Water-separated ion pair

In solvents containing low concentrations of water in acetic acid, dioxane, or sulfolane, most of the alcohol is formed by capture of water with retention of configuradon. This result has been explained as involving a solvent-separated ion pair which would arise as a result of concerted protonation and nitrogen elimination. ... 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

These KIEs were measured using lithium thiophenoxide as the nucleophile. The contact ion-pair was converted into the solvent-separated ion-pair complex by adding between 1.0% and 7.5% water to dry diglyme (Fang and Westaway, 1991). 

Quaternary ammonium and phosphonium permanganates exist as intimate ion pairs in nonpolar solvents such as methylene chloride and toluene (1). However, in more polar solvents, such as acetone, nmr studies indicate that they are better described as being solvent separated ion pairs (37). In water, these salts separate completely and exist as individual ions. 

Studies of dimensiosolvatic effects have continued with an attempt to quantify them for solvolyses of 2-bromoadamantane in water-alcohol mixtures. Product selectivities S = fc(ether)/fc(alcohol) were measured at various concentrations of water in an alcohol and at various temperatures. The reciprocals of the averages of S values for 1.0 0.8 alcohol-water mixtures at all the experimental temperatures (120-150 °C) were proposed as measures D of dimensiosolvatic effects when a solvent molecule intervenes into contact ion pair to form solvent-separated ion pair. The scale runs from Z) = 1.0 (by definition) to D = 10.0 for r-butyl alcohol and is essentially a measure of the bulkiness of solvent molecules. 

Polar protic solvents also possess a pronounced ability to separate ion pairs but are less favorable as solvents for enolate alkylation reactions because they coordinate to both the metal cation and the enolate ion. Solvation of the enolate anion occurs through hydrogen bonding. The solvated enolate is relatively less reactive because the hydrogen-bonded enolate must be disrupted during alkylation. Enolates generated in polar protic solvents such as water, alcohols, or ammonia are therefore less reactive than the same enolate in a polar aprotic solvent such as DMSO. 

Data for solvolyses of 1 -bromoadamantane (54, X = Br in Scheme 2.19) in ethanol-water in Table 2.3 show that S is approximately independent of solvent composition, but the selectivity is inverse. Why is S < 1 for competing nucleophilic substitutions when ethanol is normally more nucleophilic than water A credible explanation is that the products are formed by front-side collapse of a solvent-separated ion pair (52 in Scheme 2.18) - the caged structure prevents rear-side approach, so attack must occur from the front-side, and the proportion of water in solvent-separated ion pairs must be greater than in the bulk solvent. 

Solvent-separated ion pairs ions linked electrostatically, separated by more than one water molecule. 

This explanation is all very well for the liquid sodium chloride type of case, but deviations from the predictions of the Nernst-Einstein equation occur in dilute aqueous solutions also, and here the + and - ions are separated by stretches of water, and ion pairs do not form significantly until about 0.1 M. 

Living polymers can only exist in aprotic solvents. They are killed by water, oxygen and a high number of electrophilic substances. Operating in absence of killing impurities one obtains stable living species they are ionic species whose exact form (free ion, contact ion-pair, solvent separated ion-pair depends upon the concentration and the nature of the monomer, the counterion and the solvent polar... 

R. Contreras and J. S. Goraez-Jeria, /. Phys. Chem., 88, 1905 (1984). Proton Transfer in Water Polymers as a Model for In-Time- and Solvent-Separated Ion Pairs. 

The potential of mean force (PMF) for sodium chloride in water has been simulated by numerous investigators for many different models. Smith and Haymet noted that to dissociate a model sodium ion and chloride ion at room temperature and pressure, two free energy barriers appear to have to be surmounted. The first barrier is associated with the formation of a shared layer of water molecules between the ions, now constituting a so-called solvent separated ion pair , and the second barrier associated with the final unlinking of the solvations shells of the two ions. 

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 this scheme R X, R //X , and R" -I- X represent contact ion pairs, solvent-separated ion pairs, and the free ions, which can all lead to products. Nevertheless, relatively little is known about the details of the corresponding free energy surfaces in solution. Abraham suggested a profile similar to Fig. 4 for the solvolysis of f-butyl chloride (TBC). However, about all that is well established in water is AG = 19.5 kcal/mol, and the transition state is structurally close to the contact ion pair with about 80% charge separation. - ... 

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... 

Figure 6. (a) (CHjjjC Cl" contact and (6) solvent-separated ion pairs in water at C-Cl separations of 3.00 and 5.75 A. Water molecules with oxygens within 4.5 A of any atom of the solute are shown. Taken from arbitrary configurations of the MC simulations. 

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