Solvent-separated pair

The lack of solvent separated pairs raises the question whether some alternative mode of solvation should be considered. The ester group of the penultimate unit of the polymer, or the one preceeding it, could act as a solvating agent. The idea of intramolecular solvation was proposed by several workers in the field 37) and it is supported by the results of nmr studies of polymethyl methacrylate formed under various experimental conditions 38). Hypothetical structures such as those depicted below were proposed 39 h... 

The measurement of A vs concentration provides no evidence as to the nature of the ion pairs which form, i.e. whether they are contact or solvent separated species. Also, the mobility of the ion pairs does not influence the results. Contact ion pairs are likely to be more mobile than those separated by solvent since the latter include a section of at least one polymer chain. However, it is possible to envisage mechanisms, involving concerted motion of the cation and anion of a solvent separated pair, which would allow the effective movement of the neutral pair. This is also true for contact vs solvent separated triples. Measurements to be discussed below, involving the dc polarisation of cells, are capable of distinguishing between mobile and immobile pairs. 

Fig. 2.6 Ion pairs. A two-dimensional representation of (a) a solvent-separated pair of ions, each still retaining its intact shell of solvating solvent molecules (b) a solventsharing ion pair, which has lost some of the solvent between the partners, so that one layer of solvent shared between them separates them (c) a contact ion pair, the cation and anion being contiguous.

In fact, the A0+-value of (SrB)+ being 12.1 and that of 26.1 may be taken to Indicate a double solvent separated Ion pair structure for the Sr-salt and a mixed contact-solvent separated pair structure for the BaB2 The double solvated... 

The association of a cation that is surrounded by a tight solvation shell with an anion proceeds smoothly until the solvent shell comes into contact with the anion. At this stage either the structure of the ion pair, separated by solvent molecules, is preserved (Figure 7.1a) or the solvation shell is squeezed out in a process that leads to a contact pair. This implies that at least two types of ion pair may coexist in solution, each having its own physical and chemical properties such two-step associations have been revealed by various relaxation experiments. However, ions that weakly interact with the solvent and do not surround themselves with tight solvation shells form contact pairs only. This situation is encountered in poorly solvated liquids and for bulky ions. Those cations that interact strongly with solvent molecules tend to form solvent-separated pairs, especially when combined with large anions. 

Why is a loose, solvent-separated pair more reactive than a tight contact pair A contact pair involves a small, bare cation which becomes partially dissociated in the transition state as shown in the following diagram. This hinders the propagation. The partial dissociation may be unnecessary for the separated pair because the cation is large (owing to its solvation). 

The fluorenyl salts seem to be particularly susceptible to the formation of solvent separated pairs. Their formation is favoured by the presence of a large charge-delocalized anion plus a small compact cation which can be strongly solvated (or presumably vice-versa). The solvating agent should be highly polar, preferably small in size, or with the ability to offer multiple coordination as with the polyglycol-dimethylethers. With polystyryl salts, the formation of solvent-separated ion-pairs is less extensive. The absorption spectra are not particularly conclusive because the absorption... 

The ease of formation of solvent-separated pairs increases in the two series Cs < Rb < < Na" " < Li" and dioxane < oxepane < tetra-... 

Detailed kinetic studies on other monomers than styrene are not numerous. The data are assembled in Table 5. a-Methylstyrene has received the most extensive study with data available for several counter-ions and three solvents [20, 118, 136,142]. The kp values in the table are those extrapolated to 25°C, the actual temperature range studied was lower owing to the unfavourable equilibrium between monomer and polymer at room temperature. Some inaccuracy may result from this extrapolation. In dioxane [142] the activation energy decreases from 13 to 8 kcal mole between Na and Rb , evidently the characteristics of a contact-ion-pair. In tetrahydropyran [20] the kp value of the lithium compound is much higher than the others, presumably caused by a contribution from solvent-separated pairs. This is confirmed by the fact TABLE 5... 

C (cf. styrene —75°C) and is negative above this temperature. The potassium compound gives only evidence for one species with a constant activation energy of 5.2 kcal mole" between —60 and +10°C the proportion of solvent separated pairs must be negligible. 

The extent of ion pair formation is apparently dependent on the nature of the cation. In dioxane, 2-methyl-THF and THF, Na-fluorenyl is considered as being present as contact ion pairs, whereas in pyridine or DMSO only solvent-separated pairs (together with free ions) are observed. In the case of Li-fluorenyl, solvent-separated pairs are formed in 2-methyl-THF and especially in THF but not in dioxane and toluene. In the solvents pyridine and DMSO formation of solvent-separated ion pairs (together with free ions) is ain quantitative ). 

Ion pairing between Li and 1 and Led was also investigated by analysis of the functional dependence of Li NMR chemical-shift values on the total concentrations of L15I and of le reduced Liblred (Fig. 5). Evidence for the formation of solvent-separated pairs was provided by the small value of the molar paramagnetic contact shift (8c) in Li NMR spectra of paramagnetic solutions of Lidlred (Fig. 5b). ... 

Solvent separated pairs e-...ion e-. ..atom SSIP radical ion Hydration Solvation... 

In the same figure we also show the distance dependence of the effective interaction energy between the two spheres. The results have been obtained by eomputer simulations where the spheres are methane molecules. Note the pronounced minimum between the two spheres at contact and then the maximum at intermediate distances. The interaction energy falls off to zero as the two spheres move away. In some cases, one finds a second minimum at a distance beyond the maximum [10]. Such a minimum at a larger separation is referred to as a solvent separated pair and arises due to the structuring around the hydrophobic spheres. 

Figure 12.3. Global minimum energy structures determined by full optimization at the B3LYP/6-31G level of a trifluoromethanesulfonic acid molecule (a) and apara-toluene sulfonic acid molecule (b) each with 6 water molecules. Both acids show the dissociated proton exists as a solvent separated pair consisting of what resembles an Eigen cation and sulfonate ion. The 0-0 and 0-H (in brackets) distances for each acid reveal that aromatic anion is the stronger base. First presented in Ref. [23],...

Heptafulvenes having electron-donating substituents at the 8-positions are of interest in that they may be regarded as heptafulvenes with reversed polarisation. An example is the bis(dimethyl-amino) derivative (XLVII), which is thermochromic, solutions changing from red at room temperature to blue at -170 . Spectra indicate that two species are involved, presumably solvent separated pairs and contact pairs. This compound is sensitive to hydrolysis and to oxygen [391 j. 

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