Penetrated ion pairs

The ESR spectrum of the sodium salt of dibenzoyl ketyl in THF shows splitting due to Na, whereas in the ESR spectrum of the sodium salt of the ketyl of 2,2,6,6-tetramethylhexane-3,4-dione in THE, there is no splitting whatever due to Na (Luckhurst and Orgel 1963). Although dibenzoyl ketyl can give rise to the contact (or penetrate) ion pair, the formation of such a type of species from the 3,4-dione is difficult, owing to steric hindrance. This semidione exists in the form of a free ion (or an ion pair separated by solvent molecules) compare the two structures given in Scheme 3.48. 

Organic ion radicals exist together with counterions and often form ion pairs. Since the pioneering works of Grunwald (1954), Winstein with co-authors (1954) and Fuoss and Sadek (1954), the terms contact, tight, or intimate ion pair and solvent-separated or loose ion pair have become well known in the chemical world. More recently, Marcus (1985) and Boche (1992) introduced other colloquial expressions, the solvent-shared ion pair and the penetrated ion pair. 

In solvent-separated ion pairs, the solvation shells of the cation and the anion touch each other in solvent-bridged ion pairs, the ions share solvent molecules. In contact ion pairs, the cation and the anion are bound directly to each other and are surrounded by a common solvation shell. In penetrated ion pairs, an empty space between edge groups in one ion of a salt is occupied to a certain degree by a counterion. The two latter types of ion pair may have quite a different electronic distribution than the corresponding naked ions. The following examples show the influence of ion-pair formation. 

The presence of the inverted region, however, has not yet been demonstrated [29], With the appropriate design (see below) some of these ions can be considered as a new type of an ion pair [42], a penetrated ion pair (PIP) in which one of the ions is buried inside the other. Research into PET in organized media is very active [40-41] and encompasses a variety of topics from semiconductors [43] and zeolites [44], through various photoconductive polymers [45] to PET-initiated polymerizations and depolymerizations [46] that are generally outside of the scope of this review. 

Another type of ion pairs, called penetrated ion pairs [402], has been found by studying the conductivity of tetraalkylammonium tetrafluoroborates (with variable alkyl-chain lengths) [399, 403], and the UV/Vis spectroscopic behaviour of salts with a trimethinium cyanine cation and the tetrakis(phenylethynyl)borate anion [404], in nondissociating solvents of low relative permittivity. Clearly, in solutions of such low relative permittivity any ionic species will be highly associated. However, it has been found that the ion pairs formed can be smaller than the sum of the van der Waals radii of the components. Clearly, the ions of the ion pair interpenetrate each other depending on their molecular structure in the first case, the BF4 ion penetrates into the voids between the alkyl chains of the tetraalkylammonium ion, and in the second case the cyanine cation penetrates into the crevices of the borate ion. 

Yang, X., Zaitsev, A., Sauerwein, B., Murphy, S., and Schuster, G.B., Penetrated ion pairs photochemistry of cyanine dyes within organic borates,. Am. Chem. Soc., 114,793,1992. 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

These results have been initially considered as evidence for specific ion adsorption at ITIES [71,72]. Its origin was ascribed to extensive ion pair formation between ions in the aqueous phase and ions in the organic phase [71] [cf. Eq. (20)], or to a penetration into the interfacial region [72]. The former model, which has been considered in this context earlier [60], allows one to interpret the enhanced capacity in terms of Eq. (22). Pereira et al. (74) presented more experimental data demonstrating the effect of electrolytes and proposed a simple model, which is based on the lattice-gas model of the liquid liquid interface [23]. Theoretical calculations showed that ion pairing can lead to an increase in the stored... 

Absorption of some highly ionized compounds (e.g., sulfonic acids and quaternary ammonium compounds) from the gastrointestinal tract cannot be explained in terms of the transport mechanisms discussed earUer. These compounds are known to penetrate the Upid membrane despite their low Upid-water partition coefficients. It is postulated that these highly lipophobic drugs combine reversibly with such endogenous compounds as mucin in the gastrointestinal lumen, forming neutral ion pair complexes it is this neutral complex that penetrates the Upid membrane by passive diffusion. 

Although the cation-anion interaction of metallocenium ions is very weak, the counteranion is likely to remain in proximity with the metal cation to form a contact ion pair in low-permittivity solvents such as toluene (commonly used in polymerization reactions). If the metal cation allows the counteranion to penetrate into the first coordination sphere, it can form an inner-sphere ion pair (ISIP). When the counteranion is relegated to the second coordinating sphere, the ion pair becomes an outer-sphere ion pair (OSIP), which is more ionic in nature than ISIPs. A schematic representation of the relationship between ISIPs and OSIPs is depicted in Scheme 2. This simple scheme shows us the principal elements that affect the cation-anion interactions (e.g., counteranion (Y ), ancillary ligands (L ), transition metal (M), and alkyl ligand (R)), and the subtle balance between these elements in the dynamic equilibria. 

Figure 17.1 Trails of ionization left by heavy charged particles (initial energy of 10 MeV/ nucleon) as they penetrate through a photographic plate (nuclear emulsion). The ions interact with the atomic electrons in the emulsion creating ion pairs to expose the emulsion and the hacks become visible after the Him is developed. Notice the straight-line tracks. (From Knoll, 2000.)...

Dye Fixation. On the dried fabric, the dye is only deposited on the fiber surface. It must penetrate into the fiber during a fixation step and be incorporated in the fiber by chemical reaction (reactive dyes), aggregation (vat, sulfur dyes), ion-pair formation (acid, cationic dyes), or in the form of a solid solution (disperse dyes). 

It should be mentioned that Marcus [5] uses the terms inner-sphere ion pair and outher-sphere ion pair for CIP and SSIP, respectively. Depending on the degree of penetration of the solvation shells one may further differentiate between SSIP and solvent shared ion pairs . However, a clear experimental assignment has not yet been performed. Therefore, we will use the designation CIP and SSIP... 

This observation, as well as other facts, led to the conclusion that ion pairs may exist in various forms which could differ greatly in their reactivities. The nature of ion pairs is revealed by various studies (34), and the ESR technique applied to paramagnetic ion pairs provides probably the most penetrating and powerful tool for such investigations. Indeed, the very first direct evidence for the existence of ion pairs as independent molecular species was furnished by Weissman s ESR studies of sodium naphthalenide (I, 2). In the following years the ESR method revealed many interesting phenomena which acquaint us with the thermo-... 

Studies on the acetate complexes of Nd(III) by Choppin et al. using the hypersensitive transition showed that the formation of the monoacetato complex did not alter the intensity of the band but produced a small red shift. Formation of the diacetato complex produced marked changes in intensity and red shift. These changes were interpreted as due to the penetration of the second acetate anion into the inner coordination sphere and causing a structural change [203]. These results may be interpreted as (i) initial addition of acetate forms an ion-pair complex, (ii) further addition of acetate results in entry of acetate into the inner coordination sphere of Nd(III) ion. Complex formation of Nd(III) with EDTA and NTA ligands was studied by the analysis of the bands due to 4G7/2 4Io/2 and... 

Using the principle of ion pair formation between ammonium cations and the phosphate anions of lipids, Matile et al.33 prepared 8, an amphiphilic polyamine dendrimer. Rather than acting as a membrane channel, 8 was expected to form reversible membrane defects in the lipid bilayer. The steroid moiety was expected to act as the hydrophobic anchor for bilayer orientation and steric bulk was expected to prevent the polyamine penetrating the bilayer. Proton transport was assessed in unilamellar vesicles using the pH-fluorescence technique in which the external pH was increased to 7.8 relative to the internal pH at 7.4. The results demonstrated that 8 was almost as active as gramicidin, and maximal flux was achieved in ca. 20 s. 

Fuoss adopted the concept of the electrostatic contact ion-pair [60] and considered the anion as a point charge that may also penetrate the cation-conducting sphere of radius a. The final expression for the ion-pairing equilibrium constant,... 

The ionization of an ionogen and its subsequent dissociation according to Eq. (2-13) can be further elaborated. Between the ion pair immediately formed on heterolysis of the covalent bond and the independently solvated free ions, there are several steps of progressive loosening of the ion pair by penetration of solvent molecules between the ions. At least four varieties of ion interactions representing different stages of dissociation have been postulated [96, 134, 138, 141] cf. Eq. (2-19) and Fig. 2-14. 

Whereas the spectral behavior of solvent-separated ion pairs and free ions is very similar, the UV/Vis spectra of contact and solvent-separated ion pairs are usually different from each other, as has been shown with sodium fluorenide [141, 164]. Due to the penetration of solvent molecules between the ion-pair couples, the direct influence of the metal cation on the r-electron system of the carbanion is lost. With increasing dissociation, the absorption maximum of sodium fluorenide in tetrahydrofuran solution is shifted bathochromically in the direction of the absorption maximum of the free... 

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