Definitions

# Cation-pi interaction

Cation-$pi$ interaction is a noncovalent molecular interaction between the face of an electron-rich $pi$ system (e.g. benzene, ethylene) with an adjacent cation (e.g. Li+, Na+). This unusual interaction is an example of noncovalent bonding between a monopole (cation) and a quadrupole ($pi$ system). Cation-$pi$ interaction energies are of the same order of magnitude as hydrogen bonds or salt bridges and play an important role in molecular recognition.

## Origin of the Effect

Benzene, the model $pi$ system has no permanent dipole moment, as the contributions of the weakly polar carbon-hydrogen bonds cancel due to molecular symmetry. However, the electron-rich $pi$ system above and below the benzene ring hosts a partial negative charge. In order to counterbalance this sandwiching negative charge, a positive charge is associated within the plane in which all benzene atoms lie. An (electric) quadrupole (a pair of dipoles, which do not cancel each other) results. The negatively charged $pi$ system can then interact favorably with positively charged ions.

## Influences on the Strength of the Cation-$pi$ Interaction

The cation-$pi$ interaction is comparable in strength to hydrogen bonding and can in some cases be a decisive intermolecular force. Several criteria influence the strength of the bonding: the nature of the cation, the subsitutents on the $pi$ system, as well as the solvent.

### Nature of the Cation

From electrostatics (Coulomb's law), smaller and more positively charged cations lead to larger electrostatic attraction. The following table shows a series of Gibbs free energy changes for the interaction of benzene with several alkaline metals in the gas phase. The influence of the ionic radius, $r_\left\{mathrm\left\{ion\right\}\right\}$, is evident.

M+ Li+ Na+ K+ Rb+
-$Delta$G [kcal/mol] 38 27 19 16
$r_\left\{mathrm\left\{ion\right\}\right\}$ [pm] 76 102 138 152

### Substituents on $pi$ system

The electronic properties of the substituents on the $pi$ system also have an influence on the strength of the attraction. Electron withdrawing groups (e. g. Cyano -CN) decrease the amount of negative charge in the $pi$ system and thus weaken the interaction. On the contrary, electron donating substituents (e.g. amino –NH2) increase the charge separation of the quadrupole and strengthen the cation-$pi$ binding. This relationship is illustrated quantitatively in the margin for several substituents.

### Influence of the solvent

Additionally, the nature of the solvent also determines the relative strength of the bonding. Most data on cation-$pi$ interaction is acquired in the gas phase, as the attraction is most pronounced in that case. Any intermediating solvent molecule will attenuate the effect, which is why it becomes less pronounced with increasing solvent polarity.

## Cation-$pi$ Interaction in Nature

Nature’s building blocks consist of aromatic moieties, too. Amino acid side chains of tryptophane and tyrosine or the DNA bases are capable of binding to cationic species (not only metal ions, but also charged amino acid side chains, ...). Therefore, cation-$pi$ interactions can play an important role in stabilizing the three dimensional structure of a protein. A very impressive example is given by the nicotinamide acetylcholine receptor whose molecular recognition mechanism of its substrate acetylcholine (a positively charged molecule) nearly entirely bases on cation-$pi$ interaction.

## Anion-$pi$ interaction

In many respects, anion-$pi$ interaction is opposite to cation-$pi$ interaction, although the underlying principles are identical. Significantly less examples are known to date. In order to attract a negative charge, the charge distribution of the $pi$ system has to be reversed. This is achieved by placing several strong electron withdrawing substituents along the $pi$ system (e. g. hexafluorobenzene). The anion-$pi$ effect is advantageously exploited in chemical sensors for specific anions.