Carboxylate anion electrostatic stabilization

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. 

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. 

FIGURE 17.9 Electrostatic stabilization may account for the acidity of carboxylic acids. The highly polar carbonyl group stabilizes the carboxylate anion. 

The fluorescence spectrum of the tris-acridine cryptand A-13 shows the characteristic monomer and excimer bands. Upon complexation with various organic anions (carboxylates, sulfonates, phosphates), the monomer band increases at the expense of the excimer band. The stability of the complexes depends on the contribution of the electrostatic and hydrophobic forces and on the structural complementarity. Stability constants of the complexes ranging from 103 to 107 have been measured. In particular, A-13 binds tightly to mono- and oligonucleotides, and it can discriminate by its optical response between a pyridimic and a purinic sequence. 

Polyammonium macrocycles and macropolycycles have been studied most extensively as anion receptor molecules. They bind a variety of anionic species (inorganic anions, carboxylates, phosphates, etc.) with stabilities and selectivities resulting from both electrostatic and structural effects. 

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