Charge-localized anions

Anionic SN2 Reactions Involving Charge-localized Anions... 

The reactive species under these conditions consist of tight ion pairs involving the alkoxide anion from the carbohydrate (charge localized anion). The less reactive long chain methyl laurate leads to a later TS along the reaction coordinates and the magnitude of the microwave effect is therefore increased. 

The structure of the Ion pair complex Is also sensitive to the anion, especially the extent of charge delocalization and the presence of substituents close to the anionic site. Loose Ion pair formation requires a considerable increase In the inter-ionic Ion pair distance, and a charge localized anion, therefore. 

Polar mechanisms via polar transition states. If the polarity of a system is enhanced from the ground state to the transition state, acceleration can result from an increase in material-wave interactions during the course of the reaction. The most frequently encountered examples are unimolecular or bimolecu-lar reactions between neutral molecules (because dipoles are developed in the TS) and anionic reactions of tight ion-pairs, i.e. involving charge-localized anions (leading to ionic dissociation in the TS). 

Aicoholate anions have negative charge localized on a single oxygen. 

Wiberg split the stabilization of the energy barrier into two parts (a) electrostatic energy in the planar form and (b) delocalization. Electrostatic stabilization lowers the energy of the planar form because the charge is spread over three atoms rather than being localized on one carbon in the rotated form. An estimation of the electrostatic stabilization was made by calculating a model, methane, for the localized anion and yielded a 23 kealmol-1... 

Temkin was the first to derive the ideal solution model for an ionic solution consisting of more than one sub-lattice [13]. An ionic solution, molten or solid, is considered as completely ionized and to consist of charged atoms anions and cations. These anions and cations are distributed on separate sub-lattices. There are strong Coulombic interactions between the ions, and in the solid state the positively charged cations are surrounded by negatively charged anions and vice versa. In the Temkin model, the local chemical order present in the solid state is assumed to be present also in the molten state, and an ionic liquid is considered using a quasi-lattice approach. If the different anions and the different cations have similar physical properties, it is assumed that the cations mix randomly at the cation sub-lattice and the anions randomly at the anion sub-lattice. 

Fig. 5. Reaction scheme proposed for intramolecular exchange process in [V3Oi0]5. Local anionic charges are ignored. Subscripts t and c refer to terminal and central vanadium atoms. (Reprinted with permission from Andersson, I., Petterson, L., Hastings, J. J., Howarth, O. W., J. Chem. Soc. Dalton Trans. 1996, 3357.)...

The anion-radicals depicted in Scheme 3.62 were investigated by ESR and electron adsorption spectroscopy (Gregorius et al. 1992). The para isomer appears to behave completely different from the meta isomer. In full agreement with the results from MO theoretical calculations, the unpaired electron is delocalized over the whole para isomer, but confined to a stilbene unit in the meta isomer. The remaining parts in the meta isomer are uncharged. This spontaneous charge localization is not a consequence of steric hindrance, but follows from the role of the m-phenylene unit as a conjuga-tional barrier. 

Nelsen et al. (2007) have revealed one more aspect of solvent control over charge localization. Solvents with marked electron-donor properties contribute to charge localization in cation-radicals, whereas anion-radicals experience the same changes in better electron-accepting solvents. Thus, naked (non-ion-paired) anion-radicals of 4,4 -dinitrostilbene and 4,4 -dinitrotolane show the spectra of delocalized species in HMPA and THF, but essentially spectra of localized species in DMF, DMSO, and MeCN. 

The indole anion is resonance stabilized, with negative charge localized mainly on nitrogen and C-3. It can now participate as a nucleophile, e.g. in alkylation reactions. However, this can lead to iV-alkylation or C-alkylation at C-3. Which is the predominant product depends upon a number of... 

Loose ion pairs of such charge-localized oxyanion salts as potassium t-butoxlde may be difficult to form. This alkoxide is a tetrameric aggregate in THF (20). and crown addition breaks it down to the more reactive monomeric form. It is unlikely that with benzo-15-crown-5 a 2 1 crown-K loose ion pair can be formed similar to that found with potassium picrate or potassium fluorenyl. However, external complexation itself will slightly stretch the . bond, and this can have a profound effect on the anion reactivity (21). 

Now we assume that only electric work has to be done. We neglect for instance that the ion must displace other molecules. In addition, we assume that only a 1 1 salt is dissolved in the liquid. The electric work required to bring a charged cation to a place with potential -ip is W+ = etf). For an anion it is W = -ertp. The local anion and cation concentrations c and c+ are related with the local potential ij) by the Boltzmann factor c = c0 ee /fesT and c+ = co e e /kBT. Here, cq is the bulk concentration of the salt. The local charge density is... 

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