accuracy Quantum Mechanical Studies of π − π Interactions in Benzene Dimers . In: Journal of Physical Chemistry A . tape 110 , no. 37 , September 2006, ISSN  1089-5639 , p. 10656-10668 , doi : 10.1021 / jp0610416 .
  9. ^ A b Steven E. Wheeler: Local Nature of Substituent Effects in Stacking Interactions . In: Journal of the American Chemical Society . tape 133 , no. 26 , July 6, 2011, ISSN  0002-7863 , p. 10262-10274 , doi : 10.1021 / ja202932e .