Dipolar ion pair

One characteristic feature of theories that incorporate DD interactions is a density-dependent dielectric constant s(p ) > Ss, induced by a varying concentration of dipolar ion pairs. Friedman [3] suggested such a variation of the dielectric constant to be responsible for phase separation. As a byproduct, WS theory provides a generalization of the well-known Onsager expression for the dielectric permittivity of dipolar Quids to a system comprising free ions [221],... 

The electrolyte solution is stimulated by an electromagnetic field applied over the microwave region, and the dielectric response of the sample is then measured. The complex dielectric response is frequency sensitive and dependent on the square of the dipole moment of each species in solution. Both solvent molecules and dipolar ion-pairs contribute to the signal. Additional contributions arise from the polarizability... 

Another important source of information is dielectric spectroscopy, because dipolar ion pairs contribute to the static dielectric constant of the solution [42, 43], In polar solvents the dielectric spectra reflect two modes caused by the reorientation of solvent and of ion-pairs. In non-polar solvents one solely observes ion pair reorientation. For Bu4N-iodide (BU4NI) in dichloromethane (CH2CI2) an increase of the total dielectric constant e with the concentration of the salt is found as result of the ion pair formation. A decrease in the particle density of the solvent causes a minute decrease of the solvent contribution. The dielectric constant does, however, not increase linearly with the salt concentration. A decreasing slope at high salt concentrations may result from the redissociation of the ion pairs but at a quantitative level, redissociation alone is... 

In the following we give a short account on this theory for demonstrating that interactions between dipolar ion pairs and free ions and/or other pairs can be incorporated in a natural and transparent way [31, 32], The theories rest on the Poisson-Boltzmann equation. With the presumption of electro neutrality, the expansion in first order of f3 yields the Helmholtz equation or linearized Poisson-Boltzmann equation,... 

In solvents of low dielectric constant, where the Coulomb interactions are particularly strong, electrical conductance and dielectric spectra suggest that the ion distribution involves dipolar ion pairs, which then interact with the free ions and with other dipolar pairs. The ion pairs cause an increase of the dielectric constant, which in turn stabilizes the free ions, thus leading to redissociation at high salt concentrations. Extending the approach of Debye-Hiickel and Bjerrum, theory accounts for ion pairing, ion-ion pair and ion pair-ion pair interactions and rationalizes the basic features of the ion distribution in accordance with experiments and MC-simulations. 

The same approach can be applied to investigate the explosivity conditions of the H20-NaCl system. We have selected the Anderko-Pitzer (AP) equation of state,which is based on realistic physical hypotheses. It describes H20-NaCl by means of statistical thermodynamic models developed for dipolar hard spheres. This assumption is reasonable at high temperatures, where NaCl is known to form dipolar ion pairs. However, for this reason, this equation of state is only applicable above 573 K, 300°C. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

Both of the above-mentioned catalyst types get the anions into the organic phase, but there is another factor as well. There is evidence that sodium and potassium salts of many anions, even if they could be dissolved in organic solvents, would undergo reactions very slowly (dipolar aprotic solvents are exceptions) because in these solvents the anions exist as ion pairs with Na or and are not free to attack the substrate (p. 443). Fortunately, ion pairing is usually much less with the quaternary ions and with the positive cryptate ions, so the anions in these cases are quite free to attack. Such anions are sometimes referred to as naked anions. 

The above stereochemical results have been explained on the basis of the Criegee mechanism with the following refinements (1) The formation of 14 is stereospecific, as expected from a 1,3 dipolar cycloaddition. (2) Once they are formed, 16 and IS remain attracted to each other, much like an ion pair. (3) Compound 16 exists in syn and anti forms, which are produced in different amounts and can hold... 

Table 5.4 Relative reactivities (normalized to iodide) for a series of nucleophiles under phase transfer, homogeneous dipolar aprotic, and homogeneous protic conditions, and the hydration number of the quaternary onium-anion ion pair [43]...

The action of added ion-pairs may also be visualized as the establishment of an ion-pair atmosphere about a dipolar transition state. Simple thermodynamic treatment predicts linearity of log k3 in cs, but it has been shown that contributions of ion-pair-ion-pair repulsion, higher aggregation, and the effect of salt on the dielectric constant introduce curvature in the log k3-cs plot that is in the direction of a k3-cs dependence. 

Wilcox and co-workers (145) reported that the stereoselectivity of 1,3-dipolar cycloaddition reactions can be controlled in a predictable manner when ion pairs are located at a proper position near the reaction site (Scheme 11.40). He has employed an A-phenylmaleimide substrate having a chiral center in the substituent at ortho position of the phenyl group. Due to serious steiic hindrance, this phenyl group suffers hindered rotation around the aryl-nitrogen bond (rotation barrier 22 kcal/mol). Four diastereomeric cycloadducts are possible in the cycloaddition step with a nitrile oxide. When the cycloaddition reaction is carried out in... 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

Binding of NO to the spin-coupled copper pair of tyrosinase (Schoot Uter-kamp and Mason, 1973 Malmstrom, 1978) yields a dipolar coupled pair in which both copper ions bind NO the EPR spectra of the NO-complexed enzyme exhibits both broadened = 1 transitions and Sj = 2 transitions. 

H. J. Neusser In reply to Prof. Woste let me mention that a situation similar to the high Rydberg states I discussed has been proposed for a dipolar bound electron in molecular anions. Here large distances of the electron and small binding energies are expected. First experimental indications were found. Ion pair states represent another interesting example. 

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