Close ion pair

An explanation was suggested for these solvent and support effects and this is represented in Fig. 19. Thus, in solvents with greater dielectric permittivity, e, the cationic complex is situated further from the clay surface and the stereoselectivities are therefore more similar to those obtained in homogeneous phase. On the other hand, in solvents with low e, close ion pairs are formed and the surface has a larger effect on the reaction. 

In order for a solvated ion to migrate under an electric field, it must be prevented from forming close ion pairs with its counterions by the solvating solvent. The effectiveness of the solvent molecule in shielding the interionic Coulombic attraction is closely related with its dielectric constant. The critical distance for the ion pair formation q is given by eq 4 according to Bjerrum s treatment, with the hypothesis that ion-pair formation occurs if the interionic distance is smaller than... 

The reduction of bivalent aryltin chloride by sodium anthracenide in THF gave a new way to a bulky distannylanion in the form of a stable very close ion-pair (equation 54)71. The X-ray crystal structure reveals a normal Sn-Sn distance (2.81 A) in the distannylanion and a Sn—Na bond length of 3.24 A. 

The chemistry of the elements is mainly that of ionic salts in the solid state and solvated cations. Although some lithium and even sodium compounds are soluble in organic solvents, such compounds as (LiCH3)4 have essentially ionic Li+ for sodium and potassium compounds, close ion pairing can occur, as discussed in later sections. 

The ability of clathrochelates to form ion pairs and covalently attached complexes is utilized in biochemistry [315-321], The stereoselectivity of the redox reactions of plastocyanine and horse heart cytochrome C with several cage complexes was reported in Ref 319. Studies on stereoselective electron transfer in different systems provide information on the importance of close ion pair association of a cage complex with protein in chiral discrimination. 

Notably, the close ion pairs 211 and ent-2ll are chiral species this holds even if the carbanionic framework were completely planar. Enantiomerization results in migration of the lithium cation from one face of the carbanion to the other one, and is facilitated by the formation of a stabilized separated ion pair 212. If no further chiral elements are interacting, such as chiral ligands at the lithium cation, the e.r. in the substitution products directly reflects the e.r. in the carbanionic intermediates 211/ent-211.In most cases, the rate of racemization is much greater than the rate of the substitution step, and the formation of racemic products is the result. 

Enantiomeric substitution products can be obtained with the corresponding enantiomeric ligands with the same rate of selectivity. In general, this is true in most cases, but sometimes different enantiomeric excess values are observed, depending on the ligand or the leaving group used. This memory effect can only be explained if the substitution does not proceed via a fully symmetrical it-allyl complex, but via a close ion pair [45]. 

The IR spectra of aqueous solutions of CsCl suggest the formation of aquated Cs /Cl close ion-pairs, solvated by 5.0 + 0.4 H2O molecules per ion-pair. ... 

Diastereomeric salt formation results from complete proton transfer between CSA and solute. There is rapid exchange between the free acid and base and that constituting a close ion-pair. Solvation of such systems can be problematic since by its nature a salt requires a reasonably polar solvent. However, such solvents tend to dissociate the close ion-pairs to give solvent-separated ion-pairs in which the stereochemically dependent interaction responsible for induction of anisochronicity is lost. This limitation can be overcome in some cases by using mixed achiral solvents such as dg-benzene and ds-pyridine. 

Chemisorption of Mo(C0)5 on magnesia is also considered to afford a Mo(C0)5 species initially [22] the binding of this fragment has been proposed to be via nucleophilic addition of a surface oxide to yield [(0C)5Mo(C02)] which then forms a close ion pair with a magnesium ion [36], Other, possibly oligomeric, metal carbonyl species are formed after heating to higher temperatures, while thermolysis at x 250°C results in complete decarbonylation. 

If this electrostatic treatment of the substituent effect of poles is sound, the effect of a pole upon the Gibbs function of activation at a particular position should be inversely proportional to the effective dielectric constant, and the longer the methylene chain the more closely should the effective dielectric constant approach the dielectric constant of the medium. Surprisingly, competitive nitrations of phenpropyl trimethyl ammonium perchlorate and benzene in acetic anhydride and tri-fluoroacetic acid showed the relative rate not to decrease markedly with the dielectric constant of the solvent. It was suggested that the expected decrease in reactivity of the cation was obscured by the faster nitration of ion pairs. 

The physical picture in concentrated electrolytes is more apdy described by the theory of ionic association (18,19). It was pointed out that as the solutions become more concentrated, the opportunity to form ion pairs held by electrostatic attraction increases (18). This tendency increases for ions with smaller ionic radius and in the lower dielectric constant solvents used for lithium batteries. A significant amount of ion-pairing and triple-ion formation exists in the high concentration electrolytes used in batteries. The ions are solvated, causing solvent molecules to be highly oriented and polarized. In concentrated solutions the ions are close together and the attraction between them increases ion-pairing of the electrolyte. Solvation can tie up a considerable amount of solvent and increase the viscosity of concentrated solutions. 

Thus, in contrast to an ionization process from a neutral substrate, which initially generates an intimate ion pair, deamination reactions generate a cation which does not have an anion closely associated with it. Furthermore, the leaving group, molecular nitrogen, is very stable so that little, if any, nutleophilic participation is needed for bond cleavage. The... 

Cub and the iron atom of cytochrome Og are also situated close to each other and are thought to share a ligand, which may be a cysteine sulfur (Figure 21.19). This closely associated pair of metal ions is referred to as a buiuclear center. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

However, mechanistic features not involved in the simplified mechanism of Scheme 2 can also play a role. In particular, interaction of the bromonium-bromide ion pair with its close solvent environment, which cannot be readily estimated from kinetics or product formation in bromination. An example of this control is shown in the following paragraph. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

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