Ammonium-centered

Zwitterionic micelles of the sulfobetaine C16H33N+Me2(CH2)3SO 3 have effects very similar to those of cationic micelles (Table 7). This result is understandable if the substrate binds close to the quaternary ammonium center and the anionic sulfate moiety extends into the aqueous region. 

Figure 5. Some pseudorotaxane components containing units capable of exhibiting specific properties. The wirelike molecule 11H+, with an ammonium center and a photoactive anthracene unit as a stopper [40] the bipyridinium-containing wire 12 +, with a photo- and redox-active [Ru(bpy)3]2+ moiety [41] crown ether 13, incorporating a binaphthyl unit [42] cyclophane 14 +, which contains a [Ru(bpy)3]2+ complex and...

Figure 10. Radial distribution functions for nitrate-ammonium center-of-mass to center-of mass pairs (a) and the N-O H pairs (b) as functions of temperature at (a, c) 4.2 K and (b, d) 350 K. Adapted with permission from Ref 104. Labels Ni and N4 refer, respectively, to nitrate and ammonium nitrogen-atoms.

The sulfonate moiety is only weakly nucleophilic in water, and the effects of micellized SB3-16 upon hydrolyses are generally very similar to those of cationic micelles (Tables IV-VI). This behavior suggests that in both systems the substrates bind close to the quaternary ammonium centers and that the sulfonate moiety, like a micellar-bound counteranion, is in the water and therefore not interacting strongly with the substrate (65). The only exceptions to this generalization were observed with hydrolyses of some acid chlorides (Table V). In these reactions, the balance between bond making and breaking seems to be very sensitive to the reaction medium (58). 

The most interesting point is the selectivity of association and extraction toward different amines. As is seen from Table 1, the association constants for primary amines slightly decrease or remain nearly constant upon increasing the length of the radical. In combination with increasing ion-pair distribution constants, this leads to overall enhancing extraction. However, association and extraction are sensitive to the steric bulkiness at the ammonium center, at least for such an extreme case as /-butylamine. And, most... 

The work of Park and Simmons [92] has inspired many efforts into the research of ammonium- and polyammonium-based anion receptors and are the subject of numerous reviews [93,94] Here we will only look at receptors containing quaternary ammonium centers. These groups bind anions via electrostatic interactions only. 

---