Stabilization coulombic repulsion

Ding C F, Wang X B and Wang L S 1998 Photoelectron spectroscopy of doubly charged anions intramolecular Coulomb repulsion and solvent stabilization J. Phys. Chem. A 102 8633... 

Fig. 10. Formation of the bipolaron (= diion) state in poly-p-phenylene upon reduction In the model it is assumed that the ionized states are stabilized by a local geometric distortion from a benzoid-like to a chinoid-Iike structure. Hereby one bipolaron should thermodynamically become more stable than two polarons despite the coulomb repulsion between two similar charges...

Dicarbonyl functions have been built into macrocyclic structures, and pKa values for the resulting macrocycles [60] have been determined (Alberts and Cram, 1979). When the open-chain model [62] is compared with the macrocycles [60], identical first pK values were found (pKa = 8.6). Thus for the diketones [60], no macrocyclic effect is noticeable. But for the dissociation of a second proton from the mono-aniorts of [60] much higher pKa values are found. To a certain extent. Coulomb repulsions (see Section 2) are probably the reason for this behaviour, but the large difference in the pKa values (ApKa = 2.9, see Table 26) argues for a special stabilization of the mono-anion. Again hydrogen bonds are not unreasonable. 

Ionic compounds such as halides, carboxylates or polyoxoanions, dissolved in (generally aqueous) solution can generate electrostatic stabilization. The adsorption of these compounds and their related counter ions on the metallic surface will generate an electrical double-layer around the particles (Fig. 1). The result is a coulombic repulsion between the particles. If the electric potential associated with the double layer is high enough, then the electrostatic repulsion will prevent particle aggregation [27,30]. 

The problems of the constancy of a and the site of reaction are closely linked. It is very convenient to assume that the charge on the micellar head groups is extensively neutralized by counterions which bind specifically to the micellar surface. In this way micellar stability is associated with a balance between hydrophobic attractions between apolar groups and coulombic repulsions of the ionic head groups which will be reduced by favorable interactions with the counterions in both the Stem and the diffuse Gouy-Chapman layers. It is the behavior of the counterions which is important in considerations of their chemical reactivity. 

Scheme 9.1 Schematic representation of electrostatic stabilization a coulombic repulsion between metal colloid particles.

Electrosteric stabilization can be also obtained from the couple ammonium (Bu4N+)/polyoxoanion (INWnNb C>62)- The significant steric repulsion of the bulky Bu4N+ countercation, when associated with the highly charged polyoxo-anion (coulombic repulsion), provides efficient electrosterical stability towards agglomeration in solution of the resultant nanocatalysts [2, 5, 6]. 

The DLVO theory, a quantitative theory of colloid fastness based on electrostatic forces, was developed simultaneously by Deryaguin and Landau [75] and Verwey and Overbeek [76], These authors view the adsorptive layer as a charge carrier, caused by adsorption of ions, which establishes the same charge on all particles. The resulting Coulombic repulsion between these equally charged particles thus stabilizes the dispersion. This theory lends itself somewhat less to non-aqueous systems. 

Examples of these effects can be found. In the case of the systems shown below, the cis isomer is known to be the more stable isomer127. This is especially striking for the dianion since there exists a large coulombic repulsion between the sulfur atoms. Here, however, the possibility of counterion coordination should not be disregarded as a potential source of the greater stabilization of the cis isomer. 

The stability of nuclides is characterized by several important rules, two of which are briefly discussed here. The first is the so-called synunetry rule, which states that in a stable nuclide with low atomic number, the number of protons is approximately equal to the number of neutrons, or the neutron-to-proton ratio, N/Z, is approximately equal to unity. In stable nuclei with more than 20 protons or neutrons, the N/Z ratio is always greater than unity, with a maximum value of about 1.5 for the heaviest stable nuclei. The electrostatic Coulomb repulsion of the positively charged protons grows rapidly with increasing Z. To maintain the stability in the nuclei, more... 

When the effective on-site Coulomb repulsive energy (Geff) of the solid composed of Tt-radical molecules is smaller than the bandwidth (W), then the solid becomes a half-filled metal provided that the molecules stack uniformly without dimerization and can be described by a band picture. So far, no such radical molecules have been prepared. In order to decrease Ues and stabilize radical molecules chemically, a push-pull effect and an extension of the re-system have been implemented, though a large U ff and high reactivity (polymerization) are stiU crucial for the metallic transport. Table 2 summarizes selected organic conductors of neutral 7t-radical molecules. 

In contrast to cyclopropane, the cyclobutane C C bonds are only slightly bent, so that in order for the proton to form a bond, it must come close to the positively charged carbon nuclei, leading to increased Coulombic repulsion. Similarly, an attempt to bond one of the carbons does not lead to an ion with any apparent stabilization. This attempt to bond to one of the carbons leads to a relatively unstable... 

In aqueous suspension, the stability is discussed in reference to the DLVO (Deryaguin-Landau-Verway-Overbeek) theory. Within this framework, all solid substances have a tendency to coagulate due to their large van der Waals attractive force. The coulombic repulsive force among colloidal particles more or less prevents this tendency. These two opposite tendencies determine the stability of suspensions. What kind of parameters are concerned in the present nonaqueous system, for which little is known about the stability This is an interest in this section. 

The 3D structure of 11P-HSD-2 shows that NAD+ has stabilizing interactions between the ribose hydroxyl and aspartic acid-91, serine-92, and threonine-112. Replacement of NAD+ with NADP+ reveals a coulombic repulsion between the 2 -phosphate group and aspartic acid-91. However, 11P-HSD type 2 lacks a nearby amino acid with a positively charged side chain that could compensate for the negative charge on aspartic acid-91. This explains the preference of 11 P-HSD-2 for NAD+. 

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