Counterion solvation

For more complex mechanisms, picturesque names such as square, ladder, fence [18] or cubic schemes [20] have been selected. In redox polymer films, additional transport of counterions, solvation, and polymer reconfiguration are important and four-dimensional hyper-cubes are needed to describe the reactions [21]. 

Another feature of ethylene oxide polymerization is the participation of the polymer chain in counterion solvation. From this it follows that (1) the reaction is autoaccelerated at the initial stages, and (2) the sensitivity of kinetic parameters to the nature of solvent is low. 

Wyoming-type montmorillonite [layer charge x = 0.75, with 1/3 being tetrahedral charge sites (4)] bearing one monolayer of adsorbed water has a water/counterion molar ratio of 5 1/3, equivalent to a 10.4 m solution. For two or three water monolayers, the H20/counterion ratio increases proportionately and the equivalent solution molalities are 5.2 m or 3.46 m, respectively. Thus, from the perspective of counterion solvation, interlayer water on montmorillonite should be similar to a very concentrated aqueous solution. 

Many refinements were introduced into the various proposed explanations of steric control in anionic polymerizations over the last twenty years. Two important features of these mechanisms are (1) coordinations of the chain ends with the counterions, and (2) counterion solvations. 

Thus, because this reaction would be even more exothermic, it would be predicted based on the Hammond Postulate [107] that there would be less charge transfer onto the benzylic carbon in the transition state. The fact that p values in THF are much larger than in hydrocarbon solution can also be explained by the effect of solvent on the counterion solvation of the counterion would decrease the interaction of the counterion with the carbanion which would result in more delocalization of charge. Or in terms of the Winstein spectrum of ionic species [3, 108], solvation of the counterion and an increase in the dielectric constant of the medium would shift the structure towards a more ionic species. 

Keywords liquid-liquid extraction interface ionophore calixarene counterion solvation molecular dynamics immiscible liquids phase separation recognition organization. 

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... 

As expected, HTMAB made a respectable showing in these experiments. Trioctylmethylammonium chloride (TOMAC) and trioctylmetliylammonium bromide (TOMAB) outperformed all other catalysts. It was postulated that the three octyl groups were the proper length for solvation of the polymer while at the same time small enough to avoid sterically hindering the reaction. In order to determine if TOMAB could be used to catalyze PET depolymerization for more than one treatment cycle, the catalyst was recovered upon completion of one treatment and added to a second run for 60 min. Tetraethylammonium hydroxide (TEAOH) was studied as a catalyst in order to demonstrate the effect of hydroxide ion as a counterion. The percent PET conversion for the second cycle was 85.7% compared to a conversion of 90.4% for the first treatment cycle. 

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

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. 

It is unlikely that free carbanions exist in solution. Like carbocations, they usually exist as either ion pairs or they are solvated. " Among experiments that demonstrated this was the treatment of PhCOCHMe with ethyl iodide, where was Li ", Na", or K" . The half-lives of the reaction were for Li, 31 x 10 Na, 0.39 X 10 and K, 0.0045 x 10 , demonstrating that the species involved were not identical. Similar results were obtained with Li, Na, and Cs triphenylmethides (PhsC M Where ion pairs are unimportant, carbanions are solvated. Cram " demonstrated solvation of carbanions in many solvents. There may be a difference in the structure of a carbanion depending on whether it is free (e.g., in the gas phase) or in solution. The negative charge may be more localized in solution in order to maximize the electrostatic attraction to the counterion. ... 

It has been pointed out already that formation of a radical anion by a redox process in solution produces an ion pair and that any hopping of the electrons will thus be bound to the migration of the cation, which then becomes rate-limiting (Gerson et al., 1972, 1974, 1990). The, ion-pair structure of the radicals is mainly affected by the size of the counterions and the ion-solvating capability of the solvent (Hogen-Esch, 1977 Szwarc, 1968). 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

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