Electrostatic factor

A Schejter, I Aviram, T Goldkorn. The contribution of electrostatic factors to the oxidation-reduction potentials of c-type cytochi omes. In C Ho, ed. Electron Transport and Oxygen Utilization. New York Elsevier North-Holland, 1982, pp 95-109. 

Dack (4) has suggested that solvents be elassified on the basis of the product ep., the electrostatic factor. There seems to be no theoretical basis for this function [indeed, the dipole moment appears as ix in physical theory, as in Eqs. (8-10) and (8-6)], but ep. does contain more information than either of the quantities alone,... 

These changes in regio- and stereochemistry are likely due to conformation changes and electrostatic factors within the cavity. The intrazeolite oxidations can be improved by use of fluorocarbon solvents, owing to an enhanced lifetime of 102 and to improved occupancy of the cavity by hydrocarbons in this solvent.176... 

Adsorption on Siliceous Minerals. The adsorption of polyacrylamides on siliceous minerals in the presence of monovalent ions has been discussed previously (9, 10). While PAM adsorption is unaffected by monovalent ions since it is not governed by electrostatic factors, HPAM adsorption is increased due to reduction in electrostatic repulsion by charge screening. 

A combination of steric and electrostatic factors is presumably decisive with regard to the form of the acid most stable in sulfuric acid solution. The simple protonated form XX of benzoic acid is stabilized by resonance structures sterically prohibited in mesitoic acids. The ortho methyl groups of mesitoic acid would interfere with a coplanar dihydroxymethylene group. On the other hand, the inductive and resonance effects of the methyl groups help stabilize the acylium ion form of mesitoic acid as in the formulae XXI. In the case of 2,4,6-tribromobenzoic acid the steric effect and its abetting electronic effects are not sufficient, and this acid behaves like benzoic acid.17 >177... 

Both steric and electrostatic factors appear to favor the trans configuration, in which the bulkier and anionically charged F atoms are separated as far as possible. Nevertheless, 27a is actually found to be the energetically favored isomer by 0.80 kcal mol-1, a counterintuitive result that has been attributed to steric attraction. 83... 

To illustrate the dependence of hypervalency on steric and electrostatic factors, let us consider cu-additions of neutral or ionic ligands to [PtF4]2+, a duodectet-rule-conforming Pt(VI) species with Lewis-like formula... 

The competition between the OsvCl6 reaction with a neutral compound (A) and a negatively charged one (A-), respectively, which is the experimental situation in some of the spin trapping reactions mentioned above, was analysed by the Marcus treatment for some model cases in dichloromethane or acetonitrile. These data are shown in Table 7, giving the details of the calculations in order to illustrate the use of equations (20) and (21) and the importance of the electrostatic factors, particularly in dichloromethane. The assumptions behind the calculations are given in the table heading and footnotes. 

To see how the data can be used to provide insights into the spin trapping process, PBN would correspond to A with Ea = 1.5 V, and acetate ion to A with E° = 1.5 V (Table 5 gives 1.6 V in acetonitrile, and 1.5 V is therefore somewhat too low, but then it is presumably adequate for dichloromethane). In dichloromethane, the OsvCl6-PBN reaction is estimated to be very fast, more than 6 powers of ten faster than the OsvClg-acetate ion reaction, whereas in acetonitrile the absolute rates are still high but the ratio is only about 50. This difference resides only in the difference between electrostatic factors and illustrates the problems of understanding ET reactions in solvents of even lower dielectric constant such as benzene. 

DR. DAVID RORABACHER (Wayne State University) A point which is frequently overlooked is that the calculations generally applied for determining the extent of ion-pair (or outer-sphere complex) formation in substitution reactions may be overly simplistic. There are many types of interactions which tend to perturb the extent of outer-sphere complex formation relative to the purely statistical calculation commonly made which takes into account only the reactant radii and electrostatic factors. 

Since not only the electron-transfer step but also adsorption and some of the chemical steps involved in an electrode reaction take place in the layer, the whole process should be strongly influenced by polar factors. The orientation of polar-adsorbed species, such as ion-radicals in particular, is electrostatically influenced, and consequently, the stereochemistry of their reactions is also controlled by such kind of electrostatic factor. All these phenomena have been summarized in several monographs. The collective volume edited by Baizer and Lund (1983) is devoted to organic electrochemistry. This issue is closer to the scope of our consideration than its latest version edited by Lund and Hammerich (2001) (these editors have changed the invited authors and, consequently, the chapters included). 

The dielectric constants play a particular role in the characterization of solvents. Their importance over other criteria is due to the simplicity of electrostatic models of solvation and they have become a useful measure of solvent polarity. Since both the dielectric constant r and the dipole moment p are important complementary solvent properties, it has been recommended that organic solvents should be classified according to their electrostatic factor EF (defined as the product of 8 and p). 

Our proposed explanation for the induced pH sensitivity of the adsorbed sensitizing dye depended only on geometrical and electrostatic factors, not chemical... 

The rate constants for the association of proteins with one another and with other macromolecules are profoundly influenced by the geometry of the interaction and by electrostatic factors. Only a small part of each protein may be involved in the formation of a protein-protein complex, which imposes a bad steric factor on the reaction. Accordingly, protein-protein association rate constants may be as low as 104 s 1 M x (Table 4.1). But there is very fast association at > 5 X 109 s-1 AT 1 at low ionic strength for proteins that have complementary charged surfaces, such as bamase with its polypeptide inhibitor barstarfwhose, properties are discussed in Chapter 19), thrombin with its polypeptide inhibitor hirudin, and ferricytochrome c with ferrocytochrome b5. 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

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