Anionic system

Just as anionic polymerizations show certain parallels with the free-radical mechanism, so too can cationic polymerization be discussed in terms of the same broad outline. There are some differences from the anionic systems, however, so the fact that both proceed through ionic intermediates should not be overextended. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

The discovery (116) of the base-promoted degradation of the isomeric closo-Q,. . cages provided one of the most important carborane anion systems, the isomeric nido-Q,. ][ , anions,... 

The author anticipates that the further development of transition metal catalysis in ionic liquids will, to a significant extent, be driven by the availability of new ionic liquids with different anion systems. In particular, cheap, halogen-free systems combining weak coordination to electrophilic metal centers and low viscosity with high stability to hydrolysis are highly desirable. 

In earlier works (4), we and others (5) have formulated and computed such non BO coupling strengths for several of the anion systems that have been studied experimentally including ... 

Li- analyt-gauss four-electron atomic system, importance of the orthogonalisation variation-iteration on an anionic system 27,28... 

Ne analyt-gauss variation-iteration on a ten electron anionic system 28... 

In the case of anionic systems, following the ideas of Diinitz [6], crystal structure (C) is a good model of an intermediate in which a nucleophile attacks a silicon atom included in a pentacoordinated anionic structure. Such an intermediate occurs during the nucleophilic displacement illustrated in Scheme 3. 

This result theoretically rationalizes our guidelines for achieving large %-d exchange interaction (1) the on-site Coulomb repulsions for both donor and anion molecules should be reduced, and (2) the transfer integral between them should be increased. Iron tetrahalides FeX (X = Cl, Br) satisfy both of these requirements and they are frequently used as magnetic anion systems. In fact, the... 

This provides us with an avenue for the direct evaluation of Fukui function without considering the cationic and anionic systems. However, this approach is not generally accepted due to many inherent limitations, and Fukui functions are evaluated from finite difference formula (Equation 12.10) using atomic charges. Once the Fukui function is evaluated following a particular scheme, condensed-to-atom softness can easily be evaluated from the relation (following Equation 12.7)... 

Several workers have attempted to use the common ion technique to depress [Pn+] and thus to achieve a monoeidic Pn+A system, as was done so successfully for anionic systems. However, because generally the solvents used for cationic polymerisations are much more polar, the KD of the chain-carriers and of the common-ion salts are considerably greater than in the anionic systems. Therefore the electro-chemical situation is likely to be complicated by triple ion formation and the effects of ionic strength on the KD and on the rate-constants, so that any results obtained by extrapolations to infinite ionic strength need to be scrutinised most carefully. 

Fig. 6 Histograms of C-0 distances in the -COO( ) and -COOH groups belonging to hydrogen bonded systems of the type [COOH]A---[COO ]A (A=anionic system) [column III in Table 2]...

A recent very good review of many potential anionic homoaromatic species was given by Balaban et al. (1987). Unlike the cationic systems discussed previously, where the homoaromaticity of several species has been firmly established, no anionic systems are currently recognized as being homoaromatic. 

By analogy with the homoaromatic homotropylium cation, a prototype anionic system would be the homocyclopentadienyl system [138a]. However, Olah et al. (1978) demonstrated by H and 13C NMR that this species is not the homoaromatic [138a] but exists in the form of the planar [138b]. Theoretical MINDO/3 (Olah et al., 1978) and STO-3G RHF (Birch et al., 1980) results are in agreement with the experimental results. 

Grutzner and Winstein (1968) proposed one of the more interesting homoaromatic anionic systems — the bicycloaromatic system [147]. Their evidence was equivalences in the lH NMR spectrum that could not be separated over a temperature range of -35°C to 100°C. If this molecule is undergoing a rapid conversion, then the barrier is less than 11.8 kcal mol-1. 

Electrochemical and Optical Sensing of Anion Systems by Transition Metal Based Receptors, Coord. Chem. Rev. 205, 131—55. 

In Fig. 6 we show how the electronic energies of the neutral and anionic systems vary along the C-S bond length with all other internal geometry parameters relaxed to minimize the neutral or anion energy, respectively. These plots allow us to examine where the anion s energy surface lies relative to that of the neutral, which obviously, is directly germane to attachment of a free electron to the a orbital of the C-S bond. 

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