Ion pairing reactions

Since the formation of high Rydberg states is not a major mechanism in Hj recombination, the presence of electric fields has litde or no effect on the total cross section. This is also the case with the ion-pair reaction since this proceeds via a... 

The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects for the free ion and ion-pair reactions in Table 15 show how ion pairing affects the structure of the transition state for the S/y2 reactions between benzyldimethylpheny-lammonium nitrate and sodium para-substituted thiophenoxides in methanol at 20 °C. The... 

A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

This limiting velocity ( obsd caic) = (3 1) X 10 s is only 0.06% of the value of fesoiv = 0.049 s. This provides evidence that the ion pair reaction intermediate is trapped by solvent less than once for every 1000 times the... 

The racemization of chiral substrate or the exchange of bridging and nonbridging oxygens during solvolysis (Scheme 8) may occur through an ion-pair reaction... 

Much is known about the lifetimes of carbocation intermediates of solvolysis, and these data have proven critical in the design of experiments to estimate absolute rate constants for reorganization of ion pairs. Consider reorganization of an ion-pair reaction intermediate that exchanges the positions of the nucleophilic atoms of the leaving group (, Scheme 9) and that occurs in competition with diffusional separation to free ions (k-d) which is much faster than addition of solvent to the ion pair. Ion-pair separation is irreversible and will result in formation of solvolysis reaction products s ). Reorganization of the ion pair will result in formation of isomerization reaction product and the yield of this reaction product will provide a measure of the relative rate constant... 

Solvolysis and isomerization may either proceed through a common ion-pair reaction intermediate, or the isomerization reaction may proceed by a separate concerted reaction pathway that avoids formation of this intermediate kcon Scheme 10). Hammett reaction constants of = —4.9 and pjt = —5.5 for reactions of X-4-0(S)CPb were calculated from the data in Table 1. The larger negative value... 

The rate constants for addition of solvent to X-4 approach the value of 10 s so that the ion pair reaction intermediate will undergo addition of... 

Another historically important reaction is the reorganization of chiral ion pair intermediates of solvolysis of a chiral substrate that leads to racemization of substrate during solvolysis. This reorganization competes with other reactions of the ion pair intermediate of solvolysis of a chiral substrate, so that the relative rate constant for ion-pair racemization can be obtained by determining the relative rates of formation of products from partitioning of the ion pair reaction intermediate, including the enantiomer of substrate (Scheme 14). 

Equation (16) gives the relationship between the rate constant ratio iso = 0.53 and the rate constants from Scheme 15 for partitioning of the ion pair reaction intermediate. Racemization and isomerization will proceed at similar rates, if inversion of the ion pair is much faster than the other reactions... 

Including multiple anions into speciation calculations greatly increases their complexity because the anions also imdergo multiple ion pairing reactions. The most important involve the formation of ion pairs with the major ions, Na, Ca, Mg, ... 

In nondissociating solvents, the main process consists of ion-pair reaction that results in the disengagement of nitrous acid and formation of trinitromethylated products ... 

With divalent counterions the two terminal carbanlons of the bolaform salts are associated with the same cation. The behavior of the n 2 salt again deviates from that of the other salts (31). In THF at 25°C the compound di(9-n-butyl fluorenyl)barlum (the two carbanlons are not connected by a chain) is a tight Ion pair while at -100°C the only stable species appears to be the mixed tight-loose Ion pair (reaction 11). 

Hummel and Luthjens [398] formed electron—cation pairs in cyclohexane by pulse radiolysis. With biphenyl added to the solvent, biphenyl cations and anions were formed rapidly on radiolysis as deduced from the optical spectra of the solutions. The optical absorption of these species decreased approximately as t 1/2 during the 500 ns or so after an 11ns pulse of electrons. The much lower mobility of the molecular biphenyl anion (or cation) than the solvated electron, es, (solvent or cation) increases the timescale over which ion recombination occurs. Reaction of the solvated electron with biphenyl (present in a large excess over the ions) produces a biphenyl anion near to the site of the solvated electron localisation. The biphenyl anion can recombine with the solvent cation or a biphenyl cation. From the relative rates of ion-pair reactions (electron-cation, electron—biphenyl cation, cation—biphenyl anion etc.), Hummel and Luthjens deduced that the cation (or hole) in cyclohexane was more mobile than the solvated electron (cf. Sect. 2.2 [352, 353]). 

Look up the equilibrium constant for the ion-pairing reaction Zn2+ + SOj ZnS04( <7) in Appendix J. 

Indirect evidence concerning intramolecular electron transfer has also been obtained by the observation of low-energy charge transfer absorption bands in mixed-valence complexes (reaction 8)14 even for outer-sphere electron transfer within ion pairs (reaction 9).15 The theoretical work of Hush makes it possible to use the energies and integrated intensities of such bands to estimate rates of intramolecular electron transfer.16... 

Spirin and co-workers (98, 101) have reported the polymerization of isoprene, butadiene and styrene in triethylamine. With styrene at —34° the rate is first order in polystyryllithium in the concentration range 0.4 to 3.2 x 10 2 molar. The observed propagation constant, 2.5 x 10-2 litre/mole sec should refer to the ion-pair reaction. With isoprene and butadiene in triethylamine the plots of rate against polyalkenyllithium concentration curve somewhat at higher concentrations. This could be due to some self-association because the concentrations used were rather high. Some uncertainty applies to the data on polyisoprene polymerization since the experiments may have been carried out at +30° (101) or at —30° (05). 

The poly(styryl)Iithium active center was found 179) to partially dissociate into the free ion in benzene-THF solution when the mole fraction of THF in the solvent mixtures was > ca. 50%. Solvents or solvent mixtures of lesser polarity generally do not lead to the formation of significant concentrations of the highly reactive free ions, i.e., the ion pair reaction appears to dominate. 

In Scheme 1-71, the reversible reaction is shifted to the right when the anion, X, is larger and the cation, M+, is smaller. For example, this shift to the right is 100% in the presence of Na+, PI v, and only 35% in the presence of Na+, F (Hamon Astruc 1988). The equilibrium takes place as an exchange reaction between the two ion pairs. Reactions of this type are based on the symbiotic-effect premise The interaction between a hard cation and a hard anion or between two soft ions is stronger than that between two ions of different types. 

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