Adjacent anion sites

Phosphonium groups are well known for their ability to stabilize adjacent anionic sites (i.e., Wittig reagents), but the results with the dicationic species indicate that phosphonium groups can also destabilize adjacent cationic groups producing their superelectrophilic reactivities. 

Fig. 4. Contour plot of the potential-energy surface for oxygen-vacancy migration, showing the curved path between adjacent anion sites of a BO octahedron (in the ab plane), b Calculated migration energy as a function of the tolerance factor (from both the A and B site simulations)...

Figure 6.20 Curved path for oxygen vacancy migration between adjacent anion sites [25].

The term metalla-/3-diketonate refers to an anionic diacyl complex, such as L M(RCO)(R CO) , in which the two acyl ligands occupy adjacent coordination sites of the metal atom. Resonance stabilization of the negative charge by 7r-delocalization onto the two acyl ligands gives Lewis structures 1 and 2. The two acyl ligands acquire an acyl/carbene... 

The phosphonium cationic center is well known for its ability to stabilize adjacent anionic centers (i.e. Wittig reagents), however we have shown it to be capable of activating (or destabilizing) cationic electrophilic sites.15 As noted... 

The coupling between iron and anion binding is a critical component of transferrin function. In the absence of anion, transferrins bind Fe " weakly and nonspecifically. The putative anion site identified crystal-lographically is in an electrostatically positive region, adjacent to the side chain of an arginine and the amino terminus of an a helix. The anion site does not bridge the two domains, and so does not function directly as a latch that closes the cleft around the iron. It is possible that the anion may partially compensate for the presence of several basic residues in the cleft that enhance iron binding. 

To prevent such anionic effects, so-called anionic sites are conventionally used as additives for ISE membranes. In fact, upon addition of 84, the SHG response of membrane 80 to KSCN was improved (data not shown). This result suggests that the lipophilic anion assists the surface orientation of the 80-K complexes by inhibiting the uptake of SCN" from the adjacent aqueous solution. 

The foregoing bases are usually employed as their salts with maleic, tartaric or hydrochloric acid. They usually depress the CNS, producing drowsiness and other side-effects that may preclude the use of a particular drug, but responses of different patients vary. Since antihistamine activity is shown by many compounds of varied structure it is suggested that they do not fit the histamine receptor completely as histamine does, but merely cover the anionic site, with their aryl residues adsorbed on some adjacent area. These compounds do not influence histamine-induced gastric secretion, and it was concluded that the latter effect is produced by a different type of histamine receptor, termed the H2 receptor. Most other effects of histamine are attributed to the Hi receptor. 

The rhombohedral unit cells for rhodium and iridium trifluorides (44) contain two formula units. The structure can be related to the first structure type by considering anion positions, which here correspond to a hexagonal, close-packed array. There are no vacant anion sites and the cations occupy one-third of the octahedral holes. This leads to M—F—M angles of 132°, characteristic for filling adjacent, octahedral holes in a hexagonal close-packed lattice. Alternatively, the structure can be described as a linking of octahedra through all corners, but the octahedra are now tilted with respect to each other. 

The activation parameters for decomposition of several thiazole carboxylic acids have been reported and they are tabulated in Table 51. The lower enthalpy of activation for thiazole-2-carboxylic acid (V) compared to its 5-isomer (VI) may be explained on the basis of stability of the developing anionic site in the activated complex. In the 2-isomer the anionic site is developed between the two electron withdrawing atoms, nitrogen and sulfur. In contrast, only the nitrogen atom is adjacent to the developing anionic site in the 5-isomer. Decarboxylation of 2-thiazo-lylacetic acid apparently gives VII as the initial product which isomerizes to... 

Pedulli and co-workers have also studied anion-radicals in the thiazole system, including those from 2-nitrothiazole, 2,2 -bithiazolyl, 2-arylthiazoles, and bis(2-thiazolyl) ketone. Here the anion-radicals were generated by alkali-metal reduction in ethereal solvents and found to exist in contact ion-pairs. Whenever the organic component contained two adjacent binding sites, chelation of the counterion occurred and was revealed by examining the variation of its hyperfine splitting with temperature and solvent. The interaction of the ion-pairs with macrocyclic polyethers was also studied by ESR for certain of the chelating radicals. ... 

The EPR spectra in the Ams — 1 region for diradical anions 252 1 272 1 and diradical dianions 332 2 -352 2 in 2-MeTFlF/TFlF matrix showed typically four-line dipolar pattern assigned to S — 1 states with the values of D hc that were similar (but not identical) to those for the corresponding diradicals 11, 13, and 14.88 In conjunction with the EPR studies of selected diradical anions, this provided evidence for localization of spin density in diradical anions 252 1 272 1 and diradical dianions 332 2 -352 2 at the two adjacent triarylmethyl sites (Fig. 22).88 Also, the IT versus T plot of the EPR Ams — 2 signal drops off with decreasing temperature in the 10-80 K range. 

Ionomers are polymers that are functionalized with ionic groups (usually anionic sites) attached at various points along polymeric backbones that are not extensively crosslinked (1-2). Such materials have a tendency to form ionic domains in which the anionic groups and their associated cations are microphase separated from the typically hydrophobic portions of the polymer. Thus, the ionic domains formed are isolated by a medium of low dielectric constant (i.e. the polymeric backbone) although, in some cases, hydrophilic channels have been reported to connect adjacent ionic domains (3). The size and structures of these domains vary with the nature of the cation, the stoichiometry of the polymer, the degree of solvation of the system and the method of preparation. They can be as small as ion-pairscor small multiplets, but in some cases they have been reported to be in the 20-100 A" diameter range. 

Inoki et al. reacted poly(vinyl propionate), PVPr, with lithium diisopropy-lamide (LDA) in order to create anionic sites along the polymer chain, since LDA can replace a hydrogen atom from a methylene or methine group adjacent to a carbonyl group. Subsequent addition of MMA leads to the formation of a graft copolymer PVPr-g-PMMA [81] (Scheme 28). 

---