Ion-pair generation

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

Extraneously added water was found to be an inhibitor. The authors suggested that initiation might occur by the reaction of monomer with the ion-pair generated either directly from the dimer (EtAlCl2)2 or by a reaction of two EtAlCl2 molecules. The evidence presented, however, was insufficient to establish a satisfactory mechanism. 

Several sol-gel entrapped catalysts are likely to soon find commercial applications. A variety of transition metal catalysts physically entrapped in silica matrices as ion pairs generated from the metal halides and quaternary ammonium or phosphonium salts developed in the mid-1990s by Avnir and Blum resulted in truly heterogeneous, stable and... 

The excitation of these complexes generates intimate ion pairs which are well suited for the quantitative study of ion-pair and radical-pair dynamics. The behaviour of the ion pairs generated by this method parallels the behaviour of ion pairs generated by usual solvolytic reactions215. 

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

The competition between the various reactions depends on many factors, including the distance between the ions. Radical ion pairs generated by PET can be contact radical ion pairs (CRIP) or solvent separated radical ion pairs (SSRIP ... 

The mechanisms proposed by both of these groups to explain the details of the chemistry of esters that give rise to short-lived nitrenium ions are not the only mechanisms that could fit these data. Further developments in this area will require application of picosecond spectroscopic methods to ion pairs generated by laser flash photolysis. 

The fact that both the cis and trans isomers of diaryl epoxides give cis-ozonide made mandatory the possibility that back electron transfer within the radical ion pair generated a carbonyl ylide in which geometric equilibration can occur before oxygenation (130), although no direct evidence for this process could be obtained spectroscopically (133). 

Recombination fluorescence has been used to study the decay of radical ion pairs generated photolytically.288 Simulation of quantum beats caused by hyperftne interaction in the R and R+ enable the values of hfc to be determined for very short-lived species. In the case of one R excellent agreement with the value of hfc as determined by ESR is reported. The primary reaction in the photolysis of 1-arylalkyl radicals (128) is the heterolytic cleavage of the jS-halogen (X), generating the radical cation (129).289... 

The specific ionization of a particles in air is shown in Fig. 6.3 the number of ion pairs produced per millimetre of air increases strongly with the distance and falls off rather sharply near the end of the range of the a. particles. The increase is due to the decreasing velocity of the a. particles. As the energy decreases by about 35 eV per ion pair generated, an a particle with an initial energy of 3.5 MeV produces about 10 ion pairs. At the end of its path it forms a neutral He atom. 

In the context of the potential Cope rearrangement of hexa-1,5-diene radical cations (Section 2.4.1), we mentioned the triplet recombination of radical ion pairs generating a biradical [202, 203]. Because of continuing interest in this type of reaction we briefly mention two additional examples involving radical cationic systems discussed in this review, viz., the isomeric 1,2-diphenylcyclopropane radical cations, cis- and trans- 3 , and norbornadiene radical cation, 91 +. 

Actually, Equation (4-11) does not describe adequately the acid-base reaction (4-10) as we usually determine it experimentally. Reaction (4-10) can be considered to take place in two stages the first is the proton transfer or ion-pair generation reaction, and the second is the formation of free ions. These two stages for an acid can be expressed as... 

When ion-pair formation is significant, ion-pair generation must be differentiated from formation of free ions as indicated by Equations (4-13) and (4-14). The equilibrium constants corresponding to these two equations can be written just as for other equilibria, either in terms of activities (not including transfer activity coefficients) or approximately in terms of concentrations. If uncharged entities (including ion pairs) are assumed to have activity coefficients of unity, the constant for ion-pair generation SH2 A from an acid HA in a solvent SH, is written... 

In the following sections we consider the equilibria involved in the titration of a base B with perchloric acid, a reaction of practical interest the titration of acids with acetic acid as solvent is unimportant. The acidity of the solutions can be determined by an indicator or by the change in potential of an electrode responsive to free solvated protons. In water these two methods give the same results. In acetic acid, measurements of potential depend on the extent of formation of free solvated protons, whereas indicators respond to the extent of proton formation whether ion-paired or not thus two different measures of acidity are possible. The constant for ion-pair generation sometimes may be large, but the constant for dissociation to free ions never is. We discuss here the change in acidity during a titration as it would be obtained potentio-metrically and consider the behavior of indicators in Section 4-11. 

Farid s group describes the results of an investigation of the quantitative aspects of electron transfer sensitised arene photoreactions in this article the fate of the radical ion pair generated by irradiation of an electron acceptor in the presence of an arene donor was probed by measurement of the quantum yield of separation of the ion pair. Combination of this with the rate constant for separation of the ion pair allowed the determination of the rate constant for back electron transfer... 

Exciplex emission and photofragmentation reactions of contact ion pairs generated via quenching of cyanoaromatic singlet states by aminoalcohols is an example of a detailed study of an electron transfer reaction involving chemical change. ... 

The distinct spatial distribution of ion pairs generated by both techniques permits different aspects of geminate recombination to be investigated. 

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