Cation-rr interactions

Molecules with polar atoms or charged groups can form intramolecular interactions such as internal H-bonds or cation-rr interactions. As a consequence energy minimization in vacuo often results in electrostatically collapsed conformations due to the dominance of these intramolecular interactions. Several compounds in... 

Figure 1.19 Schematic drawing of the cation-rr interaction showing the contact between the two. The quadrupole moment of benzene, along with its representation as two opposing dipoles, is also shown.

Because ion pairs are formed in low polarity aprotic solvents, the anion of organic cations also contributes to the complexation. It has been found that the stability of complexes between aromatic receptors and ammonium and iminium ions is decreased in the order picrate>trifluoroacetate>r>Br >Cr>tosylate>acetate. Thus, the lower the cation-anion attraction, the stronger is the cation-rr interaction. ... 

S. Bartoli, S. Roelens, Electrostatic attraction of counterion dominates the cation-rr interaction of acetylcholine and tetramethylammonium with aromatics in chloroform, J. Am. Chem. Soc., 1999, 121, 11908-11909. 

Gokel. C.W. Barbour, L.J. Ferdani, R. Hu. J. Lariat ether reeeptor systems show experimental evidenee for alkali metal cation-rr interactions. Acc. Chem. Res. 2002, 35. 878-886. 

In the following section we provide an overview of the four factors mentioned above, revealing the diverse behavior of cation-rr interactions. 

In the context of assessing the size effect we also reviewed the impact of curvature of polycyclic systems in terms of their role in cation-jt interactions. B3LYP/6-311-f-G calculations were performed to assess the effect of curvature and remote electronic perturbations on the cation-rr interactions of a large series of aromatic hydrocarbons and their hetero analogs shown in Scheme 18.4 [46]. 

Thus we have explored several aspects of solvation in terms of how they affect cations and cation-rr interaction. Although the extent of effect which solvent has on each system is quite case specific, it remains an important factor in reliable quantum chemical calculations of model systems. We demonstrate how even subtle and minor variations in solvation seem to have a significant alteration in the manifestation of the interaction as seen by studies of different model systems. 

One of the main non-covalent interactions in biology is the cation-s interaction. Reviews on cation-rr interactions by Dougherty coworkers [162, 163] and other... 

Dougherty DA. Cation-rr interactions in chemistry and biology a new view of benzene, Phe, Tyr, andTrp. Science. 1996 271 163-8. 

Mahadevi AS, Sastry GN. Cation-rr interaction its role and relevance in chemistry, biology, and materials science. Chem Rev. 2013 113 2100-38, and see references therein. 

Donini O, Weaver DF. Development of modified force field for cation-amino acid interactions ab initio-AerrweA empirical correction terms with comments on cation-rr interactions. JComputChem. 1998 19 1515 25. 

Caldwell JW, Kollman PA. Cation-rr interactions Nonadditive effects are critical in their accurate representation. J Am Chem Soc. 1995 117 4177-8. 

The clear trend of Figure 3.8—Li+ > Na " > K " > Rb" —is reminiscent of the hydration trends we discussed in Section 3.2.2. The hydration trends were rationalized with an electrostatic and size model, and an electrostatic model of the cation-rr interaction has also proven to be quite powerful. How can we develop an electrostatic model with benzene as one of the partners ... 

B. The relationship between dipoles and quadrupoles, and an illustration of six bond dipoles giving rise to a molecular quadrupole. Note that the left image is top down on the benzene, while the right image is edge on. C. Substituent effects on the cation-rr interaction. These are calculated values. See also the analogous electrostatic potential surfaces in Appendix 2. 

While the hydrophobic effect is a necessary contributor to these remarkable assembly processes, the high architectural specificity is achieved by using the more directed interactions—the hydrogen bond, the ion pair, and the cation-rr interaction. To first order, the hydrophobic effect provides the initial driving force for assembly and pays the entropy cost of bringing together many molecules, and then the more specific interactions define the precise structure. 

Ruan, C., Yang, Z. and Rodgers, M.T. (2007) Cation/rr interactions with a rr-excessive nitrogen heterocycle structures and absolute binding energies of alkati metal cation-pyrrole complexes. Int. J. Mass Spectrom., 261, 233-247. 

The host behaviour of a 1,3-alternate calix-bis-crown has been studied in the solid state and has already been discussed. However, on a related theme, it has recently been established that in solution ealix[4]arene-crown eompounds act as hosts for alkali metals and that subsequent cation-rr interactions are responsible for changes in the fluorescenee bands of the host the fluoreseenee quantum yields of such complexes have been shown to decrease as the alkali metal component is made larger. ... 

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