Cage ion pairs

The separated reactants RuL32+ and Q diffuse together (k 2) to form an ion pair ( RuI 32+ q) in which electron transfer from RuL32+ to Q takes place with a first-order rate constant k23 The products RuL3 + and Q are thus produced in close proximity (solvent cage, ion pair). They may then diffuse apart 34) or go back to reactants (k32) or undergo "back" electron transfer... 

The oxidative photocleavage of l,2-bis(4-methoxyphenyl)cyclobutane is also accelerated in the presence of Mg(C104)2. However, no salt effect was found for 1,2-diphenylcyclobutane because the high-speed back electron transfer prevents interaction of the caged ion pair with the added salt (Pac et al. 1987). 

In-cage ion pairs can also be formed by initial carbon halogen photoinduced homolysis (see also Section 6.6.2), followed by an electron transfer step (C X homolysis, Scheme 6.93). Chlorobenzene, for example, is converted photochemically to phenol in aqueous solutions (Scheme 6.95). The hydrogen atom cannot be... 

The authors proposed a mechanism similar to the one in Scheme 1 (see p. 213). The excited singlet state reacts to give both the in-cage ion pair, 34, and the radical pair, 35, but the question of whether these are two separate species or two resonance contributors was left open. Toluene and the methylether are derived from this singlet pair the quantum yields are 0.19 and 0.11, respectively, in 50 50 t-butyl alcohol/water for the unsubstituted bromide. The triplet radical pair is formed from the excited triplet state with a quantum yield of 0.2 in 50 ... 

Evidence that the actual methylation of the anion can be divided into SnI, Eq. (3), and Sx2 types, Eq, (4), is provided by a whole series of investigations. " The terms S l and 8 2 must be taken to mean reactions with, respectively less or greater nucleophilic participation of the anion in the transition state. The importance of oriented ion pairs" in the solvents of low polarity frequently used in reactions involving diazomethanc, e.g., the ions formed by a diazoalkane and benzoic acid in ether, should be emphasized. The expression oriented ion pair means that, because of insufficient solvation, the ions are not individually solvated but exist as ion pairs within a solvent cage. The orientation within the ion pair is defined electrostatically, and this orientation fixes the path for the productdetermining step. Several indications (cf, foo otes 22-24) in the literature indicate the occurrence of carbonium ions and oriented ion pairs in Broensted-type equilibria of the type of Eq. (2). 

The Lewis acid complex 4 can cleave into an ion-pair that is held together by the solvent cage, and that consists of an acylium ion and a Lewis acid-bound phenolate. A fr-complex 6 is then formed, which further reacts via electrophilic aromatic substitution in the ortho- or para-position ... 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

Although the electrostatic field on the polyelectrolyte surface effectively impedes back ET, it is unable to retard very fast back ET or charge recombination of the primary ion pair within the photochemical cage. The overall quantum yield of photoinduced ET is actually controlled in most cases by the charge recombination. Hence, its retardation is the key problem for attaining high quantum yields in the photoinduced ET. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

Both the basic and nucleophilic reactions within the cage are totally inhibited by the presence of one equivalent of Na + PFg in THF. Double ion exchange between the two ion pairs is favored which removes superoxide from the cage Eq. (10) ... 

In dichloromethane, the acidic ESE cation radical undergoes a rapid proton transfer (k = 1.9 x 109 s ) to the CA anion radical within the contact ion pair (CIP) to generate the uncharged radical pair (siloxycyclohexenyl radical and hydrochloranil radical) in Scheme 6. Based on the quantum yields of hydro-chloranil radical (HCA ), we conclude that the oxidative elimination occurs by geminate combination of the radical pair within the cage as well as by diffusive separation and combination of the freely diffusing radicals to yield enone and hydrochloranil trimethylsilyl ether, as summarized in Scheme 6. 

Such variation in the lifetimes of the ion pairs, which depends on the mode of activation, primarily arises from the difference in the spin multiplicities (see above). None the less, the long-lived ion-radical pair allows the in-cage proton transfer from the cation radical ArMe+ to the CA- anion radical to effectively compete with the back electron transfer,205 i.e.,... 

Hardy effect.248-249 The internal return part of the ionization equilibrium is particularly hard to detect since it is almost completely independent of the concentration of anything in the bulk of the solution outside of the solvent cage. The extent of internal return will depend on the reactivity of the cage walls and their resistance to the escape of either ion. Unless internal return has been eliminated by the use of an extremely reactive cage wall, the measured rate is not that of the ionization but the lesser rate of ion pair dissociation. In the case of the acetolysis of a, a-dimethylallyl chloride (XXXIX), internal return is detectable by virtue of the fact that the chloride ion can return to either of two allylic carbon atoms.248... 

The role of the solvent as modulator of interactions between caged ion-radical pairs was discussed in Section 3.3.3. A more general problem is... 

At high pressures, a non-covalent ionic complex can be regarded as a microsolvated ion. It represents the simplest model for ions generated in a dynamic environment, such as in a solvent cage in solution. The main difference is that the behavior of a microsolvated ion is not perturbed by those environmental factors (solvation, ion pairing, etc.) which normally affect the fate of intimate ion-dipole pairs in solution. Hence, a detailed study of the dynamics and the reactivity of microsolvated ions may provide valuable information on the intrinsic factors governing the reaction and how these factors may be influenced by the solvent cage in solution.4 493... 

Quaternary onium salts were the first phase-transfer catalysts used subsequently, a number of compounds (linear polyethers, polypodands, crown-ethers, cryptands, cage-compounds, etc.) were found effective for the anion activation in two-phase systems. These structurally different systems must satisfy at least two fundamental conditions in order to behave as phase-transfer catalysts i) solubility in the organic phase ii) steric hindrance around the cationic center leading to a good cation-anion separation within the ion-pair. 

The formation of a contact ion pair between the anion-radical and the cation-radical facilitates the proton transfer. This reaction step proceeds in a solvent cage. Adding sodium perchlorate breaks this ion pair up ... 

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