Anions solvation

The same conclusion was reached in a kinetic study of solvent effects in reactions of benzenediazonium tetrafluoroborate with substituted phenols. As expected due to the difference in solvation, the effects of para substituents are smaller in protic than in dipolar aprotic solvents. Alkyl substitution of phenol in the 2-position was found to increase the coupling rate, again as would be expected for electron-releasing substituents. However, this rate increase was larger in protic than in dipolar aprotic solvents, since in the former case the anion solvation is much stronger to begin with, and therefore steric hindrance to solvation will have a larger effect (Hashida et al., 1975 c). 

As we have recently shown [3], the stability of GIC with Bronstcd acids is governed by ionization potential of the intercalated anion. Solvation of the anion substantially increases ionization potential and hence stabilizes GIC. Such a possibility was predicted by Inagaki [4] who proposed to use organic molecules as additional cointercalants for the purpose of stabilization. Afterwards, we successfully used glacial acetic acid and water to synthesize stable products [5],... 

In crystalline SCO complexes the influence of anions, solvate molecules, H-bonding effects and other intermolecular interactions will also influence the nature of SCO and the cooperativity, as has been discussed above. 

Anion solvation in alcohol clusters has been studied extensively (see Refs. 135 and 136 and references cited therein). Among the anions that can be solvated by alcohols, the free electron is certainly the most exotic one. It can be attached to neutral alcohol clusters [137], or a sodium atom picked up by the cluster may dissociate into a sodium cation and a more or less solvated electron [48]. Solvation of the electron by alcohols may help in understanding the classical solvent ammonia and the more related and reactive solvent water [138], By studying molecules with amine and alcohol functionalities [139] one may hope to unravel the essential differences between O- and N-solvents. One should note that dissociative electron attachment processes become more facile with an increasing number of O—H groups in the molecule [140],... 

Liotta and Grisdale (1975) have reported on the relative nucleophilicities of anions whose potassium salts were solubilized into acetonitrile by 18-crown-6 [3]. The results (Table 28) show the sequence Nj > OAc- > CN- > F- > Cl-x Br- > I- > SCN-, which is very different from the reactivity scale in water CN- > I- >SCN- > Nj > Br- > Cl- > OAc- > F-. Furthermore, the relative nucleophilicities in acetonitrile vary only by a factor of 30, whereas in water they differ by as much as a factor of 1000. The fact that gas-phase nucleophilicities also span a narrow range led the authors to conclude that anion solvation is much less important in acetonitrile than in water. The values recently reported by Lemmetyinen et al. (1978) for the relative nucleophilicities of anions towards methyl methanesulfonate in benzene show the same sequence as in protic solvents, however. The authors offered no explanation for this peculiar behaviour. 

Halide ion mobilities follow the expected trends (Fig. 9). Chloride ion, with the smallest crystallographic radius of the three halides considered, is the most mobile in solvents such as nitrobenzene and dimethylformamide, where anion solvation is expected to be small. In these solvents the Walden products are large. 

The relatively high mobilities of bromide and iodide in pyridine may be the result of poor anion solvation by the diffuse positive end of the pyridine dipole. The other aryl solvent shown in Fig. 9, nitrobenzene, appears to solvate somewhat more readily, though the nature of anion interaction is not clear. 

Ionization of a covalent compound may be defined as the process leading to the formation of solvated ions independent of their presence as associated ions or as free entities (Fig. 6). In a medium of low dielectric constant the formation of associated ions is favored. It is therefore conceivable to consider the overall process of ionization as consisting of two steps, i.e., the formation of associated ions due to cation-coordination and anion-solvation and the dissociation of the associated ions in solution as a dielectric effect. 

Besides water, other solvent molecules have been investigated to a small extent in connection with anion solvation. We mention here two examples of solvation by formic acid, concerning the ions Cl- 230> and CIO4 235). In the first case coordination numbers up to four were considered. For the one to one complexes the energy of interaction is larger for Cl- than for CIO4. 

Calculations were then refined by introducing electrophilic assistance by or Li", solvation of H by water, and optimization of the angle of nucleophilic attack. These conections are introduced either separately or simultaneously. It is found that the stereochemical influence of anion solvation is negligible, compared with that of the other two factors. In all cases, the shapes of the curves are only slightly modified, as can be seen by comparing Fig. 5 and 6 with Fig. 3 and 4. 

Anion-solvation experiments were done using benzophenone as a probe molecule. These experiments were suggested by Bernard Hickel and were based on work of Marig-nier and Hickel and Ichikawa et al. in low-temperature alcohols [7 9]. The concentration of benzophenone was in the range of 0.25 M. This concentration was shown to be sufficient so that it would react with all of the solvated-electron precursors thus, virtually no solvated electrons would be formed under these conditions [20]. The data were analyzed by considering the time dependence of the spectra and the kinetics and evaluating a global fit for these data [20,21]. These experiments were done in a series of alcohols, as a function of temperature, and in alcohol-alkane mixtures. 

We shall discuss separately the results for electron solvation and anion solvation at room temperature in different alcohols to provide a basis for the discussion of solvation mechanisms. This will be followed by a discussion of solvation at lower temperatures and in alcohol-alkane mixtures to further highlight the similarities and differences between anion and electron solvation. 

Anion solvation has been studied by observing the shift in the absorption spectrum of the benzophenone anion in various solvents and as a function of temperature. The benzo-phenone anion was formed from the reaction of the benzophenone molecule and a precursor to the solvated electron. Approximately 0.25 M benzophenone is put into the solution so that all the presolvated electrons will react with the benzophenone and virtually none will form the solvated electron. This process occurs much more quickly than the solvation processes that are observed [14,20]. 

Figure 6 Cartoon of the effect of structure on benzophenone anion solvation.

The results for the solvation of the electron in mixed systems are greatly different from the results for the anion solvation. All evidence, which we summarize below, is consistent with... 

Experiments done by Kenney-Wallace and Jonah showed a fast initial solvation of the electron followed by a shift/increase of the spectrum towards the blue on a time scale that is similar to (but somewhat slower than) the rate of the solvation of the electron in the neat alcohol [16,17]. There was no long-term alteration of the spectrum on the time scale that would be needed to allow an alcohol molecule to diffuse to the solvated electron (a nanosecond or two, as seen in the kinetics for the benzophenone anion solvation). 

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