Anions electrostatic stabilization

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... 

Various anionic compounds such as halides, carboxylates or polyoxoanions, generally dissolved in aqueous solution, can establish electrostatic stabilization. Adsorption of these compounds onto the metallic surface and the associated countercations necessary for charge balance produces an electrical double-layer around the particles (Scheme 9.1). The result is a coulombic repulsion between the particles. At short interparticle distances, if the electric potential associated with the double layer is sufficiently high, repulsive forces opposed to the van der Waals forces will be significant to prevent particle aggregation. 

Figure 5.4 The chemical basis of the large free-energy change associated with ATP hydrolysis. Hydrolysis is accompanied by relief of the electrostatic repulsion between the negative charges on ATP by charge separation the resulting phosphate anion is stabilized by resonance, while the other product, ADP2+ releases a proton into a medium where [H+] is very low ( 10 7 M).

HS may alter the reactivities of bound substrates in a way similar to that of anionic surfactants (inhibiting base-catalyzed and accelerating acid-catalyzed reactions). These effects were attributed to electrostatic stabilization of the transition state for the acid catalysis in which the substrate becomes more positively charged, and to destabilization of the transition state for base-catalyzed hydrolysis in which the substrate becomes more negatively charged. 

However, the DLVO model cannot completely explain the stabiUzation properties of imidazoUum ILs towards the lr(0) nanoparticles as it treats counterions as mono-ionic point charges and was not designed to account for sterically stabiUzed systems. Together with the electrostatic stabilization provided by the intrinsic high charge of the IL, a steric type of stabilization can also be envisaged. This is due to the presence of anionic and cationic supramolecular aggregates of the type [(BMl),t(X),( ] [(BMl),t (X)J"T where BMl is the l- -butyl-3-methyUmidazoUum cation and X is the anion. 

The monoanionic species is most reactive, but its associated rate constant for intramolecular general acid catalysis is only 65 times greater than that for the unionized species. Most of the large rate enhancement in comparison with the dimethyl ester is due to participation by one carboxyl group, as is the case with the unionized acetal [77]. If the carboxylate anion of the monoanionic species is electrostatically stabilizing the incipient carbonium ion in the reaction [8], its effect on the rate must be small. 

Enolization of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl- 1-methylpyridinium ion (94) have been measured13811 and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate.13811 For (94), qh = 1.9 x 102 M-1 s 1, about 1.1 x 106 times that for a typical methyl ketone such as acetone. Thermodynamically, it is >108 times more acidic (pAa values of 11.1 vs 19.3). These increases in kinetic and thermodynamic acidity are derived from through-bond and through-space effects, and the implications for enzymatic catalytic sites with proximal, protonatable nitrogen are discussed. The results for (94) suggest a pAa value of 8.8 for (95), a value that cannot be measured directly due to competing hydrolysis. 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

Another type of ion pairing effect likely to influence the electron transfer reactivity of dianions is the so called triplet association (triplet being here taken in the sense of triple association between two cations and one dianion). When Coulombic interactions in such multiple ions are considered, the electrostatic stabilization is enhanced when the dianion charges are close together, an unexpected observation if the electrostatic repulsions within the dianion are considered [43]. Multiple association of this type probably affects the reductive reactivity of the anionic species. 

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