Tight and loose ion-pairs

Mohammed OF, Adamczyk K, Banerji N, Dreyer J, Lang B, Nibbering ETJ, Vauthey E (2008) Direct femtosecond observation of tight and loose ion pairs upon photoinduced bimolecular electron transfer. Angew Chem Int Ed 47 9044... 

The effect of dielectric constant. D, on the formation constant of a loose ion pair complex Is probably not large in spite of the increase in the interlonlc ion pair distance. The microscopic rather than the macroscopic dielectric constant determines to a large extent the difference between the coulonb attraction energy of two Ions In a tight and loose ion pair. 

The related 1,3-diphenyl-2-azaallyl carbanion also exists in solution as an ion pair. Although the absorption maximum in the electronic spectrum is sensitive to the choice of solvent and counterion, surprisingly, no case has been found to date of a solution which simultaneously shows two absorption bands attributable to the coexistence of tight and loose ion pairs. 

Finally, we mention that Li T measurements have been used to study the rate of interconversion between tight and loose ion pairs of lithium fluorenide [44]. The quadrupolar relaxation contribution was derived from Li and Li T data, and a change of the Li quadrupole splitting constant, QSC, obtained via Li quadrupolar and dipolar relaxation rates (see Section 4) with ion pair structure, was indicated. 

In contrast, an ion pair whose constituent ions are separated by one or several solvent or other neutral molecules is described as a loose ion pair, symbolically represented as X+ Y . The members of a loose ion pair can readily interchange with other free or loosely paired ions in the solution. This interchange may be detectable (e.g., by isotopic labeling), thus affording an experimental distinction between tight and loose ion pairs. 

Using the symbolism of the preceding section, one finds the k to be a function of Et, A, EA(, AH, AS,i(f as well as AVj, AVj and AV, j. The last three parameters denote activation volumes of propagation of tight and loose ion-pairs, and the volume change arising from conversion of tight into loose ion-pairs. The experimental data were presented in the form of plots of log k vs. pressure at constant temperature and plots of log k vs. 1/T at constant pressure. The former plots were linear, while the latter were curved. 

Figure 3.21 Mechanism of dynamic reversible chain transfer between tight" and loose ion pairs employing a zinc dipolymeryl species, Zn(PA)(PB). as a chain-transfer mediator.

The TS for anionic SN2 reactions involves loose ion pairs as in a charge delocalized (soft) anion. On the another hand, the GS could involve a neutral electrophile and either tight or loose ion pairs depending on the anion structure (hard or soft) (Eq. 32). 

Apart from free solvated radical ions (FRI), evidence was gathered for two kinds of ion pairs, which are referred to as tight ion pair (or contact ion pair, CIP) and loose ion pair (or solvent separated ion pair, SSIP) (Scheme 1, Eq. (1)) [3]. It is important to point out that CIP and SSIP are not the only species in solution. There are myriads of spatial cation-anion relationships that lie between them [4]. The SSIP is a pair of two ions of opposite sign with intact solvent shells. This is lacking in the CIP, anion and cation are in direct contact, the whole aggregate being surrounded by solvent molecules. 

It is obvious from this theoretical approach that the energetic stabilization of ion-pairs induced by interaction vith the electric field E becomes increasingly important as the size of the ions and their dipole moments (jj) increase. The more polar ion-pairs are more stabilized by E, clearly increasing from tight to loose ion-pairs, i.e. vith their dissociation and polarity. 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

An ion pair in which the constituent ions are separated by one or more solvent (or other neutral) molecules. If and Y represent the constituent ions, a loose ion pair is usually symbohzed by X+ Y. The constituent ions of a loose ion pair can readily exchange with other ions in solution this provides an experimental means for distinguishing loose ion pairs from tight ion pairs. In addition, there are at least two types of loose ion pairs solvent-shared and solvent-separated. See Ion Pair Tight Ion Pair Solvent-Shared Ion Pair Solvent-Separated Ion Pair... 

An ion pair in which the constituent ions are not separated by a solvent or other intervening molecule. Tight ion pairs are also referred to as contact ion pairs. If and represent constituent ions, then a tight ion pair would be symbolized by X+Y. An example of a tight ion pair would be the case in which an enzyme stabilizes a carbonium ion with juxtaposed negatively charged side-chain groups. See Loose Ion Pair Ion Pair Solvent-Shared Ion Pair Solvent-Separated Ion Pair. 

The bare negative ions discussed in Problem 2.23 have a greatly enhanced reactivity. The small amounts of salts that dissolve in nonpolar or weakly polar solvents exist mainly as ion-pairs or ion-clusters, where the oppositely charged ions are close to each other and move about as units. Tight ion-pairs have no solvent molecules between the ions loose ion-pairs are separated by a small number of solvent molecules. 

For complex III, the Na+ Is probably as accessible to solva-tlon by solvent molecules as is the Na In the tight Fl-,Na+ Ion pair. Hence, no externally bound solvent molecules need to be removed. This may be different In other systems. For example, the formation constant of a loose Ion pair complex between FI", Na+ and tetraglyme (tetraethylene glycol dimethyl ether) Is nearly four times lower In dloxane than In THF (10). This may be caused by specific solvent effects rather than by the difference In solvent dielectric constant. The flexible glyme ligand wraps Itself around the Na+ Ion, and this may make It more difficult for solvent molecules to remain bound to Na+ In the glyme-separated Ion pair. 

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