Contact and solvent-separated ion pairs

Figure 4.7 Distribution function for contact and solvent-separated ion-pairs.

Counterions can affect the strocture of hydration regions, and conversely hydration regions can affect ion binding. We have already touched on this subject in discussing contact and solvent-separated ion pairs in Section 4.2.8. 

Contact- and solvent-separated ion pairs form whenever solvolysis proceeds to the free carbocation. However, these intermediates are generally only thought of as significant when their formation can be detected by experiment. We have focused on several different reactions of ion pairs that leave detectable signatures. 

An aspect of general interest in organometallic chemistry is the equilibrium between contact and solvent-separated ion pairs, because metal cations which are sun ounded by an individual solvent cage are expected to show different reactivity towards basic centres than those closely attached to carbanions or amines. At the same time, the anionic centre is less shielded in an SSIP than in a CIP and thus expected to be more reactive. In solution, the differentiation by NMR methods between both structural motifs relies in most cases on chemical shift interpretations and, if possible, on heteronuclear Overhauser (NOE) measurements. The latter method is especially powerful in the case of lithium organic compounds, where H, Li or even H, Li NOE can be detected by one- and two-dimensional experiments. ... 

Numerous X-ray investigations have unravelled the solid state structure of contact and solvent-separated ion pairs. It was therefore considered to be of interest to evaluate also the potential of solid state NMR as a tool for the investigation of this structural problem. In addition to the study of chemical shifts discussed above (Section II.B), the quadrupole coupling constant of the nuclide Li, x( Li), was expected to be an ideal sensor for the bonding situation around the lithium cation because, due to its dependence on the electric field gradient, the quadrupolar interaction for this spin-3/2 nucleus is strongly influenced by local symmetry, as exemplified in Section II.C.3. This is also shown with some model calculations in Section ILF. 

Consider propagation by polystyryl sodium ion pairs in a 1 M styrene solution in tetrahydrofuran. For an ion pair concentration of 2.0 x 10-3 M, calculate the relative contributions of contact and solvent-separated ion pairs to the propagation process. Use appropriate data from Table 5-12. 

Hogen-Esch, T. E and J. Smid Studies of contact and solvent-separated ion pairs of carbanions. I. Effect of temperature, counterion, and solvent. J. Am. Chem. Soc. 88, 307 (1966). 

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination... 

Direct observation of more than one type of ion pair was described by Hogen-Esch and Smid [257] for the fluorenyl salt. Differences in spectra enabled them to differentiate between contact and solvent-separated ion pairs. Buncel and Menon measured the equilibrium contact separated ion pairs of diphenylmethyl-and triphenylmethylcarbanions with Li+ and K+ in... 

Characteristic wavelengths of light absorbed by contact and solvent separated ion pairs of diphenylmethyl- and triphenylmethylcarbanions in ethers Effect of solvent and counter-ion. Temperature 298 K. 

In the simplest case, with rapid initiation and participation of a single type of active centre, the rate of propagation is equal to the polymerization rate, and kp is the overall polymerization rate constant. Rapid initiation can be established in ionic processes the presence of several kinds of centres means unequal numbers of monomer molecule additions to different centres. Long macromolecules will be formed on "rapid centres, shorter ones on "slow centres. A practical example of this situation is anionic living polymerization with the participation of contact and solvent-separated ion pairs, and of free ions. 

Based on this dependence and on the temperature course, the concept of propagation on contact and solvent separated ion pairs can be quantified (see Fig. 33). In solvents that are not too polar the plot of In + vs. T is non-linear, revealing the complexity of the constant. The reactivity of solvent-... 

Contact and solvent-separated ion pairs can be distinguished in anionic systems the interionic distance of the former is usually 1-3 A, which increases to 4 or even 7 A in solvent-separated ion pairs [21]. There is apparently no further minimum in the potential energy diagram. The reactivity of solvent-separated ion pairs and free ions in anionic systems are similar, being a few orders of magnitude more reactive than contact ion pairs. In contrast, contact ion pairs in cationic systems are separated by 4-6 A, and therefore resemble the solvent-separated species of anionic systems in terms of structure, as well as their relative reactivity and ability to dissociate. The existence of solvent-separated ion pairs in cationic polymerization is questionable and has not yet been proven spectroscopically. 

Whereas the spectral behavior of solvent-separated ion pairs and free ions is very similar, the UV/Vis spectra of contact and solvent-separated ion pairs are usually different from each other, as has been shown with sodium fluorenide [141, 164]. Due to the penetration of solvent molecules between the ion-pair couples, the direct influence of the metal cation on the r-electron system of the carbanion is lost. With increasing dissociation, the absorption maximum of sodium fluorenide in tetrahydrofuran solution is shifted bathochromically in the direction of the absorption maximum of the free... 

It should be pointed out that the formation of a carbenium ion in the course of an S l reaction does not in fact occur as simply as described in Eq. (5-100), but takes place via intermediate contact- and solvent-separated ion pairs before the free solvated ions are formed cf Eqs. (2-19) and (2-20) in Section 2.6. 

A covalent compound dissociates into free ions in stages involving the formation of at least two types of ion pairs, contact and solvent-separated ion pairs cf. Section 2.6 and Eqs. (2-19) to (2-21) [289, 333]. 

The contact and solvent-separated ion pairs are also called intimate and loose ion pairs, respectively.) Since ionic dissociation-association reactions in general are very fast, quasi-equilibrium of the species can reasonably be assumed. 

In particular, the relative stability of contact and solvent-separated ion pairs has been the object of much debate in the literature. This was started from a striking finding by Pettitt et al. [239] who observed an attractive minimum for contact Cl pairs in water by extended RISM calculations. 

In another paper Abraham provided a thermodynamic analysis that placed f-Bu Cl and the separated ions at about the same free energy, 14.5 kcal/mol above the reactant. This implies a AG of about 5 kcal/mol for collapse of the contact ion pair back to reactants, although the uncertainties in Abraham s analysis are at least 5 kcal/mol. Few related data are available, except some dynamic NMR results for the collapse of trityl and tropylium chloride ion pairs. " In both the NMR work and Abraham s analysis, solvent-separated ion pairs are not included as distinct entities. However, the interconversion of contact and solvent-separated ion pairs has been observed in ultraviolet studies of delocalized carbanions in THF. ... 

Quantitatively the MC simulations predict a 2.1-kcal/mol barrier between the contact and solvent-separated ion pairs, with the latter about 4 kcal/mol lower in free energy. These results should be considered preliminary on several counts. In particular, greater study of the dependence of the results on the cutoff procedure for the potential functions is desirable. Also, the constancy of the potential function parameters for between 2.5 and 7 A is a questionable approximation. However, some compensation in changes between the ion-ion and ion-water interactions is expected, that is, if the ion-ion interaction has a covalent component, the increased attraction would be somewhat offset by... 

The species formed between A1C13 and either Cp2TiCl2 or Cp2Ti(CH2SiMe3)Cl in chloroalkane solution have been studied by means of multinuclear NMR spectroscopy in order to establish the influence of the contact and solvent-separated ion pairs on the activity for ethylene polymerization. The species involved in the equilibria, the changes as a function of temperature, concentration, the ratio of reagents, and the nature of the solvent were... 

Under some circumstances ion-ion interactions can be more important than ion-dipole interactions. This is especially true when the valence of the ion is greater than one, and the electrolyte concentration is high. Then, the formation of ion pairs and higher aggregates is possible. Two types of ion pairs have been recognized, namely, contact ion pairs in which the cation and anion are in physical contact, and solvent-separated ion pairs in which one or two solvent molecules are situated between the cation and anion. Ion pairing must be considered in developing a complete picture of an electrolyte solution. 

Problem 8.13 Consider propagation by polystyryl sodium ion pairs in a IM styrene solution in tetrahydrofuran at 20° C. For an ion pair concentration of 2.0x 10 M, calculate the relative contributions of contact and solvent-separated ion pairs to the propagation process. [Data At 20°C, rate constant for contact ion pair, k = 24 L/mol-s rate constant for solvent-separated ion pair, k, = 5.5x10 L/mol-s equilibrium constant for interconversion between contact ion pair and solvent separated ion pair, Kcs — 2.57x10 . ]... 

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