Polarized ion pair

The ion-pair model stipulates that formation of an ion-pai. occurs in the aqueous mobile phase (16, 18, 20). The retention time is governed by the extraction coefficient of the ion-pair. A longer alkyl chain on the pairing agent simply makes a less polar ion-pair, with a resulting higher extraction coefficient, and the retention of the ion-pair increases as a result of its greater affinity for the stationary phase. 

This pictorial representation of a possible mechanism of action of a polar ion-pairing reagent has allowed prediction of what anions will be useful in decreasing the retention of a given protein. Alternatively, a nonpolar anion such as hexane sulfonate could be expected to associate with an ammonium ion present in the sample, with an increase in the nonpolar surface area and hence retention time. For example, apolipopro-tein C-III, which is readily eluted by an organic solvent gradient from a Cig column in the presence of phosphate anions, is retained indefinitely if butanesulfonate is used as the counterion. 

Fig. 4.19 Polarization effects (a) idealized ion pair with no polarization, (b) mutually polarized ion pair, (c) polarization sufficient to form covalent bond. Dashed lines represent hypothetical unpolarized ions.

CH2F2 is a polar molecule Ar is not. CsCl is a polar ion pair and is scattered more strongly by the polar CH2F2. 

It is obvious from this theoretical approach that the energetic stabilization of ion-pairs induced by interaction vith the electric field E becomes increasingly important as the size of the ions and their dipole moments (jj) increase. The more polar ion-pairs are more stabilized by E, clearly increasing from tight to loose ion-pairs, i.e. vith their dissociation and polarity. 

Positive ions dissociative ionization and dissociative rearrangements Negative ions electron capture and dissociative electron capture Both polarities ion-pair formation. 

The free volume model presumes that ion movement needs a free volume that is related to the polymer chain segment and ion types. However, it does not consider microstructure and cannot explain effects of polarization, ion pairs, and solvation degree on ionic conductivity. 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

The general formula for the initiator species can be written H B, where the degree of separation or ion pairing depends on the polarity of the medium and the possibility of specific solvation interactions. If we represent the equilibrium constant for the reactions in (6.DD) and (6.EE) by K, the initiator concentration can be written as... 

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. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

EA Carter, JT Elynes. Solute-dependent solvent force constants for ion pairs and neutral pairs m a polar solvent. J Phys Chem 93 2184-2187, 1989. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Figure 8-7 illustrates schematically the electronic natures of the polar ground state (an ion pair) and the less polar excited state. This is, therefore, a case of p.g. 

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). 

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