Free solvation

A. Frumkin, and B. Damaskin, Real free solvation energy of an electron in a solution in equilibrium with the electrode and its dependence on the solvent nature,/. Electroanal. Chem. 79, 259-266 (1977). 

At electrode potentials more negative than approximately - 2.8 V (SHE), free solvated electrons appear in the solution as a result of (dark) emission from the metal. At this potential the electrochemical potential of the electrons according to Eq. (29.6) is about —1.6 eV, which is at once the energy of electron hydration in electron transfer from vacuum into an aqueous phase. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

At moderate to high salt concentrations (>0.01 M) the adsorbability of Cd was lower from chloride than from nitrate solutions as was the increase in distibution coefficent with increasing pH, possibly as a result of the formation in solution of complexes of the type CdCl n, which have a lower affinity for the clay than the free solvated ion. Adsorbability of Cd and Co increased with increasing pH, particularly at high salt... 

It should be noted that de-excitation of exciplexes can lead not only to fluorescence emission but also to ion pairs and subsequently free solvated ions. The latter process is favored in polar media. Exciplexes can be considered in some cases to be intermediate species in electron transfer from a donor to an acceptor (see Section 4.3). 

Extended kinetic measurements of dediazoniations in trifluoroethanol, water and other solvents107,108, and a statistical treatment109, demonstrated that a mechanism with two steady-state intermediates, namely initial formation of a tight ion-molecule pair (35) followed by the free (solvated) aryl cation (36), fits the experimental results significantly better than a mechanism with 36 only (Scheme 8). 

Several aromatic molecules undergo no efficient photochemistry of their own. Thus, these molecules are well suited for use as electron transfer sensitizers. According to the substrate and the conditions chosen, they may form either complexes (or tight radical ion pairs) or free, solvated radical ions (Figure 3.9). 

Aqueous ethanolyses of adamantylideneadamantyl halides show Grunwald-Winstein sensitivity parameters (m) of 0.74 ( 0.06), 0.90 ( 0.01), and 0.88 ( 0.03) for the chloride, bromide, and iodide compounds, respectively. All reaction products are formed with retention of both the ring structure and the stereochemistry of the reaction centre. Observed common-ion rate depressions are consistent with a reaction pathway via a free solvated homoallylic carbenium ion. 

Jolvation is an important factor affecting such reactions. In some systems Hcognizably free solvated ions may be detected, as when deoxygenated... 

Proton NMR spectroscopy can similarly be used to monitor the concentrations of such salts, and has the advantage of being able to elucidate structures simultaneously 24,25), Independent conductimetric measurements can provide immediate information on the state of ionic aggregation in solution, in particular providing quantitative data concerning the proportion of free solvated ions to those existing as ion paired entities (26). 

This, however, can be an oversimplification if the reaction conditions (solvent, temperature eta) are such that the propagating ion pair is either partially dissociated or more highly aggregated. Usually the reaction medium is a moderately polar solvent, e.g. dichloromethane or 1,2-dichloro-ethane, and the concentrations of active centres employed are sufficiently small to discount contributions from more highly aggregated species. The problem, therefore, with few exceptions revolves around a fairly simple equilibrium involving only ion pairs and free solvated ions, each with its own reactivity, e.g. 

Experiments in the presence of tetrahydrofuran show that the polymerization rates are always proportional to the square root of added initiator concentration (0.1% to 15% THF). 34% of the initiator is found to lead to active chains. The dependence on monomer concentration is strange. The monomer reacts according to a first order law but the rates are usually dependent on initial monomer concentration. The square root dependence on initiator concentration can be explained if reaction proceeds via free (solvated) ions in labile equilibrium with the undissociated solvated species XLi wTHF, if the latter is still the major species present. The mechanisms suggested formally explain the observed... 

The positive and negative ions formed by electron transfer are held together strongly by the force of electrostatic attraction. They may however separate to form free, solvated ions in polar solvents and then various secondary reactions can take place. Radical cations often undergo cycloadditions with the neutral (Figure 4.9). 

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

Equation (12.8) states that the apparent and intrinsic differential activation energies AAG obs and AAG /inlr differ by the differential Gibbs free solvation energy AGS, with AGs relating the saturation solubilities of N-Ac-L-Phe-OEt in a solvent to acetone as the standard state [Eq. (12.9)], so that, with AG = AH - T AS, Eq. (12.10) follows. 

Back-transformed to the corresponding %abs scale, this model is still slightly more accurate than the model derived by Zhao et al., which is based on Abraham s linear free solvation parameters. By its construction, our model avoids the prediction of negative %abs-values or those greater than 100, which can easily occur in most other methods (see Fig. 11.4). 

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. 

In 1983, Rentzepis published a paper [38] dealing with the charge-transfer interaction of chloranil (9) and aromatic hydrocarbons, e.g. naphthalene (11). Nanosecond spectroscopy of this system [39] could verify some intermediates of the proposed mechanism [21, 40] (Scheme 3) that is the triplet excited acceptor and the free solvated radical ions (A- )s and (D+ ),. 

In this last case, the ratio of enhancements of the longitudinal and transverse relaxation rates, relative to those of the "free" solvate cation in solution, gives access to the value of the reorientational correlation time of the cation interacting with the surface. 

Furthermore, on the excited surface both formation of a complex, to the stabilization of which both an exciton resonance term and an electron transfer term contribute, and full electron transfer to yield a radical ion pair may be envisaged [3], Diffusion of the charged species leads to free solvated radical ions (FRI), but for a sizeable fraction of the systems which will be discussed in the following, the reaction takes place at the stage of the contact ion pair (CIP), and then distinction between the properties of the polar exciplex and of the radical ion pair may not be unambiguous. 

In eqn. (65), A+B is a contact (externally solvated) ion pair, A+ B" is a solvent-separated ion pair (with solvent molecules between the ions), and A+, B are free (solvated) ions S stands for solvent. The initiating efficiency of a Lewis acid AB generally increases with a shift of the equilibria (65) from left to right. In the system monomer-initiator-(solvent) the concentration of active formations suitable for initiation and propagation is a function of the equilibrium position. In non-polar, poorly solvating media, only weakly... 

Current opinions concerning ion-pair modelling share wide consensus on the electrostatic description of the pairing interaction (even if the appropriate values of a and R are still debated), but call attention to the repulsion at r < a, and to the importance of free solvation energy change and other short-term interactions. 

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. 

In recent years, it has been found that the molar absorptivity of concentrated copper sulfate solutions does show a shght concentration dependence. This concentration dependence has been attributed to ion-pair formation occurring through the operation of Coulombic forces between the copper and sulfate ions. This is perhaps ironic because Bjerrum s concept of ion pairs is being used to contradict his conclusion that there are only free ions in copper sulfate solutions. Nevertheless, there is a fundamental difference between the erroneous idea that a copper sulfate crystal dissolves to give copper sulfate molecules, which then dissociate into free ions, and the modem point of view that the ions of an ionic crystal pass into solution as free solvated ions which, under certain conditions, associate into ion pairs. 

Secondly, the nature of the interaction between the monomer-initiator adduct and solvent will be of importance. Co-ordination of a donor solvent to the ammonium ion might be expected to reduce its deactivating inductive influence on the anion. This is analogous to the frequently observed difference in reactivity between free (solvated) and paired ions. Bulky substituents on nitrogen would prevent the formation of a tight solvation shell around it. In this connection two observations, made by Kern et al.43) on formaldehyde polymerization, are very relevant. 

The forms which are actually important in a given polymerization will depend on the natures of the species U and V, the solvating ability of the medium, and the temperature. It is not unusual to find two of these forms coexisting in significant quantities in a given polymerization. In general, more polar media favor solvent-separated ion pairs or free solvated ions. Free solvated ions will not exist in hydrocarbon media, where other equilibria may occur between ion pairs and clusters of ions. 

The initiation and propagation processes are influenced by equilibria between various degrees of association of the active center and its counterion. As a minimum, it is necessary to conceive of the existence of contact (associated) ion pairs, solvent-separated ion pairs, and free solvated ions. A simplified reaction scheme [3] is presented in reaction (9-37). 

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