Pair Phenomena

To study the role of ion pairs, it is necessary to investigate electron-transfer reversibility in a solution and compare the results obtained with redox potentials of a donor and an acceptor. As a rule, ion-pairing phenomena define electrode processes too (Baizer and Lund 1983). However, the known equations for equilibrium calculations cannot take ion pairing into consideration because the equations do not contain ion-pair terms. One has to rely on experiments, which are able to take into account the equilibrium of electron transfer in solutions. 

Allylic systems have also provided fertile ground for investigation of ion-pair phenomena. Young, Winstein, and Goering established the importance of ion pairs in solvolysis of these compounds. They showed that ion pairs are responsible for the rearrangement of a,a-dimethylallyl chloride to y,y-dimethyl-allyl chloride (Equation 5.8).24 Goering s labeling methods have subsequently supplied a number of details about allylic ion-pair structure.25... 

Spectroscopic studies have also been devoted to the question of ion-pairing phenomena in silyl anions. It was found that in arylated silyl anions the 13C NMR as well as the 7Li NMR chemical shifts are only slightly influenced by the polarity of the solvent36,51. This is in clear contrast to the NMR-spectroscopic behavior of aryl-substituted carbanions, which show a marked solvent dependence. This was interpreted in terms of a significant covalent interaction between silicon and lithium in metalated silanes36. Further evidence for a significant covalent nature of the Si—Li bond arises from the observation of a scalar... 

Logically it would be expected that ion pairing phenomena would also affect the product distribution in base-promoted -elimination from 2-alkyl halides. In such systems one must distinguish between positional orientation, which refers to the relative proportions of 1-and 2-alkenes formed, and geometrical orientation, relating to the cis and trans 2-alkenes produced. 

The debate as to the exact model to describe the ion-pair phenomena will no doubt continue. Difficulties in devising a model arise from conflicting conclusions based on a large amount of experimental data. However, it is important to emphasise that theory guides experimentation. Therefore the importance of having a model is to understand the factors that control chromatographic retention, and thus, to aid in the prediction of the separating ability of a mobile phase. 

In the first part of this paper some peculiarities of electron transfer employing anions will be pinpointed as for instance ion pairing phenomena and electron photoejection. In the following section, the correlation between electron transfer rate constants and the reaction free energy, in cases where anions are reacting partners, will be considered. 

The studies related to the interactions of electronically excited arene molecules with tertiary amines have provided a basis for the present understanding of exciplexes and radical ion-pair phenomena [41,82], PET reactions of amines yield planar amine radical cations (Eq.20) which are deprotonated to give a-amino radicals (Eq.21) and usually cross-coupling (Eq.22) between radical pairs of donor-acceptor terminates the photoreaction [32a, 83]. Mechanistic studies revealed contact ion pair (CIP) intermediate for these reactions [84, 85]. 

Let us first ignore ion-pairing phenomena. With this assumption, the chloride transfer equilibria (13) correspond to the chloride transfer equilibria between two carbocations which were described in Scheme 5 and thus provide a comparison of the chloride affinities of metal halides and of carbocations. One would expect the right side of this equilibrium to be favored if MCI, is the stronger Lewis acid and the left side when R+ is the stronger Lewis acid. 

The ketyl radicals of thioxanthen-9-one (166) and -9-thione (167) have been described by several groups, and various ion-pairing phenomena were reported.The hyperfine splittings indicated in gauss, for the ion-pair of 166 with potassium in DME are those of Urberg and Kaiser, and those for the ion-pair of (167) with sodium in the same solvent are due to Aarons and Adam, who found a marked temperature dependence of the counterion hyperfine splittings that indicated is for 23°C. 

In previous sections the factors complicating comparisons between carbonium ions formed from diazonium ions with those formed in other solvolytic processes have been emphasized these are, chiefly, ion pair phenomena and conformational control of rearrangements and elimination. Some of the attempts to exclude or take explicit account of these factors are now described. 

The pairing of anion radicals with their counter cations is a wide-spread and now well documented phenomenon (Szwarc, 1972). In contrast, ion-pair phenomena in cation radical systems are not common, but see p. 222. Shifts in -values caused by halide-ion interaction with the tetramethoxybenzene cation radical have been reported (Sullivan, 1973), and halide-ion splittings of metalloporphyrin cation radical esr spectra have been demonstrated (Fajer et al., 1973). 

A very detailed study of the involvement of ion and ion-pair phenomena in the catalysis of lacZ j3-D-galactosidase of E. coli considers the nature of the D-galactosyl-enzyme complex, the number and nature of the molecular events leading to it, and the formation of the D-galactosyl-enzyme complex being faster than the hydrolysis of D-galactosyl derivatives in free solution. 

From data on relative rates of oxidation in methanol, ethanol and r-butanol in the presence of the corresponding alkoxides (Table 9) a correlation of the rate of oxidation with the pjST of the alcohol was inferred ". However, on changing the cation, large variations in rates were observed (Table 9), strongly suggesting that ion-pairing phenomena are involved. 

This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, United States Department of Energy under contract No. W-31-109-Eng-38. The authors wish to thank Dr. C. Bock and Prof. D. Stehlik for copies of their simulation program, and Drs. M. C. Thumauer, J. R. Norris, and H. Levanon for helpful discussions on radical pair phenomena. 

A review of nuclear magnetic properties of alkali metal nuclei is followed by a brief survey of solvation of alkali metal cations, their hydration especially. The chemistry of alkali metal anions is then evoked. Chemical shifts and relaxation rates will be described with emphasis on the predominant factors contributing to these observables. A brief history of alkali metal NMR will be followed by selected applications to ion pairing phenomena, inclusion complexes, preferential solvation, the chelate effect, and polyelectrolytes. 

A kinetic study of the nucleophilic substitution of Y-phenyl diphenylphosphinoth-ioates (125 X = S) by alkali metal ethoxides (MOEt M = Li, Na, K) in anhydrous ethanol at 298 K was reported (Scheme 39). Plots of pseudo-first-order rate constants ( obsd) versus [MOEt] showed distinct upwards (KOEt) and downwards (LiOEt) curvatures, respectively, pointing to the importance of ion-pairing phenomena and a differential reactivity of dissociated EtO and ion-paired MOEt. The reactivity of MOEt towards the 4-nitro compound (125 X = S, Y = 4-N02> increases in the order LiOEt < EtO < NaOEt < KOEt, which differs from the reactivity order LiOEt > NaOEt > KOEt > EtO reported previously for the reaction of 4-nitrophenyl diphenylphosphinate (125 X = O, Y = 4-NO2). Yukawa-Tsuno analysis revealed that the reactions of (125 X = S) and its P=0 analogue (125 X=O) with MOEt proceeded through the same concerted mechanism, which involved M+ ions increasing the electrophilicity of the reaction centre via TS (126). The P=0 compounds (125 X = O) were approximately 80-fold more reactive towards the dissociated EtO than the P=S compounds (125 X = S) (regardless of the electronic nature of substituent Y) but were up to 3100-fold more reactive towards ion-paired LiOEt. ... 

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