Ion Pairing Interactions

Oppositely charged ions attract each other strongly. In the gas phase the binding between a simple cation and a simple anion can be worth well over 100 kcal/mol. The major contributor to the binding is an electrostatic interaction. We will be discussing electrostatics extensively in this section, and it is important to be clear on its usage here. By an electrostatic interaction, we mean a strictly Coulombic attraction or repulsion between charges or partial 

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Except for the high molecular weight range, nearly all substances can be separated by reversed-phase (RP) HPLC. The many different separation mechanisms in RP HPLC, based on hydi ophobic, hydi ophilic and ion-pairing interactions, and size exclusion effects together with the availability of a lai ge number of high quality stationary phases, explain its great populai ity. At present approximately 90% of all HPLC separations are carried out by reversed-phase mode of HPLC, and an estimated 800 different stationai y phases for RP HPLC are manufactured worldwide. 

Ions and ion pairs interact strongly with the solvent, and hence an ionic polymerization is greatly influenced by the environment. Solvation tends to separate the ions and thus the system approaches a state which would be expected in a hypothetical solution deprived of gegen ions. At the same time formation of a solvation shell around the growing center probably slows down the addition. This effect is particularly notable in the termination step and will be discussed further in the next section of this paper. 

Berkowitz, M. Karim, O. A. McCammon, J. A. Rossky, P. J., Sodium chloride ion pair interaction in water computer simulation, Chem. Phys. Lett. 1984,105, 577-580. 

EA are the ionization potential and electron affinity of the donor anions and acceptor cations, respectively, in the gas phase and wp represents the ion-pair interaction. (Note the ionization potentials in the gas phase parallel the anodic potentials in solution for structurally related electron donors the same interrelationship applies to electron affinities and cathodic potentials.) Accordingly, these coloured crystals are also referred to as charge-transfer salts (Wei etal., 1992). 

An alternative strategy for catalyst immobilisation uses ion-pair interactions between ionic catalyst complexes and polymeric ion exchange resins. Since all the rhodium complexes in the catalytic methanol carbonylation cycle are anionic, this is an attractive candidate for ionic attachment. In 1981, Drago et al. described the effective immobilisation of the rhodium catalyst on polymeric supports based on methylated polyvinylpyridines [48]. The activity was reported to be equal to the homogeneous system at 120 °C with minimal leaching of the supported catalyst. The ionically bound complex [Rh(CO)2l2] was identified by infrared spectroscopic analysis of the impregnated resin. 

Fig. 7.23 Schematic of the active site of catechol sulphotransferase (SULTl A3), with dopamine in the active site. Phe 142, 81 and 24 form a hydrophobic pocket whilst Glu 146 provides an ion pair interaction with the sub-...

Using a similar approach, Chechik and Crooks (73), modified the PAMAM dendrimer-encapsulated palladium nanoparticles with perfluoropolyether tails utilizing non-covalent ion-pair interactions. The catalytic hydrogenation of six substrates under biphasic conditions (toluene/ perfluoro-2-butyltetrahydrofuran FC-75) was investigated. Allyl alcohol, methyl acrylate, vinyl isopropenyl ether, and... 

The energy due to coulombic interactions in a crystal is calculated for a particular structure by summing all the ion-pair interactions, thus producing an infinite series. The series will include terms due to the attraction of the opposite charges on cations and anions and repulsion terms due to cation/cation and anion/anion interactions. Figure 1.57 depicts some of these interactions for the NaCl structure. The Na ion in the centre is immediately surrounded by 6 Cl" ions at a distance of r, then by 12 cations at a distance... 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Schlitzer, D. S. Novak, B. M. Trapped Kinetic States, Chiral Amplification and Molecular Chaperoning in Synthetic Polymers Chiral Induction in Polyguanidines through Ion Pair Interactions, J. Am. Chem. Soc. 1998, 120, 2196-2197. 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

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