# Pi-interaction

In chemistry, π-effects or π-interactions are a type of non-covalent interaction that involves π systems. Just like in an electrostatic interaction where a region of negative charge interacts with a positive charge, the electron-rich π system can interact with a metal (cationic or neutral), an anion, another molecule and even another π system. Non-covalent interactions involving π systems are pivotal to biological events such as protein-ligand recognition.

## Types

The most common types of π-interactions involve:
* Metal–π interactions: involves interaction of a metal and the face of a π system, the metal can be a cation (known as cation–π interactions) or neutral
* Polar–π interactions: involves interaction of a polar molecule and quadrupole moment a π system.
* Aromatic–aromatic interactions (π stacking): involves interactions of aromatic molecules with each other.
** Arene–perfluoroarene interaction: electron-rich benzene ring interacts with electron-poor hexafluorobenzene.
* π donor–acceptor interactions: interaction between low energy empty orbital (acceptor) and a high-energy filled orbital (donor).
* Anion–π interactions: interaction of anion with π system
* Cation–π interactions: interaction of a cation with a π system
* C–H–π interactions: interaction of C-H with π system: These interactions are well studied using experimental as well as computational techniques.

## Anion–π interactions

Anion and π–aromatic systems (typically electron-deficient) create an interaction that is associated with the repulsive forces of the structures. These repulsive forces involve electrostatic and anion-induced polarized interactions. This force allows for the systems to be used as receptors and channels in supramolecular chemistry for applications in the medical (synthetic membranes, ion channels) and environmental fields (e.g. sensing, removal of ions from water).
The first X-ray crystal structure that depicted anion–π interactions was reported in 2004. In addition to this being depicted in the solid state, there is also evidence that the interaction is present in solution.

## π-effects in biological systems

π-effects have an important contribution to biological systems since they provide a significant amount of binding enthalpy. Neurotransmitters produce most of their biological effect by binding to the active site of a protein receptor. Xation-π interactions are important for the acetylcholine (Ach) neurotransmitter. The structure of acetylcholine esterase includes 14 highly conserved aromatic residues. The trimethyl ammonium group of Ach binds to the aromatic residue of tryptophan (Trp). The indole site provides a much more intense region of negative electrostatic potential than benzene and phenol residue of Phe and Tyr.

## In supramolecular assembly

π systems are important building blocks in supramolecular assembly because of their versatile noncovalent interactions with various functional groups. Particularly, ${\ce {\pi - \pi}}$ , ${\ce {CH-\pi}}$ and ${\ce {\pi -cation}}$ interactions are widely used in supramolecular assembly and recognition.
${\ce {\pi-\pi}}$ concerns the direct interactions between two π-systems; and ${\ce {cation-\pi}}$ interaction arises from the electrostatic interaction of a cation with the face of the π-system. Unlike these two interactions, the ${\ce {CH-\pi}}$ interaction arises mainly from charge transfer between the C–H orbital and the π-system.